The determinants of clinical outcome and clinical response to CRT are not the same.

The purpose of treatment is to alter outcomes favourably. From a clinical perspective, these outcomes may include symptoms, quality of life, disability, morbidity and mortality. However, a good outcome does not mean that the intervention was effective and a seemingly poor outcome could have been worse without intervention. Patients may have a good outcome either because their disease was due to run a benign course, or because they responded to the intended treatment or because they responded to some other ancillary treatment. Clearly, there is a link between response and outcome but it is loose and uncertain. The clinical substrate being treated is often a stronger determinant of outcome than the response to the intervention. The concepts of outcome and response can both be useful when deciding whether or not to implant a device but they are not the same and their determinants will differ. Patients with dilated cardiomyopathy and those with inter-ventricular mechanical dyssynchrony have a better prognosis than those who do not have these features, but this is true whether or not they receive cardiac resynchronisation therapy (CRT). Many characteristics predict outcome in patients considered for CRT but none consistently predict response. On the other hand, guidelines recommend CRT in populations that have not yet been adequately studied, such as those with atrial fibrillation. Clinicians should follow the criteria for patient selection in the landmark trials when selecting patients for CRT, should extrapolate with caution from these and should be extremely cautious in the interpretation of observational data.