NONINVASIVE ESTIMATION OF VENOUS ADMIXTURE - VALIDATION OF A NEW FORMULA

We have developed a computer program that estimates venous admixture ( intra-pulmonary shunt) from four measurements: haemoglobin concentrati on, end-tidal carbon dioxide tension (PE'(CO2)), fractional inspired o xygen concentration (FIO2) and pulse oximetry (Sp(O2)). The formula wa s tested on patients in an intensive therapy unit by using it to estim ate shunt while it was measured simultaneously by a standard, invasive method. A total of 101 measurements were made in 29 patients. After c orrecting the systematic errors in the assumed differences between PE' (CO2) and arterial PCO2 and between Sp(O2) and co-oximetrically measur ed Sa(O2), and correcting for a trend in the arteriovenous oxygen conc entration difference (C(a-(v) over bar)(O2)) with shunt, the bias of t he non-invasive minus invasive shunt differences was negligible, with no significant dependence on shunt. The limits of agreement were then +/-16% shunt overall (+/-13% within patients). When Sa(O2) was used in stead of Sp(O2), the limits were +/-11% (+/-8% within patients).