Background The ESCAPE trial was a multicentre randomised controlled trial investigating the effectiveness of Chest Pain Unit (CPU) care. The process of CPU implementation and the activity of individual CPUs varied substantially between hospitals. The study reported here explored the organisational factors that influenced this variation.

Method A multiple case study approach was taken treating each site as a ‘case’. Six intervention sites were studied. Qualitative data were collected through interviews with key personnel at each site.

Results Activity of individual CPUs was not adequately explained by simple structural differences between hospitals, such as their size or location, or between CPUs, such as staffing and hours. Analysis suggested that the more active CPUs tended to have more of the following characteristics: being ‘primed’ by previous initiative or experience; appropriate leadership; a positive climate for innovation; established relationships between key staff/departments; role clarity amongst staff; an enthusiast for the development; and continuity of staffing. Role conflict, particularly between specialist nurses and others, was reported and had potential to interfere with development.

Conclusion Organisational factors were identified that could have impacted on the outcomes of the ESCAPE trial through, for example, delays in discharge, and missed recruitment opportunities. Complex interventions such as the ESCAPE trial are prone to the effects of local organisational issues, some of which could be predicted and planned for. Findings from single centre studies of complex interventions should be treated with caution before a decision is taken to implement in a new setting.