Episodic ventilation lowers the efficiency of pulmonary CO2excretion

The ventilation pattern of many ectothermic vertebrates, as well as hibernating and diving endotherms, is episodic where breaths are clustered in bouts interspersed among apneas of varying duration. Using mechanically ventilated, anesthetized freshwater turtles ( Trachemys scripta), a species that normally exhibits this episodic ventilation pattern, we investigated whether episodic ventilation affects pulmonary gas exchange compared with evenly spaced breaths. In two separate series of experiments (a noninvasive and an invasive), ventilation pattern was switched from a steady state, with evenly spaced breaths, to episodic ventilation while maintaining overall minute ventilation (30 ml·min−1·kg−1). On switching to an episodic ventilation pattern of 10 clustered breaths, mean CO2excretion rate was reduced by 6 ± 5% (noninvasive protocol) or 20 ± 8% (invasive protocol) in the first ventilation pattern cycle, along with a reduction in the respiratory exchange ratio. O2uptake was either not affected or increased in the first ventilation pattern cycle, while neither heart rate nor overall pulmonary blood flow was significantly affected by the ventilation patterns. The results confirm that, for a given minute ventilation, episodic ventilation is intrinsically less efficient for CO2excretion, thereby indicating an increase in the total bodily CO2store in the protocol. Despite the apparent CO2retention, mean arterial Pco2only increased 1 Torr during the episodic ventilation pattern, which was concomitant with a possible reduction of respiratory quotient. This would indicate a shift in metabolism such that less CO2is produced when the efficiency of excretion is reduced.