The dose-dependent effects of halothane on right ventricular contraction pattern and regional inotropy in swine

The right ventricle (RV) is comprised of two embryologically distinct units, the inflow and outflow tracts, which normally contract sequentially and differ in the magnitude of increased inotropy during sympathetic nervous stimulation. The present study examined the dose-response effects of halothane on the RV contraction pattern and regional contractility in seven open-chest pigs instrumented for measurement of inflow and outflow tract pressures and segment lengths. The RV contraction pattern was evaluated by comparing the phase of inflow and outflow tract shortening, and regional contractility was determined by calculation of preload recruitable stroke work (PRSW) slope. Using this methodology, an inflow-outflow tract contraction phase difference of -27 degrees (inflow tract shortened earlier) was evident at baseline, but was abolished 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 alterations in cardiac sympathovagal balance played a role in the RV response to halothane, an additional four animals were studied after pretreatment with hexamethonium, propranolol, and atropine. In these animals, there was no difference in the regional contraction phase either at baseline or during halothane administration, and dose-related depression of PRSW by halothane was again similar in both regions. However, when halothane effects on regional PRSW in animals with autonomic blockade were compared to those of neurally intact animals, a 20% greater depression of outflow tract PRSW by 0.5 MAC halothane was evident. This study demonstrates that halothane abolishes the normal sequential pattern of RV contraction without exerting markedly variant negative inotropic effects within different regions of the RV, and provides evidence to suggest that alterations in cardiac sympathovagal balance may contribute to the effect of halothane on RV contraction dynamics.