Cardiocardiac excitatory reflexes during myocardial ischemia.

Myocardial ischemia represents a stimulus capable of exciting the receptive endings of both the vagal and sympathetic sensory fibers which innervate the heart. As a consequence, multiple and simultaneous reflexes can occur, mediated by the vagal and the sympathetic afferent limbs which, in turn, can modify both the vagal and the sympathetic outflows. Reflexes mediated by vagal afferent fibers are predominantly inhibitory, while those mediated by sympathetic afferent fibers are largely excitatory. In the experimental laboratory the various reflexes can be artificially isolated and analyzed, while in the clinical setting this is not feasible. However, in recent years a computer analysis of cardiovascular short-term variability seems to furnish new indices of vagal and sympathetic modulatory activities. Such analysis in the frequency domain thus seems to provide a tool for both experimental and clinical evaluation of the neural mechanisms accompanying various physiological and pathophysiological events, including acute myocardial ischemia.