Association Between Hospital Volumes and Clinical Outcomes for Patients With Nontraumatic Subarachnoid Hemorrhage

BACKGROUND: Previous studies of patients with nontraumatic subarachnoid hemorrhage (SAH) suggest better outcomes at hospitals with higher case and procedural volumes, but the shape of the volume‐outcome curve has not been defined. We sought to establish minimum volume criteria for SAH and aneurysm obliteration procedures that could be used for comprehensive stroke center certification. METHODS AND RESULTS: Data from 8512 discharges in the National Inpatient Sample (NIS) from 2010 to 2011 were analyzed using logistic regression models to evaluate the association between clinical outcomes (in‐hospital mortality and the NIS‐SAH Outcome Measure [NIS‐SOM]) and measures of hospital annual case volume (nontraumatic SAH discharges, coiling, and clipping procedures). Sensitivity and specificity analyses for the association of desirable outcomes with different volume thresholds were performed. During 8512 SAH hospitalizations, 28.7% of cases underwent clipping and 20.1% underwent coiling with rates of 21.2% for in‐hospital mortality and 38.6% for poor outcome on the NIS‐SOM. The mean (range) of SAH, coiling, and clipping annual case volumes were 30.9 (1–195), 8.7 (0–94), and 6.1 (0–69), respectively. Logistic regression demonstrated improved outcomes with increasing annual case volumes of SAH discharges and procedures for aneurysm obliteration, with attenuation of the benefit beyond 35 SAH cases/year. Analysis of sensitivity and specificity using different volume thresholds confirmed these results. Analysis of previously proposed volume thresholds, including those utilized as minimum standards for comprehensive stroke center certification, showed that hospitals with more than 35 SAH cases annually had consistently superior outcomes compared with hospitals with fewer cases, although some hospitals below this threshold had similar outcomes. The adjusted odds ratio demonstrating lower risk of poor outcomes with SAH annual case volume ≥35 compared with 20 to 34 was 0.82 for the NIS‐SOM (95% CI, 0.71–094; P=0.0054) and 0.80 (95% CI, 0.68–0.93; P=0.0055) for in‐hospital mortality. CONCLUSIONS: Outcomes for patients with SAH improve with increasing hospital case volumes and procedure volumes, with consistently better outcomes for hospitals with more than 35 SAH cases per year.