Heart rate and blood pressure response in head up tilt test (HUTT) in neurocardiogenic syncope (vasovagal syncope or neurally mediated syncope) has been classified by Sutton et al.
Most common is the mixed type (type I) in which there is a fall in both heart rate and blood pressure. Fall in blood precedes the fall in heart rate which falls at the onset of syncope. It can fall below 40 per minute and can be associated with asystole more than 3 seconds, though these cut off points may not be present always. In cardioinhibitory type (type II) there is a fall in heart rate. In type IIA, there is no asystole, but heart falls below 40 per minute and remains so for over 10 seconds without any asystole of more than 3 seconds. In cardioinhibitory type IIB, there is asystole of more than 3 seconds. In vasodepressor or type III, heart rate does not fall more than 10% from its peak at the onset of syncope.
Two exceptions which have been described are chronotropic incompetence and postural orthostatic tachycardia syndrome (POTS). In the former, there is no increase in heart rate during the tilt test or any little increase being less than 10 percent of the pre tilt heart rate. In POTS, on the other hand there is an increase in heart rate more than 28 beats per minute compared to the pre tilt heart rate, which is maintained throughout the duration of the tilt.
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