Background: Few studies have examined mortality rates in relation to the workload of hospital staff. We investigated this issue in one adult intensive-care unit (ICU) in the UK.
Methods: We measured ICU workload per shift during each patient's stay for all admissions between 1992 and 1995 that met criteria for adjustment of mortality risk by the APACHE II equation (n=1050). APACHE II data were validated by one observer. Measures of workload in each patient's stay included occupancy, total ICU nursing requirement as defined by the UK Intensive Care Society, and the ratio of occupied to appropriately staffed beds. Over the period, staffing was appropriate for between 4.1 and 5.3 occupied beds (1.3 nurses per patient).
Findings: There were 337 deaths, 49 more (95% CI 34-65) than predicted by the APACHE II equation. Median occupancy was 5.8 beds, and median nursing requirement was 1.6 per patient. On multiple logistic regression analysis, adjusted mortality was more than two times higher (odds ratio 3.1 [1.9-5.0]) in patients exposed to high than in those exposed to low ICU workload, defined by average nursing requirement per occupied bed and peak occupancy; the unadjusted odds ratio for this comparison was 4.0 (2.6-6.2). After exclusion of measures of nursing requirement, adjusted mortality increased with the ratio of occupied to appropriately staffed beds during each patient's stay. All logistic regression models fitted the data satisfactorily.
Interpretation: Variations in mortality may be partly explained by excess ICU workload. This methodology may have implications for planning and clinical governance.