Tests Available for the Evaluation of Syncope
Tilt Table Testing
A cardiovascular cause of syncope (loss of consciousness) can be identified in some patients with the use of the head-up tilt table test. Tilt table testing with or without isoproterenol can produce hypotension and/or bradycardia in patients with neurocardiogenic mechanisms of syncope, also known as vasovagal, vasodepressor, or neurally mediated syncope. Vasovagal syncope is the most common cause of syncope in otherwise healthy young persons. Tilt table testing is used to evaluate patients who have had syncope (loss of consciousness). It is simple to perform, and in most cases is quite safe.
How the test is done
In a tilt table study, the patient is secured to a table that is mechanically and slowly tilted to an upright position (60-degree incline). While the pulse, blood pressure, electrocardiogram, and occasionally blood oxygen saturation are monitored, the patient is left in a “motionless standing position” for up to 45 minutes. If the patient remains asymptomatic during this phase of the study, isoproterenol is infused intravenously at 1 mg/min and increased until heart rate is 120% of baseline. Isoproterenol is similar to adrenaline and increases the sensitivity of the tilt table test, at the cost of some decrease in specificity. The infusion is continued for 15 minutes or until symptoms occur (syncope, near-syncope). A positive study is one in which symptoms (near or total loss of consciousness) are reproduced in conjunction with a significant drop in blood pressure and/or heart rate.
How the test works
During an upright tilt – or for that matter while standing – a person’s cardiovascular system has to adjust to prevent a significant portion of the blood volume from pooling in the veins in the legs. These adjustments consist of a mild increase in heart rate and a constriction of venous vessels in the legs. When a normal individual is placed in an upright tilt, these cardiovascular adjustments occur very quickly – there is no significant drop in the blood pressure and typically there is either no change or a slight increase in heart rate.
In patients with two varieties of syncope – orthostatic hypotension and vasovagal syncope – the cardiovascular adjustment to an upright tilt does not function appropriately. In orthostatic hypotension the body’s ability to adjust to an upright posture is grossly abnormal. When these individuals stand (or when the have tilt study), their pulse increases markedly, and their blood pressure drops precipitously. These patients are unable to adjust to the upright position. Patients with orthostatic hypotension rarely require a tilt table study for diagnosis. Doctors can make the diagnosis in the office by determining the blood pressure when the patient is lying down and then when standing.
The tilt table study results are frequently abnormal in people with vasovagal syncope. The sensitivity for detection is approximately 80%. Frequently these patients initially adjust to an upright tilt, but within 15 to 30 minutes they experience a rather sudden and marked change in vital signs manifested by a dramatic drop in blood pressure and/or heart rate. The resultant decrease in cerebral perfusion leads to near syncope or syncope. Recovery occurs within seconds after the patient is returned to a prone position.
The mechanism of vasovagal syncope involves to a reflex that causes sudden dilation of peripheral blood vessels and/or a slowing of the heart rate. Either or both contribute to a dramatic fall in cardiac output. Numerous triggering events can initiate this so-called “vasomotor” reflex, including fear, pain, and noxious stimuli (such as the sight of blood). During the tilt table test, the upright tilt produces stress on the cardiovascular system that acts as the trigger. The tilt table study helps to determine whether a person has a hyperactive vasomotor reflex or a propensity to develop vasovagal syncope.
When should the test be used?
The tilt table test is used predominantly to establish a diagnosis of vasovagal or neurocardiogenic syncope. It is generally not needed for establishing a diagnosis of orthostatic hypotension. The tilt table test should be used judiciously to establish or rule out the diagnosis of vasovagal/neurocardiogenic syncope. For patients who have clear-cut vasovagal syncope, the tilt table study reproduces symptoms in about 70 – 80%. In other words, 20 – 30% of these patients have false negative results. Conversely, up to 15% of the general “non-syncopal” population are tilt table test positive. Thus, false positive results can be a problem. The tilt study, therefore, should not be considered a gold standard in the diagnosis of vasovagal syncope. The gold standard is till the physician’s careful and complete medical history. In fact, if the clinical history is strongly indicative of vasovagal syncope, performing the test to confirm the diagnosis may – if the study turns out to be negative – confuse rather than clarify the diagnosis. For this reason, doctors should not routinely order tilt studies in patients in whom the history strongly supports a diagnosis of vasovagal syncope. The test is best used in patients whose histories are suggestive of vasovagal history, but for whom there is still doubt. In such patients, a positive tilt study can help to establish the diagnosis of vasovagal syncope.
