AUTONOMIC CARDIAC CONTROL .1. ESTIMATION AND VALIDATION FROM PHARMACOLOGICAL BLOCKADES

Pharmacological blockades have been used to estimate the relative cont ributions of the autonomic branches to cardiac chronotropic control. S ystematic biases in these estimates, however, can arise from both meth odological and physiological factors. Selective blockades can be inter preted by two inferential models, and a single blockade condition can yield estimates of autonomic control for both autonomic branches. The residual autonomic control of the heart after blockade of a single div ision provides an index of the functional contribution of the unblocke d branch. In contrast, the change in chronotropic state of the heart a fter blockade of the same division reflects the subtractive loss of th at branch and thus provides an index of the normal contribution of the blocked branch. We demonstrate that the systematic biases that can ar ise in blockade studies introduce distortions of the subtractive and r esidual estimates that are of equal magnitude but opposite sign. Conse quently, the discrepancy between the subtractive- and residual-model e stimates provides a measure of bias in blockade studies and permits th e derivation of validity indices that can facilitate interpretations o f blockade data.