DEXMEDETOMIDINE ALTERS THE HEMODYNAMIC-EFFECTS OF DESFLURANE AND ISOFLURANE IN CHRONICALLY INSTRUMENTED DOGS

Background: Previous studies have shown that desflurane and isoflurane produce similar hemodynamic actions. This investigation examined the cardiovascular effects of desflurane and isoflurane in the presence or absence of dexmedetomidine, a highly selective alpha2-adrenergic agon ist that may be clinically useful as a premedicant or anesthetic adjuv ant. Methods: Four groups, comprising 40 experiments, were performed u sing ten dogs that were chronically instrumented for measurement of ao rtic and left ventricular pressure, the maximum rate of increase of le ft ventricular pressure (dP/dt(max)), diastolic coronary blood flow ve locity, cardiac output, and subendocardial segment length. On separate experimental days, systemic and coronary hemodynamics were recorded, and plasma concentrations of catecholamines were measured with or with out oral dexmedetomidine Pretreatment (30 mug/kg) in the conscious sta te and after 15 min of equilibration at 1.0, 1.3, and 1.6 end-tidal MA C desflurane or isoflurane in a random fashion. Results: In conscious dogs, dexmedetomidine significantly decreased heart rate, cardiac outp ut, percent segment shortening (%SS), left ventricular dP/dt(max), myo cardial oxygen consumption (as estimated by the pressure-work index), and plasma norepinephrine concentration. Concomitant increases in syst emic and diastolic coronary vascular resistance were observed. Pretrea tment with dexmedetomidine decreased peak increases in heart rate duri ng desflurane and isoflurane anesthesia. Mean arterial pressure was re duced less by desflurane than by isoflurane in the absence of dexmedet omidine. This difference was abolished in dogs pretreated with dexmede tomidine. Desflurane, but not isoflurane, decreased cardiac output in dexmedetomidine-pretreated dogs when compared with untreated dogs. Con comitantly, systemic vascular resistance was greater in desflurane-ver sus isoflurane-anesthetized dogs pretreated with dexmedetomidine. No d ifferences in myocardial contractility, as assessed by left ventricula r dP/dt(max) and %SS, were observed between desflurane and isoflurane groups in the absence or presence of dexmedetomidine. Conclusions: The results indicate that the cardiovascular actions of desflurane or iso flurane are similar in the absence or presence of dexmedetomidine; how ever, some differences between anesthetic groups were noted. In the pr esence of dexmedetomidine, systemic vascular resistance during desflur ane anesthesia was higher when compared with that during isoflurane an esthesia, indicating that desflurane produces less pronounced direct e ffects on peripheral vascular tone. The concomitant greater reductions in cardiac output are consistent with greater impedance to left ventr icular outflow in desflurane-anesthetized dogs pretreated with dexmede tomidine, because no differences in contractile function were observed between volatile anesthetics. In contrast, cardiac output during isof lurane anesthesia after pretreatment with oral dexmedetomidine is bett er maintained secondary to the peripheral vasodilator actions of this agent.