Detection of venous air embolism by continuous mixed venous oximetry in dogs

Continuous mixed venous oxygen saturation (SvO 2) was evaluated as a monitor of venous air embolism in a canine model. Nineteen dogs were anesthetized, paralyzed, and mechanically ventilated. Invasive monitoring included SvO 2, systemic and pulmonary artery blood pressures, and thermodilution cardiac outputs. Air boluses of 0.25 and 0.5 ml/kg were injected in six dogs and 1 ml/kg in all. All 1 ml/kg emboli were detected by greater than or equal to 5% decreases in the SvO 2. The SvO 2 decreased from 82 +/- 8% to 72 +/- 11% (mean +/- SD), an average decrease of 9 +/- 5% (p = 0.004). Time to the SvO 2 nadir was 2.6 +/- 2.5 min. Of the 0.5 and 0.25 ml/kg emboli, 50% and 17% were detected, respectively. Cardiac output decreased from 2.9 +/- 0.8 to 2.1 +/- 0.8 L/min after the 1 ml/kg emboli (p = 0.02). The 1 ml/kg emboli increased pulmonary artery pressures and decreased systemic blood pressure in 100% and 75% of animals, respectively. Peak changes in pulmonary artery pressure occurred at 1.2 +/- 0.8 min. In the present study, time to maximum change was greater for SvO 2 than for pulmonary artery pressure changes. Use of fiberoptic pulmonary artery catheters for continuous measurement of SvO 2 can add a new diagnostic modality to venous air embolism detection in patients who require a pulmonary artery catheter for other medical indications.