Causes of oxyhaemoglobin saturation instability in the postoperative period

Abstract

Healthy, young patients tend to have stable patterns of oxygen saturation (Sa02) in the perioperative period, but unstable patterns of oxygen saturation data in the postoperative period have been described which are associated with wide fluctuations in the value of Sa02 over short periods of time and with a propensity for severe desaturation. This investigation was designed to study the cause of these unstable, hypoxaemic patterns. Sa02 was measured by pulse oximetry in an "at risk" patient group (n = 20) before and after operation and displayed as sequential distribution diagrams and centile plots. In 16 of these patients Sp02 was determined also at different values of Pl02 (21-50 kPa) before and after operation. Instability of Sa02, assessed over periods of 1 h, was common (n = 18) after operation; it was caused by both acute desaturations and a fluctuating baseline Sp02. The Pl02 vs Sa02 relationship was displaced after operation, with both lateral (rightwards) and downwards displacement. The result was to bring the Sa02 to or near to the steep part of the curve. In patients who displayed an unstable pattern of Sp02 over a short period of time (<5 min), this was abolished by increasing Pl02. We postulate that the displacement of the Pl02 vs Sa02 curve increases the propensity to desaturation in those patients with impaired control by placing the steep part of the curve near to a Pl02 value of 21 kPa. (Br. J. Anaesth. 1993; 71: 481-487) © 1993 British Journal of Anaesthesia.