Effects of changing inspiratory to expiratory time ratio on carbon dioxide elimination during high-frequency jet ventilation

Clinical observations indicate that gas exchange during high-frequency ventilation can be improved by decreasing the inspiratory to expiratory time ratio (I/E). Decreasing I/E at a fixed frequency may therefore allow a reduction of tidal volume (VT) without affecting gas exchange. This decreased VT may result in lower airway pressures, and thereby reduce the incidence of barotrauma. In order to quantify the effects of I/E on gas exchange, the relationship between carbon dioxide elimination and VT was studied in 6 mongrel dogs at I/E of 1/2, 1/1, and 2/1. Ventilation was provided by a jet-type high-frequency ventilator using a frequency of 5 Hz and VT of 17 to 100% of the anatomic dead space. Carbon dioxide elimination was found to vary inversely with (I/E)^0.9, corroborating the clinical observations. These data add further evidence that I/E may be a useful parameter in optimizing high-frequency ventilation and provide a quantitative relationship that can be used to predict the effect that altering I/E will have on gas exchange.