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Syncope is a common clinical condition occurring even in otherwise healthy people
without underlying cardiovascular disease. Neurally mediated syncope is by far the
most common cause of syncope in individuals without any structural heart disease.
Based on traditional wisdom, loss of sympathetic tone with relaxation of vascular
smooth muscle is the key mechanism underlying the pathophysiology of syncope, especially
in patients without an acute decrease in heart rate. However, this concept has recently
been challenged. Some microneurographic studies indicate that sympathetic withdrawal
may not always be a prerequisite even for the development of classic “vasodepressor”
forms of syncope. Conversely, a decrease in cardiac output appears to be a determinant
factor for syncope in most circumstances. This presentation discusses the relative
contribution of cardiac output versus sympathetic vasoconstriction in neurally mediated
syncope in otherwise healthy individuals. It is suggested that a moderate to severe
fall in cardiac output with or without vasodilatation may contribute to syncope.
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