Rarely helpful. Less than 2% have hyponatremia, hypocalcemia, hypoglycemia or renal failure causing seizures.

Drugs that may alter lab results: N/A
Disorders that may alter lab results: N/A

N/A

• If history and physical suggestive of ischemic, valvular or congenital heart disease – echocardiogram, cardiac catheterization
• If CNS disease suspected – EEG, head CT, head MRI. These tests should not be ordered unless there are hints of CNS disease on history or physical examination.

Lung scan or helical CT of the thorax if history and physical examination suggestive of pulmonary embolism

• ECG monitoring, either in the hospital or ambulatory (Holter), is useful in 4–15% of patients. Arrhythmias are frequently documented, but rarely associated with syncope. Monitoring should be done in patients with heart disease, and in patients with recurrent syncope. Patient-activated intermittent loop recorders, which the patient activates after regaining consciousness, can record 4–5 minutes of retrograde ECG rhythm. These have been helpful in patients with recurrent syncope with a diagnostic yield between 24–47%.
• Electrophysiologic studies (EPS) have been positive in 18–75% of patients. Induction of ventricular tachycardia and dysfunction of the His-Purkinje system are the two most common abnormalities. Although there is the problem of knowing whether the arrhythmia noted or induced during the study is the cause of syncope, EPS should be done in patients with heart disease or recurrent syncope.
• The following findings are probable causes of syncope:
  • Sustained ventricular tachycardia
  • Sinus node recovery time 3 seconds or more
  • Pacing-induced infranodal block
  • H–V interval greater than 100 msec
• Carotid hypersensitivity should be considered in patients with syncope on head turning, especially with head turning while wearing a tight collar, and in patients with neck tumors and neck tissue scars. The technique is not standardized. One side should be massaged at a time for 20 seconds with constant monitoring of pulse and blood pressure. Atropine should be readily available.
• Tilt testing, with and without isoproterenol infusion, is a provocative test for vasovagal syncope which is not standardized, but has been reported positive (symptomatic hypotension and bradycardia) in 26–87% of patients. However, the test has been reported positive in 0–45% of control subjects. The role of this test in the workup of patients with syncope of unknown origin is not known, and should be done only in patients in which cardiac causes of syncope have been excluded. Patients with a positive tilt test often respond to beta-blocker treatment.
• Psychiatric evaluation should be considered in patients with multiple episodes of syncope (greater than 5 per year) who do not have heart disease. Anxiety, depression, alcohol and drug abuse can be associated with syncope.