Emergency department crowding and mortality for patients presenting to emergency departments in New Zealand.

Objective: The association between ED crowding and mortality has been established internationally, but not in New Zealand. The aim was to determine which measures of crowding were associated with mortality for new patients presenting to New Zealand EDs. The primary outcome was mortality for patients within 7 days of arrival in the ED.

Methods: This was a retrospective cohort study, using administrative data from 2006 to 2012. The crowding conditions at the time of presentation of each patient were recreated. Multivariable Cox proportional hazard modelling was used to determine the probability of death within 7 days of the presentation to ED. Each crowding measure was added independently to the optimum mortality model to determine how each crowding metric influenced the model.

Results: Twenty-five of 28 (89%) eligible acute hospitals in New Zealand were included, with 5 793 767 ED visits by 2 214 865 individuals. Seven-day mortality was higher for patients arriving at times when there was more than 10% hospital access block (hazard ratio [HR] 1.10, 95% confidence interval [CI] 1.05, 1.17) or non-compliance with the 4-h emergency access target (HR 1.07, 95% CI 1.01, 1.12). ED occupancy did not influence the model importantly, while the number of arrivals in the previous 6 h was associated with lower mortality (HR 0.90, 95% CI 0.84, 0.97).

Conclusion: Access block had the strongest association with 7-day mortality. That ED occupancy and the number of arrivals were not associated with increased mortality suggests that system issues related to long ED stays may be most important in the link between ED crowding and mortality.