Patterns of Orthostatic Intolerance

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Normal ] Postural Tachycardia Syndrome ] Vasovagal Syncope ] Dysautonomic Orthostatic Intolerance ]

Patterns of orthostatic intolerance are best defined by an orthostatic stress test i.e. a means by which upright stress can be imposed in a controlled fashion and the physiological response monitored in detail. There are 3 types of stress test used in practice: standing, upright tilt table testing, and lower body negative pressure (LBNP). Standing is clearly the most physiologic orthostatic stress test but individual differences and patient motion may make this difficult. LBNP use is usually confined to research laboratories. Therefore the standard of orthostatic assessment is the head-up tilt table test. Although head-up tilt was used to evoke autonomic reflexes in early NASA experiments, it was first used as a provocative agent in 1986. This device comprises a table driven by an electrical motor with a supportive footboard enabling positioning of a patient at varying angles of upright tilt. Although it would seem that an angle of 90o is most physiologic, this usually induces too many "false positives" (patients with no history of orthostatic intolerance who have intolerance induced during testing). Therefore lesser angles such as 60o or 70o are customarily used. Often cardiologists use another classification whose focus is more narrowly applied to forms of syncope designated c ardioinhibitory (decreased heart rate with sustained BP), vasodepressor (decreased BP with sustained heart rate), and mixed in which both BP and heart rate decrease. The patterns of head up tilt table testing shown in the figure are based on a more a physiologic classification of results into vasovagal (simple faint), dysautonomia (classically a fall in BP with little change in heart rate) and POTS (classically an increase in heart rate with little falll in BP).



Patterns of heart rate (upper traces) and blood pressure (lower traces) change during orthostatic challenge

Normal

 

 

Vasovagal Syncope
POTS
 

 

Dysautonomic - orthostatic Hypotension
 

 


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Physiology of Orthostasis
Patterns of Orthostatic Intolerance
Acute Orthostatic Intolerance and Syncope
Chronic Orthostatic Intolerance