Orthostatic Stress Testing & instrumentation

 Up ] Search ] [ Orthostatic Stress  Testing & instrumentation ] Circulatory Measurements ] Circulatory Autonomic Testing ] Initial Orthostatic Hypotension ] Research Studies ] Comments ] Links ] Fainting and Related Phenomena a Lay Review ] Orthostatic Intolerance ] Orthostatic Hypotension ]

 

Contents:

  1. Standing

  2. Head up tilt table testing

  3. Lower Body Negative Pressure (LBNP)

  4. Return to Home Page

   Standing
  The most logical test for orthostatic tolerance is standing itself. Othostatic tolerance comprises compensatory mechanisms including the mechanisms of neurovascular control and also the skeletal muscle pump. During significant orthostatic stress the muscle pump may need to work overtime. When the pump is encumbered as in standing at attention or inhibited during tilt testing, the neurovascular aspects of orthostatic control are made more evident. 
 

 Head-up or Upright 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.

 

 

 

goto top

  • 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.

   

Lower Body Negative Pressure

Apparatus of Victor Convertino PhD.

Lower body negative pressure (LBNP) is also of NASA devise. Testing is primarily done while horizontal although some investigators have combined LBNP with HUT to achieve a device capable of producing hypotension in almost asll subjects ill or healthy. Horizontal use closely simulates hemorrhage and its use in this regard is unparalleled. progressive shifts of blood and interstitial fluid into the lower body, below the iliac crests is attained by using progressively increasing negative pressure. This is usually in the range from -10mmHg to -60mmHg or more. One main difference with standing or upright tilt is that in upright testing grav ity fills the lower body below the hemostatic indifference point usually located at about the diaphragm. This includes the splanchnic circulation. With LBNP, filling occurs below the iliac crests and, as in hemorrhage, the splanchnic circulation empties as a primary compensation for simulated hemorrhage.

 

   

 

goto top

 

 


 


Up
Search
Orthostatic Stress  Testing & instrumentation
Circulatory Measurements
Circulatory Autonomic Testing
Initial Orthostatic Hypotension
Research Studies
Comments
Links
Fainting and Related Phenomena a Lay Review
Orthostatic Intolerance
Orthostatic Hypotension