Timothy C. Hain, MD Page last modified: May 16, 2012
|Tilt Table||Positive tilt table test showing large fluctuation in systolic blood pressure accompanying change in angle from supine (0 deg) to near upright (70 degrees)|
Tht tilt table test is used to detect dizziness associated with either low blood pressure (orthostatic hypotension) or poorly regulated pulse (usually too high - -POT). The Valsalva test is done using similar equipment, and it may indicate impaired autonomic control.
Blood pressure is regulated using negative feedback. On the input side, pressure is sensed by the baroreceptors in the carotid sinus. There is also input from gravity receptors in the inner ear, and one's internal understanding about position.
This signal goes to the brain, is processed in the brainstem, and then used to control heart rate (both parasympathetic and sympathetic), heart force (sympathetic), and blood vessel size (sympathetic). In general, regulation can be disrupted by lesions in the input, central processing, and output.
There are also some extra inputs that change the level of blood pressure outside of the desired set point - -excitement of course increases blood pressure and pulse.
The tilt table test changes the load on the heart by making it work harder, and tests the ability of the system to keep heart rate and blood pressure stable.
Persons may exhibit several failure patterns resulting in fainting or lightheadeness.
Patients are strapped, in a supine position to a tilt table as shown above from our practice at Chicago Dizziness and Hearing. They are instrumented with a pulse oximeter and EKG leads to monitor blood pressure, heart rate and EKG.
Blood pressure and pulse are recorded every minute for 5 minutes. Next, the patient is tilted 70 degrees, head-up and recording continues for another 25 minutes.
The test is terminated if there is
Criteria for a positive test include:
The TTT is largely a measure of parasympathetic afferent (baroreceptor) and sympathetic efferent limb function. There is a larger drop of blood pressure compared to normal on standing if sympathetic tone fails.
The "VASIS" classifications (Brignole, Menozzi et al. 2000) are used to interpret the tilt table test.
The Valsalva test is done while lying flat. It is largely a measure of parasympathetic (baroreceptor) input and vagal output to control the pulse. It consists of having the subject take a deep breath and strain for 20 seconds, to increase intrathoracic pressure. The pressure should be 40-50 mm.
Note that the Valsalva test can also stimulate the inner ear -- see this page for detail about how this occurs and what it looks like.
|Valsalva test illustration (from http://www.answers.com/topic/blood-pressure).|
The normal physiological response consists of 4 phases, marked on the diagram above.
In persons with autonomic disoders, the compensation part of phase 2 may be absent. Blood pressure may falls continuously and a faint occur (such as in cough syncope). Similarly, the overshoot is absent.
Several other measures can be easily obtained:
The Valsalva ratio is a measure of cardiac slowing during the Valsalva.
It is computed as:
Valsalva ratio = (longest R-R interval in phase 4) / (shortest R-R interval in phase 2). A Valsalva ratio > 1.20 is normal. A ratio <= 1.20 is abnormal.
The VR is a measure of parasympathetic afferent (baroreceptor) and efferent (parasympathetic and sympathetic) autonomic nervous system function.
Another measure that may be of interest is the 30:15 ratio.
After 15 minutes of supine rest, subjects are stood up while the heart rate is continuously monitored. The 30:15 ratio is the ratio of the RR interval at beat 30 while standing to the RR interval at beat 15. On two repetitions, the highest ratio is accepted.
The 30:15 ratio is also a measure of parasympathetic afferent (baroreceptor) and efferent cardiac autonomic nervous system function.
The heart rate response to deep breathing
Subjects are asked to breath at a rate of 6 breaths/min in the seated position. After a rest period of 5 minutes, the HR is monitored for at least 1 minute. The HR change with breathing is the mean difference between the maximal and minimum HR over at least 6 cycles.
Individuals with POT (postional orthostatic tachycardia) have little respiratory variability, but large pulse changes on standing.
There are several variants to the TTT, mainly aimed at increasing sensitivity. These are generally performed in hospital settings, under the supervision of cardiologists, because they involve administering substances that may precipitate angina.
The cardiac system may be "stressed" by administering nitroglycerin, isoproterenol. There may also be an attempt to prevent syncope by administering a beta-blocker. For these variants of the TTT, IV access is required both to administer the drug as well as to provide a quick access to the circulation should resucitation be needed. It is the author's opinion that these procedures are generally not warranted.
Tilt table testing is sometimes not fully covered by health insurance. It is common to have payment denied.
|© Copyright April 6, 2012 , Timothy C. Hain, M.D. All rights reserved. Last saved on April 6, 2012|