Premium
Prior exercise and changes in plasma volume cause differential baroreflex responses to head‐up tilt
Author(s) -
Charkoudian Nisha,
Kenefick Robert W.,
Caruso Elizabeth M,
Luippold Adam J.,
Heavens Kristen R.,
Cheuvront Samuel N.
Publication year - 2016
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.30.1_supplement.752.1
Subject(s) - supine position , medicine , orthostatic intolerance , hypovolemia , heart rate , baroreflex , blood pressure , orthostatic vital signs , tilt table test , endocrinology , cardiology
Prior exercise and loss of plasma volume each independently cause cardiovascular changes that can result in hypotension and orthostatic intolerance (OI). However, the profile of volume regulatory factors differs substantially between these conditions. We hypothesized that during isotonic hypovolemia (diuretic‐induced, 3.01±0.23 % body weight loss; n = 6, DIUR), sympathetic neural responses to tilt would be greater than in euhydrated, normothermic subjects (n = 6, EX) who had previously exercised in dry heat (40°C). We measured heart rate (HR), blood pressure (BP) and muscle sympathetic nerve activity (MSNA) during supine rest and head‐up tilt (HUT) to 30 and 45°. Plasma volume decreased 10.2±4.4% in DIUR, and 3.4±3.6% in EX (P<0.01). Supine HR was higher in EX compared to DIUR (82±12 vs. 64±9 beats/min, P < 0.01). However, supine MSNA was not different between groups, either as bursts/min (EX: 24±9 vs. DIUR: 24±10, P>0.05) or bursts/100hb (EX: 30±11 vs. DIUR: 40±19, P>0.05). In contrast to our hypothesis, MSNA responses to tilt were smaller in the DIUR group, both at 30° (EX: 12±7 vs. DIUR 3±4 bursts/100hb, P < 0.02) and at 45°HUT (deltas: EX: 14±8 vs. 3±3 bursts/100hb, P < 0.02). HR responses to tilt were larger in the DIUR group (P < 0.05). Systolic BP tended to be lower in DIUR during tilt (P = 0.05), but diastolic and mean BP values were not different between groups. These data provide an interesting example of differential responsiveness of two arms of the baroreflex (HR vs. MSNA) to successfully maintain BP during tilt in healthy humans based on pre‐tilt differences in cardiovascular and fluid volume status. Support or Funding Information Funded by USAMRMC; author views not official US Army or DOD policy.