Acid-base changes and ventilator mode during maintenance ventilation.

Assist-control ventilation was compared to intermittent mandatory ventilation (IMV) for respiratory support of 35 patients without known respiratory disease, who had undergone coronary bypass surgery. Spontaneous respiratory rates and minute ventilation did not differ significantly between the two groups. Blood gas samples obtained during the assist-control mode had a significantly lower PaCO2 and higher pH than blood gases measured during high-rate IMV. There were six cases of severe alkalemia during assist-control ventilation, all respiratory in origin. These were attributable to the higher respiratory-rate settings and the extra respirator-delivered tidal volumes when spontaneous rates were higher than the set rates. IMV was associated with fewer ventilator-induced acid-base changes.