Interventions supporting cardiac rehabilitation completion: Process evaluation investigating theory-based mechanisms of action
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Objective: A randomised trial informed by the Health Action Process Approach evaluated interventions to improve cardiac rehabilitation completion. This study investigated indirect effects of the interventions on cardiac rehabilitation adherence via targeted constructs. Methods: In this quantitative theory-based process evaluation, participants in all three trial arms (usual care; mailouts; mailouts plus telephone support) completed a questionnaire at 12 months follow-up assessing intention, goal priority, outcome expectancies, risk perception, self-efficacy, social support, action planning, and coping planning. Consecutive sampling was used until the target sample size (167 per arm) was met. Completion of cardiac rehabilitation at the same time point was self-reported. We used multiple regression mediation models to explore indirect effects. Results: In total, 594 participants completed the cardiac rehabilitation questionnaire; 588 were analysed (6 excluded due to missing data). For mailouts alone, there were no significant indirect effects. There were small indirect effects of mailouts plus telephone support on intention via goal priority, outcome expectancies, and self-efficacy, with a negative effect via severity risk perception. There were also small indirect effects on cardiac rehabilitation completion via self-efficacy and action planning. Conclusions: Findings suggest that mailouts plus telephone support increased the likelihood of completing cardiac rehabilitation by enhancing self-efficacy and action planning, and increased intention by enhancing goal priority, self-efficacy, and outcome expectancies, with an unintended consequence of a negative effect via risk perceptions. Conducting theory-based process evaluations alongside trials of behaviour change interventions can clarify mechanisms of action, which can inform efforts to refine interventions and to replicate and generalize findings to other jurisdictions.