Syncope refers to the sudden 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.
The loss of blood flow is related to a fall in blood pressure. Recall the factors that maintain blood pressure:
- BP = CO x Arterial Resistance; since CO = HR x SV
- BP = HR x SV x Arterial Resistance; since Arterial Resistance is proportional to 1/radius4
- BP = HR x SV x 1/R4; note SV depends on preload, afterload, contractility
Causes of syncope:
- Sudden changes in heart rate: bradycardia, tachycardia
- Obstruction to blood flow: aortic stenosis, pulmonic stenosis, mitral stenosis, tricuspid stenosis, pulmonary hypertension, pulmonary embolism, obstructive HCM, cardiac tamponade
- Right to left shunting: Tetralogy of Fallot, Eisenmenger’s Syndrome
- Reduced preload: dehydration, hemorrhage, hypotensive drugs
- Vascular or neurogenic dysfunction: carotid sinus hypersensitivity (vasovagal), postmicturition, post-tussive.