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Determining response dynamics of hypoxic air hunger may provide information of use in clinical practice, and will improve understanding of basic dyspnea mechanisms. It is hypothesized that air hunger arises from projection of reflex brainstem ventilatory drive (corollary discharge) to forebrain centers. If perceptual response dynamics are unmodified by events between brainstem and cortical awareness, this hypothesis predicts air hunger will exactly track ventilatory response. Thus during sustained hypoxia, initial increase in air hunger would be followed by progressive decline reflecting biphasic reflex ventilatory drive. To test this prediction, we applied a sharp onset 20min step of normocapnic hypoxia and compared dynamic response characteristics of air hunger with that of ventilation in 10 healthy subjects. Air hunger was measured during mechanical ventilation (V o E=9±1.4l/min, PETCO2=37±2, PETO2=45±7torr); ventilatory response was measured during separate free-breathing trials in the same subjects. Discomfort caused by urge to breathe was rated every 30s on a visual analog scale. Both ventilatory and air hunger responses wer