OUR PROCEDURES - Tilt Study (Autonomic Challenge Test)

The tilt table study is used to evaluate patients who have had syncope (loss of consciousness). It is an extremely simple study, and in most cases is quite safe.

How the test is done

In a tilt table study, the patient is strapped to a table, which is then mechanically tilted to an upright position. While monitoring the pulse, blood pressure, electrocardiogram, and sometimes blood oxygen saturation, the patient is left in a “motionless standing position” for 20 to 30 minutes. When the patient's syncope is reproduced during the test, a "positive" tilt table study is said to have occurred .

How the test works

During an upright tilt – or for that matter while standing – a person's cardiovascular system has to adjust itself in order to prevent a significant portion of the blood volume from pooling in the legs. These adjustments consist of a mild increase in heart rate, and a constriction of the blood vessels in the legs. When a normal individual is placed in an upright tilt, these cardiovascular adjustments occur very quickly, and there is no significant drop in the blood pressure.

However, in patients with two varieties of syncope – orthostatic hypotension and vasovagal syncope – the cardiovascular adjustment to an upright tilt does not function normally.

In orthostatic hypotension the body’s ability to adjust to an upright posture is grossly abnormal. When these individuals stand (or when they have a tilt study), their pulse increases markedly, and their blood pressure drops precipitously. These patients are simply unable to adjust to the upright position. Patients with othostatic hypotension rarely require a tilt table study for diagnosis, however – doctors can easily make the diagnosis in their office simply by taking the blood pressure first while the patient is lying down, and then while standing.

The tilt table study is also frequently abnormal in people with vasovagal syncope, but in a more subtle way. In general, these patients initially adjust normally to an upright tilt, but within 20 – 30 minutes they have a rather sudden and marked change in vital signs – their blood pressure drops dramatically; their pulse also drops, and they pass out. They recover within seconds after the table is brought back down, and they are returned to a lying-down position.

Vasovagal syncope is due to a reflex that causes sudden dilation of the blood vessels in the legs, and a slowing of the heart rate, both of which contribute to a dramatic fall in blood pressure. Numerous triggering events can initiate this so-called “vasomotor” reflex, including things like 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, then, tests whether a person has a hyperactive vasomotor reflex. It tests, in other words, a person's propensity to develop vasovagal syncope.

When should tilt table testing be used?

While the tilt table study can be used to diagnose orthostatic hypotension, this test should virtually never be necessary in patients with this condition, which is easily diagnosed in any physician’s office. The main utility of the tilt table study is in diagnosing vasovagal syncope. Observing a typical vasovagal episode during an upright tilt can solidify a diagnosis that was previously uncertain, and for this reason the tilt table study can occasionally be quite useful.

However, the test should be used judiciously. In fact, it should be used only in a minority of patients with vasovagal syncope. In people who have clear-cut vasovagal syncope, the tilt table study reproduces their symptoms only about 70 – 75% of the time. In other words, 25 - 30% of these patients have "false negative" studies. The tilt study, therefore, should not be considered a “gold standard” in the diagnosis of vasovagal syncope. The “gold standard” is still 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 solidify what otherwise was a clear picture. For this reason, doctors should probably not routinely order tilt studies in patients who clearly have vasovagal syncope.

The test is best used in patients whose histories are suggestive of vasovagal episodes, but where there is still some doubt. In these patients, a positive tilt study can go a long way in pinning down the diagnosis of vasovagal syncope.