THE DOSE-DEPENDENT EFFECTS OF HALOTHANE ON RIGHT-VENTRICULAR CONTRACTION PATTERN AND REGIONAL INOTROPY IN SWINE

The right ventricle (RV) is comprised of two embryo-logically distinct units, the inflow and outflow tracts, which normally contract sequent ially and differ in the magnitude of increased inotropy during sympath etic nervous stimulation. The present study examined the dose-response effects of halothane on the RV contraction pattern and regional contr actility in seven open-chest pigs instrumented for measurement of infl ow and outflow tract pressures and segment lengths. The RV contraction pattern was evaluated by comparing the phase of inflow and outflow tr act shortening and regional contractility was determined by calculatio n of preload recruitable stroke work (PRSW) slope. Using this methodol ogy, an inflow-outflow tract contraction phase difference of -27 degre es (inflow tract shortened earlier) was evident at baseline, but was a bolished by 1.0 and 1.5 minimum alveolar anesthetic concentration (MAC ) halothane; PRSW slope of both the inflow and outflow tracts, however , demonstrated similar dose-related change. To determine whether alter ations in cardiac sympathovagal balance Flayed a role in the RV respon se to halothane, an additional four animals were studied after pretrea tment with hexamethonium, propranolol, and atropine. In these animals, there was no difference in the regional contraction phase either at b aseline or during halothane administration, and dose-related depressio n of PRSW by halothane was again similar in both regions. However, whe n halothane effects on regional PRSW in animals with autonomic blockad e were compared to those of neurally intact animals, a 20% greater dep ression of outflow tract PRSW by 0.5 MAC halothane was evident. This s tudy demonstrates that halothane abolishes the normal sequential patte rn of RV contraction without exerting markedly variant negative inotro pic effects within different regions of the RV, and provides evidence to suggest that alterations in cardiac sympathovagal balance may contr ibute to the effect of halothane on RV contraction dynamics.