Comparative study of the postoperative flow in the saphenous vein and internal mammary artery bypass grafts Robert I. Hamby, M.D. Agop Aintablian, M.D. Bo George Wisoff, M.D. Marvin L. Hartstein, M.D. New Hyde Park, Jamaica, and Stony Brook, N. Y.
Coronary artery revascularization is an accepted surgical approach for a select group of patients with symptomatic coronary artery disease. The two types of grafts commonly utilized for coronary artery bypass are saphenous vein autografts 1 and the internal m a m m a r y artery. 2 The advantages and disadvantages of the internal mammary artery bypass graft, as compared to the saphenous vein, have been discussed by others. 3-8 Comparative flow studies between the internal mammary artery and an autogenous vein used for coronary bypass have been investigated in the dog. 3-~ No similar study has been reported in man. The present study was designed to evaluate the postoperative coronary bypass flow in two groups of randomly selected patients receiving either autogenous saphenous vein or internal m a m m a r y artery bypass grafts to the left anterior descending artery. Methods
Any patient referred because of angina pectoris, and considered on the basis of selective coronary angiography a surgical candidate for a revascularization procedure, was considered for the present study. Selective coronary angiography had to demonstrate significant disease of the left anterior descending artery considered by the surgical From the Department of Medicine, Cardiology Division, and Department of Surgery, Cardiovascular Division, Long Island Jewish-Hillside Medical Center, New Hyde Park, Queens Hospital Center Affiliation, Jamaica, N. Y., and the School of Medicine, Health Sciences Center, State University of New York at Stony Brook, Stony Brook, N. Y. Received for publication Oct. 21, 1975. Reprint requests: Robert I. Hamby, M.D., Cardiology Division, Long Island Jewish-Hillside Medical Center, New Hyde Park, N. Y. 11040.
306
team suitable for revascularization. Only patients requiring a single or double bypass procedure were selected for this study.On the basis of these requirements, 88 patients were accepted and randomly divided on the basis of the hospital chart number into two groups. One group of 45 patients was selected for a saphenous vein bypass graft, whereas the remaining 43 patients were selected for an internal mammary artery bypass graft to the left anterior descending artery. Surgery, as previously described} was performed with total cardiopulmonary bypass, cooling to 32 ° , and electrically induced fibrillation. Intermittent aortic cross-clamping and bulldog clamp control of the coronary arteries assured a dry field. The left ventricle was continuously vented into the venous drainage via the right superior pulmonary vein. Internal m a m m a r y pedicles, when used, were taken down prior to bypass. Graft to coronary anastomoses were performed with one continuous suture and through a simple aortic incision over a partial occlusion clamp. Intraoperative bypass flow in the saphenous vein graft was measured with a precalibrated electromagnetic flow probe (Statham Instruments, Inc.). Zero blood flow was obtained by clamping the graft just distal to the flow probe and adjusting the flowmeter. All measurements were carried out in normothermia after the patient was off cardiopulmonary bypass and the blood pressure and heart rate stabilized. Flow in the internal mammary artery was measured prior to cardiopulmonary bypass by permitting the transected end of the internal m a m m a r y artery to bleed into a graduated cylinder for 15 seconds.
March, 1977, Vol. 93, No. 3, pp. 306-315
Postoperative flow in bypass grafts
Such a method to determine flow in the internal mammary is similar to the technique usually performed.2.8 Postoperative bypass flows were determined 2 weeks after surgery by angiographic methods described below. Of the 45 original patients undergoing saphenous vein bypass surgery to the left anterior descending artery, nine refused to be restudied and in one patient the graft was closed when studied. In the remaining 35 patients, the saphenous vein graft was patent. However, in eight of these, the angiographic study was not technically adequate for determining bypass flows by the method utilized. Six of these eight patients had studies with poor photographic results and the remaining two patients had overlaping opacified vessels adjacent to the distal portion of the graft. Because of these factors, postoperative flows were not feasible by the method utilized. Of the 43 patients randomly selected for internal m a m m a r y artery bypass surgery, five were eliminated from the study because of low intraoperative flows (--< 50 ml. per minute) in three and technical problems in isolating the internal m a m m a r y artery in two. It is generally recommended ~. 8 t h a t intraoperative flows in the internal m a m m a r y artery of less than 50 ml. per minute are not suitable for utilizing as grafts. Five other patients who underwent internal mammary artery bypass surgery refused postoperative study. In two restudied patients the internal mammary artery graft was closed; in another six the angiographic study was not technically adequate for determining bypass flow by the method utilized. Therefore, this study reports observations in 27 patients with saphenous vein and 25 with internal m a m m a r y artery bypass graft surgery to the left anterior descending artery. Selective cineangiographic studies of the bypass graft to the left anterior descending artery were performed in a right anterior oblique projection at that angle, indicating the long axis of the distal third of the graft to be parallel to the plane of the x-ray table. Cineangiograms were taken with a 35 mm. camera at 60 frames per second while 1 to 2 ml. of 75 per cent sodium meglumine diatrizoate was injected over 1 second with a power injector (Vitamonte-Hobbs). In all of the patients, care was taken to insure t h a t the catheter did not occlude the proximal orifice when performing the selective bypass angiogram. The
American Heart Journal
catheter was also promptly withdrawn after the completion of the injection. Cine filming continued until all the contrast medium washed out of the graft. The x-ray equipment included a dual field 6 inch 3,000 gain, 9 inch 6,000 gain image intensifier (General Electric) with a 35 mm. (Photomechanism) synchronous camera, utilizing a grid control x-ray tube. When the selective cineangiogram was completed, a grid of known dimension was positioned at the approximate location of the left ventricle and a short film strip taken to permit correction of errors due to magnification. Blood flow in the bypass graft was determined by cinedensitometric methods similar in principle to those described by Rutishauser and co-workers TM and Smith and associates. 11 The determination of blood flow in saphenous vein bypass grafts by similar methods has previously been describedY Cinedensitometric flow measurement is based on the derivation of a pair of indicator dilution curves from two separate points (S, and $2) along the bypass graft. From two such curves the mean transit time between points S, and $2 can be determined. When the distance between these two points and the mean diameter of the vessel is known, the flow can be determined by the following equation:
Q=
x~-x
--As x 6 0 At
where Q = flow in milliliters per minute, d = mean graft diameter (in millimeters), As = distance (in millimeters) between S~ and $2, and At -- mean transit time (in seconds) between the two points. This method does not require either knowledge of the amount of contrast medium injected or the concentration of contrast medium injected in the bypass graft in absolute units. However, the method is not valid if any vascular branches are present between the two indicator dilution points. A Quantimet 720 image-analyzing computer system (Imanco) incorporated with a Tagarno cine projector was utilized for recording the indicator dilution curves at two separate points in the distal third of the bypass graft. A high-resolution vidicon camera, using 720 line noninterlaced scan raster and a 10.6 frame per second scan rate, optically coupled the densitometric system to the cine projector. Each scan line was digitized into
307
H a m b y et al.
Table
I. Clinical profile of the two bypass
groups Saphenous vein group Number Age {yr.) Body surface area (M. ~) Duration of s y m p t o m s {yr.) Hypertension (%) Diabetes mellitus (%) Congestive heart failure
Internal mammary artery group
27 52 _+ 11" 1.88 ± 0.18" 4.1 ± 3.4*
25 55 ± 5 1.85 + 0.12 3.7 ± 3.3
29 11 13
28 12 12
(%) Cholesterol (mg./100 ml.) Triglyceride (mg,/100
ml.) Myocardialinfarctionby ECG (%) Enlarged heart on x-ray (%)
247 ± 51" 148 ± 47*
230 ± 37 160 ± 79
42
30
17
20
*Figures represent means -- 1 S.D.
approximately 1,000 picture points. The video signal resulting from the scan of each cine frame coupled to a densitometer module permits the operator to adjust threshold of detection in order to select features in the video image for quantitation based on their density or gray level. Setting the threshold for the detection of contrast medium is simplified by a real-time system display capability in which the operator can observe the effect of varying the threshold level. All video signals above the threshold level appear as an intensified overlay upon the original video image. Variation in the light transmission of the image from cine frame to frame does not affect data accuracy, which would normally result if the video signal varied about a fixed threshold. An auto sensitivity circuit maintains a constant video signal output level from the scanner through a compensating effect on system gain. Data from the densitometer is further controlled by a variable frame module. Only detected features within the variable frame (or detecting window) are processed. Horizontal and vertical dimensions of the variable frame (or window) are in picture points easily established by using a digiswitch. In the present study the variable frame or detecting window was 50 by 20 picture points where, depending on the magnification factor, one picture point was equivalent to 0.15 mm. The position of the variable frame within the image
308
area can likewise be controlled by a digi-switch. However, in order to automatically track the moving graft from cine frame to cine frame without operator control, between two points, $1 and S~ on the bypass graft, a 9,830 HP software system (Hewlett Packard) was utilized. This system utilizes the area value to track the moving graft and keeps it centered within the frame (or window) boundaries established by the operator at the onset of the analysis. The density value of the centered graft is determined at $1 and $2 and stored in the software system which may later be transferred to magnetic tape for processing. Following analysis of "n" cine frames, two dyedilution curves from point $1 and $2 at a distance AS are inscribed on an X-Y plotter (Hewlett Packard) simultaneously with the integral of each curve in order to determine the difference between the mean circulation time of the two indicator-dilution curves (At). Prior to an actual run the mean vessel diameter (d) between points S~ and $2 must be determined. A variable frame (or window) of 100 to 200 picture points high with a width adjusted to encompass the v.essel width is positioned over the distal segment of the bypass graft, using a cine frame in which the bypass graft is fully outlined by contrast material. The densitometer module is adjusted to outline at proper threshold setting the vessel encompassed by the variable frame. The sum of all Video picture points above the threshold and within the variable frame is automatically given as an area value and, when divided by the height of the variable frame, gives the mean graft diameter in picture points. Proper calibration converts picture points to millimeters. In a similar manner, for the purpose of this study, the mean diameter of the left anterior descending artery adjacent to the graft anastamosis was determined. Results were expressed as the mean _+ one standard deviation of the mean with statistical comparison between the saphenous vein bypass and internal mammary artery groups determined with the Student's unpaired t test. Results
The saphenous vein group (SVG), consisting of 27 patients, was compared clinically with the 25 patients in the internal m a m m a r y artery group (IMAG). Table I displays various clinical parameters in the two groups of patients. No significant
March, 1977, Vol. 93, No. 3
Postoperative flow in bypass grafts O0
150"
| 2.0-
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Flow(ml/min) In O.R.
O0
p-=:0.001
.
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-- ' ~ _
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INTERNAL MAMMARY SAPHENOUS ARTERY VEIN (N=25) (N=27) Fig. 1. Intraoperative bypass flow for the individual patients in each group studied. The horizontal bar is the mean for each group. Flow in the internal mammary artery was determined by a time volumetric collection, whereas flow in the saphenous vein graft was determined with a flowme~er Isee Methods for details). d i f f e r e n c e s w e r e n o t e d b e t w e e n t h e g r o u p s for age, d u r a t i o n o f s y m p t o m s , o r f r e q u e n c y o f h y p e r t e n s i o n , d i a b e t e s m e l l i t u s , c o n g e s t i v e h e a r t failure, t r a n s m u r a l m y o c a r d i a l i n f a r c t i o n , o r c a r d i o megaly. Serum cholesterol and triglyceride r e v e a l e d no g r o u p differences. I n t r a o p e r a t i v e b y p a s s flow s t u d i e s w e r e n o t s i g n i f i c a n t l y differe n t w h e n t h e t w o g r o u p s w e r e c o m p a r e d (Fig. 1). T h e m e a n i n t r a o p e r a t i v e b y p a s s flow in t h e S V G w a s 75 __ 27 ml: p e r m i n u t e ; t h e m e a n flow a t t h e t i m e o f s u r g e r y in t h e I M A G w a s 77 -- 24 m l . p e r minute.
American Heart Journal
INTERNAL MAMMARY ARTERY (N=25)
SAPHENOUS VEIN (N=27)
Fig. 2. A comparison between the internal mammary artery group and the saphenous vein group of the relation of graft diameter to the diameter of the left anterior decending artery (LAD). A value above 1 indicates that the graft diameter is larger than the diameter of the LAD. The horizontal bar is the mean value for each group.
At the time of postoperative angiographic study, the heart rate and mean aortic pressure w e r e 90 + 16 b e a t s p e r m i n u t e a n d 88 + 7 r a m . Hg, r e s p e c t i v e l y , in t h e S V G , n o t s i g n i f i c a n t l y different from the IMAG, where the heart rate w a s 90 _+ 15 b e a t s p e r m i n u t e a n d m e a n a o r t i c p r e s s u r e was 94 __ 14 m m . Hg. T h e m e a n g r a f t d i a m e t e r s were 3.0 _ 0.5 a n d 1.9 ~_ 0.3 m m . f o r t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). Relating the body surface area to the mean diameter of the internal mammary artery revealed a poor relationship ( R = 0.45, p < 0.025). T h e r a t i o o f t h e g r a f t d i a m e t e r t o t h e left a n t e r i o r d e s c e n d i n g a r t e r y d i a m e t e r w a s 1.9 ___ 0.3 a n d 1.2 __+ 0.2 for t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). I n 80 p e r c e n t o f ~he patients with internal mammary artery grafts, t h e r a t i o was less t h a n 1.4, w h e r e a s in 89 p e r c e n t of t h e p a t i e n t s w i t h s a p h e n o u s vein b y p a s s g r a f t s t h e r a t i o w a s o v e r 1.4 (Fig. 2). O f n o t e w a s t h e f i n d i n g t h a t a l m o s t 25 p e r c e n t ( 6 / 2 5 ) o f t h e p a t i e n t s in t h e i n t e r n a l m a m m a r y a r t e r y g r o u p had a graft of a smaller diameter than the d i a m e t e r of t h e l e f t a n t e r i o r d e s c e n d i n g a r t e r y (see Fig. 2, g r a f t d i a / L A D dia. < 1). Fig. 3 d e m o n -
309
H a m b y et al.
Fig. 3. Selective angiograms of the patients with saphenous vein grafts to the LAD (upper) and three patients with IMA grafts to LAD (lower). The arrow labeled I points to th e graft and the arrow labeled 2 points to the LAD. The numbem to the left indicate graft diameter (numerator) and LAD diameter (denominator).
strates three typical cases f r o m each of t h e groups studied, d e m o n s t r a t i n g t h e relationship of g r a f t d i a m e t e r to the d i a m e t e r of the left a n t e r i o r descending artery. In three of the 25 p a t i e n t s in the I M A G , angiographic studies d e m o n s t r a t e d atherosclerotic changes at the p r o x i m a l p o r t i o n of the internal m a m m a r y artery, in one of w h o m t h e changes involved at least 50 per cent of t h e wall lumen. T h e p o s t o p e r a t i v e flow, as d e t e r m i n e d b y roentgendensitometric methods, was 68 _+ 27 ml. per m i n u t e in the SVG a n d 46 _+ 16 ml. per m i n u t e i n the I M A G (p < 0.01). In 68 per c e n t of the patients with i n t e r n a l m a m m a r y a r t e r y grafts, the p o s t o p e r a t i v e flow was 50 ml. p e r m i n u t e or less, as c o m p a r e d with 33 per cent of the patients with s a p h e n o u s vein grafts (Fig. 4). As shown in Fig 5, in i n t r a o p e r a t i v e flow for t h e I M A G , with b u t one exception, o v e r e s t i m a t e d t h e subsequently calculated r o e n t g e n d e n s i t o m e t r i c flow determination, whereas t h e c o m p a r i s o n
310
between intraoperative flow with the a n g i o g r a p h ically determined flow in the SVG revealed a wide scatter a r o u n d the line of unity. Discussion
A l t h o u g h others 7. 8 h a v e p r e s e n t e d the a d v a n tages in utilizing the i n t e r n a l m a m m a r y a r t e r y over the saphenous vein for b y p a s s surgery, several problems were evident in t h e s m a l l group of p a t i e n t s r a n d o m l y selected for i n t e r n a l m a m m a r y a r t e r y surgery. In the original 43 patients selected for i n t e r n a l m a m m a r y a r t e r y graft surgery, three h a d free flows a t the t i m e of surgery of less t h a n 50 ml. per m i n u t e . I n t r a o p e r ative flows of less t h a n 50 ml. per m i n u t e in the internal m a m m a r y a r t e r y are n o t recommended 2. 8 as suitable for grafting. Therefore, in b o t h of these p a t i e n t s a s a p h e n o u s vein was utilized. I n two other patients, t e c h n i c a l problems arose in isolating t h e artery, necessitating the use of saphenous vein a u t o g r a f t s. I n two
March, 1977, Vol. 93, No. 3
Postoperative flow in bypass grafts
patients, graft closure was n o t e d at the time of restudy, requiring reoperation and the use of a saphenous vein graft. Of great interest were t h r e e patients in w h o m the proximal portion of the internal m a m m a r y a r t e r y d e m o n s t r a t e d atherosclerotic changes. In one of these, the a t h e r o m a s significantly compromised the wall lumen. Frazier and co-workers 13 have also observed atherosclerotic involvement of the i n t e r n a l m a m m a r y artery and c o m m e n t e d t h a t it m a y be m o r e frequent t h a n previously implied. T h e s e authors, on the basis of their experience, have recommended preoperative and intraoperative evaluation of the internal m a m m a r y a r t e r y prior to its use as a bypass graft. We concur with this a n d recommend selective internal m a m m a r y arteriographic studies at the time of the initial c o r o n a r y angiogram in any patient considered a c a n d i d a t e for internal m a m m a r y a r t e r y bypass surgery. In one patient not included in the present study, a left subclavian steal was present which was n o t clinically apparent. This negated the use of t h e left internal m a m m a r y a r t e r y for c o r o n a r y bypass surgery. It was also evident t h a t because of the tedious and time-consuming dissection required in isolating the internal m a m m a r y artery, such surgery could n o t be utilized in patients requiring p r o m p t institution of cardiopulmonary bypass, as in h y p o t e n s i o n during induction of anesthesia or emergency bypass surgery in the early stages of an acute m y o c a r d i a l infarction. T h e roentgenographic indicator-dilution technique for determining blood flow, in the present study, has been shown to be valid b o t h in a model circulation as well as in vivo experiments, when compared to flowmeters. 1°. 14 In vitro studies b y us have shown t h a t in tubes of the d i a m e t e r comparable to saphenous veins, there is an excellent correlation for flows of less t h a n 150 ml. per minute. Flows above 150 ml. per m i n u t e t e n d e d to be underestimated by the angiographic m e t h o d . Determination of flow in a bypass graft to the left anterior descending a r t e r y is ideally suited for this method, since the graft is an u n b r a n c h e d vessel and generally has a uniform d i a m e t e r with a relatively straight distal portion. Cinedensitometric determination of flow requires goodquality cineangiography, not only for deriving t h e indicator-dilution curves needed, b u t for obtaining an accurate m e a s u r e m e n t of graft diameter.
American Heart Journal
150.
Flow(ml/min) Roentgenographic Method 100• p'~O.01
~:
! 50-
INTERNAL MAMMARY SAPHENOUS ARTERY VEIN (N=25) (N=27) Fig. 4, Bypass flow. as determined postoperatively by roentgenographic methods (see Methods for details) in each group studied. The horizontal bar is the mean for each group.
When the diameter of various sizes of t u b i n g filled with H y p a q u e was determined by the m e t h o d utilized in this study, there was excellent agreement between the known diameters and the measured diameter. Changes in graft d i a m e t e r during systole and diastole, a l t h o u g h present, were either of small m a g n i t u d e or not perceptible. One of the major problems with cinedensitometric m e t h o d is the influence of b a c k g r o u n d density on the m e a s u r e m e n t of flow. COntrast medium in a vessel over a high b a c k g r o u n d density will show a different change in t o t a l field density, as compared to a low b a c k g r o u n d density
31 1
H a m b y et al.
\,
150-
/
• InternalMammaryArteryGraft o SaphenoosVeinGraft
100
° ~ o °
FI0w(ml/min) R0entgen0graphic Method
o J
0 0 ~ 0 m~
o
,~ oo 0
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Fig. 5. The relationship between intraoperative flow determination and postoperative roentgenographic flow determination for each patient in the groups of patients. The straight line is the line of unity, i.e., intraoperative flow equals postoperative roen~genographic flow. Note in the saphenous vein bypass group a wide scatter around the line of unity, whereas in the IMA group, in all but one patient, the points are below the line of unity. area. Thus, uneven background density along the course of a vessel will adversely affect the determination of flow in that vessel. Others have circumvented this problem by various methods.i0.12 In the present study, several cardiac cycles were observed before the graft was opacified and only those areas revealing equal background density were utilized for flow measurement. Because of this limitation, in 12 of 66 patients (18 per cent) flow studies were not performed, as it was not feasible to place the detecting window over two areas of the graft with equal background densities throughout the cardiac cycle. Prior studies 14 demonstrated t h a t the injection of contrast medium into the coronary artery may alter the coronary blood flow pattern. Rutishauser and co-workers, 1~using small injectate volumes, as in the present study, demonstrated good agreement for flow measured by roentgenographic technique in dogs, when compared to electromagnetic flowmeter determination.
312
The intraoperative flows in both the SVG and IMAG were comparable to those reported by others. 1'-2° However, the intraoperative flow in the internal m a m m a r y artery graft reported by Green 8 in his patients was considerably higher than in our group of patients. Such a difference in flow may, in part. be explained by differences in technique and the use of papaverine by Green prior to recording flows. There was a random relation between the intraoperative flows and the postoperative cinedensitometric flow studies in the group of patients with saphenous vein bypass grafts (Fig. 5). This is not an unexpected finding, since the physiological conditions {open chest, anesthesia, drugs) at the time o f the flow studies were not comparable. However, Fig. 5 shows t h a t for the group of patients with internal m a m m a r y artery grafts the postoperative cinedensitometric flows almost invariably underestimated the flows determined at the time of surgery. The most reasonable explanation for this finding is the method used for measuring flow in the internal
March, 1977, Vol. 93, No. 3
Postoperative flow in bypass grafts mammary artery at the time of surgery. Free graft flow, measured by a timed volumetric collection just prior to making the anastomosis, as commonly utilized, ~. ~ is a poor indicator of flow after completion of the anastomosis, since it represents flow taken with a negligible resistance at the distal ostia and an infinite runoff. Thus, such a flow determination would necessarily overestimate the flow taken after the anastomosis since, in the latter situation, the native coronary vascular resistance and runoff would tend to reduce the final flow. For the two groups studied, the mean differences between the intraoperative and postoperative flows were 7 and 31 ml. per minute for the SVG and IMAG, respectively. The difference in graft diameters between the two groups studied was readily apparent (Fig. 2 and 3). In all the patients with saphenous vein, the graft was always of larger diameter than the left anterior descending artery. In contrast, in six (24 per cent) of the patients with internal mammary artery bypasses, the graft diameter was less than the diameter of the left anterior descending artery. It was found t h a t the size of the internal mammary artery could not be predicted from the size of the patient. At the time of the postoperative flow study, there was no difference in the mean aortic pressure or heart rate for both groups of patients. The significantly higher flow in the SVG, as compared to the IMAG, may then possibly be due to the morphological difference between the two types of grafts utilized. The flow in the bypass graft is determined by the pressure gradient and vascular resistance across the entire vascular bed. Since the graft is placed in series with the native coronary bed, it will add to the total resistance of the vascular system as a direct result of its length and diameter. The greater length and smaller diameter of the internal m a m m a r y artery will increase the total resistance to a greater extent than the saphenous veins. Random selection of patients into two groups was necessary to eliminate any influence of the native coronary circulation as to vessel size and runoff. A review of all the preoperative coronary angiograms in the patients studied revealed no differences in the mean diameter of the left anterior descending artery at the anastomotic site or qualitative evaluation of runoff when the two groups of patients were compared. Thus, the lower flow in
American Heart Journal
the internal mamary artery group may, in part, be due to the higher resistance it contributes to the vascular system than the wider saphenous vein grafts. Another possibility which may explain the higher flow in the saphenous vein grafts, as compared to internal m a m m a r y artery grafts, may be that proposed by Wakabayashi and coworkers 6 from observations made in dog experiments. These authors observed a higher flow in grafts originating in the ascending aorta than the internal mammary artery graft or a graft originating in the descending aorta. In all three situations, the mean arterial pressure at the origin of the grafts was similar, whereas the mean diastolic pressures were significantly higher in the grafts originating from the ascending aorta and lowest in the internal mammary artery grafts. The diastolic pressure was sustained in the graft arising from the ascending aorta, whereas, at the orifice of the internal mammary artery, a rapid rise and fall in the diastolic pressure occurs, resulting in a higher mean diastolic pressure in the former and a lower mean diastolic pressure in the latter. Thus, the contour change in the pressure pulse as it is propagated along the aorta 21 may influence and determine the pressure gradient across the graft-coronary vascular bed. Since the coronary flow occurs mainly during diastole, the higher mean diastolic pressure in grafts originating from the ascending aorta would favor a higher flow, as compared to the internal m a m m a r y artery graft. An evaluation of the flow patterns in the grafts from the ascending aorta, as compared to internal mammary artery grafts, indicated that the difference in mean flow was due mainly to differences in diastolic flow.~ Morphological degenerative changes, including intimal fibrous proliferation, have been repeatedly described in saphenous vein grafts both experimentally3-~and clinically. 22, 23 Such changes will, in themselves, compromise the wall lumen of the venous graft. Thus, the differences in saphenous vein versus internal m a m m a r y artery flow noted in the early postoperative period may not be sustained after a longer follow-up period. In a small number of patients thus far followed this appears to be true in some cases. However, as already observed by us, as well as Frazier and coworkers, 13 the internal m a m m a r y artery is not immune to atherosclerotic changes, especially at
313
Hamby et al.
its p r o x i m a l p o r t i o n . T h u s , t h e u n c e r t a i n t y a b o u t t h e l o n g - t e r m f a t e of t h e g r a f t s u t i l i z e d for a o r t o c o r o n a r y b y p a s s s u r g e r y , as well as t h e i n h e r e n t p r o b l e m s o b s e r v e d i n t h e c l i n i c a l a p p l i c a t i o n s of these grafts, 7, 8, 19 w a r r a n t r e s e r v a t i o n s i n m a k i n g g e n e r a l r e c o m m e n d a t i o n s for t h e a d o p t i o n of o n e t y p e of graft over t h e other.
Summary P o s t o p e r a t i v e c o r o n a r y b y p a s s flow w a s e v a l u a t e d i n t w o g r o u p s of r a n d o m l y s e l e c t e d p a t i e n t s w i t h grafts to t h e left a n t e r i o r d e s c e n d i n g a r t e r y (LAD). S a p h e n o u s v e i n b y p a s s grafts were p l a c e d i n 27 p a t i e n t s a n d i n t e r n a l m a m m a r y a r t e r y grafts i n 25 p a t i e n t s . P o s t o p e r a t i v e flow s t u d i e s were p e r f o r m e d i n b o t h g r o u p s w i t h r o e n t g e n d e n s i t o m e t r i c m e t h o d s b a s e d o n t h e t r a n s i t t i m e of radiopaque media along the graft plus the m e a n graft d i a m e t e r . T h e r e was n o s i g n i f i c a n t difference b e t w e e n t h e t w o g r o u p s of p a t i e n t s for age, d u r a t i o n of s y m p t o m s , or t h e f r e q u e n c y of h y p e r tension, diabetes mellitus, prior m y o c a r d i a l i n f a r c t i o n , or c a r d i o m e g a l y . I n t r a o p e r a t i v e b y pass flows were 7 5 - 2 7 a n d 77 + 24 m l . p e r m i n u t e for t h e s a p h e n o u s v e i n g r o u p ( S V G ) a n d i n t e r n a l m a m m a r y artery group (IMAG), respectively. T h e r e was n o s i g n i f i c a n t d i f f e r e n c e i n t h e h e a r t r a t e or m e a n a o r t i c p r e s s u r e a t t h e t i m e of t h e r o e n t g e n d e n s i t o m e t r i c flow s t u d y . T h e m e a n graft d i a m e t e r s were 3.0 ± 0.5 a n d 1.9 _+ 0.3 m m . for t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). T h e r a t i o s of graft d i a m e t e r to L A D d i a m e t e r were 1.9 ± 0.3 a n d 1.2 ± 0.2 for t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). T h e r o e n t g e n d e n s i t o m e t r i c p o s t o p e r a t i v e flows were 68 ± 27 ml. per m i n u t e i n t h e S V G a n d 46 ± 16 m l . p e r m i n u t e i n t h e I M A G (p < 0.01). T h e p r e s e n t s t u d y i n d i c a t e s t h a t flow i n s i g n i f i c a n t l y h i g h e r i n s a p h e n o u s vein t h a n i n i n t e r n a l m a m m a r y a r t e r y b y p a s s e s a n d t h a t t h e difference i n flow m a y i n p a r t be e x p l a i n e d o n t h e basis of t h e g r a f t d i a m e ter. The authors wish to thank Irwin Hoffman, M.D., for his critical review of this manuscript and Brenda Hamby for the kind assistance given.
REFERENCES 1. Favaloro, R. G., Effier, D. R., Groves, L. K., Sheldon, W. C., and Riahi, M.: Direct myocardial revascularization with saphenous vein autograft. Clinical experience in 100 cases, Dis, Chest 56:279, 1969. 2. Green, G. E.: Internal mammary artery-to-coronary
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3.
4.
5.
6.
7. 8. 9.
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11.
12.
13. 14.
15. 16.
17.
18.
artery anastomosis. Three-year experience with 165 patients, Ann. Thorac. Surg. 14:260, 1972. Barner, H. B., Kaiser, G. C., Jellinek, M., Hahn, J. W., Amako, H., Ohtsubo, M., and Willman, V. L.: Aortocoronary vein graft and internal mammary-coronary anastomosis, Arch. Surg. 105:908, 1972. Rossiter, S. J., Brody, W. R., Kosek, J. C., Lipton, M. J., and Angell, W. W.: Internal mammary artery versus autogenous vein for coronary artery bypass graft, Circulation 50:1236, 1974. Hirose, T., and Marshall, D.: A comparative study of the direct internal mammary-to coronary artery anastomosis and the aortocoronary vein bypass graft, Ann. Thorac. Surg. 16:471, 1973. Wakabayashi, A., Beron, E., Lou, M. A., Mino, J. Y., Da Costa, I. A., and Connolly, J. E.: Physiological basis for the systemic-to coronary artery bypass graft, Arch. Surg. 100:17, 1970. Mills, N. L., and Ochsner, J. L.: Technique of internal mammary-to coronary arterial bypass, Ann. Thorac. Surg. 17:237, 1974. Green, G. E.: Rate of blood flow from the internal mammary artery, Surgery 70:809, 1971. Wisoff, B. G., Hartstein, M. L., Antablian, A., and Hamby, R. I.: Risk of coronary surgery. Two hundred consecutive patients with no hospital deaths, J. Thorac. Cardiovasc. Surg. 69:669, 1975. Rutishauser, W., Simon, H., Stucky, J. P., Schad, N., Nosida, G., and Wellauer, J.: Evaluation of roentgen cinedensitometry for flow measurement in models and in the intact circulation, Circulation 36:951, 1967. Smith, H. C., Sturm, R. E., and Wood, E. H.: Videodensitometric system for measurement of vessel blood flow, particularly in the coronary arteries, in man, Am. J. Cardiol. 32:144, 1973. Smith, H. C., Frye, R. L., Davis, G. D., Pluth, J. R., Sturm, R. E., and Wood, E. H.: Simultaneous indicator dilution curves at selected sites in the coronary circulation and determination of blood flow in coronary artery-saphenous vein grafts by roentgen videodensitometry, in Heintzen, P. H., editor: Roentgen-, cine- and videodensitometry: Fundamentals and applications for blood flow and heart volume determination, Stuttgart, 1971, Georg Thieme Verlag, pp. 152-157. Frazier, B. L., Flemma, R. J., Tector, Ao J., and Korns, M. E.: Atherosclerosis involving the internal mammary artery, Ann. Thorac. Surgo 18:305, 1974. Bfirsch, J., Johs, R., Kirbach, H., Schnfirer, C., and Heintzen, P.: Accuracy of videodensitometric flow measurement, in Heintzen, P. H., editor: Roentgen-, cine- and videodensitometry: Fundamentals and applications for blood flow and heart volume determination, Stuttgart, 1971, Georg Thieme Verlag, pp. 119-132. Guzman, S. V., and West, J. W.: Cardiac effects of intracoronary artery injections of various roentgenographic contrast media, AM. HEARTJ. 58:597, 1959. Rutishauser, W., Bussman, W., Noseda, G., Meier, W., and Willauer, J.: Blood flow measurement through single coronary arteries by roentgen densitometry. Part 1. A comparison of flow measured by a radiologic technique applicable in the intact organism and by electromagnetic flowmeter, AM. HEARTJ. 109:12, 1970. Grondin, C. M., Miere, C. M., Castonguay, Y. R., Lepage, G., and Grondin, P.: Blood flow through aorta-to coronary artery bypass grafts and early postoperative patency. A study of 100 patients, Ann. Thorac. Surg. 12:574, 1971. Danielson, G. K., Gau, G. T., and Davis, G. B.: Early
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results of vein bypass grafts for coronary artery disease, Mayo Clin. Proc. 48:487, 1973. 19. Bainer, H. B.: Blood flow in the internal mammary artery, AM. HEART J. 86:570, 1973. 20. Green, G. E., Spencer, F. C., Tice, D. A., and Stertzer, S. H.: Arterial and venous microsurgical bypass grafts for coronary artery disease, J. Thorac. Cardiovasc. Surg. 60:491, 1970. 21. Luchsinger, P. C., Snell, R. E., Patel, D. J., and Fry,
American Heart Journal
D. L.: Instantaneous pressure distribution along the human aorta, Circ. Res. 15:503, 1964. 22. Grondin, C. M., Meere, C., Castonguay, Y., Lepage, G., and Grondin, P.: Progressive and late obstruction of an aortocoronary venous bypass graft, Circulation 43:698, 1971. 23. Vlodaver, Z., and Edwards, J. E.: Pathologic changes in aortic-coronary arterial saphenous vein grafts, Circulation 44:719, 1971.
31 5
Coronary artery revascularization is an accepted surgical approach for a select group of patients with symptomatic coronary artery disease. The two types of grafts commonly utilized for coronary artery bypass are saphenous vein autografts 1 and the internal m a m m a r y artery. 2 The advantages and disadvantages of the internal mammary artery bypass graft, as compared to the saphenous vein, have been discussed by others. 3-8 Comparative flow studies between the internal mammary artery and an autogenous vein used for coronary bypass have been investigated in the dog. 3-~ No similar study has been reported in man. The present study was designed to evaluate the postoperative coronary bypass flow in two groups of randomly selected patients receiving either autogenous saphenous vein or internal m a m m a r y artery bypass grafts to the left anterior descending artery. Methods
Any patient referred because of angina pectoris, and considered on the basis of selective coronary angiography a surgical candidate for a revascularization procedure, was considered for the present study. Selective coronary angiography had to demonstrate significant disease of the left anterior descending artery considered by the surgical From the Department of Medicine, Cardiology Division, and Department of Surgery, Cardiovascular Division, Long Island Jewish-Hillside Medical Center, New Hyde Park, Queens Hospital Center Affiliation, Jamaica, N. Y., and the School of Medicine, Health Sciences Center, State University of New York at Stony Brook, Stony Brook, N. Y. Received for publication Oct. 21, 1975. Reprint requests: Robert I. Hamby, M.D., Cardiology Division, Long Island Jewish-Hillside Medical Center, New Hyde Park, N. Y. 11040.
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team suitable for revascularization. Only patients requiring a single or double bypass procedure were selected for this study.On the basis of these requirements, 88 patients were accepted and randomly divided on the basis of the hospital chart number into two groups. One group of 45 patients was selected for a saphenous vein bypass graft, whereas the remaining 43 patients were selected for an internal mammary artery bypass graft to the left anterior descending artery. Surgery, as previously described} was performed with total cardiopulmonary bypass, cooling to 32 ° , and electrically induced fibrillation. Intermittent aortic cross-clamping and bulldog clamp control of the coronary arteries assured a dry field. The left ventricle was continuously vented into the venous drainage via the right superior pulmonary vein. Internal m a m m a r y pedicles, when used, were taken down prior to bypass. Graft to coronary anastomoses were performed with one continuous suture and through a simple aortic incision over a partial occlusion clamp. Intraoperative bypass flow in the saphenous vein graft was measured with a precalibrated electromagnetic flow probe (Statham Instruments, Inc.). Zero blood flow was obtained by clamping the graft just distal to the flow probe and adjusting the flowmeter. All measurements were carried out in normothermia after the patient was off cardiopulmonary bypass and the blood pressure and heart rate stabilized. Flow in the internal mammary artery was measured prior to cardiopulmonary bypass by permitting the transected end of the internal m a m m a r y artery to bleed into a graduated cylinder for 15 seconds.
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Such a method to determine flow in the internal mammary is similar to the technique usually performed.2.8 Postoperative bypass flows were determined 2 weeks after surgery by angiographic methods described below. Of the 45 original patients undergoing saphenous vein bypass surgery to the left anterior descending artery, nine refused to be restudied and in one patient the graft was closed when studied. In the remaining 35 patients, the saphenous vein graft was patent. However, in eight of these, the angiographic study was not technically adequate for determining bypass flows by the method utilized. Six of these eight patients had studies with poor photographic results and the remaining two patients had overlaping opacified vessels adjacent to the distal portion of the graft. Because of these factors, postoperative flows were not feasible by the method utilized. Of the 43 patients randomly selected for internal m a m m a r y artery bypass surgery, five were eliminated from the study because of low intraoperative flows (--< 50 ml. per minute) in three and technical problems in isolating the internal m a m m a r y artery in two. It is generally recommended ~. 8 t h a t intraoperative flows in the internal m a m m a r y artery of less than 50 ml. per minute are not suitable for utilizing as grafts. Five other patients who underwent internal mammary artery bypass surgery refused postoperative study. In two restudied patients the internal mammary artery graft was closed; in another six the angiographic study was not technically adequate for determining bypass flow by the method utilized. Therefore, this study reports observations in 27 patients with saphenous vein and 25 with internal m a m m a r y artery bypass graft surgery to the left anterior descending artery. Selective cineangiographic studies of the bypass graft to the left anterior descending artery were performed in a right anterior oblique projection at that angle, indicating the long axis of the distal third of the graft to be parallel to the plane of the x-ray table. Cineangiograms were taken with a 35 mm. camera at 60 frames per second while 1 to 2 ml. of 75 per cent sodium meglumine diatrizoate was injected over 1 second with a power injector (Vitamonte-Hobbs). In all of the patients, care was taken to insure t h a t the catheter did not occlude the proximal orifice when performing the selective bypass angiogram. The
American Heart Journal
catheter was also promptly withdrawn after the completion of the injection. Cine filming continued until all the contrast medium washed out of the graft. The x-ray equipment included a dual field 6 inch 3,000 gain, 9 inch 6,000 gain image intensifier (General Electric) with a 35 mm. (Photomechanism) synchronous camera, utilizing a grid control x-ray tube. When the selective cineangiogram was completed, a grid of known dimension was positioned at the approximate location of the left ventricle and a short film strip taken to permit correction of errors due to magnification. Blood flow in the bypass graft was determined by cinedensitometric methods similar in principle to those described by Rutishauser and co-workers TM and Smith and associates. 11 The determination of blood flow in saphenous vein bypass grafts by similar methods has previously been describedY Cinedensitometric flow measurement is based on the derivation of a pair of indicator dilution curves from two separate points (S, and $2) along the bypass graft. From two such curves the mean transit time between points S, and $2 can be determined. When the distance between these two points and the mean diameter of the vessel is known, the flow can be determined by the following equation:
Q=
x~-x
--As x 6 0 At
where Q = flow in milliliters per minute, d = mean graft diameter (in millimeters), As = distance (in millimeters) between S~ and $2, and At -- mean transit time (in seconds) between the two points. This method does not require either knowledge of the amount of contrast medium injected or the concentration of contrast medium injected in the bypass graft in absolute units. However, the method is not valid if any vascular branches are present between the two indicator dilution points. A Quantimet 720 image-analyzing computer system (Imanco) incorporated with a Tagarno cine projector was utilized for recording the indicator dilution curves at two separate points in the distal third of the bypass graft. A high-resolution vidicon camera, using 720 line noninterlaced scan raster and a 10.6 frame per second scan rate, optically coupled the densitometric system to the cine projector. Each scan line was digitized into
307
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Table
I. Clinical profile of the two bypass
groups Saphenous vein group Number Age {yr.) Body surface area (M. ~) Duration of s y m p t o m s {yr.) Hypertension (%) Diabetes mellitus (%) Congestive heart failure
Internal mammary artery group
27 52 _+ 11" 1.88 ± 0.18" 4.1 ± 3.4*
25 55 ± 5 1.85 + 0.12 3.7 ± 3.3
29 11 13
28 12 12
(%) Cholesterol (mg./100 ml.) Triglyceride (mg,/100
ml.) Myocardialinfarctionby ECG (%) Enlarged heart on x-ray (%)
247 ± 51" 148 ± 47*
230 ± 37 160 ± 79
42
30
17
20
*Figures represent means -- 1 S.D.
approximately 1,000 picture points. The video signal resulting from the scan of each cine frame coupled to a densitometer module permits the operator to adjust threshold of detection in order to select features in the video image for quantitation based on their density or gray level. Setting the threshold for the detection of contrast medium is simplified by a real-time system display capability in which the operator can observe the effect of varying the threshold level. All video signals above the threshold level appear as an intensified overlay upon the original video image. Variation in the light transmission of the image from cine frame to frame does not affect data accuracy, which would normally result if the video signal varied about a fixed threshold. An auto sensitivity circuit maintains a constant video signal output level from the scanner through a compensating effect on system gain. Data from the densitometer is further controlled by a variable frame module. Only detected features within the variable frame (or detecting window) are processed. Horizontal and vertical dimensions of the variable frame (or window) are in picture points easily established by using a digiswitch. In the present study the variable frame or detecting window was 50 by 20 picture points where, depending on the magnification factor, one picture point was equivalent to 0.15 mm. The position of the variable frame within the image
308
area can likewise be controlled by a digi-switch. However, in order to automatically track the moving graft from cine frame to cine frame without operator control, between two points, $1 and S~ on the bypass graft, a 9,830 HP software system (Hewlett Packard) was utilized. This system utilizes the area value to track the moving graft and keeps it centered within the frame (or window) boundaries established by the operator at the onset of the analysis. The density value of the centered graft is determined at $1 and $2 and stored in the software system which may later be transferred to magnetic tape for processing. Following analysis of "n" cine frames, two dyedilution curves from point $1 and $2 at a distance AS are inscribed on an X-Y plotter (Hewlett Packard) simultaneously with the integral of each curve in order to determine the difference between the mean circulation time of the two indicator-dilution curves (At). Prior to an actual run the mean vessel diameter (d) between points S~ and $2 must be determined. A variable frame (or window) of 100 to 200 picture points high with a width adjusted to encompass the v.essel width is positioned over the distal segment of the bypass graft, using a cine frame in which the bypass graft is fully outlined by contrast material. The densitometer module is adjusted to outline at proper threshold setting the vessel encompassed by the variable frame. The sum of all Video picture points above the threshold and within the variable frame is automatically given as an area value and, when divided by the height of the variable frame, gives the mean graft diameter in picture points. Proper calibration converts picture points to millimeters. In a similar manner, for the purpose of this study, the mean diameter of the left anterior descending artery adjacent to the graft anastamosis was determined. Results were expressed as the mean _+ one standard deviation of the mean with statistical comparison between the saphenous vein bypass and internal mammary artery groups determined with the Student's unpaired t test. Results
The saphenous vein group (SVG), consisting of 27 patients, was compared clinically with the 25 patients in the internal m a m m a r y artery group (IMAG). Table I displays various clinical parameters in the two groups of patients. No significant
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Postoperative flow in bypass grafts O0
150"
| 2.0-
O0
Flow(ml/min) In O.R.
O0
p-=:0.001
.
GRAFTDIAMETER LAD DIAMETER
-- ' ~ _
!
°: J
I00'
O0
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O0 O0 m
m
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GRAFT
0000
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INTERNAL MAMMARY SAPHENOUS ARTERY VEIN (N=25) (N=27) Fig. 1. Intraoperative bypass flow for the individual patients in each group studied. The horizontal bar is the mean for each group. Flow in the internal mammary artery was determined by a time volumetric collection, whereas flow in the saphenous vein graft was determined with a flowme~er Isee Methods for details). d i f f e r e n c e s w e r e n o t e d b e t w e e n t h e g r o u p s for age, d u r a t i o n o f s y m p t o m s , o r f r e q u e n c y o f h y p e r t e n s i o n , d i a b e t e s m e l l i t u s , c o n g e s t i v e h e a r t failure, t r a n s m u r a l m y o c a r d i a l i n f a r c t i o n , o r c a r d i o megaly. Serum cholesterol and triglyceride r e v e a l e d no g r o u p differences. I n t r a o p e r a t i v e b y p a s s flow s t u d i e s w e r e n o t s i g n i f i c a n t l y differe n t w h e n t h e t w o g r o u p s w e r e c o m p a r e d (Fig. 1). T h e m e a n i n t r a o p e r a t i v e b y p a s s flow in t h e S V G w a s 75 __ 27 ml: p e r m i n u t e ; t h e m e a n flow a t t h e t i m e o f s u r g e r y in t h e I M A G w a s 77 -- 24 m l . p e r minute.
American Heart Journal
INTERNAL MAMMARY ARTERY (N=25)
SAPHENOUS VEIN (N=27)
Fig. 2. A comparison between the internal mammary artery group and the saphenous vein group of the relation of graft diameter to the diameter of the left anterior decending artery (LAD). A value above 1 indicates that the graft diameter is larger than the diameter of the LAD. The horizontal bar is the mean value for each group.
At the time of postoperative angiographic study, the heart rate and mean aortic pressure w e r e 90 + 16 b e a t s p e r m i n u t e a n d 88 + 7 r a m . Hg, r e s p e c t i v e l y , in t h e S V G , n o t s i g n i f i c a n t l y different from the IMAG, where the heart rate w a s 90 _+ 15 b e a t s p e r m i n u t e a n d m e a n a o r t i c p r e s s u r e was 94 __ 14 m m . Hg. T h e m e a n g r a f t d i a m e t e r s were 3.0 _ 0.5 a n d 1.9 ~_ 0.3 m m . f o r t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). Relating the body surface area to the mean diameter of the internal mammary artery revealed a poor relationship ( R = 0.45, p < 0.025). T h e r a t i o o f t h e g r a f t d i a m e t e r t o t h e left a n t e r i o r d e s c e n d i n g a r t e r y d i a m e t e r w a s 1.9 ___ 0.3 a n d 1.2 __+ 0.2 for t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). I n 80 p e r c e n t o f ~he patients with internal mammary artery grafts, t h e r a t i o was less t h a n 1.4, w h e r e a s in 89 p e r c e n t of t h e p a t i e n t s w i t h s a p h e n o u s vein b y p a s s g r a f t s t h e r a t i o w a s o v e r 1.4 (Fig. 2). O f n o t e w a s t h e f i n d i n g t h a t a l m o s t 25 p e r c e n t ( 6 / 2 5 ) o f t h e p a t i e n t s in t h e i n t e r n a l m a m m a r y a r t e r y g r o u p had a graft of a smaller diameter than the d i a m e t e r of t h e l e f t a n t e r i o r d e s c e n d i n g a r t e r y (see Fig. 2, g r a f t d i a / L A D dia. < 1). Fig. 3 d e m o n -
309
H a m b y et al.
Fig. 3. Selective angiograms of the patients with saphenous vein grafts to the LAD (upper) and three patients with IMA grafts to LAD (lower). The arrow labeled I points to th e graft and the arrow labeled 2 points to the LAD. The numbem to the left indicate graft diameter (numerator) and LAD diameter (denominator).
strates three typical cases f r o m each of t h e groups studied, d e m o n s t r a t i n g t h e relationship of g r a f t d i a m e t e r to the d i a m e t e r of the left a n t e r i o r descending artery. In three of the 25 p a t i e n t s in the I M A G , angiographic studies d e m o n s t r a t e d atherosclerotic changes at the p r o x i m a l p o r t i o n of the internal m a m m a r y artery, in one of w h o m t h e changes involved at least 50 per cent of t h e wall lumen. T h e p o s t o p e r a t i v e flow, as d e t e r m i n e d b y roentgendensitometric methods, was 68 _+ 27 ml. per m i n u t e in the SVG a n d 46 _+ 16 ml. per m i n u t e i n the I M A G (p < 0.01). In 68 per c e n t of the patients with i n t e r n a l m a m m a r y a r t e r y grafts, the p o s t o p e r a t i v e flow was 50 ml. p e r m i n u t e or less, as c o m p a r e d with 33 per cent of the patients with s a p h e n o u s vein grafts (Fig. 4). As shown in Fig 5, in i n t r a o p e r a t i v e flow for t h e I M A G , with b u t one exception, o v e r e s t i m a t e d t h e subsequently calculated r o e n t g e n d e n s i t o m e t r i c flow determination, whereas t h e c o m p a r i s o n
310
between intraoperative flow with the a n g i o g r a p h ically determined flow in the SVG revealed a wide scatter a r o u n d the line of unity. Discussion
A l t h o u g h others 7. 8 h a v e p r e s e n t e d the a d v a n tages in utilizing the i n t e r n a l m a m m a r y a r t e r y over the saphenous vein for b y p a s s surgery, several problems were evident in t h e s m a l l group of p a t i e n t s r a n d o m l y selected for i n t e r n a l m a m m a r y a r t e r y surgery. In the original 43 patients selected for i n t e r n a l m a m m a r y a r t e r y graft surgery, three h a d free flows a t the t i m e of surgery of less t h a n 50 ml. per m i n u t e . I n t r a o p e r ative flows of less t h a n 50 ml. per m i n u t e in the internal m a m m a r y a r t e r y are n o t recommended 2. 8 as suitable for grafting. Therefore, in b o t h of these p a t i e n t s a s a p h e n o u s vein was utilized. I n two other patients, t e c h n i c a l problems arose in isolating t h e artery, necessitating the use of saphenous vein a u t o g r a f t s. I n two
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Postoperative flow in bypass grafts
patients, graft closure was n o t e d at the time of restudy, requiring reoperation and the use of a saphenous vein graft. Of great interest were t h r e e patients in w h o m the proximal portion of the internal m a m m a r y a r t e r y d e m o n s t r a t e d atherosclerotic changes. In one of these, the a t h e r o m a s significantly compromised the wall lumen. Frazier and co-workers 13 have also observed atherosclerotic involvement of the i n t e r n a l m a m m a r y artery and c o m m e n t e d t h a t it m a y be m o r e frequent t h a n previously implied. T h e s e authors, on the basis of their experience, have recommended preoperative and intraoperative evaluation of the internal m a m m a r y a r t e r y prior to its use as a bypass graft. We concur with this a n d recommend selective internal m a m m a r y arteriographic studies at the time of the initial c o r o n a r y angiogram in any patient considered a c a n d i d a t e for internal m a m m a r y a r t e r y bypass surgery. In one patient not included in the present study, a left subclavian steal was present which was n o t clinically apparent. This negated the use of t h e left internal m a m m a r y a r t e r y for c o r o n a r y bypass surgery. It was also evident t h a t because of the tedious and time-consuming dissection required in isolating the internal m a m m a r y artery, such surgery could n o t be utilized in patients requiring p r o m p t institution of cardiopulmonary bypass, as in h y p o t e n s i o n during induction of anesthesia or emergency bypass surgery in the early stages of an acute m y o c a r d i a l infarction. T h e roentgenographic indicator-dilution technique for determining blood flow, in the present study, has been shown to be valid b o t h in a model circulation as well as in vivo experiments, when compared to flowmeters. 1°. 14 In vitro studies b y us have shown t h a t in tubes of the d i a m e t e r comparable to saphenous veins, there is an excellent correlation for flows of less t h a n 150 ml. per minute. Flows above 150 ml. per m i n u t e t e n d e d to be underestimated by the angiographic m e t h o d . Determination of flow in a bypass graft to the left anterior descending a r t e r y is ideally suited for this method, since the graft is an u n b r a n c h e d vessel and generally has a uniform d i a m e t e r with a relatively straight distal portion. Cinedensitometric determination of flow requires goodquality cineangiography, not only for deriving t h e indicator-dilution curves needed, b u t for obtaining an accurate m e a s u r e m e n t of graft diameter.
American Heart Journal
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When the diameter of various sizes of t u b i n g filled with H y p a q u e was determined by the m e t h o d utilized in this study, there was excellent agreement between the known diameters and the measured diameter. Changes in graft d i a m e t e r during systole and diastole, a l t h o u g h present, were either of small m a g n i t u d e or not perceptible. One of the major problems with cinedensitometric m e t h o d is the influence of b a c k g r o u n d density on the m e a s u r e m e n t of flow. COntrast medium in a vessel over a high b a c k g r o u n d density will show a different change in t o t a l field density, as compared to a low b a c k g r o u n d density
31 1
H a m b y et al.
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Fig. 5. The relationship between intraoperative flow determination and postoperative roentgenographic flow determination for each patient in the groups of patients. The straight line is the line of unity, i.e., intraoperative flow equals postoperative roen~genographic flow. Note in the saphenous vein bypass group a wide scatter around the line of unity, whereas in the IMA group, in all but one patient, the points are below the line of unity. area. Thus, uneven background density along the course of a vessel will adversely affect the determination of flow in that vessel. Others have circumvented this problem by various methods.i0.12 In the present study, several cardiac cycles were observed before the graft was opacified and only those areas revealing equal background density were utilized for flow measurement. Because of this limitation, in 12 of 66 patients (18 per cent) flow studies were not performed, as it was not feasible to place the detecting window over two areas of the graft with equal background densities throughout the cardiac cycle. Prior studies 14 demonstrated t h a t the injection of contrast medium into the coronary artery may alter the coronary blood flow pattern. Rutishauser and co-workers, 1~using small injectate volumes, as in the present study, demonstrated good agreement for flow measured by roentgenographic technique in dogs, when compared to electromagnetic flowmeter determination.
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The intraoperative flows in both the SVG and IMAG were comparable to those reported by others. 1'-2° However, the intraoperative flow in the internal m a m m a r y artery graft reported by Green 8 in his patients was considerably higher than in our group of patients. Such a difference in flow may, in part. be explained by differences in technique and the use of papaverine by Green prior to recording flows. There was a random relation between the intraoperative flows and the postoperative cinedensitometric flow studies in the group of patients with saphenous vein bypass grafts (Fig. 5). This is not an unexpected finding, since the physiological conditions {open chest, anesthesia, drugs) at the time o f the flow studies were not comparable. However, Fig. 5 shows t h a t for the group of patients with internal m a m m a r y artery grafts the postoperative cinedensitometric flows almost invariably underestimated the flows determined at the time of surgery. The most reasonable explanation for this finding is the method used for measuring flow in the internal
March, 1977, Vol. 93, No. 3
Postoperative flow in bypass grafts mammary artery at the time of surgery. Free graft flow, measured by a timed volumetric collection just prior to making the anastomosis, as commonly utilized, ~. ~ is a poor indicator of flow after completion of the anastomosis, since it represents flow taken with a negligible resistance at the distal ostia and an infinite runoff. Thus, such a flow determination would necessarily overestimate the flow taken after the anastomosis since, in the latter situation, the native coronary vascular resistance and runoff would tend to reduce the final flow. For the two groups studied, the mean differences between the intraoperative and postoperative flows were 7 and 31 ml. per minute for the SVG and IMAG, respectively. The difference in graft diameters between the two groups studied was readily apparent (Fig. 2 and 3). In all the patients with saphenous vein, the graft was always of larger diameter than the left anterior descending artery. In contrast, in six (24 per cent) of the patients with internal mammary artery bypasses, the graft diameter was less than the diameter of the left anterior descending artery. It was found t h a t the size of the internal mammary artery could not be predicted from the size of the patient. At the time of the postoperative flow study, there was no difference in the mean aortic pressure or heart rate for both groups of patients. The significantly higher flow in the SVG, as compared to the IMAG, may then possibly be due to the morphological difference between the two types of grafts utilized. The flow in the bypass graft is determined by the pressure gradient and vascular resistance across the entire vascular bed. Since the graft is placed in series with the native coronary bed, it will add to the total resistance of the vascular system as a direct result of its length and diameter. The greater length and smaller diameter of the internal m a m m a r y artery will increase the total resistance to a greater extent than the saphenous veins. Random selection of patients into two groups was necessary to eliminate any influence of the native coronary circulation as to vessel size and runoff. A review of all the preoperative coronary angiograms in the patients studied revealed no differences in the mean diameter of the left anterior descending artery at the anastomotic site or qualitative evaluation of runoff when the two groups of patients were compared. Thus, the lower flow in
American Heart Journal
the internal mamary artery group may, in part, be due to the higher resistance it contributes to the vascular system than the wider saphenous vein grafts. Another possibility which may explain the higher flow in the saphenous vein grafts, as compared to internal m a m m a r y artery grafts, may be that proposed by Wakabayashi and coworkers 6 from observations made in dog experiments. These authors observed a higher flow in grafts originating in the ascending aorta than the internal mammary artery graft or a graft originating in the descending aorta. In all three situations, the mean arterial pressure at the origin of the grafts was similar, whereas the mean diastolic pressures were significantly higher in the grafts originating from the ascending aorta and lowest in the internal mammary artery grafts. The diastolic pressure was sustained in the graft arising from the ascending aorta, whereas, at the orifice of the internal mammary artery, a rapid rise and fall in the diastolic pressure occurs, resulting in a higher mean diastolic pressure in the former and a lower mean diastolic pressure in the latter. Thus, the contour change in the pressure pulse as it is propagated along the aorta 21 may influence and determine the pressure gradient across the graft-coronary vascular bed. Since the coronary flow occurs mainly during diastole, the higher mean diastolic pressure in grafts originating from the ascending aorta would favor a higher flow, as compared to the internal m a m m a r y artery graft. An evaluation of the flow patterns in the grafts from the ascending aorta, as compared to internal mammary artery grafts, indicated that the difference in mean flow was due mainly to differences in diastolic flow.~ Morphological degenerative changes, including intimal fibrous proliferation, have been repeatedly described in saphenous vein grafts both experimentally3-~and clinically. 22, 23 Such changes will, in themselves, compromise the wall lumen of the venous graft. Thus, the differences in saphenous vein versus internal m a m m a r y artery flow noted in the early postoperative period may not be sustained after a longer follow-up period. In a small number of patients thus far followed this appears to be true in some cases. However, as already observed by us, as well as Frazier and coworkers, 13 the internal m a m m a r y artery is not immune to atherosclerotic changes, especially at
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its p r o x i m a l p o r t i o n . T h u s , t h e u n c e r t a i n t y a b o u t t h e l o n g - t e r m f a t e of t h e g r a f t s u t i l i z e d for a o r t o c o r o n a r y b y p a s s s u r g e r y , as well as t h e i n h e r e n t p r o b l e m s o b s e r v e d i n t h e c l i n i c a l a p p l i c a t i o n s of these grafts, 7, 8, 19 w a r r a n t r e s e r v a t i o n s i n m a k i n g g e n e r a l r e c o m m e n d a t i o n s for t h e a d o p t i o n of o n e t y p e of graft over t h e other.
Summary P o s t o p e r a t i v e c o r o n a r y b y p a s s flow w a s e v a l u a t e d i n t w o g r o u p s of r a n d o m l y s e l e c t e d p a t i e n t s w i t h grafts to t h e left a n t e r i o r d e s c e n d i n g a r t e r y (LAD). S a p h e n o u s v e i n b y p a s s grafts were p l a c e d i n 27 p a t i e n t s a n d i n t e r n a l m a m m a r y a r t e r y grafts i n 25 p a t i e n t s . P o s t o p e r a t i v e flow s t u d i e s were p e r f o r m e d i n b o t h g r o u p s w i t h r o e n t g e n d e n s i t o m e t r i c m e t h o d s b a s e d o n t h e t r a n s i t t i m e of radiopaque media along the graft plus the m e a n graft d i a m e t e r . T h e r e was n o s i g n i f i c a n t difference b e t w e e n t h e t w o g r o u p s of p a t i e n t s for age, d u r a t i o n of s y m p t o m s , or t h e f r e q u e n c y of h y p e r tension, diabetes mellitus, prior m y o c a r d i a l i n f a r c t i o n , or c a r d i o m e g a l y . I n t r a o p e r a t i v e b y pass flows were 7 5 - 2 7 a n d 77 + 24 m l . p e r m i n u t e for t h e s a p h e n o u s v e i n g r o u p ( S V G ) a n d i n t e r n a l m a m m a r y artery group (IMAG), respectively. T h e r e was n o s i g n i f i c a n t d i f f e r e n c e i n t h e h e a r t r a t e or m e a n a o r t i c p r e s s u r e a t t h e t i m e of t h e r o e n t g e n d e n s i t o m e t r i c flow s t u d y . T h e m e a n graft d i a m e t e r s were 3.0 ± 0.5 a n d 1.9 _+ 0.3 m m . for t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). T h e r a t i o s of graft d i a m e t e r to L A D d i a m e t e r were 1.9 ± 0.3 a n d 1.2 ± 0.2 for t h e S V G a n d I M A G , r e s p e c t i v e l y (p < 0.001). T h e r o e n t g e n d e n s i t o m e t r i c p o s t o p e r a t i v e flows were 68 ± 27 ml. per m i n u t e i n t h e S V G a n d 46 ± 16 m l . p e r m i n u t e i n t h e I M A G (p < 0.01). T h e p r e s e n t s t u d y i n d i c a t e s t h a t flow i n s i g n i f i c a n t l y h i g h e r i n s a p h e n o u s vein t h a n i n i n t e r n a l m a m m a r y a r t e r y b y p a s s e s a n d t h a t t h e difference i n flow m a y i n p a r t be e x p l a i n e d o n t h e basis of t h e g r a f t d i a m e ter. The authors wish to thank Irwin Hoffman, M.D., for his critical review of this manuscript and Brenda Hamby for the kind assistance given.
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Postoperative flow in bypass grafts
results of vein bypass grafts for coronary artery disease, Mayo Clin. Proc. 48:487, 1973. 19. Bainer, H. B.: Blood flow in the internal mammary artery, AM. HEART J. 86:570, 1973. 20. Green, G. E., Spencer, F. C., Tice, D. A., and Stertzer, S. H.: Arterial and venous microsurgical bypass grafts for coronary artery disease, J. Thorac. Cardiovasc. Surg. 60:491, 1970. 21. Luchsinger, P. C., Snell, R. E., Patel, D. J., and Fry,
American Heart Journal
D. L.: Instantaneous pressure distribution along the human aorta, Circ. Res. 15:503, 1964. 22. Grondin, C. M., Meere, C., Castonguay, Y., Lepage, G., and Grondin, P.: Progressive and late obstruction of an aortocoronary venous bypass graft, Circulation 43:698, 1971. 23. Vlodaver, Z., and Edwards, J. E.: Pathologic changes in aortic-coronary arterial saphenous vein grafts, Circulation 44:719, 1971.
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