EVALUATION OF MYOCARDIAL-CONTRACTILITY IN THE CHRONICALLY INSTRUMENTED DOG WITH INTACT AUTONOMIC NERVOUS-SYSTEM FUNCTION - EFFECTS OF DESFLURANE AND ISOFLURANE

Characterization of changes in myocardial contractility using indices derived from ventricular pressure-segment length loops in animals with intact autonomic nervous system (ANS) function is complicated by alterations in systemic hemodynamics mediated by baroreceptor reflexes and by spontaneous respiration and its effects on ventricular pressure and filling. This investigation examined and compared the effects of desflurane and isoflurane on myocardial contractility in dogs with intact ANS reflexes using techniques designed to overcome these potential difficulties. Two groups comprising 18 experiments were performed using nine dogs chronically instrumented for measurement of aortic and left ventricular pressure. the maximum rate of increase of left ventricular pressure (dP/dt), subendocardial segment length, cardiac output and intrathoracic pressure. A brief occlusion of the inferior vena cava was used to alter preload to generate pressure-length loops prior to onset of baroreceptor reflex-mediated increases in heart rate. Respiratory variation in ventricular pressure was negated by calculation of ''transmural pressure'' via instantaneous subtraction of intrathoracic pressure from corresponding left ventricular pressure. Contractility was then evaluated in the conscious and anesthetized states using transmural pressure-length loops and calculation of the preload recruitable stroke work (PRSW) relationship. Dogs were anesthetized with 1.25, 1.5, or 1.75 MAC desflurane or isoflurane and measurements were repeated after 30 min of equilibration at each anesthetic concentration. Desflurane and isoflurane produced similar declines in PRSW slope [M(w); 41 +/- 6 (5.5 +/- 0.8) during 1.75 MAC desflurane compared to 43 +/- 5 mmHg (5.7 +/- 0.7 kPa) during 1.75 MAC isoflurane], indicating that these agents cause similar depression of contractile state at equivalent MAC. The PRSW relationship derived from transmural pressure-length loops using brief inferior vena caval occlusion may facilitate the examination of the impact of other volatile and intravenous anesthetics on myocardial contractility using a load-independent index of contractile state in the presence of intact ANS reflexes.