Effects of hypovolemia on aortic baroreflex control of heart rate in humans.
BACKGROUND
To test the hypothesis that hypovolemia can acutely increase the sensitivity of chronotropic baroreflex response, eight men (21-45 yr old) underwent measurements of heart rate response to aortic baroreceptor stimulation under normovolemic and hypovolemic conditions.


METHODS
Hypovolemia was acutely induced by a bolus injection of 30 mg of furosemide. The sensitivity of the aortic-cardiac baroreflex was determined with a approximately 15 mmHg elevation in mean arterial pressure (MAP) induced by steady-state infusion of 30 to 97 micrograms.min-1 phenylephrine (PE) combined with approximately 13 mmHg lower body negative pressure (LBNP) to counteract central venous pressure elevations, and 17-19 mmHg neck pressure (NP) to offset increases in carotid sinus transmural pressure. The aortic-cardiac baroreflex gain was assessed by determining the ratio of the change in heart rate to the change in MAP (delta HR/delta MAP) between baseline and aortic baroreceptor isolated conditions (i.e., PE + LBNP + NP stage).


RESULTS
When compared to normovolemia (3182 +/- 163 ml), furosemide-induced hypovolemia (2812 +/- 101 ml) resulted in an average 12% reduction in plasma volume (p = 0.05). Hypovolemia increased the average gain of the aortic-cardiac baroreflex by 68% (0.71 +/- 0.26 to 1.19 +/- 0.37 beats.min-1.mmHg-1; p = 0.0349) while it had no effect on the calculated response of the carotid-cardiac baroreflex.


CONCLUSIONS
These results indicate that greater aortic baroreflex sensitivity observed in individuals who are physically untrained or have been exposed to microgravity may be explained by smaller vascular volume rather than differences in autonomic function associated with adaptations to lower aerobic capacity.
Convertino
V A
VA
Physiology Research Branch, Clinical Sciences Division, Armstrong Laboratory/AOCY, Brooks Air Force Base, TX 78235-5117, USA.
Baumgartner
N
N
D000328:Adult; D000337:Aerospace Medicine; D001011:Aorta / Q000503:physiopathology*; D017704:Baroreflex / Q000502:physiology*; D001810:Blood Volume; D002316:Cardiotonic Agents / Q000494:pharmacology; D002496:Central Venous Pressure / Q000187:drug effects; D004232:Diuretics / Q000494:pharmacology; D005665:Furosemide / Q000494:pharmacology; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology; D010807:Physical Endurance; D012769:Shock / Q000139:chemically induced / Q000503:physiopathology*
D002316:Cardiotonic Agents; D004232:Diuretics; D010656:Phenylephrine; D005665:Furosemide