Airway Deadspace, End‐Tidal CO2, and Christian Bohr

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Abstract
To calculate alveolar deadspace, an important measure of ventilation/perfusion mismatching, it is necessary to measure airway of anatomical deadspace (VDaw) and physiological deadspace. VDaw is usually measured graphically or by similar means, but sometimes it is estimated from a formula, based on Christian Bohr''s work, in which end-tidal PCO2 [CO2 tension] is used as a measure of alveolar PCO2. In 58 patients undergoing anesthesia and positive pressure ventilation, there were large errors in this estimate of VDaw compared to a graphical method. At tidal volumes of 400-500 ml, the median error was 34 ml; at larger tidal volumes, the median error increased to 74 ml (P < 0.001). The size of the error was correlated to the slope of phase III, the part of the CO2 tracing representing alveolar CO2, at both ventilator settings (P < 0.01). Evidently, estimates of VDaw based on end-tidal PCO2 are unreliable, and their use will lead to a large part of the alveolar deadspace being wrongly accredited to VDaw.