Arterial and transcutaneous oxygen tension measurement during hypotensive anaesthesia

The values of oxygen tension (PO2) as measured by transcutaneous (at two different sites) and invasive methods were compared during hypotensive anaesthesia with sodium nitroprusside (SNP) in 15 patients. Generally the PO2 significantly decreased during and increased after SNP-induced hypotensive anaesthesia (p less than 0.001). A positive correlation was found between the ratios of change of invasive arterial oxygen tension (PaO2) and supraclavicular transcutaneous oxygen tension (tcPO2) both during (r = 0.64) and after (r = 0.83) hypotension. A similar correlation existed between the ratios of change of PaO2 and forearm tcPO2 only after (r = 0.66) but not during (r =0.33) hypotensive anaesthesia. It is concluded that any of the above methods can be used to give an assessment of PO2 changes during SNP-induced hypotension provided the prehypotensive value is measured. For quantitative measurement of PO2 the invasive method is the first choice. Supraclavicular tcPO2, however, can give an accurate result, while the forearm tcPO2 is the least sensitive.