Fish Physiologyand Biochemistryvol. 1 no. 2 pp 75-83 (1986) Kugler Publications, Amsterdam/Berkeley
Elasmobranch pericardial function. I. Pericardial pressures are not always negative D.C. Abel* ~, J.B. Graham 1, W.R. Lowell I , R. Shabetai 2 I Physiological Research Laboratory, A-O04, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, U.S.A. ZDepartment o f Medicine, University o f California, San Diego, and Medical Service, Veterans Administration Medical Center, San Diego, CA 92161, U.S.A. Keywords: elasmobranch, heart, horn shark, pericardioperitoneal canal, pericardium, pressure-volume
Abstract
Acute studies have led to the generalization that negative pericardial pressure is necessary for optimal cardiac function in elasmobranchs. We chronically instrumented horn sharks with pericardial catheters to test the hypothesis that ejection of pericardial fluid through the pericardioperitoneal canal (PPC) during routine handling could have accounted in part for previous measurements of exclusively negative pressures ( - 0 . 3 to - 9 . 1 cm H2 O) in elasmobranchs. Maximum and minimum pericardial pressures measured immediately following routine handling (acute pressures) were more negative than those measured in resting horn sharks at intervals from 1 to 27 days following handling (chronic pressures). Chronic pericardial pulse pressure was less than acute. Entirely positive pericardial pressures were observed on occasion. Handling of chronically catheterized horn sharks resulted in ejection of 21 per cent (range = 10-26, n = 5) of the initial pericardial fluid volume through the P P C and reduced pericardial pressure. Operating pericardial fluid volume o f horn sharks averaged 2.0 ml.kg- ~ (range = 1.6-2.6, n = 9). The PPC opened after 4.3 _+ 0.2 ml.kg-' (x +_ S.E.) of elasmobranch saline had been slowly infused into the pericardium, corresponding to an average pressure of 1.3 _+ 0.2 cm H 2 0 (n = 10). The presence of the PPC plus a comparatively large pericardial fluid volume allows horn sharks to regulate pericardial pressure. Our analysis of pericardial pulse pressure, which can be an index of cardiac activity, suggests in contrast to previous studies that the elasmobranch heart can have relatively high stroke volumes at pericardial pressures near ambient. Thus, for venous return in resting or even moderately active elasmobranchs, it is more important that pericardial pressure be pulsatile than at a mean level which is negative.
Introduction
Heart function in elasmobranch fishes (sharks and rays) is believed to differ from that of other vertebrates because their relatively rigid pericardium allows the maintenance of a negative (subambient) pericardial pressure that becomes more negative during ventricular ejection (Sudak 1965a; Satchell
1971). Negative pressure is considered necessary to enhance return from the central venous sinuses by aspiration. If negative pressure is abolished by puncturing the pericardium, venous return and hence cardiac output drop drastically (Johansen t965; Sudak 1965b; Hanson 1967; Johansen and Burggren 1980). However, this conclusion remains less than certain for two reasons. Because only
* Present Address: Department of Biology, Collegeof Charleston, Charleston, SC 29424.
76 pericordiol forobronch
cork ftoo! ~
~'al~t
/ ~
~
iol
==
-5
i
\
o
o
o
c
y
TANK
STAND WITH TRANSDUCERS (1)
RACK WITH RECORDER AND CRT
Fig. 1. Experimental apparatus for measurement of chronic pericardial pressures in horn sharks. Fluid-filledtubes from pressure
transducers are attached to stopcocks on cork float leading from pericardialand orobranchialcatheters. Zero referencetube is shown below water surface. Pressure transducers on brick pedestal are connected to cathode ray tube (CRT) and 4-channel physiological recorder. ECG electrodes are not pictured. acute experiments, in many cases on anesthetized, supine, emergent specimens, have been performed, the continued presence and importance of negative pericardial pressure to cardiac function in elasmobranchs under more physiological conditions has not been examined. Furthermore, the pericardial and peritoneal spaces of elasmobranchs are connected.by a duct, the pericardioperitoneal canal (PPC), which may function as a pressure-regulating valve (Shabetai et al. 1985). Normally closed, the PPC allows one-way flow of pericardial fluid to the peritoneal cavity (Smith 1929; Satchell 1971), which lowers pericardial pressure (Shabetai et al. 1985). Because the pericardium is not entirely insulated from external pressures, it is possible that most of the previously reported acute pressure data were obtained after pericardial fluid had been ejected through the PPC as a result of handling. Thus, we hypothesize that routine pericardial pressures may not be as negative as those measured immediately following handling, and test this by measuring acute and chronic pericardial pressure in horn
sharks, Heterodontusfrancisci, fitted with an indwelling pericardial catheter.
Materials and methods Maintenance and instrumentation
Male and female horn sharks (0.5-2.5 kg) were captured off La Jolla, CA, and maintained in large aerated aquaria with continuously flowing seawater (14-24°C) in ambient photoperiod at Scripps Institution of Oceanography. They were fed cut mackerel once or twice weekly. At least 24 h before experimentation, post-absorptive sharks were transferred to laboratory holding tanks (3501) adjacent to physiological monitoring equipment (Fig. 1). To determine the effects of routine handling on pericardial pressure, we anesthetized each shark in its holding tank with tricaine methanesulfonate (MS222, Crescent Chemical Co., 0.05-0.06 g. 1-i), and kept it submersed and ventilated with aerated
77 anesthetic mixture while an acute pressure (see below) was measured. This was done by inserting a 19 ga. needle connected to a PE 190 tube filled with elasmobranch saline (Prosser 1973) through the skin and coracoid bar into the pericardial space (Shabetai et al. 1985) (Fig. 1). The shark was then fitted with a chronic catheter that permitted longterm monitoring of pericardial pressure. The needle used for acute measurement was removed and the hole enlarged to allow insertion of the tip of a PE 190 chronic catheter with terminal side holes. The shark was then removed from the water and placed supine on a V-board with continuous ventilation. The skin around the catheter entry site was dried with acetone and warm air, and the catheter epoxled in place. One ECG electrode (40 ga. tefloncoated silver wire, Cooner Wire Co.) was inserted into the pericardium with the pericardial catheter. Another was inserted through the skin and implantod into a lateral wall o f the pericardium by needle (23 ga.). The needle was then removed leaving the electrode in place. Catheters and wires were sutured to the shark and their ends attached to a cork float. The shark was then allowed to recover in its holding tank filled with seawater at ambient temperatures (14-24°C).
Acute and chron& pericardial pressure Acute pressure is defined as that measured immediately after induction of anesthesia and during handling and emersion before attaching the chronic catheter. Chronic pressure is that measured once or twice daily beginning from the day following catheter implantation for up to 27 days, and thus approximates the normal range of operating pressure. Pressure transducers (Statham P23ID) were calibrated weekly against l0 cm H 2 0 (sensitivity = 0.1 cm HEO). Care was taken when connecting the catheter and electrodes to the monitoring equipment to disturb the shark minimally, and five to ten minutes elapsed before data collection. Recordings were made over a 5 - 6 h period without disturbing the shark to validate this method of daily measurement. Because there was little variation in the pericardial pressure record for any one period,
a single random or representative pulse (wave) of pressure was selected for data analysis of both acute and chronic maximum and minimum pericardial pressures and the difference between them, termed "pericardial pulse pressure" (Shabetai et aL 1985). Means were compared using t-tests. If a leak was discovered in the chronic catheter system, the experiment was terminated and that day's results discarded. Pericardial pressures were referenced to a fluid-filled tube in the tank below the surface. Positioning of the zero line anywhere in the tank accurately describes ambient pressure for referencing pericardial pressure regardless of the depth o f the shark if both zero and reference pressure lines are connected to the same transducer, in accordance with Torricelli's law (Shortley and Williams 1967).
Pressure-volume characteristics o f chronically catheterized horn sharks Pressure-volume experiments The relationship between pericardial pressure and volume was assessed by incremental infusion (0.2-0.5 ml) of fluid into the pericardium of conscious, submersed, chronically catheterized horn sharks from which all fluid was first aspirated from the pericardium. Heart rate was continuously monitored and pericardial pressure was recorded after each infusion. When a volume sufficient to open the P P C had been infused (indicated by a plateau in pericardial pressure), all fluid was removed (see next paragraph) and its volume determined. This volume, representing the maximum pericardium capacity for increases in either heart size or pericardial fluid volume, will be termed " m a x i m u m pericardial fluid volume." The corresponding mean pericardial pressure is defined as " P P C opening pressure." Operating pericardial fluid volume To determine the fraction o f the maximum pericardial fluid volume present under routine conditions, we measured pericardial fluid volume periodically in nine chronically catheterized horn sharks. Accurate measurement o f pericardial fluid ( + O. 1 ml) was possible because the catheter tip is in the
78 lowest part of the pericardial space. The effectiveness of this method was verified in several postmortem examinations. Replicate measurements of maximum pericardial fluid volume in the same specimen revealed that the method is also precise (+ 0.05 ml). Following measurement, the same fluid was replaced.
I N I T I A L AND CHRONIC PERICARDIAL PRESSURES
,IMPLANTATION
Effects o f handling in chronically catheterized horn sharks
r Y v ~
0
-1 -2 -3 -4 -5 -6 -7
-1 -2 -4 6
9
HF 14
-3
HF 13
~
12 15 18 21 2 4 27
,
. . . . . . . .
3
6
9
12 15 18 21 2 4 2 7
+2 0 -2
The effects o f handling on pericardial pressure and volume were directly assessed in 5 chronically catheterized horn sharks. In these tests, pericardial pressure and volume were determined and then the shark was either lifted from the water and allowed to struggle, or grasped by the first dorsal spine and allowed to swim in place, each time for 30 to 45 s. Following this, pericardial pressure and volume were immediately measured. We also assessed the effects of handling after occluding the P P C in three specimens. After implantation o f the chronic pericardial catheter and electrodes, the abdomen was opened by midline incision, and the left lobe of the liver reflected. The PPC was then ligated in two places with surgical silk (# 00). Following verification of complete occlusion, the liver was replaced and the abdomen sutured. The handling experiment was conducted at least 24 h later.
Resulls
-4 O~
-6
==
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16
-8 -8 = o
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• 2
• 4
. 6
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4
6
8
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10 12
-4
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DAYAFTER IMPLANTATION
Fig. 2. Acute ("initial") and chronic pericardial pressures of six horn sharks ( H F I 3 - H F I S ) . Arrows at day zero indicate acute pressures. Pressures for each subsequent day are chronic. Maximum (circles) and minimum (dots) values o f a single pulse o f pericardial pressure are pictured for each day.
Lower than acute (p < 0.025; Table 1). In general, chronic maximum pericardial pressure was just above ambient, whereas chronic minimum pericardial pressure was slightly negative, although on occasion both phases were positive.
A cute and chronic pericardial pressure Fig. 2 shows daily maximum and minimum pericardial pressure-and pericardial pulse pressure for six of the ten chronically catheterized horn sharks. Acute pericardial pressure just after anesthesia and handling (day zero) was negative throughout the cardiac cycle and within the range of values reported for other elasmobranchs. Acute pulse pressure was high. In contrast, chronic pericardial pressure (day 1 and subsequently) was significantly higher (p < 0.025) than acute pressure, and in seven sharks chronic pulse pressure was significantly
Pericardial pressure-volume characteristics Pressure-volume experiments The pericardial pressure-volume curve for the horn shark is similar to those reported in acute experiments for a variety of species under anesthesia by Shabetai et al. (1985), and is typified by the curve of maximum and minimum pressures shown for a chronically catheterized specimen in Fig. 3. Following aspiration (zero volume), pericardial pressure is very negative (as low as - 4 0 cm HzO
79 Table 1. Acute and chronic pericardial pressures (cm HzO) and pericardial pulse pressures measured for 10 horn sharks Shark #
Acute pressure
Chronic pressure x _+ S.E. (n)
HF13 HF14 HFl5 HFI6 HFI 7
-4.8 -2.4 -4.8 -3.5 - 1.8
0.0 +0.5 + 1.0 +0.8 -0.7
_+ _+ _+ _+ _+
0.1 0.1 0.2 0.4 0.3
(25)* (45)* (17)* (17)* (10)*
HFI 8 HFI9 HF20 HF21 HF22
-1.8 -2.8 -3.5 --2.0
+0.4 +0.2 +0.3 +0.1 +0.6
_+ +_ _+ +_ _
0.1 0.1 0.2 0.2 0.3
(9)* (9)* (4)* (9) (6)*
PRESSURE VOLUME CURVE WITH CHRONIC VOLUMES
A, M a x i m u m pressure
-4 []
o Moximum
I-/ ~/
• Minimum
0. ~ 6
V
.t
B. Minimum pressure HFI 3 HFI4 HF15 HFI6 HF17 HFI8 HFI9 HF20 HF21 HF22
() -6.1 -4.2 -8.8 -7.5 -3.4 -3.8 -4.0 -7.0
-1.2 -0.6 -0.4 -0.6 - 1.7 -0.4 -0.7 -0.4
--6.0
- 0 . 8 _+ 0.2 (9) - 0 . 4 _+ 0.3 (6)*
C. Pulse pressure HFI3 HF14 HFI5 HFI6 HFI 7 HFI 8 HFI9 HF20 HF21 HF22
+ 0.1 _+ 0.2 + 0.3 _+ 0.1 _+ 0.4 _+ 0.1 _+ 0.1 _+ 0.3
(25)* (45)* (17)* (17)* (10)* (9)* (9)* (4)*
1.3 1.8 4.0 4.0
1.2 1.1 1.4 1.4
+ 0.1 + 0.1 ___ 0.2 _+ 0.1
(25) (45) + (17) + (17) +
1,6 2.0 1.2 3.5
1.0 0.8 1.0 0.8 0.9 1.1
_+ _+ _+ + _+ _+
(10) + (9) + (9) (4) + (9) (6) +
-4.0
0.1 0.2 0.1 0.2 0.1 0.1
* Acute < chronic (p < 0.025; t-tes0 + Acute > chronic (p < 0,025; t-test)
in some specimens), and pulse pressure is large (up to 40 cm HzO). As fluid is infused m a x i m u m pressure generally rises at a slow rate until P P C opening pressure is reached (approximately 1 cm H 2 0 in Fig. 3). Minimum pressure rises quickly at first then slowly until the P P C opens. At P P C opening (average = 0.8 to 3.0 cm HzO, Table 2), continued infusion does not result in further pres-
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2
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PERICARDIAL FLUID VOLUME,ml Fig. 3, Pressure-volume curve for horn shark HF14. Circles and dots are the ma xi mum and minimum values of a single pulse of pericardial pressure, At zero volume pericardial pressures are very negative with a large pulse. Pressures rise and pulse pressure decreases as pericardial fluid volume is increased. At P P C opening pericardial pressures reach a ma xi mum with zero pulse pressure. Also shown in relation to this curve are the mean pericardiat pressures and corresponding volumes (squares) chronically measured for HFI4.
sure rise because the added volume is vented into the peritoneum, Pericardial pulse pressure is maximum at zero volume, and decreases as volume increases. Maximum pericardial fluid volumes obtained following pericardial volume infusions averaged 4.3 ml. kg- m(range = 3 . 7 - 5.0 ml. kg- ]; n = 10).
Heart rate changes Heart rate generally decreased following complete aspiration of the pericardial fluid, and increased upon subsequent infusion. In a typical case heart rate was 21 b p m at zero pericardial fluid volume and 48 b p m at maximum volume, and the response was reversible and repeatable. Heart rates of five horn sharks are also shown in Table 4. The high variability in this data set which masks the above trend can be attributed at least in part to seasonal temperature effects.
80 Table 2. Chronic pericardial fluid volumes and pressure-volume characteristics for 10 sharks Fish #
HFI3 HF14 HF15 HFI6 HFI7 HFI8 HFI9 HF20 HF21 HF22
Body wt (kg)
0.9 1.3 0.7 1.1 1.0 0.9 1.4 1.1 0.6 1.0
Pericardial fluid volume
Maximum pericardial fluid volume
Daily pericardial fluid volume as % maximum fluid volume
PPC opening pressure, cm H 2 0 _+ S.E. (n)
Daily, ml x ± S.E. (n)
ml per kg body wt
ml, ~" ± S.E, (n)
ml per kg body wt
1.4 3.1 1.8 1.7 No 2,2 3.5 2.4 1.0 1.6
1.6 2.4 2.6 1.5 -2.4 2.5 2.2 1.7 1.6
3.4 5,9 3.1 4.6 3.7 3.5 6.3 5.5 3.0 3.8
3.8 4.5 4.4 4.2 3.7 3.9 4.5 5.0 5.0 3.8
42 53 59 36
34 42
0.9 1.6 3.0 1.3 1.2 1.2 0.4 1.6 0.8 1.3
4.3
48
1.3
+ 0.1 +_ 0.1 ± 0.0 ± 0.2 data +_ 0.1 +_ 0.2 _+ 0.1 ± 0.1 _+_ 0.2
x
(6) (14) (2) (6) (10) (5) (4) (7) (5)
+ 0.2 ± 0.I _+ 0.1 ± 0.1 ± 0.0 ± 0.2 +_ 0.2 _+ 0.2 ± 0,1 _+ 0.2
(5) (4) (4) (4) (3) (4) (4) (5) (8) (5)
2.0
Pericardial fluid volumes The mean pericardial fluid volume of horn sharks was 2.0 +_ 0.1 ml.kg -~, x _+ S.E., Table 2), which represents an average of 48 per cent of the maximum pericardial fluid volume. Values for pericardial fluid volume and mean pericardial pressure observed for horn shark HF14 have been superimposed on its pressure-volume curve in Fig. 3 to show the range o f pericardial pressures and volumes at which it was operating under resting conditions and their relationship to the curve. These results are typical o f those obtained for other horn sharks.
Factors promoting PPC function: handling and pericardial volume loading Handling lowered pericardial pressure and volume in all sharks except those with ligated PPC. Ten to 26 per cent of the starting pericardial fluid volume was ejected through the PPC o f five horn sharks as a result of struggling (Table 3). Fig. 4 illustrates the record from one handling experiment in which 1.1 ml (78 per cent o f the starting volume) was ejected, resulting in a decrease in mean pericardial pressure from about - 0 . 5 to - 1 . 8 cm H 2 0 , and an increase in pulse pressure. We also observed that horn sharks would not
62 56 44
_+ 0.2 _+ 0.1 _+ 0.2 + 0.0 _+ 0.1 + 0.0 ± 0.3 +_ 0.1 +_ 0.2 _+ 0.1
(5) (4) (4) (4) (3) (4) (4) (5) (9) (3)
Table 3. The effects of handling episodes on pericardial fluid volume in 5 horn sharks Fish # Body wt (kg.)
Pericardial fluid volume (ml)
% Initial volume ejected
Before handling After handling ~ _+ S.E. (n) HFI8 HFI9 HF20 HF21 HF22
0.9 1.4 1.1 0.6 1.0
2.3 3.8 2.8 1.9 2.0
_+ 0.1 + 0.2 + 0.3 + 0.1 +_ 0.1
(5) (4) (4) (4) (4)
1.7 3,4 2.1 1.5 t.5
± ± + ± ±
0.2 0,2 0.2 0.1 0.1
26 10 25 21 25
tolerate the maximum pericardial volume load attained during pericardial pressure-volume experiments. The pressure-volume curve from a shark that began 20 s of vigorous swimming following a volume infusion into its pericardial space is shown in Fig. 5. After swimming, pericardial pressure was similar to control, suggesting that vigorous swimming acts in a manner similar to struggling to vent the pericardial fluid and that in this example virtually the entire infusion volume had been ejected through the PPC.
81
EFFECTS OF HANDLING BEFORE
AFTER
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PERICARDIAL FLUID VOL; 1.4ml Fig. 4.
Effects
of handling
episode
on
pericardial
pressure
and
volume
in a horn
shark.
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PERIC,~I,~DIALFLUID VOL= 0.3 rnl Before
handling
pericardium
contained
1.4 ml
fluid with near ambient pressure and approximately 1 cm H20 pulse pressure. After handling fluid volume was 0.3 ml with decreased pericardial pressure and increased pulse pressure.
EFFECTS
Discussion
OF VOLUME LOADING
+ 15
End infusion. Slort swimming (20 see)
+1.0
+0.5 w
o
-0.5
- LO
o Molimunl • Minimum
-15
-20
~ 0
1
2
, 3
4
5
6
VOLUME INFUSED, m l
Fig. 5. Effects of volume loading of the pericardium of a horn shark. Following incremental infusion of elasmobranch saline into the pericardium, shark began 20 s of burst swimming, resulting in decreased pericardial pressure due to ejection of fluid through the PPC.
The importance of negative pericardial pressure for optimal cardiac function in elasmobranch fishes is widely accepted (Satchell 1971 ; Johansen and Burggren 1980; Shabetai et al. 1985). However, studies of horn sharks equipped with chronic pericardial catheters suggest that negative pericardial pressure is not necessary under all conditions and that an exclusively negative pericardial pressure may result from ejection of pericardial fluid through the PPC as a consequence of handling or after a period of rapid swimming. It is possible that a more negative pericardial pressure than we measured could be found in other elasmobranchs, or under other conditions. Our results were obtained exclusively from the horn shark, a nocturnally active species that remained inactive during daytime pressure measurements (we" were unable to measure pericardial pressure in swimming horn sharks because of the noise associated with the use of fluid-filled catheters). Our data suggest that pericardial pressure may
82 change when the shark is active, or differ in more active species, when activity induces ejection of pericardial fluid through the P P C (see below). Results from HF17, a very active individual that had relatively negative pressures before dislodging its chronic catheter, support this hypothesis (Fig. 2). It is also possible that negative surface pressures associated with a hydrodynamic pressure gradient during swimming in teleosts (Dubois et al., 1974) and sharks (Aleyev, 1977) could produce negative pressure in the pericardial region (Freadman, 1983). An exclusively negative pericardial pressure may also occur only as a consequence of ejection of fluid through the PPC, a view supported by our results. The volume of fluid in the pericardium, 2.0 mI-kg-l, is to our knowledge the largest known for any vertebrate. Humans, for example, possess only 0 . 2 - 0 . 7 ml.kg -1 (Shabetai 1981). A sufficient reserve is therefore available for ejection through the PPC. Shabetai et al. (1985) speculated that pericardial pressure sufficiently high to result in PPC opening could result from swimming or feeding, or increases in heart size associated with enlarged venous return, or an inadequate ability to absorb pericardial fluid. Only the last of these factors was examined and rejected. The large maximum pericardial fluid volume of horn sharks suggests that adequate space is available for cardiac expansion. Because the influence of pericardial pressure on cardiac dynamics under physiologic conditions is not known, the significance of the chronic pericardial pressure level that we found in horn sharks cannot be fully evaluated. However, because pericardial pulse pressure varies with stroke volume, changes in pericardial pulse pressure can be used to detect gross changes in cardiac output as pericardial pressure is varied. Table 4 shows how this measure of cardiac output changes as a result of infusion into the pericardium. Although the maximum value for the product o f pericardial pulse pressure and heart rate occurs when the pericardial cavity contains virtually no fluid, this is an unphysiological situation and the grossly distorted pericardial pressure waveform should not be included in the analysis. Although maximum cardiac output ap-
Table 4. Perieardial pulse pressure ( c m H 2 0 , heart rate, and estimate of cardiac output changes as pericardial fluid volume is varied. Data are from 5 horn sharks
% M a x i m u m pericardial fluid volume infused
Pericardial pulse pressure x-_+S.E.
Heart rate x-_+S.E.
Pulse pressure x heart rate
0 25 50 75 100
10.8_+2.6 1.1-+0.2 0.7-+0.2 0.2-+0.1 0.0-+ 0.0
34.6_+6.4 39.2+4.4 38.4_+ 3,4 41,4+4.4 40.4-+4,4
374 43 27 8 0
parently occurs when pericardial fluid volume is low, output is still relatively large at the pericardial fluid levels where chronically catheterized horn sharks were operating. No trace of cardiac output was evident at maximum pericardial fluid volume. In preliminary studies in which we have implanted electromagnetic flow probes on the ventral aorta of horn sharks, we have found similar results. It therefore appears that resting horn sharks with a physiologic pressure and volume are capable of maintaining a relatively high cardiac output. Furthermore, the presence of entirely-positive pulsatile pericardial pressures in some healthy, resting horn sharks on occasion implies that the mean pressure level may not be important when sufficiently large pericardial pressure fluctuations occur in synchrony with the cardiac cycle. These findings also suggest that pericardial pressure may have more subtle control over cardiac function than previously thought. Although positive pressures may be appropriate under resting or mildly active conditions, the pressure-volume curve indicates that a decreased pericardial pressure associated with ejection of fluid through the PPC could lead to increased cardiac output and greater tissue perfusion, factors important during more strenuous activities.
Acknowledgements We thank V. Bhargava, J.L. Roberts, G.H. Satchell, and F.N. White for helpful comments. Horn sharks were captured by R. McConnaughey and
83 R. West, R.H. Rosenblatt and G. Somero reviewed this manuscript. This study was supported by NSF Grant DCB-8416852 (Graham and Shabetai, coP.I.'s). DCA was supported by this grant, NSF Predoctoral and Hubbs Sea World Fellowships, and a grant from the Slocum-Lunz Foundation, Inc.
References cited Aleyev, Yu.G. 1977. Nekton. Dr W. Junk b.v., The Hague. Dubois, A.B., Cavagna, G.A. and Fox R.S. 1974. Pressure on the body surface of swimming fish. J. Exp. Biol. 60:581-591. Freadman, M.A. 1983. Pericardial cavity pressures in swimming fishes. Am. Zool. 23: 892. Hanson, D. 1967. Cardiovascular dynamics and aspects of gas exchange in chondrichthyes. Ph.D. dissertation, University of Washington. Johansen, K. 1965. Dynamics of venous return in elasmobranch fishes. Hval. skr. 48: 94-100. Johansen, K. and Burggren W. 1980. Cardiovascular function in lower vertebrates. In Hearts and Heart-like Organs, Vol. 1,
pp. 61-117. Comparative Anatomy and Development. Edited by G.H. Bourne. Academic Press, New York. Prosser, C.L. 1973. Comparative Animal Physiology. W.B. Saunders Co., Philadelphia. Satchell, G.H. 1971. Circulation in Fishes. Cambridge University Press, London. Shabetai, R. 1981. The Pericardium. Grune and Stratton, New York. Shabetai, R., Abet D.C., Graham J.B., Bhargava V., Keys R.S., and Witztum, K. I985. Function of the pericardium and pericardioperitoneal canal in elasmobranch fishes. Am. J. Physiol. 248: HI98-H207. Shortley, G., and Williams, D. 1967. Principles of College Physics. Prentice-Hall, New Jersey. Smith, H.W. 1929. The composition of the body fluids of elasmobranchs. J. Biol. Chem. 81: 407-419. Sudak, F.N. 1965a. lntrapericardial and intracardiac pressures and the events of the cardiac cycle in Mustelus canis (Mitchell). Comp. Biochem. Physiol. 14: 689-705. Sudak, F.N. 1965b. Some fact,ors contributing to the development of subatmospheric pressure in the heart chambers and pericardial cavity of Mustelus canis (Mitchill). Comp. Biochem. Physiol. 15: 199-215.
Elasmobranch pericardial function. I. Pericardial pressures are not always negative D.C. Abel* ~, J.B. Graham 1, W.R. Lowell I , R. Shabetai 2 I Physiological Research Laboratory, A-O04, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, U.S.A. ZDepartment o f Medicine, University o f California, San Diego, and Medical Service, Veterans Administration Medical Center, San Diego, CA 92161, U.S.A. Keywords: elasmobranch, heart, horn shark, pericardioperitoneal canal, pericardium, pressure-volume
Abstract
Acute studies have led to the generalization that negative pericardial pressure is necessary for optimal cardiac function in elasmobranchs. We chronically instrumented horn sharks with pericardial catheters to test the hypothesis that ejection of pericardial fluid through the pericardioperitoneal canal (PPC) during routine handling could have accounted in part for previous measurements of exclusively negative pressures ( - 0 . 3 to - 9 . 1 cm H2 O) in elasmobranchs. Maximum and minimum pericardial pressures measured immediately following routine handling (acute pressures) were more negative than those measured in resting horn sharks at intervals from 1 to 27 days following handling (chronic pressures). Chronic pericardial pulse pressure was less than acute. Entirely positive pericardial pressures were observed on occasion. Handling of chronically catheterized horn sharks resulted in ejection of 21 per cent (range = 10-26, n = 5) of the initial pericardial fluid volume through the P P C and reduced pericardial pressure. Operating pericardial fluid volume o f horn sharks averaged 2.0 ml.kg- ~ (range = 1.6-2.6, n = 9). The PPC opened after 4.3 _+ 0.2 ml.kg-' (x +_ S.E.) of elasmobranch saline had been slowly infused into the pericardium, corresponding to an average pressure of 1.3 _+ 0.2 cm H 2 0 (n = 10). The presence of the PPC plus a comparatively large pericardial fluid volume allows horn sharks to regulate pericardial pressure. Our analysis of pericardial pulse pressure, which can be an index of cardiac activity, suggests in contrast to previous studies that the elasmobranch heart can have relatively high stroke volumes at pericardial pressures near ambient. Thus, for venous return in resting or even moderately active elasmobranchs, it is more important that pericardial pressure be pulsatile than at a mean level which is negative.
Introduction
Heart function in elasmobranch fishes (sharks and rays) is believed to differ from that of other vertebrates because their relatively rigid pericardium allows the maintenance of a negative (subambient) pericardial pressure that becomes more negative during ventricular ejection (Sudak 1965a; Satchell
1971). Negative pressure is considered necessary to enhance return from the central venous sinuses by aspiration. If negative pressure is abolished by puncturing the pericardium, venous return and hence cardiac output drop drastically (Johansen t965; Sudak 1965b; Hanson 1967; Johansen and Burggren 1980). However, this conclusion remains less than certain for two reasons. Because only
* Present Address: Department of Biology, Collegeof Charleston, Charleston, SC 29424.
76 pericordiol forobronch
cork ftoo! ~
~'al~t
/ ~
~
iol
==
-5
i
\
o
o
o
c
y
TANK
STAND WITH TRANSDUCERS (1)
RACK WITH RECORDER AND CRT
Fig. 1. Experimental apparatus for measurement of chronic pericardial pressures in horn sharks. Fluid-filledtubes from pressure
transducers are attached to stopcocks on cork float leading from pericardialand orobranchialcatheters. Zero referencetube is shown below water surface. Pressure transducers on brick pedestal are connected to cathode ray tube (CRT) and 4-channel physiological recorder. ECG electrodes are not pictured. acute experiments, in many cases on anesthetized, supine, emergent specimens, have been performed, the continued presence and importance of negative pericardial pressure to cardiac function in elasmobranchs under more physiological conditions has not been examined. Furthermore, the pericardial and peritoneal spaces of elasmobranchs are connected.by a duct, the pericardioperitoneal canal (PPC), which may function as a pressure-regulating valve (Shabetai et al. 1985). Normally closed, the PPC allows one-way flow of pericardial fluid to the peritoneal cavity (Smith 1929; Satchell 1971), which lowers pericardial pressure (Shabetai et al. 1985). Because the pericardium is not entirely insulated from external pressures, it is possible that most of the previously reported acute pressure data were obtained after pericardial fluid had been ejected through the PPC as a result of handling. Thus, we hypothesize that routine pericardial pressures may not be as negative as those measured immediately following handling, and test this by measuring acute and chronic pericardial pressure in horn
sharks, Heterodontusfrancisci, fitted with an indwelling pericardial catheter.
Materials and methods Maintenance and instrumentation
Male and female horn sharks (0.5-2.5 kg) were captured off La Jolla, CA, and maintained in large aerated aquaria with continuously flowing seawater (14-24°C) in ambient photoperiod at Scripps Institution of Oceanography. They were fed cut mackerel once or twice weekly. At least 24 h before experimentation, post-absorptive sharks were transferred to laboratory holding tanks (3501) adjacent to physiological monitoring equipment (Fig. 1). To determine the effects of routine handling on pericardial pressure, we anesthetized each shark in its holding tank with tricaine methanesulfonate (MS222, Crescent Chemical Co., 0.05-0.06 g. 1-i), and kept it submersed and ventilated with aerated
77 anesthetic mixture while an acute pressure (see below) was measured. This was done by inserting a 19 ga. needle connected to a PE 190 tube filled with elasmobranch saline (Prosser 1973) through the skin and coracoid bar into the pericardial space (Shabetai et al. 1985) (Fig. 1). The shark was then fitted with a chronic catheter that permitted longterm monitoring of pericardial pressure. The needle used for acute measurement was removed and the hole enlarged to allow insertion of the tip of a PE 190 chronic catheter with terminal side holes. The shark was then removed from the water and placed supine on a V-board with continuous ventilation. The skin around the catheter entry site was dried with acetone and warm air, and the catheter epoxled in place. One ECG electrode (40 ga. tefloncoated silver wire, Cooner Wire Co.) was inserted into the pericardium with the pericardial catheter. Another was inserted through the skin and implantod into a lateral wall o f the pericardium by needle (23 ga.). The needle was then removed leaving the electrode in place. Catheters and wires were sutured to the shark and their ends attached to a cork float. The shark was then allowed to recover in its holding tank filled with seawater at ambient temperatures (14-24°C).
Acute and chron& pericardial pressure Acute pressure is defined as that measured immediately after induction of anesthesia and during handling and emersion before attaching the chronic catheter. Chronic pressure is that measured once or twice daily beginning from the day following catheter implantation for up to 27 days, and thus approximates the normal range of operating pressure. Pressure transducers (Statham P23ID) were calibrated weekly against l0 cm H 2 0 (sensitivity = 0.1 cm HEO). Care was taken when connecting the catheter and electrodes to the monitoring equipment to disturb the shark minimally, and five to ten minutes elapsed before data collection. Recordings were made over a 5 - 6 h period without disturbing the shark to validate this method of daily measurement. Because there was little variation in the pericardial pressure record for any one period,
a single random or representative pulse (wave) of pressure was selected for data analysis of both acute and chronic maximum and minimum pericardial pressures and the difference between them, termed "pericardial pulse pressure" (Shabetai et aL 1985). Means were compared using t-tests. If a leak was discovered in the chronic catheter system, the experiment was terminated and that day's results discarded. Pericardial pressures were referenced to a fluid-filled tube in the tank below the surface. Positioning of the zero line anywhere in the tank accurately describes ambient pressure for referencing pericardial pressure regardless of the depth o f the shark if both zero and reference pressure lines are connected to the same transducer, in accordance with Torricelli's law (Shortley and Williams 1967).
Pressure-volume characteristics o f chronically catheterized horn sharks Pressure-volume experiments The relationship between pericardial pressure and volume was assessed by incremental infusion (0.2-0.5 ml) of fluid into the pericardium of conscious, submersed, chronically catheterized horn sharks from which all fluid was first aspirated from the pericardium. Heart rate was continuously monitored and pericardial pressure was recorded after each infusion. When a volume sufficient to open the P P C had been infused (indicated by a plateau in pericardial pressure), all fluid was removed (see next paragraph) and its volume determined. This volume, representing the maximum pericardium capacity for increases in either heart size or pericardial fluid volume, will be termed " m a x i m u m pericardial fluid volume." The corresponding mean pericardial pressure is defined as " P P C opening pressure." Operating pericardial fluid volume To determine the fraction o f the maximum pericardial fluid volume present under routine conditions, we measured pericardial fluid volume periodically in nine chronically catheterized horn sharks. Accurate measurement o f pericardial fluid ( + O. 1 ml) was possible because the catheter tip is in the
78 lowest part of the pericardial space. The effectiveness of this method was verified in several postmortem examinations. Replicate measurements of maximum pericardial fluid volume in the same specimen revealed that the method is also precise (+ 0.05 ml). Following measurement, the same fluid was replaced.
I N I T I A L AND CHRONIC PERICARDIAL PRESSURES
,IMPLANTATION
Effects o f handling in chronically catheterized horn sharks
r Y v ~
0
-1 -2 -3 -4 -5 -6 -7
-1 -2 -4 6
9
HF 14
-3
HF 13
~
12 15 18 21 2 4 27
,
. . . . . . . .
3
6
9
12 15 18 21 2 4 2 7
+2 0 -2
The effects o f handling on pericardial pressure and volume were directly assessed in 5 chronically catheterized horn sharks. In these tests, pericardial pressure and volume were determined and then the shark was either lifted from the water and allowed to struggle, or grasped by the first dorsal spine and allowed to swim in place, each time for 30 to 45 s. Following this, pericardial pressure and volume were immediately measured. We also assessed the effects of handling after occluding the P P C in three specimens. After implantation o f the chronic pericardial catheter and electrodes, the abdomen was opened by midline incision, and the left lobe of the liver reflected. The PPC was then ligated in two places with surgical silk (# 00). Following verification of complete occlusion, the liver was replaced and the abdomen sutured. The handling experiment was conducted at least 24 h later.
Resulls
-4 O~
-6
==
-6
H F 15
[
~
16
-8 -8 = o
O..
• 2
• 4
. 6
, 8
i I0
i 12
14
+2
-2 -4
HF 17 J
0
2
4
6
8
i
H F 18 i
10 12
-4
. . . . . 0 2
' 4
' 6
. . . . 8
10
DAYAFTER IMPLANTATION
Fig. 2. Acute ("initial") and chronic pericardial pressures of six horn sharks ( H F I 3 - H F I S ) . Arrows at day zero indicate acute pressures. Pressures for each subsequent day are chronic. Maximum (circles) and minimum (dots) values o f a single pulse o f pericardial pressure are pictured for each day.
Lower than acute (p < 0.025; Table 1). In general, chronic maximum pericardial pressure was just above ambient, whereas chronic minimum pericardial pressure was slightly negative, although on occasion both phases were positive.
A cute and chronic pericardial pressure Fig. 2 shows daily maximum and minimum pericardial pressure-and pericardial pulse pressure for six of the ten chronically catheterized horn sharks. Acute pericardial pressure just after anesthesia and handling (day zero) was negative throughout the cardiac cycle and within the range of values reported for other elasmobranchs. Acute pulse pressure was high. In contrast, chronic pericardial pressure (day 1 and subsequently) was significantly higher (p < 0.025) than acute pressure, and in seven sharks chronic pulse pressure was significantly
Pericardial pressure-volume characteristics Pressure-volume experiments The pericardial pressure-volume curve for the horn shark is similar to those reported in acute experiments for a variety of species under anesthesia by Shabetai et al. (1985), and is typified by the curve of maximum and minimum pressures shown for a chronically catheterized specimen in Fig. 3. Following aspiration (zero volume), pericardial pressure is very negative (as low as - 4 0 cm HzO
79 Table 1. Acute and chronic pericardial pressures (cm HzO) and pericardial pulse pressures measured for 10 horn sharks Shark #
Acute pressure
Chronic pressure x _+ S.E. (n)
HF13 HF14 HFl5 HFI6 HFI 7
-4.8 -2.4 -4.8 -3.5 - 1.8
0.0 +0.5 + 1.0 +0.8 -0.7
_+ _+ _+ _+ _+
0.1 0.1 0.2 0.4 0.3
(25)* (45)* (17)* (17)* (10)*
HFI 8 HFI9 HF20 HF21 HF22
-1.8 -2.8 -3.5 --2.0
+0.4 +0.2 +0.3 +0.1 +0.6
_+ +_ _+ +_ _
0.1 0.1 0.2 0.2 0.3
(9)* (9)* (4)* (9) (6)*
PRESSURE VOLUME CURVE WITH CHRONIC VOLUMES
A, M a x i m u m pressure
-4 []
o Moximum
I-/ ~/
• Minimum
0. ~ 6
V
.t
B. Minimum pressure HFI 3 HFI4 HF15 HFI6 HF17 HFI8 HFI9 HF20 HF21 HF22
() -6.1 -4.2 -8.8 -7.5 -3.4 -3.8 -4.0 -7.0
-1.2 -0.6 -0.4 -0.6 - 1.7 -0.4 -0.7 -0.4
--6.0
- 0 . 8 _+ 0.2 (9) - 0 . 4 _+ 0.3 (6)*
C. Pulse pressure HFI3 HF14 HFI5 HFI6 HFI 7 HFI 8 HFI9 HF20 HF21 HF22
+ 0.1 _+ 0.2 + 0.3 _+ 0.1 _+ 0.4 _+ 0.1 _+ 0.1 _+ 0.3
(25)* (45)* (17)* (17)* (10)* (9)* (9)* (4)*
1.3 1.8 4.0 4.0
1.2 1.1 1.4 1.4
+ 0.1 + 0.1 ___ 0.2 _+ 0.1
(25) (45) + (17) + (17) +
1,6 2.0 1.2 3.5
1.0 0.8 1.0 0.8 0.9 1.1
_+ _+ _+ + _+ _+
(10) + (9) + (9) (4) + (9) (6) +
-4.0
0.1 0.2 0.1 0.2 0.1 0.1
* Acute < chronic (p < 0.025; t-tes0 + Acute > chronic (p < 0,025; t-test)
in some specimens), and pulse pressure is large (up to 40 cm HzO). As fluid is infused m a x i m u m pressure generally rises at a slow rate until P P C opening pressure is reached (approximately 1 cm H 2 0 in Fig. 3). Minimum pressure rises quickly at first then slowly until the P P C opens. At P P C opening (average = 0.8 to 3.0 cm HzO, Table 2), continued infusion does not result in further pres-
a Mean
.
I
I
1
2
.
.
,
3
i
4
,
i
5
I
i
i
I
6
7
8
9
PERICARDIAL FLUID VOLUME,ml Fig. 3, Pressure-volume curve for horn shark HF14. Circles and dots are the ma xi mum and minimum values of a single pulse of pericardial pressure, At zero volume pericardial pressures are very negative with a large pulse. Pressures rise and pulse pressure decreases as pericardial fluid volume is increased. At P P C opening pericardial pressures reach a ma xi mum with zero pulse pressure. Also shown in relation to this curve are the mean pericardiat pressures and corresponding volumes (squares) chronically measured for HFI4.
sure rise because the added volume is vented into the peritoneum, Pericardial pulse pressure is maximum at zero volume, and decreases as volume increases. Maximum pericardial fluid volumes obtained following pericardial volume infusions averaged 4.3 ml. kg- m(range = 3 . 7 - 5.0 ml. kg- ]; n = 10).
Heart rate changes Heart rate generally decreased following complete aspiration of the pericardial fluid, and increased upon subsequent infusion. In a typical case heart rate was 21 b p m at zero pericardial fluid volume and 48 b p m at maximum volume, and the response was reversible and repeatable. Heart rates of five horn sharks are also shown in Table 4. The high variability in this data set which masks the above trend can be attributed at least in part to seasonal temperature effects.
80 Table 2. Chronic pericardial fluid volumes and pressure-volume characteristics for 10 sharks Fish #
HFI3 HF14 HF15 HFI6 HFI7 HFI8 HFI9 HF20 HF21 HF22
Body wt (kg)
0.9 1.3 0.7 1.1 1.0 0.9 1.4 1.1 0.6 1.0
Pericardial fluid volume
Maximum pericardial fluid volume
Daily pericardial fluid volume as % maximum fluid volume
PPC opening pressure, cm H 2 0 _+ S.E. (n)
Daily, ml x ± S.E. (n)
ml per kg body wt
ml, ~" ± S.E, (n)
ml per kg body wt
1.4 3.1 1.8 1.7 No 2,2 3.5 2.4 1.0 1.6
1.6 2.4 2.6 1.5 -2.4 2.5 2.2 1.7 1.6
3.4 5,9 3.1 4.6 3.7 3.5 6.3 5.5 3.0 3.8
3.8 4.5 4.4 4.2 3.7 3.9 4.5 5.0 5.0 3.8
42 53 59 36
34 42
0.9 1.6 3.0 1.3 1.2 1.2 0.4 1.6 0.8 1.3
4.3
48
1.3
+ 0.1 +_ 0.1 ± 0.0 ± 0.2 data +_ 0.1 +_ 0.2 _+ 0.1 ± 0.1 _+_ 0.2
x
(6) (14) (2) (6) (10) (5) (4) (7) (5)
+ 0.2 ± 0.I _+ 0.1 ± 0.1 ± 0.0 ± 0.2 +_ 0.2 _+ 0.2 ± 0,1 _+ 0.2
(5) (4) (4) (4) (3) (4) (4) (5) (8) (5)
2.0
Pericardial fluid volumes The mean pericardial fluid volume of horn sharks was 2.0 +_ 0.1 ml.kg -~, x _+ S.E., Table 2), which represents an average of 48 per cent of the maximum pericardial fluid volume. Values for pericardial fluid volume and mean pericardial pressure observed for horn shark HF14 have been superimposed on its pressure-volume curve in Fig. 3 to show the range o f pericardial pressures and volumes at which it was operating under resting conditions and their relationship to the curve. These results are typical o f those obtained for other horn sharks.
Factors promoting PPC function: handling and pericardial volume loading Handling lowered pericardial pressure and volume in all sharks except those with ligated PPC. Ten to 26 per cent of the starting pericardial fluid volume was ejected through the PPC o f five horn sharks as a result of struggling (Table 3). Fig. 4 illustrates the record from one handling experiment in which 1.1 ml (78 per cent o f the starting volume) was ejected, resulting in a decrease in mean pericardial pressure from about - 0 . 5 to - 1 . 8 cm H 2 0 , and an increase in pulse pressure. We also observed that horn sharks would not
62 56 44
_+ 0.2 _+ 0.1 _+ 0.2 + 0.0 _+ 0.1 + 0.0 ± 0.3 +_ 0.1 +_ 0.2 _+ 0.1
(5) (4) (4) (4) (3) (4) (4) (5) (9) (3)
Table 3. The effects of handling episodes on pericardial fluid volume in 5 horn sharks Fish # Body wt (kg.)
Pericardial fluid volume (ml)
% Initial volume ejected
Before handling After handling ~ _+ S.E. (n) HFI8 HFI9 HF20 HF21 HF22
0.9 1.4 1.1 0.6 1.0
2.3 3.8 2.8 1.9 2.0
_+ 0.1 + 0.2 + 0.3 + 0.1 +_ 0.1
(5) (4) (4) (4) (4)
1.7 3,4 2.1 1.5 t.5
± ± + ± ±
0.2 0,2 0.2 0.1 0.1
26 10 25 21 25
tolerate the maximum pericardial volume load attained during pericardial pressure-volume experiments. The pressure-volume curve from a shark that began 20 s of vigorous swimming following a volume infusion into its pericardial space is shown in Fig. 5. After swimming, pericardial pressure was similar to control, suggesting that vigorous swimming acts in a manner similar to struggling to vent the pericardial fluid and that in this example virtually the entire infusion volume had been ejected through the PPC.
81
EFFECTS OF HANDLING BEFORE
AFTER
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PERICARDIAL FLUID VOL; 1.4ml Fig. 4.
Effects
of handling
episode
on
pericardial
pressure
and
volume
in a horn
shark.
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PERIC,~I,~DIALFLUID VOL= 0.3 rnl Before
handling
pericardium
contained
1.4 ml
fluid with near ambient pressure and approximately 1 cm H20 pulse pressure. After handling fluid volume was 0.3 ml with decreased pericardial pressure and increased pulse pressure.
EFFECTS
Discussion
OF VOLUME LOADING
+ 15
End infusion. Slort swimming (20 see)
+1.0
+0.5 w
o
-0.5
- LO
o Molimunl • Minimum
-15
-20
~ 0
1
2
, 3
4
5
6
VOLUME INFUSED, m l
Fig. 5. Effects of volume loading of the pericardium of a horn shark. Following incremental infusion of elasmobranch saline into the pericardium, shark began 20 s of burst swimming, resulting in decreased pericardial pressure due to ejection of fluid through the PPC.
The importance of negative pericardial pressure for optimal cardiac function in elasmobranch fishes is widely accepted (Satchell 1971 ; Johansen and Burggren 1980; Shabetai et al. 1985). However, studies of horn sharks equipped with chronic pericardial catheters suggest that negative pericardial pressure is not necessary under all conditions and that an exclusively negative pericardial pressure may result from ejection of pericardial fluid through the PPC as a consequence of handling or after a period of rapid swimming. It is possible that a more negative pericardial pressure than we measured could be found in other elasmobranchs, or under other conditions. Our results were obtained exclusively from the horn shark, a nocturnally active species that remained inactive during daytime pressure measurements (we" were unable to measure pericardial pressure in swimming horn sharks because of the noise associated with the use of fluid-filled catheters). Our data suggest that pericardial pressure may
82 change when the shark is active, or differ in more active species, when activity induces ejection of pericardial fluid through the P P C (see below). Results from HF17, a very active individual that had relatively negative pressures before dislodging its chronic catheter, support this hypothesis (Fig. 2). It is also possible that negative surface pressures associated with a hydrodynamic pressure gradient during swimming in teleosts (Dubois et al., 1974) and sharks (Aleyev, 1977) could produce negative pressure in the pericardial region (Freadman, 1983). An exclusively negative pericardial pressure may also occur only as a consequence of ejection of fluid through the PPC, a view supported by our results. The volume of fluid in the pericardium, 2.0 mI-kg-l, is to our knowledge the largest known for any vertebrate. Humans, for example, possess only 0 . 2 - 0 . 7 ml.kg -1 (Shabetai 1981). A sufficient reserve is therefore available for ejection through the PPC. Shabetai et al. (1985) speculated that pericardial pressure sufficiently high to result in PPC opening could result from swimming or feeding, or increases in heart size associated with enlarged venous return, or an inadequate ability to absorb pericardial fluid. Only the last of these factors was examined and rejected. The large maximum pericardial fluid volume of horn sharks suggests that adequate space is available for cardiac expansion. Because the influence of pericardial pressure on cardiac dynamics under physiologic conditions is not known, the significance of the chronic pericardial pressure level that we found in horn sharks cannot be fully evaluated. However, because pericardial pulse pressure varies with stroke volume, changes in pericardial pulse pressure can be used to detect gross changes in cardiac output as pericardial pressure is varied. Table 4 shows how this measure of cardiac output changes as a result of infusion into the pericardium. Although the maximum value for the product o f pericardial pulse pressure and heart rate occurs when the pericardial cavity contains virtually no fluid, this is an unphysiological situation and the grossly distorted pericardial pressure waveform should not be included in the analysis. Although maximum cardiac output ap-
Table 4. Perieardial pulse pressure ( c m H 2 0 , heart rate, and estimate of cardiac output changes as pericardial fluid volume is varied. Data are from 5 horn sharks
% M a x i m u m pericardial fluid volume infused
Pericardial pulse pressure x-_+S.E.
Heart rate x-_+S.E.
Pulse pressure x heart rate
0 25 50 75 100
10.8_+2.6 1.1-+0.2 0.7-+0.2 0.2-+0.1 0.0-+ 0.0
34.6_+6.4 39.2+4.4 38.4_+ 3,4 41,4+4.4 40.4-+4,4
374 43 27 8 0
parently occurs when pericardial fluid volume is low, output is still relatively large at the pericardial fluid levels where chronically catheterized horn sharks were operating. No trace of cardiac output was evident at maximum pericardial fluid volume. In preliminary studies in which we have implanted electromagnetic flow probes on the ventral aorta of horn sharks, we have found similar results. It therefore appears that resting horn sharks with a physiologic pressure and volume are capable of maintaining a relatively high cardiac output. Furthermore, the presence of entirely-positive pulsatile pericardial pressures in some healthy, resting horn sharks on occasion implies that the mean pressure level may not be important when sufficiently large pericardial pressure fluctuations occur in synchrony with the cardiac cycle. These findings also suggest that pericardial pressure may have more subtle control over cardiac function than previously thought. Although positive pressures may be appropriate under resting or mildly active conditions, the pressure-volume curve indicates that a decreased pericardial pressure associated with ejection of fluid through the PPC could lead to increased cardiac output and greater tissue perfusion, factors important during more strenuous activities.
Acknowledgements We thank V. Bhargava, J.L. Roberts, G.H. Satchell, and F.N. White for helpful comments. Horn sharks were captured by R. McConnaughey and
83 R. West, R.H. Rosenblatt and G. Somero reviewed this manuscript. This study was supported by NSF Grant DCB-8416852 (Graham and Shabetai, coP.I.'s). DCA was supported by this grant, NSF Predoctoral and Hubbs Sea World Fellowships, and a grant from the Slocum-Lunz Foundation, Inc.
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