The frequency with which congestive heart failure develops in cardiovascular patients at risk for CHF in the nondiuretic treatment arm of a blood pressure trial will tend to be inversely related to the threshold for starting or restarting a diuretic.
The ALLHAT trial presumably had a high threshold for starting a diuretic in the protocol arms without diuretics.
Though this would enhance the purity of the treatment arms, this also would also lead to episodes of congestive heart failure that would not occur in routine clinical practice, even when other blood-pressure medicines are initially used.
In clinical practice or in an open label trial, there will tend to be a lower threshold for the institution of a diuretic in the early stages of mild volume excess prior to the precipitation of full-fledged congestive heart failure. Similarly, each individual randomized double blind hypertension trial involving treatment arms without diuretics will have a particular threshold for starting a diuretic as determined by trial protocol.
This increased incidence of protocol driven heart failure may be more readily treatable when the diuretics (which had been initially withheld by trial protocol) are ultimately initiated.
This may help to explain why the increased incidence of CHF in the ALLHAT trial occurred without the typical early increase in mortality associated with CHF.