cardiovascular disorders
Syncope
Pathophysiology of Syncope
Most syncope results from insufficient cerebral blood flow. Some cases involve adequate flow but with insufficient cerebral substrate (oxygen, glucose, or both). Most deficiencies in cerebral blood flow result from decreased cardiac output (CO). Decreased COcan be caused by Cardiac disorders that obstruct outflow Cardiac disorders of systolic dysfunction Cardiac disorders of diastolic dysfunction Arrhythmias (too fast or too slow) Conditions that decrease venous return Outflow obstruction can be exacerbated by exercise, vasodilation, and hypovolemia (particularly in aortic stenosis and hypertrophic cardiomyopathy), which may precipitate syncope. Overview of Arrhythmias cause syncope when the heart rate is too fast to allow adequate ventricular filling (eg Venous return can be decreased by hemorrhage, increased intrathoracic pressure, increased vagal tone (which can also decrease heart rate), and loss of sympathetic tone (eg, from drugs, carotid sinus pressure, autonomic dysfunction). Syncope involving these mechanisms (except for hemorrhage) is often termedvasovagalor neurocardiogenic and is common and benign. Orthostatic Hypotension , a common benign cause of syncope, results from failure of normal mechanisms (eg, sinus tachycardia, vasoconstriction, or both) to compensate for the temporary decrease in venous return that occurs with standing. (eg, Overview of Stroke , The central nervous system (CNS) requires oxygen and glucose to function. Even with normal cerebral blood flow, a significant deficit of either will cause loss of consciousness . In practice, hypoglycemia is the primary cause because hypoxia rarely develops in a manner causing abrupt loss of consciousness (other than in flying or diving incidents). Loss of consciousness due to hypoglycemia is seldom as abrupt as in syncope or seizures because warning symptoms occur (except in patients taking beta-blockers); however, the onset may be unclear to the examiner unless the event was witnessed. 