CUMULATIVE RISK-ADJUSTED MORTALITY CHART FOR DETECTING CHANGES IN DEATH RATE - OBSERVATIONAL STUDY OF HEART-SURGERY

Objective: To detect changes in mortality after surgery with allowance being made for variations in case mix. Design: Observational study of postoperative mortality from January 1992 to August 1995. Setting: Re gional cardiothoracic unit. Subjects: 3983 patients aged 16 and over w ho had open heart operations. Main outcome measures: Preoperative risk factors and postoperative mortality in hospital within 30 days were r ecorded for all surgical heart operations. Mortality was adjusted for case mix using a preoperative estimate of risk based on additive Parso nnet factors. The number of operations required for statistical power to detect a doubling of mortality was examined, and control limits at a nominal significance level of P = 0.01 for detection of an adverse t rend were determined. Results: Total mortality of 7.0% was 26% below t he Parsonnet predictor (P < 0.0001). There was a highly significant va riation in annual case mix (Parsonnet scores 8.7-10.6, P < 0.0001). Th ere was no significant variation in mortality after adjustment for cas e mix (odds ratio 1-1.5, P = 0.18) with monitoring by calendar year. W ith continuous monitoring, however, nominal 99% control limits based o n 16 expected deaths were crossed on two occasions. Conclusions: Hospi tal league tables for mortality from heart surgery will be of limited value because year to year differences in death rate can be large (odd s ratio 1.5) even when the underlying risk or case mix does not change . Statistical quality control of a single series with adjustment for c ase mix is the only way to take into account recent performance when i nforming a patient of the risk of surgery at a particular hospital. If there is an increase in the number of deaths the chances of the next patient surviving surgery can be calculated from the last 16 deaths.