The degree to which elevated mortality associated with weekend or night-time hospital admissions reflects poorer quality of care ('off-hours effect') is a contentious issue. We examined if off-hours admissions for primary percutaneous coronary intervention (PPCI) were associated with higher adjusted mortality and estimated the extent to which potential differences in door-to-balloon (DTB) times-a key indicator of care quality for ST elevation myocardial infarction (STEMI) patients-could explain this association. Nationwide registry-based prospective observational study using Myocardial Ischemia National Audit Project data in England. We examined how off-hours admissions and DTB times were associated with our primary outcome measure, 30-day mortality, using hierarchical logistic regression models that adjusted for STEMI patient risk factors. In-hospital mortality was assessed as a secondary outcome. Our study found that higher adjusted mortality associated with off-hours admissions for PPCI could be partly explained by differences in DTB times.

Inclusion Criteria:

• STEMI patients admitted from 1 January 2007 to 31 December 2012
• STEMI patients aged over 18 years
• STEMI patients admitted directly to '24/7' PPCI-capable hospitals for PPCI
• Discharge diagnosis of STEMI
• Provision of PPCI based on initial reperfusion strategy

Exclusion Criteria:

• Hospitals performing less than 20 procedures per year
• Hospitals performing PPCIs only during regular hours
• Interhospital transfers
• PPCIs conducted within 6 hours on hospital arrival

Population of patients admitted to hospital in England for STEMI between 1 January 2007 and 31 December 2012.