Since 1994, a case-control study into perioperative morbidity and mortality is being undertaken in the Netherlands under the auspices of the “Committee on Anesthesia Related Severe Morbidity and Mortality.” The case-control method, considered by some epidemiologists as controversial, has never before been strictly applied to questions concerning the outcome and safety of anesthesia.

The main objectives of this research are to study severe morbidity and mortality as a function of anesthesia related risk factors, to assess frequency of these events, and to formulate suggested preventive measures that should improve patient safety.

Severe morbidity in this study is defined as coma, unintentionally persisting 24 hours after an anesthetic procedure, and mortality is defined as death during or within 24 hours of undergoing anesthesia. A case is a patient fulfilling the aforementioned criteria, a control person is a postanesthetic patient carefully matched for gender and age. The study area comprises three of the 12 Dutch provinces, representing 55% of the total population, which have an estimated 400,000 procedures annually. Of the 64 eligible hospitals, 61 agreed to participate. The study base from which cases and controls are sampled consists of all patients undergoing an anesthetic procedure, general or regional, in one of these hospitals from January 1, 1995, to January 1, 1997. The study is financially supported by the Dutch Association for Anesthesiology and the Ministry of Health. The latter provided protection of anonymity and confidentiality by indemnifying the investigators from revealing knowledge on events as collected for the purpose of the study.

Multivariate Analysis

Data on risk factors related to anesthesia, the practice of anesthesia, the patient, and the surgical procedure are collected by means of two structured questionnaires. At baseline, information on the anesthetic practice and hospital characteristics is collected. Immediately following an event, a second questionnaire is filled in by the anesthesiologist providing data on the actual anesthetic procedures during the event and on the anesthesia, surgery and patient related putative risk factors.

When a case is reported, two appropriate controls are randomly drawn from two randomly selected hospitals, for whom an identical questionnaire is obtained. Furthermore, an anesthetic and recovery form is submitted to the study center for all study subjects. All information is stripped of identifiers so as to render it anonymous and strictly confidential, but still retaining a unique identification number known only at the study center and the hospital involved.

The data will be evaluated in both a quantitative and qualitative way. The quantitative analysis follows the general approach for a case-control study. Estimates of relative risk will be obtained to disclose possible anesthesia-related risk factors that may have been causally implicated in the event. With respect to the qualitative analysis, the information gathered during the study is viewed as an extended case series. All cases will be classified as to the nature and the characteristics of the event. Cases are classified according to two different classification systems to assess appropriateness of care, to assess the contribution of anesthesia to the event, and to identify contributing factors. Each case is classified twice, and in the event of differing opinions, by a third person. The team of classifiers comprises all anesthesiologists of the Committee on Morbidity and Mortality. It is complemented by anesthesiologists trained in the different university teaching hospitals and currently employed outside the study region. So far, anesthesiologists are participating with great enthusiasm.

Of the 61 participating hospitals, 88% have submitted at least one case since the start of data collection. The total number of cases collected in 1995 is 376, including 11 comatose patients. At first, the anesthesiologists needed stimulation to submit controls but, in the second year, seem to be improving their performance with respect to this, after encouragement and informative instructional sessions. Data collection will be going on for another year. It is hoped that a preview summary may appear in the APSF Newsletter after a manuscript based on the results is accepted for publication.

Drs. Arbous, Grobbee, van Kleef and Meursing are from the Erasmus University, Rotterdam, The Netherlands.