Patterns of 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).