The P50 is reduced in critically ill patients.

A prospective study was designed to measure the P50 in 20 critically ill patients, and compare it with the P50 measured in 20 normal individuals. Arterial blood gases, lactate, haemoglobin (Hb) and phosphate (PO4) levels were also measured and compared with the P50 in the critically ill patients. The mean P50 of the critically ill patients was 24.5 mmHg (SD +/- 2.9) and was significantly lower than the mean P50 of 26.2 (SD +/- 2.2) in the normal individuals (p less than 0.05). In the critically ill patients, strong correlations were observed between the P50 and the arterial pH and base excess (BE) levels, with coefficients of 0.79 and 0.69 respectively whereas correlations between the P50 and arterial oxygen tension (PO2), carbon dioxide tension (PCO2), lactate, Hb and PO4 levels were poor, with correlations of 0.001, 0.008, 0.07, 0.13 respectively. It is concluded that the P50 is commonly reduced in critically ill patients, and has a strong correlation with arterial pH and BE.