Estimating Arterial Partial Pressure of Carbon Dioxide in Ventilated Patients: How Valid Are Surrogate Measures?

The arterial partial pressure of carbon dioxide (Pa) is an important parameter in critically ill, mechanically ventilated patients. To limit invasive procedures or for more continuous monitoring of Pa, clinicians often rely on venous blood gases, capnography, or transcutaneous monitoring. Each of these has advantages and limitations. Central venous Pco allows accurate estimation of Pa, differing from it by an amount described by the Fick principle. As long as cardiac output is relatively normal, central venous Pco exceeds the arterial value by approximately 4 mm Hg. In contrast, peripheral venous Pco is a poor predictor of Pa, and we do not recommend using peripheral venous Pco in this manner. Capnography offers measurement of the end-tidal Pco (Pet), a value that is close to Pa when the lung is healthy. It has the advantage of being noninvasive and continuously available. In mechanically ventilated patients with lung disease, however, Pet often differs from Pa, sometimes by a large degree, often seriously underestimating the arterial value. Dependence of Pet on alveolar dead space and ventilator expiratory time limits its value to predict Pa. When lung function or ventilator settings change, Pet and Pa can vary in different directions, producing further uncertainty. Transcutaneous Pco measurement has become practical and reliable. It is promising for judging steady state values for Pa unless there is overt vasoconstriction of the skin. Moreover, it can be useful in conditions where capnography fails (high-frequency ventilation) or where arterial blood gas analysis is burdensome (clinic or home management of mechanical ventilation).