Lesson 4, Topic 3
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Clinical Context: Syncope

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Introduction to syncope

Syncope refers to a sudden and transient loss of consciousness due to the temporary loss of cerebral perfusion. The metabolism of the brain, unlike other organs, is exclusively dependent on perfusion. In contrast to skeletal muscle, for example, storage of high-energy phosphate in the brain is limited, and energy supply depends largely on the oxidation of glucose extracted from the blood. Thus, cessation of cerebral blood flow causes a loss of consciousness within about 10 seconds.

Pre-syncope refers to a sudden and transient loss of postural tone (collapse) without loss of consciousness, and is related to compromised perfusion long enough in duration to affect skeletal muscular tone but not long enough in duration to cause loss of consciousness.

Causes of syncope and pre-syncope

Considering maintenance of normal blood pressure and perfusion depends on heart rate, stroke volume, and peripheral vascular tone, causes of syncope and pre-syncope include disorders or situations that disrupt these factors:

  • Sudden changes in heart rate: bradycardia, tachycardia
  • Obstruction to blood flow: aortic stenosis, pulmonic stenosis, mitral stenosis, tricuspid stenosis, pulmonary artery hypertension, pulmonary artery embolism, obstructive HCM, cardiac tamponade with pericardial effusion
  • Right to left shunting: Tetralogy of Fallot, Eisenmenger’s Syndrome
  • Reduced preload: dehydration, hemorrhage, hypotensive drugs, cardiac tamponade
  • Vascular or neurogenic dysfunction: carotid sinus hypersensitivity (vasovagal), post-micturition, post-tussive