Mixed Venous and Arterial Pco2

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Abstract
The rebreathing method of measuring oxygenated mixed venous Pco2 (Pv̄co2) was originally introduced as a bloodless way to estimate arterial Pco2 (Paco2). It has become common practice to subtract 6 mm Hg from the Pv̄co2 to obtain the Paco2 but there are many circumstances in which this leads to an overestimate of the Paco2. Measurements of Pv̄co2 and Paco2 in 19 patients have shown that a better approximation to Paco2 under normal conditions of cardiac output and arterial O2 saturation is Paco2 = 0·8 Pv̄co2. These studies also showed that the Pv̄co2 — Paco2 difference may be much wider, particularly in the presence of arterial unsaturation and a low cardiac output. The factors governing the venoarterial Pco2 difference are reviewed and their magnitude is calculated to emphasize the complementary roles of measurements of Pv̄co2 and Paco2 in the assessment of patients with cardiorespiratory disease.