Cardiovascular responses to forearm muscle metaboreflex activation during hypercapnia in humans

We characterized the cardiovascular responses to forearm muscle metaboreflex activation during hypercapnia. Ten healthy males participated under three experimental conditions: 1) hypercapnia (HCA, PetCO2: +10 mmHg, by inhalation of a CO2-enriched gas mixture); 2) muscle metaboreflex activation (MMA, by 5 min of local circulatory occlusion after 1 min of 50% maximum voluntary contraction isometric handgrip under normocapnia); and 3) HCA+MMA. We measured mean arterial pressure (MAP), heart rate (HR), and cardiac output (CO); calculated stroke volume (SV), and total peripheral resistance (TPR); and evaluated myocardial oxygen consumption (MV̇o2) and cardiac work (CW) noninvasively. MAP increased in the three experimental conditions but HCA+MMA led to the highest MAP, CO, and HR. Moreover, HCA+MMA increased SV and was associated with the highest MV̇o2and CW. HCA and MMA exhibited inhibitory interactions with MAP, HR, TPR, MV̇o2, and CW, increases of which were smaller during HCA+MMA than the sum of the increases during HCA and MMA alone (MAP: +28 ± 2 vs. +34 ± 2 mmHg, P < 0.001; HR: +15 ± 2 vs. +22 ± 3 bpm, P < 0.01; TPR: +1.1 ± 1.4 vs. +3.0 ± 1.5 mmHg·l·min−1, P < 0.05; MV̇o2: +50.25 ± 4.74 vs. +59.48 ± 5.37 mmHg·min−1·10−2, P < 0.01; CW: +59.10 ± 7.52 vs. +63.67 ± 7.71 ml mmHg·min−1·10−4, P < 0.05). Oppositely, HCA and MMA interactions were linearly additive for CO (+2.3 ± 0.4 l/min) and SV (+13 ± 4 ml). We showed that muscle metaboreflex and hypercapnia interact in healthy humans, reducing vasoconstriction but enhancing SV.