SIGGAARD-ANDERSEN ALGORITHM-DERIVED P50 PARAMETERS - PERTURBATION BY ABNORMAL HEMOGLOBIN-OXYGEN AFFINITY AND ACID-BASE DISTURBANCES

The p50 and derived indexes, calculated by using the Siggaard-Andersen algorithm from a single measurement of arterial blood gas tensions an d hemoglobin-oxygen saturation, are used to assess tissue oxygen avail ability in critical illness. We tested the accuracy of the Siggaard-An dersen p50 algorithm over a wide range of pathophysiologic conditions. Blood gases, cooximetry, and calculation of standard and in vivo p50 were performed at multiple saturations, CO, tensions, and HC concentra tions on blood with normal (standard p50 of 26.1 and 26.7 mm Hg), incr eased (19.0 and 25.4), and reduced (33.9 and 38.2) hemoglobin-oxygen a ffinity, as well as on high-affinity blood from two patients with diab etic ketoacidosis (16.7 and 20.8). Log p50 in vivo/pH plots were const ructed to determine the Bohr effect. Except in the normal affinity spe cimens (coefficient of variation < 1.7%), standard p50 values showed h igh variability (coefficient of variation > 5.9%), with saturation-lin ked bias and distortion of the Bohr effect. Standard p50 was overestim ated by up to 11 mm Hg as saturation approached 97%. Although base def icit correction of the stored specimens (6.9 < pH < 7.1) restored the Bohr effect and improved the accuracy of standard p50 calculations (co efficient of variation = 4.4% and 2.9%), saturation-linked bias persis ted. We conclude that Siggaard-Andersen p50 calculations may be mislea ding when there are disturbances of hemoglobin-oxygen affinity and aci d-base balance, owing to changes in shape of the hemoglobin-dissociati on curve. When metabolic acidosis occurs with high hemoglobin-oxygen a ffinity, as can occur in critical illness, indexes derived by the Sigg aard-Andersen algorithm on arterial blood may greatly overestimate oxy gen availability.