Effects of Nitrous Oxide on Myocardial Contractility as Evaluated by the Preload Recruitable Stroke Work Relationship in Chronically Instrumented Dogs

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Abstract
Assessment of the effects of nitrous oxide on myocardial contractile function in vivo has been complicated by lack of a reliable, easily quantified, load-independent index of contractility and by the presence of intact autonomic nervous system reflexes. Although several previous investigations in humans and experimental animals have demonstrated that nitrous oxide possesses direct negative inotropic effects, this conclusion remains controversial. This investigation reeexamined the effect of nitrous oxide on myocardial contractile function when this agent was combined with baseline isoflurane or sufentanil anesthesia in chronically instrumented dogs. Contractility was evaluated with the use of the regional preload recruitable stroke work (PRSW)-end-diastolic segment length relationship, a method that provides an accurate, relatively afterload-independent assessment of inotropic state in conscious and anesthetized dogs. Because autonomic nervous system tone may influence the response of systemic hemodynamics to anesthesia in vivo, experiments were performed in the presence of pharmacologic blockade of the autonomic nervous system. Two groups of experiments, consisting of a total of 15 experiments, were performed with 12 dogs. Dogs, chronically instrumented for measurement of systemic hemodynamics, including left ventricular pressure and subendocardial segment length, were anesthetized with isoflurane or sufentanil. Thirty percent and 70% nitrous oxide were then administered in a random fashion. Left ventricular pressure-segment length loops were generated after 30 min of equilibration after each anesthetic intervention with the use of preload reduction by partial inferior vena caval construction, and regional PRSW was calculated. Regional PRSW versus end-diastolic length slope reflected decreases in contractile state when nitrous oxide was added to isoflurane (50 .+-. 5 for isoflurane alone to 28 .+-. 2 erg .cntdot. cm-2 .cntdot. 10-3 .cntdot. mm-1 with 70% added nitrous oxide) or sufentanil (73 .+-. 8 for sufentanil alone to 52 .+-. 5 erg .cntdot. cm-2 .cntdot. 10-3 .cntdot. mm-1 with 70% added nitrous oxide). Similar decreases in left ventricular positive dP/dt50 were observed as well, reflecting decreases in contractile function. The results further suggest that the degree of functional depression produced by nitrous oxide is nearly equal when isoflurane and sufentanil groups are compared. This study demonstrates that nitrous oxide possesses direct negative inotropic actions independent of changes in autonomic nervous system tone in the chronically instrumented dog.