RESPIRATORY MUSCLE WORK COMPROMISES LEG BLOOD-FLOW DURING MAXIMAL EXERCISE

We hypothesized that during exercise at maximal O-2 consumption ((V) o ver dotO(2max)), high demand for respiratory muscle blood flow ((Q) ov er dot) would elicit locomotor muscle vasoconstriction and compromise limb (Q) over dot. Seven male cyclists ((V) over dotO(2max) 64 +/- 6 m l.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 sessio ns. Inspiratory muscle work was either 1) reduced via a proportional-a ssist ventilator, 2) increased via graded resistive loads, or 3) was n ot manipulated (control). Arterial (brachial) and venous (femoral) blo od samples, arterial blood pressure, leg (Q) over dot ((Q) over dotleg s; thermodilution), esophageal pressure, and Oz consumption ((V) over dotO(2)) were measured. Within each subject and across all subjects, a t 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 spil lover. Mean arterial pressure did not change with changes in Wb nor di d 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 change d 13% of control, and O-2 extraction did not change; thus (V) over dot O(2legs) changed 0.4 l/min or 10% of control. Total (V) over dot O-2ma x was unchanged with loading but fell 9.3% with unloading; thus (V) ov er dotO(2legs) as a percentage of total (V) over dotO(2max) was 81% in control, increased to 89% with respiratory muscle unloading, and decr eased to 71% with respiratory muscle loading. We conclude that Wb norm ally incurred during maximal exercise causes vasoconstriction in locom otor muscles and compromises locomotor muscle perfusion and (V) over d otO(2).