Decreases in Mixed Venous Blood O 2 Saturation in Cardiac Surgery Patients Following Extubation.

BACKGROUND: Decreases in mixed venous O2 saturation (Sv O2 ) 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˙ O 2 ).

METHODS: 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 (Sa O2 ) and central venous O2 saturation (Scv O2 ) with an oximetry catheter. Mixed venous O2 saturation (Sv O2 ) and cardiac output were also measured continuously with an oximetry pulmonary artery catheter. Systemic O2 delivery and V˙ O 2 were calculated according to accepted formulae.

RESULTS: Immediately following extubation, Scv O2 and Sv O2 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 Sv O2 postextubation. All patients weaned successfully.

CONCLUSIONS: Decreases in Sv O2 after discontinuation of ventilatory support in postcardiac surgery patients occur as V˙ O 2 increases in response to greater energy requirements by muscles of ventilation that are not initially matched by increases in DO2 .