Hypoxemia during surgery: learning from history, science, and current practice

Abstract

Prevention of hypoxemia remains one of the primary goals related to the safe practice of anesthesia. Development of the pulse oximeter has allowed for the reliable detection and prompt treatment of hypoxemia. As reviewed by Drs. Ehrenfeld et al. in this issue of the Journal, routine intraoperative application of pulse oximetry coupled with parallel increases in patient safety has made a significant impact on the practice of anesthesia. Careful conduct of anesthesia based on real-time knowledge of blood oxygenation (SpO2) stands in stark contrast with anesthesia practice patterns of previous eras in which short-term maintenance of anesthesia with high concentrations of nitrous oxide was regarded as effective and safe in healthy pre-oxygenated patients. With the invention and use of the oxygen electrode, it gradually became apparent that profound decreases in arterial oxygen tension occurred with the former method of anesthetic induction. Nonetheless, several clinical studies were continued wherein high concentrations of nitrous oxide with reciprocally low concentrations of inspired oxygen were explored. This practice led to the frequent occurrence of clinically significant hypoxemia. In their classic article published a half century ago, Heller et al. reported the following with respect to induction of anesthesia: ‘‘It is preferable to start with an 80/20 mixture of N2O and O2 rather than to subject the anxious wakeful patient to a preliminary period of (100%) oxygen breathing.’’ Although such approaches were developed at a time of limited availability of anesthetic agents, this practice – thankfully differs dramatically from our current approach. These days, we often advocate the avoidance of nitrous oxide, and pre-oxygenation is advocated routinely as one means to prevent hypoxemia. The large database of the current study helps us to appreciate further the frequency and risks of perioperative hypoxia, moving us forward from the days of ‘‘black gas anesthesia’’. In their report in this issue of the Journal, Ehrenfeld et al. present the incidence of hypoxemia during surgery using data they derived from a sizeable series of electronic records from two large medical centres. The authors report that hypoxemia (SpO2 90%) is infrequent (6.8%) but perhaps too common for an entity that anesthesiologists always try to avoid. Indeed, severe hypoxemia (SpO2 85%) occurred in 3.5% of patients. They also reported that transient hypoxemia occurred during all phases of anesthesia, especially during induction and emergence and particularly in patients who were American Society of Anesthesiologists (ASA) physical status III or IV. Aside from recognizing that we have not ‘‘conquered’’ intraoperative hypoxemia, what other important lessons can be learned from these data? First, we should reiterate the most obvious positive news. Acceptable levels of SpO2 ([ 90%) were maintained G. M. T. Hare, MD, PhD Department of Anesthesia, Keenan Research Centre in the Li Ka Shing Knowledge Institute and the Cara Phelan Centre for Trauma Research, St. Michael’s Hospital, Toronto, ON, Canada