REDUCED RESPIRATORY MUSCLE WORK LOWERS CARDIAC OUTPUT AND INCREASES LIMB Q DURING MAX EXERCISE 794

Abstract

We have previously reported that pulmonary VO2 (VO2tot)decreased 7-10% while leg VO2 and blood flow (Ql) increased 4-7% when work of breathing was reduced by 50% of control at max workload in fit men. Our hypothesis for this project was that the fall in VO2tot with respiratory unloading was due to a lower cardiac output (Qtot) at VO2max due to less negative intra-thoracic pressure. Four male cyclists(VO2max: 66±5ml/kg/min) performed repeat 2.5 min bouts of cycle exercise at VO2max. Inspiratory muscle work was either: 1) reduced via a proportional assist ventilator (peak esophageal pressure(Pes):-14±3cmH2O), 2) increased via resistive loads(Pes:-35±5cmH2O), or not manipulated(Pes:-28±2cmH2O). Tidal volume and breathing frequency were unchanged. Arterial (brachial) and mixed venous (pulmonary artery) blood were used to calculate Qtot via direct Fick method. Qtot and VO2tot were not different (P>0.05) between control vs loaded trials at VO2max. However, Qtot and VO2tot were lower (-7.6±1.3%, -8.1±1.5%, respectively) than control with inspiratory unloading at VO2max while a-[horizontal bar over]vDO2 was unchanged. These data suggest that unloading of the respiratory muscles and less negative intra-thoracic pressure during max exercise caused a reduced VO2tot because of a reduced Qtot. Therefore, the increase in leg blood flow at VO2max during inspiratory unloading, which we previously determined, occurred in spite of decreased cardiac output. Accordingly, Ql averaged 78±1% of Qtot while in control and 86±1% of Qtot with respiratory muscle unloading.