Myocardial Catecholamines and the Pathophysiology of Heart Failure

The sympathetic nervous system contributes importantly to the clinical expression and, perhaps, to the course of myocardial failure. The failing heart exhibits both anatomic and functional defects in its sympathetic innervation and adrenergic receptor function. The nature of these abnormalities is at least partially dependent on the etiology of the underlying myocardial injury. Both efferent cardiac sympathetic tone and circulating catecholamines are elevated during the later stages of most forms of heart failure. This increase is not merely a reflex compensatory response but is also a reflection of defects in parasympathetic function, baroreceptor afferent nerve traffic, and regulation of sympathetic tone within the central nervous system through a serotonergic pathway. Prolonged stimulation of the heart may exhaust myocardial stores of norepinephrine and may lead to the destruction of sympathetic nerve terminals. Importantly, these abnormalities of autonomic function are distributed nonuniformly across the myocardium. Such heterogeneity can have profound effects on the temporal coordination of myocardial contraction and relaxation as well as the duration and configuration of the cardiac action potential and, thus, may contribute to both the mechanical and electrophysiological derangements seen in the failing heart. Therapy that makes sympathetic responses more uniform may improve the temporal coordination of excitation and contraction between innervated and denervated segments. This hypothesis might explain why both sympathetic agonists and antagonists may improve cardiac function since both types of drugs can restore the uniformity of neural stimulation.