Decreases in mixed venous O₂ saturation (S_vO₂) have been reported to occur in postcardiac surgery patients during weaning from mechanical ventilation. Our aim was to establish whether the physiological mechanism responsible for this phenomenon was a decrease in systemic O₂ delivery (DO₂) or an increase in global O₂ consumption (V˙V˙ O ₂).

We studied 21 mechanically ventilated, postoperative cardiac patients for 30 minutes before and 60 minutes after extubation. We monitored continuously arterial O₂ saturation by pulse oximetry (S_aO₂) and central venous O₂ saturation (S_cvO₂) with an oximetry catheter. Mixed venous O₂ saturation (S_vO₂) and cardiac output were also measured continuously with an oximetry pulmonary artery catheter. Systemic O₂ delivery and V˙V˙ O ₂ were calculated according to accepted formulae.

Immediately following extubation, S_cvO₂ and S_vO₂ decreased rapidly (P < .01). Systemic O₂ consumption increased from 65 (57) mL·min⁻¹ to 194 (66) mL·min⁻¹ (P < .05) with no changes in DO₂. Consequently, systemic O₂ extraction rose from 38% (8%) to 45% (9%; P < .01). Preoperative left ventricular ejection fraction correlated with the decline in S_vO₂ postextubation. All patients weaned successfully.

Decreases in S_vO₂ after discontinuation of ventilatory support in postcardiac surgery patients occur as V˙V˙ O ₂ increases in response to greater energy requirements by muscles of ventilation that are not initially matched by increases in DO₂.