Tilt Table Testing
Some people are prone to fainting. They may have a history of fainting going on their whole lives but rarely have any serious consequences from it. These fainting spells often occur when they are upset or nervous about something. These swoons also frequently occur when they undergo medical procedures, have a blood test drawn or simply at the site of blood and needles. These nervous faints have been portrayed to great dramatic effect in the old movies of the 1930s and 1940s. Such people will often appear to have perfectly normal hearts by most tests. However, their otherwise normal heart is prone to an abnormal neural reflex.
When somebody gets excited or nervous, it is normal for their body to produce adrenaline which then makes the heart beat forcefully. In people prone to nervous fainting, this forceful heart beat is sensed by an abnormal nerve ending in their heart. This nerve then elicits a reflex whereby the brain suddenly makes the heart beat very slowly. At the same time, all the blood vessels in the body dilate. As a result of the slow heart beat and the dilated blood vessels, the blood pressure falls dramatically. Their brain no longer receives enough blood flow and the person loses consciousness. As the person falls down, more blood from the body returns to the heart, thus interrupting the reflex. Consciousness is soon regained.
Some people will get warning symptoms such as feeling warm, sweaty and nauseous before these faints. If they recognize these symptoms, they can often abort the fainting by sitting or lying down.
So what does all this have to do with tilt table tests?
Physiologists used to do experiments that involved putting a person on a table and gradually tilting it upright to study the effect of body position on their physiology. They noted that some people fainted during this procedure. Further study revealed that it was the aforementioned reflex that was responsible.
The tilt table test is now used to determine if this reflex is indeed the cause for a person fainting if there are no other evident causes.
Preparation for the test requires not eating for a minimum of six hours. The patient then lays down on a tilt table which includes a foot board. The patient is secured to the table so they won't fall off of it if they faint while tilted upright. The table is then tilted to an angle of 80 degrees for 45 minutes. Pulse, blood pressure and heart rhythm are closely monitored. Sometimes, an intravenous medicine is infused as well. A nurse and physician are both in attendance.
If the patient faints, the table is rapidly returned to the horizontal position and with this, the patient regains consciousness. The patient can return to normal activities after the test unless advised otherwise by their physician.
If the test is positive (i.e., the patient fainted or came close to fainting), the patient is diagnosed as having "vasovagal" or "neurocardiogenic" fainting. Specific medications are available to treat this condition.
Some patients who suffer from the chronic fatigue syndrome also have positive tilt table tests. The same medications used to treat vasovagal fainting may also help these chronic fatigue patients to feel better.
Because compensatory processes for positional change are mediated through the autonomic nervous system, evaluating the adequacy of this system in patients with recurrent unexplained syncope seems logical. Head-upright tilt table testing has been designed to provide controlled stress (gravity) on the system and see how it responds. Although tilt table testing has been used by physiologists for nearly half a century to explore the body's adaptation to positional change, not until 1986 was it employed as a diagnostic method in recurrent syncope. Since that time, tilt table testing has become widely used to determine a patient's predisposition to autonomically mediated hypotensive syncope.
Two basic methods of performing tilt table testing have evolved. The first uses head-upright tilt alone to cause dependent venous pooling and thereby provoke the aforementioned events in predisposed patients. No provocative drugs are used.
The second method was developed after investigators observed that serum catecholamine levels increase significantly before syncope. This led to use of isoproterenol (Isuprel) infusion during testing to increase sensitivity, although some researchers have cautioned that the increase in sensitivity may occur at the expense of specificity. Several other provocative agents, including edrophonium chloride (Enlon, Reversol, Tensilon), nitroglycerin (Nitro-Bid IV, Tridil), and adenosine triphosphate, have been investigated.
The first method of tilt table testing (i.e., using gravity as the only provocative stimulus) has been found to distinguish between asymptomatic control subjects and symptomatic syncope patients with a degree of accuracy that is considered acceptable in other clinically useful testing procedures. Recent studies have shown that gravity-only head-upright tilt table testing at angles between 60° and 80° has a specificity of about 90%. For testing using low-dose isoproterenol infusions, most available literature reports a specificity of 80% to 90%.
The American College of Cardiology recently issued an expert consensus statement on use of tilt table testing for syncope. Indications for testing are divided into three categories:
- Those in which there is general agreement that the test is warranted
- Those in which differences of opinion regarding the test's usefulness remain
- Those of potential interest but requiring more study.
Patients are to NPO past midnight and should be scheduled to perform the head-up tilt table testing as early as possible in the day.
The procedure should be thoroughly explained the patient and an informed consent for the performance of the procedure is to obtained at this time.
- The patient is positioned supine on a standard tilt table with a weight-bearing footboard.
- Attach the patient to a standard ECG monitor and automated blood pressure measuring device. A defibrillator should be immediately available.
- A peripheral intravenous line should be started at KVO utilizing normal saline.
- After baseline heart rate and blood pressure are determined, the table is slowly tilted upright to an angle of 80°.
- Heart rate and blood pressure are to be checked every 3 minutes. ECG recordings are taken continuously.
- If the patient's blood pressure and heart rate responses are abnormal and clinical symptoms are reproduced, the test is considered positive and is concluded.
- If after 30 minutes there is no response, the table is lowered to the horizontal position and an intravenous infusion of isoproterenol is started at 1 microgram/min.
- The dose is adjusted upward or downward to achieve a heart rate 20% higher than the supine baseline rate, and testing is repeated for 20 minutes.
- Repeat the tilt procedure precisely as before (same angle, same duration) while continuing to monitor vital signs.
- The study is complete at the conclusion of the testing protocol or upon reproduction of syncope / symptoms. If syncope occurs, obtain STAT heart rate and blood pressure and return patient to the supine position.
- The patient is discharged upon order of the attending physician.
There are five abnormal response patterns to head-upright tilt table testing:
The classic neurocardiogenic response is characterized by a sudden drop in blood pressure followed by a decrease in heart rate. Patients have few medical complaints between syncopal episodes.
The dysautonomic response is a gradual decrease in blood pressure to a hypotensive level, leading to loss of consciousness. Often, these patients have other signs of autonomic dysfunction, such as abnormal sweating, constipation, and thermal intolerance.
The postural orthostatic tachycardia syndrome was recently identified. This seems to be a mild form of autonomic dysfunction in which an excessive increase in heart rate cannot compensate for low peripheral vascular resistance. Patients have an increase in heart rate of 30 beats per minute or a maximum heart rate of 120 beats per minute in the first 10 minutes of upright posture. They often report near syncope, palpitations, and extreme fatigue, which may be mistakenly attributed to chronic fatigue syndrome.
Cerebral syncope has been reported by several medical centers as a relatively rare finding in patients undergoing concomitant tilt table testing and transcranial Doppler ultrasonography, which measures the degree of cerebral vasoconstriction or vasodilation. Patients pass out owing to cerebral vasoconstriction alone with no hypotension or bradycardia.
The psychogenic or psychosomatic response occurs with no observable change in heart rate or blood pressure or findings on transcranial Doppler ultrasonography or electroencephalography. Patients are often found to have psychiatric disorders, ranging from conversion reactions to major depression.
Therapy for recurrent syncope
A detailed discussion of therapy can be found elsewhere, but briefly, any therapeutic program must be tailored to meet the needs of the individual. Understanding the nature of the problem is imperative, as is instructing the patient to avoid predisposing factors (e.g., heat, dehydration). Pharmacotherapy is required only for recurrent episodes that disturb normal activities, and various agents have been reported to be useful. No single therapy is effective in all patients. Often, a combination of two agents at low doses is more effective than a single agent at a high dose.
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