Acta Physiologica Hungarica, Volume 96 (2), pp. 157–166 (2009)
DOI: 10.1556/APhysiol.96.2009.2.2
0231–424X/$ 20.00 © 2009 Akadémiai Kiadó, Budapest
Individual stability of orthostatic
tolerance response
N Goswami1*, H Lackner3, HG Hinghofer-Szalkay1,2
1Institute of Physiology, Center of Physiological Medicine, Medical University Graz, Austria
2Institute of Adaptive and Spaceflight Physiology, Wormgasse 9, Graz, Austria
3Institute of Medical Engineering, University of Technology, Graz, Austria
Received: October 7, 2008
Accepted after revision: December 15, 2008
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). 10 healthy young males were subjected to HUT+ LBNP, to achieve a pre-syncopal 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 mins. Repetitions from the 1 st to the 4 th 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.

Keywords: Graded orthostatic stress, LBNP, presyncope, hemodynamics, heart rate, orthostatic
tolerance, head-up tilt, mean arterial blood pressure

Syncope is a common clinical state. It can arise in normal subjects on standing up from
a supine position or on return from microgravity environment of spaceflight. During
simulated spaceflight missions and to study etiologies of syncope in clinical practice, it
is necessary to create, and to deal with situations that produce high levels of
cardiovascular stress.

* Corresponding author: Nandu Goswami, M.B.B.S
Institute of Physiology, Medical University Graz
Harrachgasse 21, A-8010 Graz, Austria
E-mail: nandu.goswami@meduni-graz.at

158 N Goswami et al.
Acta Physiologica Hungarica 96, 2009
Extreme cardiovascular stress can be produced by increasing orthostatic challenge
via hypergravity (human centrifuge), hypovolemia, or a combination of passive head-up
tilt (HUT) and lower body suction (LBNP). HUT+LBNP induces cardiovascular and
neuroendocrine changes which are dependent on the intensity and duration of the stress
(4). The initial effects of upright posture occur due to hydrostatic changes: Carotid
baroreceptor pressure and cardiac preload both decrease because the arterial hydrostatic
indifference point between supine and upright is located at heart level, while the venous
hydrostatic indifference point is located below heart level (7), rendering both carotid
baroreceptor unloading and reduced diastolic filling upon assumption of an upright
position. A fall in cardiac filling pressure reduces the firing rate from cardiopulmonary
receptors as well. Medullar feedback patterns from both baroreceptor sites are altered,
which triggers neurohumoral responses, including sympathetic activation (20).
Additional LBNP diminishes cardiac preload even further – e.g. LBNP of 40 mmHg
reduces cardiac output by approximately 25%, similar to passive HUT (25), so the
stimulus combination eventually leads to cerebral underperfusion and consequently a
presyncopal situation.
There exists a considerable body of evidence regarding hormonal and
hemodynamic changes due to HUT and LBNP in presyncopal state (6, 11, 21) but little
is known about individual stability of orthostatic tolerance in subjects. We investigated
whether the intra-individual orthostatic tolerance was similar across four runs in
subjects reaching orthostatically induced presyncope using HUT + LBNP. As in the
study of (21), we compared orthostatic tolerance reproducibility across trials. However,
our main focus was intra- and inter-individual reproducibility of orthostatic tolerance in
young healthy males. We also assessed whether the presyncopal signs and symptoms
were similar in the same subject across the four runs.
Procedures and Methods
Persons
The study was done in 10 healthy, non-obese, non-medicated, non-smoking males who
were free from any somatic or mental condition as assessed in a medical exam before
the test. Furthermore, to avoid effects of confounding variables such as height, gender
or athletic training on orthostatic tolerance (10) subjects were carefully selected.
Subjects were told to refrain from vigorous exercise, smoking or alcohol during any part
of the study. Each subject was studied four times with an interval of two weeks or more
between tests.
All subjects were advised to keep their fluid and salt intake according to their usual
dietary habits (2). We, however, did not attempt to monitor sodium balance as sodium
balance is a controversial issue which goes beyond classical concepts: Inter-
compartmental sodium shifts occur from and to the interstitial (“third”) compartment
which potentially invalidate the significance of day to day sodium input (dietary
analysis) or output (urinary measurements), respectively (29). We have observed earlier