Abstract 33: Seasonal Patterns in Risk-Standardized Mortality after Ischemic Stroke: A Winter and July Effect

Abstract

Background: A recent systematic review found that mortality rates were higher and efficiency of care lower in hospitals coincident with the introduction of new trainees in July, but there was significant heterogeneity in study quality, and none of the included studies focused on stroke care. Purpose: To assess seasonal variation in 30-day risk-standardized mortality rates (RSMRs) for elderly ischemic stroke patients from 1999 to 2006 and determine whether patterns differ for patients discharged from teaching versus non-teaching hospitals. Methods: The study cohort included all fee-for-service Medicare beneficiaries aged 65 years and older discharged with an ischemic stroke (ICD-9 primary codes 433, 434, 436) from 1999 to 2006 in the United States. A hierarchical logistic regression model fitted annual data to estimate the 30-day RSMR, adjusted for demographic and clinical characteristics. The annual datasets were combined and reconstructed to time series analyses, with month as a unit. The time series analysis included 96 months; RSMRs were calculated for each month. The unobserved components modeling approach was used to fit the time series data with risk-adjusted mortality as an outcome to compare seasonal patterns by month for teaching and non-teaching hospitals. Results: Of 2,824,694 ischemic stroke discharges, 51.7% were from teaching hospitals. The 30-day RSMR decreased steadily from 1999 to 2006 in teaching hospitals ( Figure 1 A). Seasonal patterns were present within each calendar year, with the highest 30-day RSMR occurring in the winter (January) and the lowest RSMR in the summer, with a secondary peak in July (p=0.004; Figure 1 B). The same patterns were also seen for non-teaching hospitals (not shown). Conclusions: Thirty-day risk-standardized mortality rates after ischemic stroke in the elderly have decreased between 1999 to 2006. Seasonal patterns are evident, with the highest RSMR in January, and a secondary peak in July. Because these patterns were similar for teaching and non-teaching hospitals, the July peak can not be explained based on the introduction of new trainees. The reasons for these seasonal patterns, including higher mortality rates in July, are unclear.