INFLUENCE OF CARDIAC ACTION ON GAS MIXING IN CLOSED-CHEST DOGS

We performed single-breath tests in closed-chest, paralyzed, and anest hetized dogs (6 with bilateral vagotomy and 6 with intact vagi) with t he heart beating and during cardiac arrest. Repeated cardiac arrest wa s achieved by ventricular fibrillation and subsequent defibrillation. Twenty-four single-breath tests per dog were performed in combinations of three inspiratory volumes (VI; 0.2, 0.5, and 0.8 liter) and four p ostinspiratory pauses (0, 5, 10, and 30 s), either with or without car diac arrest. The test gas contained four inert relatively insoluble ga ses (He, Ne, Ar, and SF6) with a sixfold range in diffusivity. Series dead space (VD) decreased with increasing postinspiratory pause, incre asing gas diffusivity, or decreasing VI. In vagotomized animals, VD wa s smaller with the heart beating than during cardiac arrest, but this relationship was reversed in animals with intact vagi. The decrease in VD due to cardiogenic mixing accounted for only 10.8% of the total de crease in VD occurring during a 30-s postinspiratory pause. The slope of phase III decreased with increasing postinspiratory pause except at VI of 0.2 liter. No significant differences were noted in the slope o f phase III between experiments performed with the heart beating or ar rested. Tracer gas retained in the residual volume after expiration in creased with increasing inspiratory pause. Gas retention was greater f or He than for SF6 but was not affected by cardiac action. These findi ngs indicate that cardiac action mainly affects the interface between inspired and alveolar gas and has little effect on mixing in the alveo lar compartment.