“…14 In fact, most patients show a marked increase in heart rate and a low frequency/high frequency ratio that reflects sympathetic activity in heart rate variability immediately before vasovagal syncope during HUT testing. [14][15][16][17][18][19]25 Consequently, if a positive response is not observed with a HUT alone, the addition of isoproterenol infusion further increases the heart rate and inotropism, mimicking the increased catecholamine discharge provoked by stressful stimulation that occurs in vasovagal syncope. Thus, isoproterenol provocation as part of a HUT has recently proved to be of great help in inducing vasovagal syncope.…”