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

Abstract

A B S T R A C T Recent studies have shown that effective pulmonary ventilation is possible with tidal volumes (VT) less than the anatomic dead-space if the oscillatory frequency (f) is sufficiently large. We systematically studied the effect on pulmonary CO2 elimination (VCO2) of varying f (2-30 Hz) and VT (1-7 ml/kg) as well as lung volume (VL) 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 PCO2 was 40±+1.5 mm Hg. In all studies, VCO2 increased monotonically with f at constant VT. We quantitated the effects of f and VT on VCo2 by using the dimensionless equation VCO2/ Vosc = a(VT/VTO)b(f/fo)c where: Vosc = f x VT, VTO = mean VT, 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 VCO2 was Vosc; however, there was considerable variability among dogs in the independent effect of VT and f on VCO2, with a doubling of VT at a constant Vosc causing changes in VCO2 ranging from -13 to + 110% (mean = +35%). Increasing VL from functional residual capacity (FRC) to the lung volume at an airway opening minus body surface pressure of 25 cm H20 had no significant effect on VCO2.

Frequency, Tidal Volume, and Mean Airway Pressure Combinations that Provide Adequate Gas Exchange and Low Alveolar Pressure during High Frequency Oscillatory Ventilation in Rabbits

“…Perhaps it is not appropriate to think of an optimal frequency or an optimal V, in isolation. Slutsky et al (3,4 ) found C02 elimination to be dependent on V,,, as determined by the product of oscillatory frequency and V,. Slutsky concluded that although V,,, is an important factor in determining CO2 elimination, frequency and V, have independent effects with V, having a Veater effect on C 0 2 elimination than frequency at any given V,,,.…”

“…We observed that many combinations of frequency and V, produced equivalent blood gas tensions and that the composite variable f x V : was a good descriptor of our mean arterial blood gas tensions during HFOV in healthy rabbits. Others have found that different combinations of frequency, V, and Paw can produce equivalent blood gas tensions (3,4) but the resultant pressure swings in the trachea and alveolus may not be equivalent. Near the resonant frequency of the respiratory system, pressure swings in the alveolus may exceed those at the airway opening during HFOV (8,9).…”

Gas Exchange during High Frequency Flow-Interruption in Rabbits before and after Bronchoalveolar Lavage

“…This characteristic is not confined to HFFI ventilation because VT larger than dead space have also been reported during jet ventilation (6,20,21). However, unlike other forms of HFV, in which VT decreases with increasing ventilation rate (9,22,23), the VT recorded HFFI were independent of ventilation rate (Fig. 3).…”

Reflections on Pediatric High-Frequency Oscillatory Ventilation From a Physiologic Perspective

“…62 Oscillator settings include oscillatory power setting (magnitude of membrane displacement), frequency (f), in Hertz (Hz), inspiratory to expiratory ratio, position of the membrane, endotracheal tube (ETT) length and diameter, and the presence of ETT leakage. 20,66,67 The ETT constitutes the major work load to the oscillator and is an important determinant of V T . 68,69 V T is proportional to the ETT inner cross-sectional area, because the impedance of the ETT exceeds the impedance of the lung.…”