Background: A volume-outcome relationship has been found for acute myocardial infarction (AMI); however, the mechanisms underlying the relationship remain unclear. In particular, it is not known whether processes of care are mediators of the volume-outcome relationship, that is, whether the mechanisms underlying the relationship are through processes of care.
Objective: We used nationwide population-based data to examine the mediating effects of processes of care on the relationships of physician and hospital volume with AMI mortality.
Methods: We analyzed all 6838 ST-elevation myocardial infarction (STEMI) patients admitted in 2008, treated by 740 physicians in 142 hospitals through Taiwan's National Health Insurance Research Database. Multilevel meditational models were performed after adjustment for patient, physician, and hospital characteristics to test the relationships among physician and hospital volume, processes of care, and 30-day STEMI mortality.
Results: Physicians with higher volume had higher use of percutaneous coronary intervention and aspirin, and lower mortality in the following year, and the processes of care were mediators of the relationship between physician volume and mortality. Low-volume hospitals had higher mortality in the following year than medium-volume hospitals. In stratified analyses the relationships only existed in nonlarge hospitals.
Conclusions: Physicians with high volume perform better on certain processes of care than those with medium and low volume, and have better outcomes for patients with AMI. The processes of care could partly explain the relationship between physician volume and AMI mortality. However, the relationships existed in nonlarge hospitals but not in large hospitals.