Effect of increased inspiratory muscle work on blood flow to inactive and active limbs during submaximal dynamic exercise.

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What is the central question of this study? Increased respiratory muscle activation is associated with neural and cardiovascular consequences via the respiratory muscle metaboreflex. Does increased sympathetic vasoconstriction originating from the respiratory musculature elicit a reduction in blood flow to an inactive limb in order to maintain blood flow to an active limb? What is the main finding and its importance? Arm blood flow was reduced whereas leg blood flow was preserved during mild leg exercise with inspiratory resistance. Blood flow to the active limb is maintained via sympathetic control of blood flow redistribution when the respiratory muscle-induced metaboreflex is activated. The purpose of this study was to elucidate the effect of increasing inspiratory muscle work on blood flow to inactive and active limbs. Healthy young men (n=10, 20±2years of age) performed two bilateral dynamic knee-extension and knee-flexion exercise tests at 40% peak oxygen uptake for 10min. The trials consisted of spontaneous breathing for 5min followed by voluntary hyperventilation either with or without inspiratory resistance for 5min (40% of maximal inspiratory mouth pressure, inspiratory duty cycle of 50% and a breathing frequency of 40 breathsmin-1 ). Mean arterial blood pressure was acquired using finger photoplethysmography. Blood flow in the brachial artery (inactive limb) and in the femoral artery (active limb) were monitored using Doppler ultrasound. Mean arterial blood pressure during exercise was higher (P<0.05) with inspiratory resistance (121±7mmHg) than without resistance (99±5mmHg). Brachial artery blood flow increased during exercise without inspiratory resistance (120±31mlmin-1 ) compared with the resting level, whereas it was attenuated with inspiratory resistance (65±43mlmin-1 ). Femoral artery blood flow increased at the onset of exercise and was maintained throughout exercise without inspiratory resistance (2576±640mlmin-1 ) and was unchanged when inspiratory resistance was added (2634±659mlmin-1 ; P>0.05). These results suggest that sympathetic control of blood redistribution to active limbs is facilitated, in part, by the respiratory muscle-induced metaboreflex.