A RECENT review of the published literature demonstrated a wide range of perioperative mortality rates, which are probably caused by differences in operational definitions and reporting sources, as well as a lack of appropriate risk stratification. 1In this issue of Anesthesiology, Sprung et al. report on another perioperative outcome, cardiac arrest. 2As with perioperative mortality, the literature is replete with studies of cardiac arrest data using a variety of definitions and reporting sources. In the current study, Sprung et al. bring us closer to the development of an appropriate risk stratification model by examining predictors of immediate survival, and survival to discharge from the hospital, following cardiac arrests during anesthesia care. The authors also show a declining rate of perioperative arrests at their institution. But, as with perioperative mortality, we must take a critical look at the effect that methodologic differences have on the interpretation of the data.
This Editorial View accompanies the following article: Sprung J, Warner ME, Contreras MG, Schroeder DR, Beighley CM, Wilson GA, Warner DO: Predictors of survival following car-diac arrest in patients undergoing noncardiac surgery: A study of 518,294 patients at a tertiary referral center. Anesthesiology 2003: 99:259-69.
Sprung et al. state that “the two most recent studies of anesthesia-attributable mortality 1,3each found somewhat higher rates” than theirs. The authors then caution against direct comparison of these studies because the study by Newland et al. included cardiac surgery. In fact, the study by Newland et al. was not a study of anesthesia-related mortality but instead a study of anesthesia-related cardiac arrests that resulted in death. Because an arrested heart is one of the criteria for death, one might assume that the difference is trivial. Keep in mind, however, that an arrested heart is not one of the criteria for a cardiac arrest as defined by Newland or Sprung. Both of these investigators define a cardiac arrest as an event requiring cardiopulmonary resuscitation with closed chest compressions or open cardiac massage. Just as “studies of perioperative mortality alone do not include patients successfully resuscitated from cardiac arrest,” these studies of cardiac arrest do not include patients who died without cardiac compression. For example, the outcomes database maintained by the Department of Anesthesiology at Montefiore Medical Center contains 253 deaths within 2 days after the procedure, but only 110 of these involved a cardiac arrest during anesthesia care. Of these 110 deaths, peer review judged that the anesthesiologist contributed to only five of them (approximately 1:36,000 anesthetics).
Of particular interest, “unstable patients whose arrest occurred after an anesthetic induction agent was given were not considered as having had an anesthesia-attributable cardiac arrest (regardless of the fact that anesthesia may have contributed)” in the current study. This effectively removes the patients at highest risk of death from the anesthesia-related mortality rates reported by Sprung et al. at the Mayo Clinic. Not only are these patients at high risk of death, but their death are also more likely to involve a human error by an anesthesiologist. 1Previous investigators who chose not to include high-risk patients have also reported lower anesthesia-related mortality rates. 4Continuing the comparison between Montefiore Medical Center and the Mayo Clinic, three of the five above-noted anesthesia-related deaths that followed a cardiac arrest at Montefiore Medical Center involved unstable patients. Therefore, following the methodology of Sprung et al. , Montefiore Medical Center has an anesthesia-related mortality rate of about 1 in 90,000 anesthetics, which is nearly identical to that of the Mayo Clinic. Similarly, the exclusion of unstable patients from anesthesia-attributable arrests at the Mayo Clinic may also explain the improved survival rate when compared with the report by Newland et al. from the University of Nebraska Medical Center. Sprung et al. should also consider their exclusion criteria when concluding that their most common etiology of anesthesia-related arrest “contrasts with other studies of anesthesia-related arrest.”
The declining incidence of cardiac arrest at the Mayo Clinic over the duration of the study period is also quite interesting. The authors do not supply enough data points for statistical process control to detect a trend, but this could probably be overcome by sampling more frequently (e.g. , monthly rather than annually). As suggested by the authors, “the decrease in the frequency of perioperative cardiac arrest may imply a significant improvement in patient care.” Keep in mind, however, that cardiac arrests are defined as events requiring cardiopulmonary resuscitation with cardiac compressions while under an anesthesiologist's care. With changing guidelines for Advanced Cardiac Life Support, the Mayo Clinic anesthesiologists may have become more successful at resuscitating patients with early defibrillation and high-dose epinephrine before cardiac massage. Another possibility is that the emergence of critical care as a subspecialty of anesthesiology has resulted in more patients being transferred directly to the intensive care unit for immediate postoperative management, effectively shortening the observation time of the study patients.
Despite the room for varying interpretation, Sprung et al. are to be congratulated for creating and maintaining an exceptional outcomes database. The Mayo Clinic continues to lead the way for researchers in anesthesiology investigating perioperative outcomes. It is now time for these leaders to join forces with other investigators to standardize the methods of data collection and analysis, so that data can be shared worldwide. Large international data pools will allow us to develop risk adjustment models and identify best practices. Before we can discuss the fruits of our labor in anesthesia care, we must ensure that we are not talking about apples and oranges.
