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.