Įvery person undergoing evaluation for syncope including presumed vasovagal syncope should have an ECG to look for evidence of underlying arrhythmogenic cardiac abnormalities, such as Wolff-Parkinson-White syndrome, prolonged QT syndrome, Brugada syndrome, or heart blocks. Patients may require radiographic studies of trauma sustained during the vasovagal event. Abnormalities on physical exam (such as tongue biting, which suggests seizure, or ongoing pallor, which suggests symptomatic anemia) or atypical history should prompt a more thorough evaluation into the underlying cause of syncope.Įvery patient with a complaint of syncope should have a complete history and physical exam, with attention to the presence or absence of pallor, cardiac abnormalities (murmurs, tachy/bradydysrhythmias, and irregular heartbeat), pulmonary abnormalities, and signs of trauma. After the event, the patient should have an entirely normal physical exam. During the event, the patient will usually be bradycardic, hypotensive, pale, and diaphoretic. Patients should return to consciousness spontaneously. Many patients describe tunnel vision, ringing in their ears, and profuse sweating. Feelings of warmth and nausea are common. Patients will classically describe a feeling of lightheadedness. Most patients with vasovagal syncope present with a history of a syncopal prodrome. Patients with syncope also frequently require a trauma evaluation to assess for injuries sustained during the syncopal event. Patients should be asked specifically about family and personal history of cardiovascular diseases, the potential for gastrointestinal blood loss, the potential for genitourinary blood loss, and medications that could be complicating the presentation (i.e., antihypertensives, antihistamines, anticholinergics, anticoagulants). History and physical exam should focus on ruling out life-threatening entities before a diagnosis of vasovagal syncope. The differential diagnosis for syncope is broad and includes cardiac arrhythmia, acute blood loss, seizure, orthostasis, dehydration, dissection, subarachnoid hemorrhage, pulmonary embolus, ruptured ectopic pregnancy, trauma (concussion, epidural hematoma), toxins, and many other pathologies. Typically, if the patient falls or is laid supine, the increase in circulating blood volume from the lower extremities combined with the decreased work necessary to get blood to the brain (i.e., the heart does not have to pump "uphill" against gravity) will cause the patient to regain consciousness rapidly.
This results in a drop in the patient's mean arterial pressure. Cerebral autoregulation results in constant cerebral blood flow over a wide range of mean arterial pressures, but when the mean arterial pressure falls below the body's ability to autoregulate, the patient loses consciousness.
Since cardiac output is the product of stroke volume and heart rate, this reflex arc affects both factors in the equation: slowing the heart rate and decreasing the amount of volume. The results in decreased preload, venous return, and ventricular volume. At the same time, decreased sympathetic activity results in decreased vascular tone in both arterioles and venules. This decrease in heart rate can be profound, with asystole that can be several seconds long. Increased vagal firing (increased parasympathetic activity) at the sinus node and the atrioventricular node causes a decrease in heart rate. The efferent limb of the reflex arc is better understood. This may trigger mechanoreceptors in the ventricle that signal via vagal afferents to the central nervous system. It is believed that this trigger, usually in combination with central hypovolemia (from upright posture or dehydration) results in increased cardiac contractility in the setting of a relatively underfilled left ventricle. Although this trigger may be emotional stress or pain, it is often unidentifiable. The afferent limb of the reflex arc begins with a trigger. Although the autonomic nervous system mediates vasovagal syncope, pathophysiological mechanisms are not completely understood. It is helpful to think about the pathophysiology of vasovagal syncope as a reflex arc, with an afferent limb and an efferent limb.
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