Spontaneous tidal exercise FVLs obtained during exercise at 50%, 75%, and 90% of peak V̇o2 are shown in Figure 1
, left, A, and during maximal exercise in Figure 1
, right, B. The inspiratory limbs exhibited a sawtooth pattern, which became more pronounced as intensity increased (Fig 1
, left, A). During maximal exercise (Fig 1
, right, B), a loud stridor and severe dyspnea appeared at the same time that both inspiratory and expiratory flow rates were clearly reduced when compared with those values obtained only 30 s earlier at the initiation of the maximal workload. A breath-by-breath analysis of breathing mechanics during the maximal workload (Fig 2
) revealed sudden and significant decreases in peak inspiratory and expiratory flows, breathing frequency, and minute ventilation at the same time the symptoms appeared. A significant CO2 retention and moderate arterial hypoxemia occurred immediately following the onset of symptoms (Table 1
). On exercise termination, the dyspnea and stridor disappeared almost immediately. As summarized in Table 2
, expiratory flow rates at both high lung volumes (ie, FEV1) and lower lung volumes (midexpiratory range of forced expiratory flow) were not different from baseline values at all time points postexercise.