Controlling Arterial Carbon Dioxide Tensions with Carbon Dioxide Gas during Mechanical Ventilation

Hyperventilation induced by mechanical ventilation has been a frequent problem, because large tidal or minute volumes are often required. Resulting hyperventilation can lead to undesirable arterial hypocapnia and respiratory alkalosis. It has been common to prevent these events by adding deadspace to the ventilator breathing circuit so the patient rebreathes CO 2 from his expired end-tidal gas. However, it is not easy to estimate the correct amount of deadspace to add to the circuit, and adjustment of the deadspace volume during therapy changes is cumbersome and often inaccurate. We therefore developed a method that does not use added deadspace but does add carbon dioxide gas to the gas that the ventilator delivers during inspiration. CO 2 is titrated into inspired gas by precision, CO 2 -calibrated flowmeters. We find that Pa CO 2 increases about 5 to 7 mm Hg for every 1 per cent increase in FI CO 2 . This method has been used in the ICUs of two Cleveland hospitals since April 1973 and permits easy normalization of pH and Pa CO 2 while permitting increases of Pa O 2 without change of FI O 2 . The method has proven safe in our hands, and it has been easier to control Pa CO 2 during mechanical ventilation by this means than by manipulating mechanical deadspace.