Associations between surgical volume and nursing organization in patient outcomes after abdominal aortic aneurysm repair

In 1979, researchers coined the term volume-outcomes effect to describe a phenomenon in which hospitals with higher volumes of a specific surgery also experienced a decrease in the mortality of patients undergoing the surgery in question. Hundreds of studies have shown this association between surgical volume and outcomes for a variety of surgeries. However, the underlying cause of this volume-outcomes relationship is unknown. The organization of nursing care has also been shown to be associated with patient outcomes, and preliminary studies examining nursing and volume suggest nursing may play a role in the volume-outcomes relationship. The purpose of this study was to determine whether a volume-outcomes effect was seen in patients undergoing abdominal aortic aneurysm (AAA) repair in four states, and if this relationship could be at least partially explained by the organization of nursing care. Additionally, we evaluated whether the volume-outcomes effect was contingent upon or interacted with the organization of nursing care. A cross sectional, secondary analysis utilized data from a national nurse survey, patient discharge abstracts from 2005-2006, and administrative data. The sample included 25,265 nurses, and 20,409 patients undergoing AAA repair surgery at 517 hospitals. The outcomes of interest were thirty day mortality, failure to rescue, prolonged length of stay, and mean length of stay. Regression models were used for modeling with detailed risk adjustment. A significant volume-outcomes effect was noted, with higher volume hospitals having decreased mortality, failure to rescue, and prolonged length of stay, and shorter length of stays. Including nurse organization variables in the models decreased the magnitude of the effect of volume on mortality and failure to rescue. Further, a significant association was found between the interaction for nurse staffing and surgical volume and both mortality and failure to rescue. In hospitals with patient-to-nurse ratios of 4:1, the effect of high volume was much more pronounced (the odds ratio is 0.29) than in hospitals with patient-to-nurse ratios of 8:1 (where the odds ratio was 0.93). These finding suggests the effect of surgical volume on mortality and failure to rescue may be at least partially dependent on the organization of nursing care