Causes of oxyhaemoglobin saturation instability in the postoperative period

Healthy, young patients tend to have stable patterns of oxygen saturation (SaO2) 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 SaO2 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. SpO2 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 SpO2 was determined also at different values of PlO2 (21-50 kPa) before and after operation. Instability of SpO2, assessed over periods of 1 h, was common (n = 18) after operation; it was caused by both acute desaturations and a fluctuating baseline SpO2. The PlO2 vs SpO2 relationship was displaced after operation, with both lateral (rightwards) and downwards displacement. The result was to bring the PlO2 to or near to the steep part of the curve. In patients who displayed an unstable pattern of SpO2 over a short period of time (< 5 min), this was abolished by increasing PlO2. We postulate that the displacement of the PlO2 vs SpO2 curve increases the propensity to desaturation in those patients with impaired control by placing the steep part of the curve near to a PlO2 value of 21 kPa.