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The accuracy of in vivo P50 at high haemoglobin saturation
1Department of Intensive Care, Nambour General Hospital, Queensland, Australia.
The in vivo P50 (P50iv) provides a useful index of haemoglobin-oxygen affinity and is calculated according to software algorithms incorporated into commercial blood gas analysers. These algorithms are known to be inaccurate at high haemoglobin saturation (SpO2 > 97%) although just how inaccurate has not been documented. This study examines the arterial blood gas profiles of patients admitted to a busy secondary referral Intensive Care Unit and stratifies them according to haemoglobin saturation in order to quantify the accuracy and potential clinical utility of the Siggaard-Andersen algorithm (SAA) for assessing P50iv in blood with SpO2 > 90%. Sicker patients, as identified by plasma pH < 7.35 or [lactate] > 2.0 mmol/l, were substratified and the SAA assessed as before. In both groups, the results show not only that the SAA is completely unreliable above 97% saturation, a fact acknowledged by Siggaard-Andersen in 1984, but it is also inaccurate in the range 92% < or = SpO2 < or = 97%, thus rendering P50iv calculations suspect in 90% of the patients in each of the study groups.
The in vivo P50 (P50iv) provides a useful index of haemoglobin-oxygen affinity and is calculated according to software algorithms incorporated into commercial blood gas analysers. These algorithms are known to be inaccurate at high haemoglobin saturation (SpO2 > 97%) although just how inaccurate has not been documented. This study examines the arterial blood gas profiles of patients admitted to a busy secondary referral Intensive Care Unit and stratifies them according to haemoglobin saturation in order to quantify the accuracy and potential clinical utility of the Siggaard-Andersen algorithm (SAA) for assessing P50iv in blood with SpO2 > 90%. Sicker patients, as identified by plasma pH < 7.35 or [lactate] > 2.0 mmol/l, were substratified and the SAA assessed as before. In both groups, the results show not only that the SAA is completely unreliable above 97% saturation, a fact acknowledged by Siggaard-Andersen in 1984, but it is also inaccurate in the range 92% < or = SpO2 < or = 97%, thus rendering P50iv calculations suspect in 90% of the patients in each of the study groups.