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LETTER TO THE EDITOR
HUT, like lower body negative pressure, is a valuable test to evaluate venous, arterial, autonomic, and cardiac response capacity. All four components, including coupling to each other, may affect the reaction to postural changes. In the absence of a rapid, easily accessible repeatedly applicable method to reproducibly evaluate in vivo human limb venous function until 1999, most data are available on the latter three constituents (1). Since we recently observed that low plasma volume decreased orthostatic tolerance and cardiac modulation of stroke volume without altering autonomic responses during stepwise inflicted head-up tilt, these observations raised the question about whether shallow venous response capacity may be involved (2).
Monahan (6) brings up the absence of not only a time control group but also the random exposure to varying levels of orthostatic stress. To begin with the latter, this would only be methodologically valid when all involved systems would return to normal within a short period of time. Although reported for most concerned variables, aldosterone remained elevated as a function of prior HUT angle for more than 20 minutes (4). Since aldosterone has been reported to impair baroreflex sensitivity, we thought it would be better to subject all participants to a comparable baroreflex unloading protocol (5).
Although we randomly allocated the two methods of venous function assessments, we agree that a time control group without HUT would have excluded the possibility that the repeated measures at short time intervals would have affected our observations. Although at larger time intervals the coefficients of variation (CV) of venous compliance and venous emptying rate at HUT 0° are 18.8% and 12.4%, respectively, systematic error at shorter time periods cannot be ruled out. Although we evaluated repeated measures as quality control within our laboratory, we never assessed venous function at a similar time frame as performed in this study. To this end, in six healthy participants, we repeatedly measured venous compliance in a comparable protocol at HUT 0°. From these data, we calculated the CV and evaluated possible changes in time nonparametrically. Venous compliance (CV, 14%) did not change throughout the protocol (Fig. 1). Therefore, although not completely proved since randomization did not take place during the study, we think that time itself did not substantially affect our observations.
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FOOTNOTES
Address for reprint requests and other correspondence: I. Krabbendam, Dept. of Obstetrics and Gynecology, Radboud Univ. Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands (e-mail: i.krabbendam{at}obgyn.umcn.nl)
REFERENCES
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