The relationship between annual procedural volume and patient outcome remains a debated issue in the field of interventional medicine. An under-explored issue in a UK context is whether or not such a relationship exists for percutaneous coronary interventions (PCI). In recent years, this procedure has replaced thrombolysis as the standard intervention method used in UK hospitals for widening patients' obstructed coronary arteries. However, the actual number of such procedures currently undertaken annually remains variable across hospitals where these interventions are performed. The UK's National Institute for Health and Care Excellence has consequently called for research into whether such differences in volume play any role in patient outcome for this particular procedure. The current study is intended to address that research need, and will do so by looking at whether the annual number of PCI procedures undertaken by individual hospitals is predictive of patient mortality post-procedure.

Using data collected as part of a UK national audit of percutaneous coronary interventions, this study will:

• Establish whether annual hospital volume is a clinically important novel risk factor for 30-day mortality post-PCI when examined relative to a newly developed risk model intended for use in clinical practice.
• Establish whether the nature of this volume-outcome relationship is best fit as a linear or non-linear model.
• Establish whether annual hospital volume is a clinically important novel risk factor for 30-day mortality post-primary PCI when examined relative to a newly developed risk model intended for use in clinical practice.

Inclusion Criteria:

• are aged between 18 and 100 (inclusive) on the date of their procedure;
• do not have pre-operative ventilation;
• undergo a PCI procedure in a UK hospital that contributes data to the National Audit of Percutaneous Coronary Intervention; and
• have a tracked mortality status at 30 days' post-procedure.

To account for potential dependencies in the data, patients' non-index (i.e. follow-up) PCI procedures occurring during the data collection period will be excluded from the study sample.

Percutaneous coronary interventions undertaken in UK hospitals are recorded in the UK National Audit of Percutaneous Coronary Intervention. Patients who contribute data to this audit will be eligible for inclusion in this study. Additional inclusion criteria, as described below, will be applied to ensure that data are appropriate for the intended analysis.