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1 Pediatrics and Physiology, New York Medical College, Valhalla, New York, United States
2 LDM Associates, United States
3 Pediatrics, New York Medical College, Valhalla, New York, United States
* To whom correspondence should be addressed. E-mail: stewart{at}nymc.edu.
Increased BP and heart rate during exercise characterizes the exercise pressor reflex. When evoked by static handgrip mechanoreceptors and metaboreceptors produce regional changes in blood volume and blood flow which are incompletely characterized in humans. We studied 16 healthy subjects aged 20-27 years using segmental impedance plethysmography validated against dye dilution and venous occlusion plethysmography to non-invasively measure changes in regional blood volumes and blood flows. Static handgrip while supine was performed for two minutes without post exercise ischemia. Measurements of heart rate and BP variability and coherence analyses were used to examine baroreflex-mediated autonomic effects. During handgrip systolic BP increased from 120±10 to 148±14 mmHg while heart rate increased from 60±8 to 82±12 bpm. Heart rate variability decreased while BP variability increased and transfer function amplitude was reduced from 18±2 to 8±2 ms/mmHg at low frequencies of approximately 0.1Hz. This was associated with marked reduction of coherence between BP and HR (from 0.76±0.10 to 0.26±.05) indicative of uncoupling of heart rate regulation by the baroreflex. Cardiac output increased by approximately 18% with a 4.5% increase in central blood volume and an 8.5% increase in total peripheral resistance suggesting increased cardiac preload and contractility. Splanchnic blood volume decreased reciprocally with smaller decreases in pelvic and leg volumes, increased splanchnic, pelvic and calf peripheral resistance, and evidence for splanchnic venoconstriction. We conclude that the exercise pressor reflex is associated with reduced baroreflex cardiovagal regulation and driven by increased cardiac output related to enhanced preload, cardiac contractility and splanchnic blood mobilization.
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