PETCO2 inversely affects MSNA response to orthostatic stress

Arterial hypocapnia has been associated with orthostatic intolerance. There fore, we tested the hypothesis that hypocapnia may be detrimental to increa ses in muscle sympathetic nerve activity (MSNA) and total peripheral resist ance (TPR) during head-up tilt (HUT). Ventilation was increased similar to1 .5 times above baseline for each of three conditions, whereas end-tidal PCO 2 (PETCO2) was clamped at normocapnic (Normo), hypercapnic (Hyper; +5 mmHg relative to Normo), and hypocapnic (Hypo; -5 mmHg relative to Normo) condit ions. MSNA (microneurography), heart rate, blood pressure (BP, Finapres), a nd cardiac output (Q, Doppler) were measured continuously during supine res t and 45 degrees HUT. The increase in heart rate when changing from supine to HUT (P < 0.001) was not different across PETCO2 conditions. MSNA burst f requency increased similarly with HUT in all conditions (P < 0.05). However , total MSNA and the increase in total amplitude relative to baseline (%Del ta MSNA) increased more when changing to HUT during Hypo compared with Hype r (P < 0.05). Both BP and Q were higher during Hyper than both Normo and Hy po (main effect; P < 0.05). Therefore, the MSNA response to HUT varied inve rsely with levels of PETCO2. The combined data suggest that augmented cardi ac output with hypercapnia sustained blood pressure during HUT leading to a diminished sympathetic response.