Oxygen cost of exercise hyperpnea: Measurement

To quantitate the O₂ cost of maximal exercise hyperpnea, we required eight healthy adult subjects to mimic, at rest, the important mechanical components of submaximal and maximal exercise hyperpnea. Expired minute ventilation (V̇E), transpulmonary and trans-diaphragmatic (Pdi) pressures, and end-expiratory lung volume (EELV) were measured during exercise at 70 and 100% of maximal O₂ uptake. At rest, subjects were given visual feedback of their exercise transpulmonary pressure-tidal volume loop (Ẇ(v)), breathing frequency, and EELV, which they mimicked repeatedly for 5 min per trial over several trials, while hypocapnia was prevented. The change in total body O₂ uptake (V̇O₂) was measured and presumed to represent the O₂ cost of the hyperpnea. In 61 mimicking trials with V̇E of 115-167 l/min and Ẇ(v) of 124-544 J/min, V̇E, Ẇ(v), duty cycle of the breath, and expiratory gastric pressure (Pga) integrated with respect to time (∫Pga · dt/min) were not different from those observed during maximum exercise. ∫Pdi · dt/min was 14% less and EELV was 6% greater during maximum exercise than during mimicking. The O₂ cost measurements within a subject were reproducible over 3-12 trials (coefficient of variation ±10%, range 5-16%). The O₂ costs of hyperpnea correlated highly and positively with V̇E and Ẇ(v) and less, but significantly, with ∫Pdi · dt and ∫Pga · dt. The O₂ cost of V̇E rose out of proportion to the increasing hyperpnea, so that between 70 and 100% of maximal V̇O₂ ΔV̇O₂/ΔV̇E increased 40-60% (1.8 ± 0.2 to 2.9 ± 0.1 ml O₂/l V̇E) as V̇E doubled. We believe that these data provide a realistic conservative estimate of the actual cost of exercise hyperpnea over a broad range of ventilatory outputs during moderately heavy-to-maximum exercise in healthy human subjects.