Estimating Arterial Partial Pressure of Carbon Dioxide in Ventilated Patients: How Valid Are Surrogate Measures?
The arterial partial pressure of carbon dioxide (Symbol) is an important parameter in critically ill, mechanically ventilated patients. To limit invasive procedures or for more continuous monitoring of Symbol, clinicians often rely on venous blood gases, capnography, or transcutaneous monitoring. Each of these has advantages and limitations. Central venous Pco2 allows accurate estimation of Symbol, differing from it by an amount described by the Fick principle. As long as cardiac output is relatively normal, central venous Pco2 exceeds the arterial value by approximately 4 mm Hg. In contrast, peripheral venous Pco2 is a poor predictor of Symbol, and we do not recommend using peripheral venous Pco2 in this manner. Capnography offers measurement of the end-tidal Pco2 (Symbol), a value that is close to Symbol when the lung is healthy. It has the advantage of being noninvasive and continuously available. In mechanically ventilated patients with lung disease, however, Symbol often differs from Symbol, sometimes by a large degree, often seriously underestimating the arterial value. Dependence of Symbol on alveolar dead space and ventilator expiratory time limits its value to predict Symbol. When lung function or ventilator settings change, Symbol and Symbol can vary in different directions, producing further uncertainty. Transcutaneous Pco2 measurement has become practical and reliable. It is promising for judging steady state values for Symbol 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).