Transcutaneous O2 and CO2 monitoring of neurosurgical patients: detection of air embolism.

Transcutaneous oxygen tension (PtcO2) and transcutaneous carbon dioxide tension (PtcCO2) were monitored in 60 patients undergoing neurosurgical procedures. Twenty-six patients were in the sitting position and underwent routine monitoring for air embolism. Seventeen episodes of air embolism were diagnosed by precordial Doppler ultrasound or transesophageal echocardiography, and the PtcO2 decreased early during the course of each episode. The mean PtcO2 decrease was 48 +/- 35 mmHg. During ten episodes the end-tidal carbon dioxide tension (PETCO2) decreased but only after the PtcO2 had already begun to decrease. PtcCO2 increased during air embolism but PETCO2 changes preceded the change in PtcCO2 by 1-2 min. Transcutaneous values during air embolism were verified with simultaneous arterial blood gas values during six air embolism episodes. A strong positive correlation was found between transcutaneous and arterial oxygen and carbon dioxide tensions. Correcting the PtcCO2 by the patient's baseline PtcCO2/PaCO2 ratio, PtcCO2 monitoring correctly reflected hypocarbia, normocarbia, and hypercarbia in 92% of the cases. PtcO2 monitoring was useful in detecting venous air embolism and may respond sooner than PETCO2. PtcCO2 monitoring was not useful as an early detector of air embolism.