Effects of frequency, tidal volume, and lung volume on CO2 elimination in dogs by high frequency (2-30 Hz), low tidal volume ventilation

Abstract

Recent studies have shown that effective pulmonary ventilation is possible with tidal volumes (V(T)) less than the anatomic dead-space if the oscillatory frequency (f) is sufficiently large. We systematically studied the effect on pulmonary CO2 elimination (V̇CO2) of varying f (2-30 Hz) and V(T) (1-7 ml/kg) as well as lung volume (V(L) in 13 anesthetized, paralyzed dogs in order to examine the contribution of those variables that are thought to be important in determining gas exchange by high frequency ventilation. All experiments were performed when the alveolar P(CO2) was 40±1.5 mm Hg. In all studies, V̇CO2 increased monotonically with f at constant V(T). We quantitated the effects of f and V(T) on V̇CO2 by using the dimensionless equation V̇VO2/V̇CO2 by using the dimensionless equation V̇CO2/V̇(OSC) = a(V(T)/V(TO)(b)(f/fo)(c) where: V̇(OSC) = f x V(T), V(TO) = mean V(T), fo = mean f and a, b, c, are constants obtained by multiple regression. The mean values of a, b, and c for all dogs were 2.12 x 10-3, 0.49, and 0.08, respectively. The most important variable in determining V̇CO2 was V̇(OSC); however, there was considerable variability among dogs in the independent effect of V(T) and f on V̇CO2, with a doubling of V(T) at a constant V̇(OSC) causing changes in V̇CO2 ranging from -13 to +110% (mean = +35%. Increasing V(L) from functional residual capacity (FRC) to the lung volume at an airway opening minus body surface pressure of 25 cm H2O had no significant effect on V̇CO2.