O-2 COST OF BREATHING - VENTILATORY VS PRESSURE LOADS

We compared the O2 cost of breathing (VO2resp) at high levels of ventilation (HV) with that against high inspiratory pressure loads (HP) using an external elastance when end-expiratory volume, work rate (W), and pressure-time product (PBAR) were matched at two levels of ventilation and elastic loading. Each of five normal subjects performed three pairs of loaded runs (one HV and one HP) bracketed by two resting runs. Mean O2 consumption from the pairs of resting runs was subtracted from that of each of the loaded runs to give VO2resp during loaded breathing. Matching for W and PBAR was within 15% in all 15 pairs of runs. During HV runs, ventilation was 398 +/- 24% of corresponding values during HP runs (P < 0.01). Although there was no difference in W (P > 0.05), the VO2resp during HV runs was 237 +/- 33% of that during HP (P < 0.01) and efficiency of HV was 51 +/- 5% of that during HP (P < 0.01). When W was normalized for the decrease in maximum inspiratory pressure with increased mean lung volume, efficiency during HV and HP runs did not differ (P > 0.05). In the second series of experiments, when both HV and HP runs were matched for W but PBAR was allowed to vary, efficiency increased by 1.42 +/- 0.42% (P < 0.05) for each percent decrease in P during HV runs but was unchanged (P > 0.05) during HP runs despite a 193 +/- 10% increase in PBAR. The results indicate that when end-expiratory lung volume, W, and PBAR are matched, the VO2resp at HV may be as much as twice that against HP loads. Alinearity of the relationship between efficiency and PBAR at a constant W may account for the reported variability of inspiratory muscle efficiency during ventilatory loading.