Head-up Tilt Table Testing (HUT) and Data Collection Instrumentation

Upright tilt testing was devised in the 1940's at NASA as a postural stressor and its use in cardiology as a test for fainting and related disorders dates  from the 1980's. In the young it is neither specific nor sensitive and to improve these indices workers have combined the test with pharmacological agents whose use is moot. However, as an orthostatic stress test that [mostly] disables the skeletal muscle pump it is excellent. Note that tilt is not exclusively a test of the autonomic nervous system but is instead an orthostatic stressor which evokes compensatory responses among them autonomic responses.

 Patients with orthostatic intolerance and in particular those with syncope (fainting) are often referred early during their evaluation to a cardiologist. The reason is that cardiac disease ranks high on the list of dangerous illnesses which provoke syncope although rarely chronic orthostatic intolerance. Typically, this form of syncope is not posturally induced but occurs in all positions. While most children and adolescents with syncope or orthostatic intolerance do not have cardiac disease, our first task is to rule out heart disease. Therefore a cardiac examination along with pertinent laboratory screening tests are often required and may include electrocardiography, echocardiography (cardiac ultrasound) to rule out structural heart disease, and event monitoring using transtelephonic devices to rule out important arrhythmias which often are the immediate precipitant for cardiac syncope. In addition a good neurologic examiniation is important and may also include some tests. Blood sampling often includes a complete blood count with leukocyte differential; erythrocyte sedimentation rate, serum levels of alanine aminotransferase, total protein, albumin, globulin, alkaline phosphatase, calcium, phosphorus, glucose, blood urea nitrogen, electrolytes, creatinine, thyroid-stimulating hormone, urinalysis., and a pregnancy test in girls.

Once cardiac and other diseases are ruled out, neurovascular diagnosis is necessary.

 

 

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  • Head-up Tilt Table Testing (HUT)

HUT is neither more nor less than an orthostatic stress test. The table can assume continuous angles from -20 degrees through completely upright as shown in the upper panels. Typically, patients are studied supine and upright at 70 degrees while noninvasively instrumented for circulatory measurements. The table is mechanically driven which removes the need for lower limb muscle activity. Low angles may activate low pressure cardiopulmonary mechanoreceptors. Upright positioning at angles greater than approximately 30 degrees activates the sinoaortic baroreflex which is essential for the maintenance of blood pressure and cerebral perfusion while standing. While normal subjects develop a degree of tachycardia (10-25 beats per minute increase in heart rate is typical) patients with POTS develop much higher heart rates associated with symptoms of orthostatic intolerance and sometimes hypotension. Patients with simple faint (neurally mediated syncope, vasovagal episodes) typically behave normally for some minutes later developing bradycardia and hypotension.

 

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