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Am J Physiol Heart Circ Physiol 281: H1040-H1046, 2001;
0363-6135/01 $5.00
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Vol. 281, Issue 3, H1040-H1046, September 2001

PETCO2 inversely affects MSNA response to orthostatic stress

J. Kevin Shoemaker1, Debbie D. O'Leary2, and Richard L. Hughson2

1 School of Kinesiology, University of Western Ontario, London, Ontario N6A 3K7; and 2 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Arterial hypocapnia has been associated with orthostatic intolerance. Therefore, we tested the hypothesis that hypocapnia may be detrimental to increases in muscle sympathetic nerve activity (MSNA) and total peripheral resistance (TPR) during head-up tilt (HUT). Ventilation was increased ~1.5 times above baseline for each of three conditions, whereas end-tidal PCO2 (PETCO2) was clamped at normocapnic (Normo), hypercapnic (Hyper; +5 mmHg relative to Normo), and hypocapnic (Hypo; -5 mmHg relative to Normo) conditions. MSNA (microneurography), heart rate, blood pressure (BP, Finapres), and cardiac output (Q, Doppler) were measured continuously during supine rest and 45° HUT. The increase in heart rate when changing from supine to HUT (P < 0.001) was not different across PETCO2 conditions. MSNA burst frequency increased similarly with HUT in all conditions (P < 0.05). However, total MSNA and the increase in total amplitude relative to baseline (%Delta MSNA) increased more when changing to HUT during Hypo compared with Hyper (P < 0.05). Both BP and Q were higher during Hyper than both Normo and Hypo (main effect; P < 0.05). Therefore, the MSNA response to HUT varied inversely with levels of PETCO2. The combined data suggest that augmented cardiac output with hypercapnia sustained blood pressure during HUT leading to a diminished sympathetic response.

muscle sympathetic nerve activity; head-up tilt; end tidal CO2; cardiac output; total peripheral vascular resistance


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