Sympathetic Neural Mechanisms in Human Cardiovascular Health and Disease

The sympathetic nervous system plays a key role in regulating arterial blood pressure in humans. This review provides an overview of sympathetic neural control of the circulation and discusses the changes that occur in various disease states, including hypertension, heart failure, and obstructive sleep apnea. It focuses on measurements of sympathetic neural activity (SNA) obtained by microneurography, a technique that allows direct assessment of the electrical activity of sympathetic nerves in conscious human beings. Sympathetic neural activity is tightly linked to blood pressure via the baroreflex for each individual person. However, SNA can vary greatly among individuals and that variability is not related to resting blood pressure; that is, the blood pressure of a person with high SNA can be similar to that of a person with much lower SNA. In healthy normotensive persons, this finding appears to be related to a set of factors that balance the variability in SNA, including cardiac output and vascular adrenergic responsiveness. Measurements of SNA are very reproducible in a given person over a period of several months to a few years, but SNA increases progressively with healthy aging. Cardiovascular disease can be associated with substantial increases in SNA, as seen for example in patients with hypertension, obstructive sleep apnea, or heart failure. Obesity is also associated with an increase in SNA, but the increase in SNA among patients with obstructive sleep apnea appears to be independent of obesity per se. For several disease states, successful treatment is associated with both a decrease in sympathoexcitation and an improvement in prognosis. This finding points to an important link between altered sympathetic neural mechanisms and the fundamental processes of cardiovascular disease.MI = myocardial infarction; MSNA = muscle sympathetic neural activity; OSA = obstructive sleep apnea; SA = sinoatrial; SNA = sympathetic neural activityThe sympathetic nervous system plays a vital role in the everyday lives of human beings. Sympathetic neural responses are essential to simple tasks such as changing posture. Movement from a supine or sitting position to an upright position requires complex adjustments in blood flow and blood pressure, and these adjustments are ultimately coordinated by sympathetic nerves in conjunction with parasympathetic modulation of heart rate. Without such adjustments, blood flow to the brain would fall below autoregulatory limits, and standing up would consistently cause syncope. Indeed, some persons with severe autonomic failure are unable to stand (or sometimes even to sit upright) without fainting.^1,2This article presents an overview of our current understanding of sympathetic neural mechanisms in human cardiovascular control. It focuses on measurements of sympathetic neural activity (SNA) that are obtained by microneurography, a technique that can directly measure the electrical activity of sympathetic nerves in intact, conscious human beings. During the past 3 decades, the information yielded by this technique has greatly increased our clinical and mechanistic understanding of sympathetic neural mechanisms in health and disease. Ongoing research using this technique continues to yield new insights into the pathophysiology of cardiovascular diseases, including hypertension, coronary artery disease, and heart failure, and into the cardiovascular risk associated with diseases such as obstructive sleep apnea (OSA) and obesity.