From Ask Dr Wiki
- Neurally Mediated Syncope includes Carotid Sinus Syndrome, Situational Syncope, and Neurocardiogenic Syncope (also known as vasovagal syncope)
- Diagnosis is usually made on the basis of a medical history and absence of structural heart disease.
- In normal patients, upright posture leads to gravity-mediated displacement of blood in the lower extremities which is followed by a reactive increase in heart rate and peripheral vascular resistance.
- Physiologically, a reduction in blood pressure with upright posture is sensed by arterial baroreceptors located primarily in the aortic arch and carotid sinus. These receptors send signals to the midbrain which modulates sympathetic and parasympathetic responses. Typically, increased arterial pressure leads to a centrally mediated decrease in sympathetic tone and reduction in heart rate.
- The heart also plays a role in this baroreceptor mediated response by virtue of mechanoreceptors, or C-fibers. The normal response to upright posture is an increase in heart rate, an increase in diastolic pressure and slight decreased systolic blood pressure.
- In patients with neurocardiogenic syncope, excessive venous pooling results in central hypovolemia with a dramatic reduction in preload to the cardiac chambers. This reduction in cardiac volume results in vigorous ventricular contractions which, in turn, activates a large number of C-mechanoreceptors that would normally respond to stretch from hypertension. This surge in ventricular activity mimics the effects of hypertension and creates a paradoxical response with bradycardia and hypotension.
