Individual stability of orthostatic tolerance response

We studied individual stability of orthostatic tolerance as well as presyncopal signs and symptoms across four runs in subjects undergoing combined head-up tilt (HUT) and lower body negative pressure (LBNP). Ten healthy young males were subjected to HUT+ LBNP, to achieve a presyncopal end-point. Four test runs were separated by two week intervals. Hemodynamic variables and orthostatic tolerance were measured. From supine control to presyncope, heart rate increased while mean arterial blood pressure and stroke index decreased significantly. Individual orthostatic tolerance ranged from 7.2 to 30.0 min. Repetitions from the 1st to the 4th trial increased orthostatic tolerance by about 3 min, from 15+/-6 (trial 1) to 18+/-7 min (trial 4) but not significantly (p>0.05). Additionally, specific signs and symptoms as subjects approached presyncope were not always identical in the same persons. While considerable difference existed in tolerance times between healthy young men, orthostatic tolerance within subjects was similar, with little individual variability. However, as the reasons for termination of the tests were not always identical in the same subjects, and many subjects showed presyncopal symptoms rather than signs, close attention must be given to monitoring not only the signs but also the symptoms in subjects reaching presyncope.