Preload Dependence of Ri t Ventricular Blood Flow: I. The Norm P Right Ventricle Cornelius M. Dyke, A.B., Louis A. Brunsting, M.D., David R. Salter, M.D., Charles E. Murphy, M.D., Anwar Abd-Elfattah, Ph.D., and Andrew S. Wechsler, M.D. ABSTRACT Right ventricular (RV) failure is commonly treated with intravascular volume expansion to increase the RV-left atrial pressure gradient and improve left-sided filling. As RV pressure rises, chamber distention occurs and wall tension increases. These studies were designed to determine if increased wall tension might impede RV myocardial blood flow in the normal canine right ventricle and thus contribute to RV failure. Hemodynamic data, the septal-RV free wall dimension, and RV myocardial blood flow were obtained at low and high levels of preload and in both the autoregulated and vasodilated (adenosine, 2 mg per kilogram of body weight per minute) states. Ele- vated filling pressure decreased RV myocardial blood flow in both the autoregulated (0.85 2 0.18 to 0.67 k 0.15 ml/ midgm; p < .05) and vasodilated (2.25 k 0.50 to 0.85 2 0.25 mllmidgm; p < .05) states but did not change the transmural distribution of blood flow to the right ventricle. Vasodilator reserve was markedly impaired in the high- preload state. These observations suggest that preload is an important determinant of RV myocardial blood flow. Volume loading to treat RV dysfunction may be limited by impairment of RV myocardial blood flow. Right ventricular (RV) failure following a cardiac opera- tion or a myocardial infarction is recognized as an impor- tant clinical problem. It is manifested by low cardiac out- put, low left atrial pressures, and high right atrial pressures [l, 2). Intravascular volume expansion is a common therapeutic intervention for RV dysfunction and serves to increase the filling pressure in both ventri- cles. The effect of high filling pressures on myocardial blood flow, however, is not clear. Previous studies ex- amining the relationship between preload and left ven- tricular myocardial blood flow suggest that high levels of preload impede coronary blood flow and adversely af- fect the pressure-flow relationships of the coronary bed [3-91. Metabolic evidence of ischemia has been demon- strated in the left ventricular subendocardium with high filling pressure [9]. However, the relationship between From the Department of Surgery, Duke University Medical Center, Durham, NC. Presented at the Thirty-third Annual Meeting of the Southern Thoracic Surgical Association, White Sulphur Springs, WV, Oct 30-Nov 1, 1986. Address reprint requests to Dr. Wechsler, Box 3174, Duke University Medical Center, Durham, NC 27710. preload and blood flow to the right ventricle is much less well defined. This study was designed to examine the effects of dif- ferent levels of preload on RV myocardial blood flow independent of coronary driving pressure, local autoreg- ulatory responses, and ventricular afterload. A canine model that allowed independent variation of RV loading conditions was used to study RV myocardial blood flow, the transmural distribution of blood flow, and the coro- nary vasodilator reserve of the RV coronary arterial bed. Our results indicate that preload is an important deter- minant of blood flow to the normal right ventricle. Material and Methods Instrumentation All animals used in these studies 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 Labo- ratory Animals” prepared by the National Academy of Sciences and published by the National Institutes of Health (NIH Publication No. 80-23, revised 1985). Seven mongrel dogs weighing between 17.5 and 27.5 kg were anesthetized with intravenous administration of sodium pentobarbital (30 mg per kilogram of body weight), intubated, and ventilated with a Bennett MA1 respirator. A fluid-filled catheter was placed in the right carotid artery to monitor arterial pressure, and the heart was exposed by median sternotomy. The pericardium was opened and the heart suspended in a pericardial sling. In preparation for cardiopulmonary bypass, the azygos vein was ligated and the right phrenic nerve sec- tioned. The venae cavae were cannulated and snared with umbilical tape to ensure total venous return to the cardiopulmonary bypass pump. The left subclavian ar- tery was cannulated with the arterial bypass line. A hemispherical ultrasonic crystal was sewn onto the RV free wall in the mid-sinus region; care was taken to avoid compromising the coronary vasculature. Another crystal was placed in the interventricular septum in such a way that the septal-RV free wall dimension could be measured. Accuracy of crystal placement was verified at autopsy after each experiment. Micromanometer-tipped catheters (Mikro-tip pressure transducer, Millar Instru- ments, Houston, TX) were placed in the pulmonary ar- tery and right ventricle. Each catheter was physically and electrically calibrated. The reservoir system illustrated in Figure 1 was used to control loading conditions to the right ventricle. The 478 Ann Thorac Surg 43:478-483, May 1987
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