Gas exchange during constant flow ventilation (CFV)

Recent studies have demonstrated that adequate alveolar ventilation can be maintained by delivering a continuous flow of inspired ventilation at high flow rate (1-3 L/min/kg) at the carina, without tidal excursions. Despite normal PaCO₂ during CFV, the efficiency of gas exchange might be adversely affected due to hyperventilation of alveoli proximal to central airways and hypoventilation of alveoli located in distant acini. We tested this hypothesis by comparing gas exchange during conventional (IPPV) and CFV in anesthetized, paralyzed mongrel dogs in a body plethysmograph. Gas exchange was assessed by the steady state multiple inert gas elimination during IPPV, and during CFV at high (3 L/min/kg) or low (1 L/min/kg) flow rate at unchanged lung volume. During CFV at high flow rate, inert gas retention - excretion (R-E) differences were markedly increased for SF₆, ethane, cyclopropane, halothane and ether, compared to IPPV. These data could be fit to a 50 compartment parallel model of the lung with only small residual error. Resulting V(A)/Q distributions showed very broad distributions of blood flow during CFV (log SD = 1.5) compared to IPPV (log SD = 0.4). Low CFV flow rate at the same lung volume produced an improvement in the elevated R-E differences and a narrowing of the V/Q distribution, but a fall in CO₂ elimination. No evidence for gas phase diffusion limitation of high molecular weight gases was found. We conclude that the efficiency of gas exchange is substantially reduced in CFV in comparison to IPPV.