This theory suggests that when sensor mechanisms detect a decrease in cardiac output and blood pressure, a reflex increase in efferent sympathetic activity develops. The Ventricular Theory: The concept of relative circulating hypovolemia causing venous pooling and decreased filling return to the heart is one main postulate of the �ventricular theory of NMS. Theories of NMS There are multiple theories related to neurally mediated syncope, including: Factors responsible for NMS are varied and not always evident. Neurally mediated syncope may represent an intermediate functional state between those conditions, and anomalies in one or several parts of the reflex arc may result in the sudden development of this syndrome. Likewise, subjects with baroreflex failure experience pronounced episodes of unopposed hypertension and tachycardia. For instance, patients with severe autonomic failure experience severe hypotension either after assuming the upright position or after food consumption (orthostatic and postprandial hypotension, respectively). The relevance of these adaptive mechanisms can be better appreciated in subjects in whom these cardiovascular reflexes are absent or severely impaired. These changes restore blood pressure and allow for adequate cerebral perfusion during standing. These changes are detected by stretch or baroreceptors which relay information to the central nervous system that in turn decreases parasympathetic tone (i.e., cardiac vagal tone, tachycardia) and increases sympathetic outflow (increase in norepinephrine secretion and in peripheral resistances). After a few minutes of standing, between 500 and 800 ml of blood are trapped in the distensible veins below the level of the heart, plasma moves to the interstitial fluid and a decrease in venous return, cardiac output and blood pressure occurs. For instance, upon standing, the increase of gravitational forces results in regional and hemodynamic changes that include a pooling of blood in the lower extremities. Changes in posture, physical exercise, food digestion, and mental activity are among many activities that alter cardiovascular homeostasis and require the involvement of neurocardiovascular reflex mechanisms. Under normal conditions, cardiovascular reflexes, such as the baroreceptor reflex, preserve arterial blood pressure and cerebral perfusion within adequate margins. Related terms and conditions include situational fainting such as cough, micturitional, defecation, diving, sneezing, and swallowing syncope, carotid sinus syncope (shaving syncope), and autonomic conditions including hyperadrenergic and hypoadrenergic states.īasic Pathophysiology Performance of vital and complex mental functions depends on adequate cerebrovascular perfusion pressure. The parasympathetic and sympathetic systems are part of the autonomic nervous system. There is general agreement that these changes in heart rate and blood pressure are due to an increase in parasympathetic tone and concomitant inhibition of sympathetic outflow. This hypotension leads to loss of consciousness if sufficiently severe or to presyncope if less severe or if the patient is lying down. Neurally mediated syncope is characterized by peripheral vasodilation and a decrease in blood pressure, or hypotension, along with bradycardia, or a slow heart rate. It has also been known as vasovagal syncope, neurocardiogenic syncope, common or emotional fainting, or reflex syncope. Neurally mediated syncope (NMS) is one of the most frequent forms of syncope. It is important to distinguish syncope from “dizziness”, which generally refers to an alteration in balance, vision, or perception of the environment, without the loss of consciousness. Repeated episodes, however, may be caused by a wide variety of medical problems, and require diagnosis and treatment. A history of an isolated episode of syncope will be found in as many as 25% of healthy young adults, especially in settings that precipitate fear, disgust or anxiety, and if not repeated does not warrant further work-up. It is a common problem, accounting for 3% of emergency room visits. Syncope is a sudden and transient loss of consciousness and postural tone, usually described as “fainting” or “passing out”.
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