SYNCOPE, which is a sudden transient loss of consciousness, results from an impairment in cerebral metabolism, which is the consequence of a brief deprivation in essential energy substrates, specifically oxygen and glucose. This deprivation in energy substrates may originate from one of only four levels (Fig 1): (1) from decrease or loss of intrinsic cerebral circulation, (2) from the heart, as a result of a transient decrease in cardiac output, (3) from a decrease in systemic arterial pressure to values inadequate to perfuse the brain, or (4) from insufficient energy substrates contained in the blood delivered to the brain. After reviewing the physiological mechanisms of syncope involved at each of these levels of potential abnormality, this article will attempt to simplify and organize the diagnostic approach to syncope into a conveniently manageable scheme.
PHYSIOLOGICAL MECHANISM Obstruction to Cerebral Blood Flow
At this level, the mechanism of the reduction in cerebral
Noble RJ. The Patient With Syncope. JAMA. 1977;237(13):1372–1376. doi:10.1001/jama.1977.03270400076031
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