Ann Thorac Surg 1998;65:663-666
© 1998 The Society of Thoracic Surgeons
Original Articles: Cardiovascular
Effect of Assisted Circulation on Left Ventricular Performance in a Canine Model
Yasuhiro Uozaki, MD,
Arata Murakami, MD,
Hidetsugu Asanoi, MD,
Shinji Ishizaka, MD,
Takuro Misaki, MD
First Department of Surgery, Toyama Medical and Pharmaceutical University, Toyama, Japan
Second Department of Internal Medicine, Toyama Medical and Pharmaceutical University, Toyama, Japan
Accepted for publication September 2, 1997.
Dr Uozaki, First Department of Surgery, Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama, 930-0194, Japan.
 |
Abstract
|
|---|
Background. Little is known about left ventricular performance during venoarterial bypass and left heart bypass (LHB) after cross-clamping the descending thoracic aorta. We evaluated the effects of venoarterial bypass and LHB on ventricular load optimization and left ventricular work efficiency.
Methods. We used the left ventricular conductance catheter and a micromanometer in 7 anesthetized mongrel dogs. We assessed preload by the end-diastolic volume, afterload by the effective arterial elastance, and left ventricular contractile properties by the slope of the end-systolic pressure–volume relationship. In addition, optimal ventricular arterial coupling (ratio of effective arterial elastance to slope of end-systolic pressure–volume relationship) and left ventricular work efficiency (ratio of external work to pressure–volume area) were calculated.
Results. The decrease in preload was much greater with LHB than venoarterial bypass. There were no significant differences in afterload and left ventricular contractility between venoarterial bypass and LHB. The ventricular arterial coupling during LHB was near 0.50 (0.69 ± 0.16) in the "best heart" condition (effective arterial elastance = slope of end-systolic pressure–volume relationship/2), whereas the work efficiency during LHB was at maximum (0.73 ± 0.12).
Conclusions. We conclude that LHB has a more beneficial effect on left ventricular performance after cross-clamping of the descending thoracic aorta.
|
Introduction
|
|---|
The surgical treatment of descending thoracic aneurysms requires use of a bypass device, such as venoarterial bypass (VAB) or left heart bypass (LHB), because cross-clamping of the descending thoracic aorta proximal to the left subclavian artery dramatically increases left ventricular afterload. Care must be taken in the selection of the type of bypass device, especially in patients with ischemic heart disease and low output syndrome. Although some reports [1][2][3][4] have demonstrated changes in cardiac function during simple aortic cross-clamping and changes in cardiac function during LHB or with the use of left ventricular assist devices after cross-clamping [5][6], there are few reports comparing left ventricular performance during VAB and LHB.
In the present study, we assessed left ventricular performance during VAB and LHB after cross-clamping the descending thoracic aorta using a left ventricular conductance catheter [7]. We assessed left ventricular function with respect to preload (end-diastolic volume), afterload (effective arterial elastance; Ea), and left ventricular contraction (left ventricular elastance; Ees), which was calculated from the left ventricular pressure–volume loop. The purpose of this study was to determine whether VAB or LHB is better for bypassing in terms of the coupling of the cardiac and vascular systems. These estimates were based on the optimal ventricular arterial coupling (Ea/Ees) [8][9] and left ventricular work efficiency (EW/PVA; EW, external work; PVA, pressure–volume area) [10][11].
|
Material and Methods
|
|---|
Operative Procedures
Seven mongrel dogs weighing 10 to 17 kg (average weight, 12.6 ± 3.27 kg) were used in this study. All animals received humane care in compliance with the "Principles of Laboratory Animal Care" formulated by the National Society for Medical Research and the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (NIH publication 85-23, revised 1985).
All dogs were anesthetized with ketamine (20 mg/kg, intramuscularly) and pentobarbital sodium (25 mg/kg, intravenously) and mechanically ventilated through an endotracheal tube. Anesthesia was maintained with 1% to 2% halothane and 40% oxygen. After a median sternotomy was performed, the pericardium was opened, the inferior vena cava and the descending thoracic aorta distal to the left subclavian artery were taped, and a micromanometer (Konigsberg P7; Konigsberg Instruments Inc, Pasadena, CA) was inserted near the left ventricular apex. To determine left ventricular volumes, an eight-electrode conductance catheter was used with appropriate electrode distances to match the left ventricular dimensions and was passed antegrade through the aortic valve. Fluid-filled polyvinyl catheters were placed in the left ventricle to calibrate the manometer. The animal was then given heparin (3 mg/kg). Venoarterial bypass and LHB were established using a centrifugal pump (Sarns 7850; Sarns Inc/3M, Ann Arbor, MI) and a membrane oxygenator (MENOX A-L2000; Kurare Inc, Okayama, Japan). An electromagnetic blood flowmeter (10 mm in diameter, FF-100T; Nihon Koden Inc, Tokyo, Japan) was placed between the pump and the oxygenator to measure blood flow. A return cannula (10F) was inserted into the right femoral artery, and withdrawal cannulas were inserted into the right atrium (for VAB) and the left atrium (for LHB). Venoarterial bypass and LHB were established by clamping the withdrawal cannulas to the other bypass circuit. The circuits were primed with 240 mL of homologous fresh blood and 10 mL of 7% sodium bicarbonate. Pacemaker leads were implanted into the left atrial appendage.
Left Ventricular Volumes
Left ventricular volumes were determined using the principles and techniques of the Baan volume catheter method [12][13][14] with some modifications. Briefly, the method is based on measuring the temporal variance of the electrical conductance of five segments of blood in the left ventricle from which total ventricular volume was calculated. A low-amplitude alternating current was applied between the proximal and distal electrodes. To obtain the desired volume variable, conductance measurements and analog computations were performed by signal-conditioner processor (Sigma 5, Leycom, Oegstgeest, the Netherlands). The analog signals were recorded on an eight-channel oscillograph (Nihon Koden). These signals were then digitized with an on-line analog-to-digital converter at a sampling rate of 333 Hz and analyzed with a personal computer system (PC-9801RX; NEC, Tokyo, Japan).
Left Ventricular Mechanics
Ventricular End-Systolic Pressure–Volume Relationship, Effective Arterial Elastance, and Ventricular Load Optimization
Hemodynamic variables measured with the conductance catheter technique were defined as described below. The inferior vena cava was transiently occluded to determine the end-systolic pressure–volume relationship. End-systole was defined as peak instantaneous value for the ratio of left ventricular pressure to volume. This relationship is described by the equation: 
where ESP and ESV are the left ventricular end-systolic pressure and volume, respectively, and Ees and V0 are the slope and the volume axis intercept, respectively, of the end-systolic pressure–volume relationship. Vascular loading was quantified by the effective arterial elastance (Ea), which is approximately equal to the total mean arterial resistance divided by cardiac cycle length. The two parameters Ea and Ees were used to determine the ventricular-to-vascular coupling ratio (Ea/Ees), as previously described by Sunagawa and colleagues [8][9].
Left Ventricular Work
According to Suga and colleagues [10][11], the total mechanical energy generated by ventricular ejection is equal to the area under the systolic pressure–volume curve, which is defined by the end-systolic pressure–volume line, the end-diastolic pressure–volume curve, and the systolic segment of the pressure–volume trajectory. Experimentally, this measurement has been shown to correlate with myocardial oxygen consumption. The pressure–volume area (PVA) of ventricular ejection is determined by two factors: end-systolic potential energy and external stroke work, calculated as 
where EDV is end-diastolic volume. We also calculated the ratio of external work (EW) to PVA, which represents the left ventricular work efficiency. Work efficiency is theoretically related to the ratio of arterial elastance to ventricular elastance (Ea/Ees). The relationship is described by the following equation [15][16][17][18]: 
Experimental Protocol
After the descending thoracic aorta was clamped, baseline measurements of the pressure-volume loop were obtained by inferior vena cava occlusion. After this, the LHB withdrawal cannula was clamped and the VAB pump was started at a constant flow (15 mL · min-1 · kg-1). Hemodynamics were allowed to stabilize and measurements were obtained. Then, the VAB withdrawal cannula was clamped and the LHB pump was started at a constant flow (15 mL · min-1 · kg-1). To evaluate the effects of VAB and LHB on left ventricular function, the end-systolic pressure–volume relationship was assessed during VAB and LHB by inferior vena cava occlusion. The femoral artery pressure was monitored to control the bypass flow. The heart rate was held constant by atrial pacing at 120 beats/min.
Statistical Analysis
Results are expressed as the mean ± standard error of the mean. To analyze hemodynamic changes, a one-factor analysis of variance for repeated measures was performed with the Scheffé post-hoc F test. Two-tailed paired Student’s t tests were used to compare changes in hemodynamic parameters between VAB and LHB. A value of p less than 0.05 was considered statistically significant.
|
Results
|
|---|
Hemodynamic Responses
Table 1 shows the hemodynamic changes during the baseline period and during VAB and LHB for the 7 dogs. There were no significant differences in the parallel conductance of the surrounding tissue, end-diastolic pressure, ESP, and V0 at baseline, after clamping of the aorta, or during either VAB or LHB.
End-diastolic volume, ESV, and stroke volume decreased significantly (p < 0.05) during LHB compared with those values during the baseline period. There were also significant (p < 0.05) differences in end-diastolic volume, ESV, and stroke volume between VAB and LHB.
Ventricular Load Optimization
The value of Ea during either VAB or LHB was significantly lower than the value for Ea during the baseline period (Fig 1). There were no significant differences in Ea and Ees during VAB and LHB. As shown in Fig 1, the Ea/Ees was 1.90 ± 0.87 during the baseline period, decreasing significantly to 0.83 ± 0.22 during VAB and to 0.69 ± 0.16 during LHB.
View larger version (13K):
[in this window]
[in a new window]
|
Left ventricular load optimization. Effective arterial elastance (Ea) during both venoarterial bypass (VAB) and left heart bypass (LHB) were significantly less (p < 0.05) than Ea during the baseline period. Left ventricular load optimization (Ea/Ees) during LHB was significantly less (p < 0.05) than Ea/Ees during the baseline period. (Ees = left ventricular elastance.)
|
|
Left Ventricular Work Efficiency
The EW during LHB decreased significantly compared with the baseline value (Fig 2). Further, EW was significantly lower during LHB compared with VAB. The value for PVA during VAB and LHB was significantly less than the value for PVA during the baseline period. As shown in Fig 2, EW/PVA was 0.49 ± 0.17 during the baseline period, 0.65 ± 0.16 during VAB, and 0.73 ± 0.12 during LHB. Compared with the baseline period, EW/PVA was significantly greater during LHB.
View larger version (13K):
[in this window]
[in a new window]
|
Left ventricular work efficiency. Left ventricular external work (EW) during left heart bypass (LHB) was significantly less (p < 0.05) than EW during the baseline period. Pressure–volume area (PVA) during both venoarterial bypass (VAB) and LHB were significantly less (p < 0.05) than PVA during the baseline period. Left ventricular work efficiency (EW/PVA) during LHB increased significantly (p < 0.05) compared with EW/PVA during the baseline period.
|
|
|
Comment
|
|---|
In the present study we attempted to determine whether VAB or LHB improved left ventricular performance after cross-clamping of the descending thoracic aorta using changes in the work efficiency (EW/PVA). The predictability of this efficiency would help quantitate changes in cardiac mechanical efficiency under normal and pathologic conditions. We used a volumetric conductance catheter to study the left ventricles in anesthetized dogs in vivo using a well-established procedure [18]. This method has been very useful in studying VAB and LHB, because the end-systolic pressure–volume relationship is a relatively load-independent index of contractile state [5][19][20].
Hemodynamic Responses
Preload, as assessed by the end-diastolic volume, decreased significantly during LHB compared with VAB. Although VAB decreases right ventricular preload by reducing venous return, a portion of the blood from the right ventricle circulates to the left atrium. In addition, blood from the bronchial artery returns to the left atrium. Therefore, during VAB the left atrium has a significant amount of blood return. If the left ventricle cannot eject a sufficient blood volume against the arterial afterload developed by pump flow during VAB, then left atrial pressure will increase. In this respect, VAB differs from LHB, which directly withdraws blood from the left atrium.
Ventricular Load Optimization
Enhanced afterload, defined by Ea and caused by clamping the descending thoracic aorta, decreased significantly during both VAB and LHB. However, both VAB and LHB affected the measures of afterload (Ea) and left ventricular contractility (Ees) equally.
Asanoi and associates [15][16][17][21][22] have determined the optimal ventricular load coupling under normal and pathologic conditions. Sunagawa and coworkers [8][9] predicted and experimentally validated that maximum left ventricular external work occurs when ventricular contractility (Ees) and arterial elastance (Ea) are matched (Ea/Ees = 1; best load) in a physiologically loaded canine heart. Similarly, Burkhoff and Sagawa [15] hypothesized that maximum mechanical efficiency was attained when arterial elastance is half the value of ventricular elastance (Ea = Ees/2; best heart). In the present studies, we demonstrated that a coupling of approximately 1 (Ea = Ees) during VAB achieved maximum left ventricular external work. In addition, a coupling in which Ea = Ees/2 during LHB achieved maximum mechanical efficiency. In this "best heart" condition, the cardiac output could be maintained without left ventricular overload.
Left Ventricular Work Efficiency
Suga and colleagues [10][11] have shown there is a correlation between the PVA and myocardial oxygen consumption. Enhanced myocardial oxygen consumption (or PVA), caused by clamping the descending thoracic aorta, decreased during both VAB and LHB. The work efficiency of cardiac contraction expresses the fraction of total mechanical energy that is converted into external work. The work efficiency increases linearly with reductions in the ratio of arterial to ventricular elastance (Ea/Ees). However, work efficiency does not change with mechanical efficiency in a similar fashion because the mechanical efficiency, which is the ratio of external work to myocardial oxygen consumption, is the product of work efficiency (EW/PVA) and the ratio of myocardial oxygen consumption to pressure–volume area [17]. In the present studies, although EW decreased during LHB, work efficiency (EW/PVA) increased with LHB.
Decreases in PVA have been shown to be much greater in the failing heart than in the normal heart [5]. Although we did not study the effects of bypass on left ventricular performance in the failing heart, we hypothesize that the PVA would be decreased. Work efficiency and its dependence on Ees and Ea could be assessed using the methods described in this study to obtain a better understanding of ventriculoarterial coupling in the failing heart.
Conclusion
Both VAB and LHB decreased afterload and pressure–volume area after cross-clamping the descending thoracic aorta. Neither VAB nor LHB affected LV contractility. However, the decrease in preload was much greater during LHB than during VAB. The ventriculoarterial coupling during LHB simulated best-heart conditions (Ea = Ees/2), and the work efficiency (EW/PVA) during LHB was at maximum. Furthermore, the effect of LHB on left ventricular performance after cross-clamping the descending thoracic aorta was more beneficial than VAB.
|
References
|
|---|
- Suga H Time course of left ventricular pressure–volume relationship under various extents of aortic occlusion. Jpn Heart J 1970;11:373-378.[Medline]
- Roberts AJ, Nora JD, Hughes WA, et al. Cardiac and renal responses to cross-clamping of the descending thoracic aorta. J Thorac Cardiovasc Surg 1983;86:732-741.[Abstract]
- Gupta KB, Bavaria JE, Ratcliffe MB, Edmunds LH, Jr, Bogen DK Measurement of end-systolic pressure–volume relations by intra-aortic balloon occlusion. Circulation 1989;80:1016-1028.[Abstract/Free Full Text]
- Mandelbaum I, Webb MK Left ventricular function during cross-clamping of the descending thoracic aorta. JAMA 1963;186:229-231.
- Kawaguchi O, Sapirstein JS, Daily WB, Pae WE, Pierce WS Left ventricular mechanics during synchronous left atrial–aortic bypass. J Thorac Cardiovasc Surg 1994;107:1503-1511.[Abstract/Free Full Text]
- Kawaguchi O, Sapirstein JS, Daily WB, Pae WE, Pierce WS Linear end-systolic pressure–volume relationship during pulsatile left ventricular bypass represents native heart function. J Thorac Cardiovasc Surg 1995;109:780-786.[Abstract/Free Full Text]
- Asanoi H, Ishizaka S, Kameyama T, Nozawa T, Miyagi K, Sasayama S Serial reproducibility of conductance catheter volumetry of left ventricle in conscious dogs. Am J Physiol 1992;262:H911-H915.
- Sunagawa K, Maughan L, Burkhoff D, Sagawa K Left ventricular interaction with arterial load studied in isolated canine ventricle. Am J Physiol 1983;245:H773-H780.
- Sunagawa K, Maughan WL, Sagawa K Optimal arterial resistance for the maximum stroke work studied in isolated canine left ventricle. Circ Res 1985;56:586-595.[Abstract/Free Full Text]
- Suga H, Igarashi Y, Yamada O, Goto Y Mechanical efficiency of the left ventricle as a function of preload, afterload, and contractility. Heart Vessels 1985;1:3-8.[Medline]
- Suga H Ventricular energetics. Physiol Rev 1990;70:247-277.[Free Full Text]
- Bann J, Aouw Jong TT, Kerkhof PLM, et al. Continuous stroke volume and cardiac output from intra-ventricular dimensions obtained with impedance catheter. Cardiovasc Res 1981;15:328-334.[Medline]
- Baan J, van der Velde ET, de Bruin HG, et al. Continuous measurement of left ventricular volume in animals and humans by conductance catheter. Circulation 1984;70:812-823.[Abstract/Free Full Text]
- Kass D, Yamazaki T, Burkhoff D, Maughan WL, Sagawa K Determination of left ventricular end-systolic pressure–volume relationships by the conductance (volume) catheter technique. Circulation 1986;73:586-595.[Abstract/Free Full Text]
- Burkhoff D, Sagawa K Ventricular efficiency predicted by an analytical model. Am J Physiol 1986;250:R1021-R1027.
- Nozawa T, Yasumura Y, Futaki S, Tanaka N, Uenishi M, Suga H Efficiency of energy transfer from pressure–volume area to external mechanical work increases with contractile state and decreases with afterload in the left ventricle of the anesthetized closed-chest dog. Circulation 1988;77:1116-1124.[Abstract/Free Full Text]
- Kameyama T, Asanoi H, Ishizaka S, Sasayama S Ventricular load optimization by unloading therapy in patients with heart failure. J Am Coll Cardiol 1991;17:199-207.[Abstract]
- Matsuwaka R, Matsuda H, Shirakura R, et al. Changes in left ventricular performance after global ischemia: assessing LV pressure–volume relationship. Ann Thorac Surg 1994;57:151-156.[Abstract]
- Suga H, Sagawa K, Shoukas AA Load independence of the instantaneous pressure volume ratio of the canine left ventricle and effects of epinephrine and heart rate on the ratio. Circ Res 1973;32:314-322.[Abstract/Free Full Text]
- Suga H, Sagawa K Instantaneous pressure–volume relationships and their ratio in the excised, supported canine left ventricle. Circ Res 1974;35:117-126.[Abstract/Free Full Text]
- Asanoi H, Sasayama S, Kameyama T Ventriculoarterial coupling in normal and failing heart in humans. Circ Res 1989;65:483-493.[Abstract/Free Full Text]
- De Tombe PP, Jones S, Burkhoff D, Hunter WC, Kass DA Ventricular stroke work and efficiency both remain nearly optimal despite altered vascular loading. Am J Physiol 1993;264:H1817-H1824.
Top
Abstract
Introduction
