Basic Res. Cardiol. "70, 420~,33 (1975) Medizinische Abteilung der Universitiit Miami~Miami, Florida, und Benedikt-Kreutz-Rehabilitationszentrum ]iir Herz- und Kreislaufkranke Bad Krozingen e. V.

Pacing studies after cardiac surgery Schrittmacheruntersuehungen Georg

naeh Herzoperation

Csapo

With 7 figures and 3 tables (Received October, 1974) Summary

The diagnostical use of pacemaker after cardiac surgery is presented. Temporary pacemaker electrodes have been inserted during surgery into the wall of atrial and ventricular myocardium. With the aid of these electrodes, an analysator wire of 6 volts, and a Medtronic 5840 type pacemaker electrophysiological studies have been performed. The diastolic and supernormal stimulation threshold, the duration of atrial and ventricular relative refractory period were measured, the clinical significance of latency was analysed. It has been found that decrease of stimulation threshold, shortening of relative refractory period, and appearance of latency phenomenon promote arrhythmias. By investigating the conduction capacity of atrioventricular conduction system, latent conduction disturbances could be revealed, and a significant difference could be demonstrated between the atrioventricular conduction of WPW syndrome and that of other kinds of PR interval shortening. In addition the optimal heart rate requirement (optimal pacing rate) following heart surgery was defined. These parameters present more precise information on the electrophysiological condition of the heart than does the generally used ECG monitoring. When these parameters are repeatedly determined, the significance of patient's arrhythmias can be evaluated more safely and accurately; in a n u m b e r of cases, even in the absence of any r h y t h m disturbances, impending arrhythmias can be predicted. "Pacemaker monitoring" of the postoperative heart patients, therefore, affords greater possibility for preventing the development of major cardiac arrhythmias. T h e f r e q u e n t o c c u r r e n c e of a r r h y t h m i a s i n the p o s t o p e r a t i v e p e r i o d a f t e r h e a r t s u r g e r y a n d i n t h e a c u t e p h a s e of m y o c a r d i a l i n f a r c t i o n , c o u p l e d w i t h o u r s t r u g g l e a g a i n s t c i r c u l a t o r y collapse d u e to c a r d i a c a r r e s t or shock, led to t h e d e v e l o p m e n t of i n t e n s i v e - c a r e u n i t s a n d s o p h i s t i c a t e d r e s u s c i t a t i o n p r o c e d u r e s . C l i n i c a l d e a t h s h o u l d n o w be c o n s i d e r e d a r e v e r s i b l e state. P r o m p t r e c o g n i t i o n a n d t r e a t m e n t of c a r d i a c arrest, h o w e v e r , is a sine q u a n o n for this r e v e r s i b i l i t y . C o n t i n u o u s m o n i t o r i n g of p a t i e n t s m a k e s this possible, w h i c h u s u a l l y i n a c a r d i a c p o s t o p e r a t i v e w a r d i n v o l v e s E C G m o n i t o r i n g , c o n t i n u o u s c o n t r o l of h e m o d y n a m i c a n d r e s p i r a t o r y p a r a m e t e r s , of acid/base a n d e l e c t r o l y t e b a l a n c e s .

Csapo, Pacing studies a]ter cardiac surgery

421

Although clinical death might be reversed, resuscitation remains an undesirable process. One of our aims is, therefore, to seek early information which would predict imminent circulatory collapse. Such information m a y suggest appropriate therapeutic measures, avoiding t h e r e b y clinical death and the subsequent complications of resuscitation. This paper presents our pacing studies aiming a more accurate approach to the electrophysiologic condition of the heart. We believe this method is broadly applicable and can provide some early information regarding the imminence of dangerous cardiac arrhythmias an circulatory eollapse following heart surgery. Materials and methods In cases of open heart surgery and in cases of a r r h y t h m i a s during thoracic surgery, t e m p o r a r y pacemaker electrodes were inserted in the myocardium, usually into the wall of the right atrium and ventricle. In some instances, electrodes were placed into the wall of the left atrium and left ventricle as well. These electrodes served primarily for postoperative standby pacing, but also suffered a means for direct atrial and ventricular monitoring, thus permitting more precise analysis of the ECG compared with conventional or other monitoring leads. Our diagnostic pacemaker investigations were also performed using these electrodes. We used a six-volts analyzator wire (Medtronic threshold m e a s u r e m e n t cable), and a Medtronic 5840 external pacemaker for the diagnostic pacing. Patients were artificially ventilated, if needed. Blood pressures were measured by means of three catheters: one inserted directly into the left atrium, another into the right atrium via the brachial vein, and a third one into the brachial artery. The results of our investigations presented in this paper have been collected from continuous ECG monitoring of 84 patients and from diagnostical use of pacing performed on 81 patients. 159 of the altogether 165 patients (97 females, 68 males, aged 6-68 years; mean 26.4 years) were postoperative patients undergoing heart surgery because of congenital (56 patients) or acquired (103 patients) heart disease. Six non-surgical patients were investigated by Elecath semi-floating electrode catheters inserted into the right atrium t h r o u g h either the brachial or the femoral vein. Our procedures consisted of determinations of (1) stimulation threshold, (2) duration of r e f r a c t o r y period, (3) optimal pacing rate, (4) conduction capacity of the AV system, and (5) estimation of the role of latency in prediction of arrhythmias. Each of these determinations is described in details below. Stimulation threshold The diastolic and supernormal stimulation threshold was determined by gradually increasing the energy of the pacemaker impulses. The diastolic threshold was the lowest mA which elicited pacemaker systoles anywhere in the electric diastole. The threshold of the supernormal phase was the subthreshold stimulus which resulted in pacemaker systole only during the terminal phase of the repolarization process, that is, during the negative afterpotential of the monophasic curve (near the end of the T wave).

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Basic Research in Cardiology, Vol. 70, No. 4

W e e s t a b l i s h e d a " n o r m a l " r a n g e in a p r e v i o u s s t u d y (3) f o r v a l u e s o b t a i n e d f o l l o w i n g t h e i m p l a n t a t i o n of e l e c t r o d e s : t h e m e a n v e n t r i c u l a r s t i m u l a t i o n t h r e s h o l d of e l e c t r i c d i a s t o l e w a s r e c o r d e d as 1.52 i 0.21 m A , a n d t h a t of t h e s u p e r n o r m a l p h a s e as 1.18 i 0.21 m A . E l e v a t i o n of t h r e s h old v a l u e s d u r i n g t h e first p o s t o p e r a t i v e d a y s - n o t c o n s i d e r e d to h a v e a n y s i g n i f i c a n t e l e c t r o p h y s i o l o g i c a l i m p o r t a n c e - m a y be c a u s e d b y i n c r e a s e d tissue i m p e d a n c e a r o u n d t h e p a c e m a k e r e l e c t r o d e s , b y t h e i r r u p t u r e, etc. (6, 12, 18, 22). O n t h e o t h e r h a n d , d e c r e a s e in t h r e s h o l d v a l u e may indicate increased myocardial irritability. However, possible physioTable 1. Changes of diastolic stimulation threshold in m A during the first 5 postoperative days No

Dg

1st day

2nd day

3rd day

4th day

5th day

1 2 3 4 5 6 7 8 9

AR MS ~- MR II-I S S MS MS ~- MR FT VSD AS AS + A R

1.3 1.6 1.4 1.7 1.3 1.7 1.8 2.2 1.2

1.3 1.4 1.4 1.8 1.2 1.6 1.8 2.0 1.3

1.2

1.4

11

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1.3

12

MR

1.4

0.9 (-- 31%) 1.1 VF Ind 1.5 1.7

1.5 1.5 1.4 1.8 1.8 1.7 1.8 2.2 1.8 Ind 3.5 B.E. 1.4 Ind 1.2 (-- 290/o)

1.7 1.6 1.5 1.8 1.8 1.7 1.9 2.2 1.9 Ind

10

1.4 1.4 1.5 1.9 1.4 1.5 1.8 2.1 1.5 Ind 1.3

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P

13

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1.5

14

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AR

1.8

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A R : Aortic regurgitation AS: Aortic stenose F T : Fallot tetralogy I H S S : Idiopathic hypertrophic subaortic stenose MR : Mitral regurgitation MS: lV[itral stenose VSD: Ventricular septal defect P V B : Premature ventricular beats VT: Ventricular tachyeardia V F : Ventricular fibrillation I n d : Inderal therapy P : Prednisolon therapy B.E. : Braking of the Electrode

P

1.6 Ind 1.4 1.5 Ind 1.5 Ind + P 1.4 P 1.6 P

Csapo, Pacing studies aster cardiac surgery

423

logical and pharmacological influences on the threshold value also have to be considered. A m o n g others, sleep, eating, mineralocorticoids, procainamide, sodium, beta blockers, hyperkalemia raise - whereas exercise, isoproterenol, epinephrine, glycocorticoids, hypokalemia lower the stimulation threshold (11, 18, 19, 24). According to P r e s t o n and J u d g e (20) a change in threshold has electrophysiologic significance only when greater than 16 %. Of 16 postoperative heart patients who were regularly monitored, six showed more than 16 % reduction in stimulation threshold. Ventricular tachycardia occurred in two of these six patients, and ventricular fibrillation in another. An additional two of these six patients had frequent PVB's. Propranolol and/or prednisolon controlled both the decreased stimulation threshold and the a r r h y t h m i a s in all six of these patients (tab. 1). On the basis of this observation we suggest preventive beta-blocker administration in cases of significant diminution of the ventricular stimulation threshold. In an additional case the onset of atrial fibrillation was preceded by significant (18~ in another by not significant (11~ decrease of atrial stimulation threshold. It was noted that change in ventricular stimulation threshold in any direction was not necessarily accompanied by either the same or even similar change in atrial stimulation threshold. The duration of repolarization process (refractory period) By determining the place of the supernormal phase in the electric heart cycle, if absent, the onset of electric diastole, the duration of the atrial and ventricular repolarization processes can be exactly measured. These are the interval between the onset of P wave and the start of atrial supermsec

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Fig. 1. The atrial and ventricular refractory periods of 10 postoperative patients without arrhythmia determined by subthreshold stimulation (see text for discussion). Dotted lines s h o w the limits of normal QT interval according to H u r s t and L o h u e (13).

424

Basic Research in Cardiology, Vol. 70, No. 4

n o r m a l p h a s e o r e l e c t r i c d i a s t o l e ( p h a s e 4 of t h e m o n o p h a s i c a c t i o n p o t e n t i a l c u r v e ) , a n d t h e i n t e r v a l b e t w e e n t h e o n s e t of Q R S c o m p l e x a n d t h e b e g i n n i n g of v e n t r i c u l a r s u b t h r e s h o l d o r t h r e s h o l d i r r i t a b i l i t y . T h e o r e t i c a l l y t h e v e n t r i c u l a r r e c o v e r y c o r r e s p o n d s t o t h e Q T i n t e r v a l (7), h o w e v e r , our diagnostic pacemaker method enables us to measure this process more p r e c i s e l y a n d a l s o t o d e f i n e t h e a t r i a l r e f r a c t o r y p e r i o d . T h e d u r a t i o n of T a b . 2. C h a n g e s of a t r i a l r e f r a c t o r y p e r i o d ( A R P ) a n d v e n t r i c u l a r r e f r a c t o r y p e r i o d ( V R P ) of t h r e e p o s t o p e r a t i v e p a t i e n t s . - I n t h e 1st a n d 2 n d cases a f t e r s h o r t e n i n g of A R P a t r i a l f i b r i l l a t i o n o c c u r r e d . I n t h e 3 r d case s h o r t e n i n g of V R P w a s followed b y a n episode of v e n t r i c u l a r fibrillation. - See t e x t for discussion. Pat.

Postoperative day

Heart rate

:PR mscc

VPR msec

ARP msec

1st

112 220 100 98 104 100 90 136 90 78

140 240 140 140 130 150 150

280 290 310 320 320 320 330 290 340 340

260 92,8 270 93,1 280 90,3 280 87,5 240 75,0 240 75,0 230 69,6 A ~ i a l fibrillation

98 100 104 80 96 100

160 160 160 160

300 300 340 340 340 320

280 260 240 240

2nd 3rd 4th 5th

1st 2nd 3rd

4th 5th

Ist 2nd 3rd 4th

5th 6th

Digitalis therapy

160 160 160 160 160 160 160 160

320 320 310 310 290 260 260 260

93,3 86,6 70,6 70,6

A t r i a l fib~llation

C a r d i o v e r s i o n w i t h DC s h o c k a n d Q u i n i d i n t h e r a p y 104 190 310 260 92 180 310 280 96 180 310 280 94 180 310 290 104 96 94 100 96 88 84 78 ( V P B )

100 • A R P VRP

280 280 290 280 280 280 270 270

83,8 90,3 90,3 93,7 87,5 87,5 93,5 90,3 96,5 I07,6 103,8 103,8

Ventmcular fibHllation - D e f i b r i l l a t i o n a n d b e t a b l o c k e r t h e r a p y 104 160 290 280 96,5 98 160 300 270 90,0 94 160 320 280 87,5

VPB: Ventrieular premature beat

425

Csapo, Pacing studies a]ter cardiac surgery

recovery varies with the heart rate; we previously reported that the refractory period of the ventricle amounted to 94.67 +_ 4.17% of the QT interval (3). The atrial refractory period in patients without a r r h y t h m i a was found 91.79 _ 1.73% of the refractory period of the corresponding ventricle (fig. 1). Change in action potential duration at a given heart rate is a sign of increased proneness to arrhythmia. It m a y be shortened by increased vagal tonus, acetylcholine, digitalis, hyperkalemia, whereas lengthenend by hypokalemia, hypocalcemia, hypoglycemia (8, 17, 26). In two patients we observed a not-rate-dependent shortening of atrial recovery prior to the onset of atrial fibrillation; in another patient the ventricular r e f r a c t o r y period shortened before an episode of ventricular fibrillation (tab. 2). These results m a y suggest that drugs with quinidine-like action might be used in patients with shortened action potential.

The "Optimal Pacing Rate" We studied effect of heart rate on h e m o d y n a m i c parameters in 17 patients with low cardiac output syndrome within five days following openheart surgery for mitral and/or aortic valve replacement. Patients with spontaneous heart rate exceeding 90/min. during the period of study were excluded from this group. rnmHg

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426

B a s i c R e s e a r c h in C a r d i o l o g y , Vol. 70, No. 4

Ventricular pacing was performed in seven patients with atrial flutter, atrial fibrillation and AV block; atrial pacing was applied in the remaining ten patients. Systolic and diastolic blood pressures, central venous pressure (CVP), and left atrial mean pressure (LAMP) were determined at spontaneous heart rate (SHR). Thereafter - in a steady state regarding other conditions (fluid balance, respiration, temperature, etc.) - during continuous monitoring of ECG, heart rate was gradually raised in increments of 10/min. up to 130-150/min. The same h e m o d y n a m i c parameters were registered at each frequency level, following a short period allowed for adaptation to the new heart rate (fig. 2). An optimal pacing rate (OPR) characterized by the highest systolic and pulse pressure at the lowest possible levels of CVP and LAMP, and by suppression of preexisting arrhythmias, if any, was established once or twice daily for maintaining each patient on that rate by demand pacing. H e m o d y n a m i c values of OPR were compared to values obtained at SHR to establish the effect of heart rate on circulation. Statistical analysis of these data revealed that artificial augmentation of heart rate in patients with relative b r a d y c a r d i a and low-cardiac-output s y n d r o m e results in significant increase of blood pressure and pulse pressure, and a significant decrease of CVP, LAMP, and a r r h y t h m i a s (tab. 3). Comparison of the correspondent h e m o d y n a m i c values obtained at OPR on the first postoperative day and at SHR on the fifth postoperative day revealed no significant differences (tab. 3); that means the h e m o d y n a m i c i m p r o v e m e n t induced by pacing on the first postoperative day was comparable to the spontaneous i m p r o v e m e n t seen during the first five postoperative days. Parallel with the i m p r o v e m e n t of cardiac performance in the postoperative period, the significance of the beneficial effect of pacing gradually decreased during this five-day period of study. The effect of atrial pacing on h e m o d y n a m i c parameters was more pronounced than that of ventricular pacing. E.g. the fall of CVP on the fifth postoperative day was significant only in cases of atrial pacing. Differences between corresponding values obtained at SHR and OPR - thus data representing the improvement of parameters on the effect of pacing established that the increase of systolic blood pressure, and the fall of CVP and L A M P were significantly higher in the subgroup of ventricular pacing (p ~ 0.05). Also the OPR performing atrial pacing was less high than in cases of ventricular pacing (p ~ 0.05). These results confirm the concept that when rate augmentation is necessary in the postoperative period the choice of w a y is atrial pacing if not impossible because of the given electrophysiologic conditions (AV block, atrial fibrillation, etc.). On the basis of this study we can conclude that heart rate requirement for optimal i m p r o v e m e n t of patient's clinical and h e m o d y n a m i c conditions following heart surgery is higher than the normal range, and also frequently surpasses that of the patient's own rate; however, we emphasize that clinical practice must not slavishly follow these statistical data; the h e a r t - r a t e requirement has to be determined for each patient individually u n d e r the existing condition. Continuous monitoring of cardiac output requires the placement of a flowmeter around the aorta. We did not do that as our purpose was to

427

Csapo, Pacing studies after cardiac surgery

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428

B a s i c R e s e a r c h in Cardiology, Vol. 70, No. 4

o b t a i n a d d i t i o n a l p a r a m e t e r s for clinical use for the control of c i r c u l a t i o n a p p l y i n g only e v e r y d a y p o s t o p e r a t i v e methods, w i t h o u t i n c r e a s i n g the p r o c e d u r e s on p a t i e n t s d u r i n g and following h e a r t surgery. F u r t h e r m o r e our results w e r e in accordance w i t h p r e v i o u s studies in which the r a t e effect on p o s t o p e r a t i v e c i r c u l a t i o n was d e t e r m i n e d by m e a s u r i n g c a r d i a c output (1, 15). Heart rate and ventricular arrhythmias The r e l a t i o n b e t w e e n h e a r t r a t e and v e n t r i c u l a r a r r h y t h m i a s p r o v e d h i g h l y significant in other p o s t o p e r a t i v e p a t i e n t s ' groups, as well: b y ECG m o n i t o r i n g of 17 p a t i e n t s we could e x a c t l y o b s e r v e the h e a r t r a t e i m m e d i a t e l y b e f o r e 177 episodes of v e n t r i c u l a r t a c h y c a r d i a a n d v e n t r i c u l a r fibrillation. In 144 i n s t a n c e s (81.3%) the h e a r t r a t e p r e c e d i n g the onset of these a r r h y t h m i a s was less t h a n 80/min. In a n o t h e r group 16 of 67 p a tients h a d a m e a n h e a r t r a t e of less t h a n 80/min. f o l l o w i n g o p e n - h e a r t surgery. Nine of these 16 p a t i e n t s h a d m a j o r v e n t r i c u l a r a r r h y t h m i a s d u r ing the p o s t o p e r a t i v e period. This w a s in c o n t r a s t to the six cases of

below 60/min patients I

I

~with

with heart rate

60 or more/min

episodes of VF

Fig. 3. Occurrence of ventricular arrhythmias (VF and VT) in postoperative patients related to their heart rate. a r r h y t h m i a s of the r e m a i n i n g 51 p a t i e n t s w i t h a m e a n h e a r t r a t e of 80/min. or m o r e (fig. 3). The v e n t r i c u l a r a r r h y t h m i a s w e r e successfully c o n t r o l l e d in seven of the nine m e n t i o n e d cases w i t h r e l a t i v e b r a d y c a r d i a by raising the h e a r t r a t e e i t h e r b y p a c e m a k e r (five cases) or by a d m i n i s t r a t i o n of I s o p r e n a l i n (two cases).

The conduction capacity of the atrioventricular conduction system By g r a d u a l l y i n c r e a s i n g the a t r i a l p a c i n g rate, the conduction c a p a c i t y of the a t r i o v e n t r i c u l a r system, or in o t h e r words its t e n d e n c y to block, can be also established. Such studies h a v e been t a k e n also in n o n s u r g i c a l

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