Mortality and intensive care volume in ventilated patients from 1995 to 2009 in the Australian and New Zealand binational adult patient intensive care database*

Abstract

The mortality outcome of mechanical ventilation, a key intervention in the critically ill, has been variously reported to be determined by intensive care patient volume. We determined the volume-(mortality)-outcome relationship of mechanically ventilated patients whose records were contributed to the Australian and New Zealand Intensive Care Society Adult Patient Database. Retrospective cohort study of 208,810 index patient admissions from 136 Australian and New Zealand intensive care units in the same number of hospitals over the course of 1995-2009. The patient-volume effect on hospital mortality, overall and at the level of patient (nonsurgical, elective surgical, and emergency surgical) and intensive care unit (rural/regional, metropolitan, tertiary, and private) descriptors, was determined by random-effects logistic regression adjusting for illness severity and demographic and geographical predictors. Annualized patient volume was modeled both as a categorical (deciles) and, with calendar year, a continuous variable using fractional polynomials. The patients were of mean age of 59 yrs (SD, 19 yrs), Acute Physiology and Chronic Health Evaluation III score 66 (32), and 39.4% female, with a hospital mortality of 22.4%. Overall and at both the patient and intensive care unit descriptor levels, no progressive decline in mortality was demonstrated across the annual patient volume range (12-932). Over the whole database, mortality odds ratio for the last volume decile (801-932 patients) was 1.26 (95% confidence interval, 1.06-1.50; p = .009) compared with the first volume decile (12-101 patients). Calendar year mortality decreases were evident (odds ratio, 0.96; 95% confidence interval, 0.94-0.98; p = .0001). Using fractional polynomials, modest curvilinear mortality increases (range, 5%-8%) across the volume range were noted over the whole database for nonsurgical patients and at the tertiary intensive care unit level. No inverse volume-(mortality)-outcome relationship was apparent for ventilated patients in the Australian and New Zealand Intensive Care Society database. Mechanisms for mortality increments with patient volume were not identified but warrant further study.