Respiratory muscle work compromises leg blood flow during maximal exercise

We hypothesized that during exercise at maximal O-2 consumption ((V) over dotO(2max)), high demand for respiratory muscle blood flow ((Q) over dot) would elicit locomotor muscle vasoconstriction and compromise limb (Q) over dot. Seven male cyclists ((V) over dotO(2max) 64 +/- 6 ml.kg(-1).min(-1)) each completed 14 exercise bouts of 2.5-min duration at (V) over dotO(2max) on a cycle ergometer during two testing sessions. Inspiratory muscle work was either 1) reduced via a proportional-assist ventilator, 2) increased via graded resistive loads, or 3) was not manipulated (control). Arterial (brachial) and venous (femoral) blood samples, arterial blood pressure, leg (Q) over dot ((Q) over dotlegs; thermodilution), esophageal pressure, and Oz consumption ((V) over dotO(2)) were measured. Within each subject and across all subjects, at constant maximal work rate, significant correlations existed (r = 0.74-0.90; P < 0.05) between work of breathing (Wb) and (Q) over dotlegs (inverse), leg vascular resistance (LVR), and leg (V) over dotO(2) ((V) over dotO(2legs); inverse), and between LVR and norepinephrine spillover. Mean arterial pressure did not change with changes in Wb nor did tidal volume or minute ventilation. For a +/-50% change from control in Wb, (Q) over dotlegs changed 2 l/min or 11% of control, LVR changed 13% of control, and O-2 extraction did not change; thus (V) over dotO(2legs) changed 0.4 l/min or 10% of control. Total (V) over dot O-2max was unchanged with loading but fell 9.3% with unloading; thus (V) over dotO(2legs) as a percentage of total (V) over dotO(2max) was 81% in control, increased to 89% with respiratory muscle unloading, and decreased to 71% with respiratory muscle loading. We conclude that Wb normally incurred during maximal exercise causes vasoconstriction in locomotor muscles and compromises locomotor muscle perfusion and (V) over dotO(2).