SUMMARY1. Ventilatory, tidal volume and frequency responses to progressive isocapnic hypoxia have been measured in twenty-nine healthy subjects by a rebreathing technique.2. A strong correlation was found between ventilatory response to hypoxia (.jI/ASao.) and frequency response to hypoxia (Af/.Sao2) (r = 082, P < 0-001). There was a lesser correlation between AQ'/LSao2and tidal volume response (LXVT//ASaO2) (r = 050, P < 0-01). These findings suggest that the wide range of ventilatory response to hypoxia among subjects is mainly determined by differences in frequency response and contrast with previous findings in studies of the response to progressive hypercapnia. 3. The breathing pattern during progressive hypoxia and hypercapnia was compared in ten subjects. Ventilation/tidal volume plots were constructed and patterns of response were further analysed in terms of inspiratory duration (TI), expiratory duration (TF) and mean inspiratory flow rate (VI).4. Increments in ventilation during hypoxia were achieved with a greater respiratory frequency and a smaller tidal volume than during hypercapnia in eight of the ten subjects studied. In two subjects no difference in breathing pattern during hypoxia and hypercapnia was observed.5. Changes in respiratory frequency during progressive hypoxia were achieved in all subjects by a progressive shortening of T1 and TE. By contrast, T1 remained constant during hypercapnia until VT had increased to 3-5 times the eupnoeic value; during hypercapnia the increase in frequency was achieved mainly by a progressive shortening of TE.

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In a body plethysmograph we have demonstrated differences in total lung capacity (TLC) derived from panting maneuvers performed at different levels in the vital capacity. In almost all cases, the discrepancies were due to the magnitude of the abdominal gas volume (AGV) and the relative magnitude of abdominal and thoracic pressure swings during the panting mandeuver. When panting was performed at functional residual capacity (FRC), the effect of AGV compression on the determination of thoracid gas volume (TGV) was small. Of 11 individuals studied 2 were known to have mild asthma. Compression and decompression of AGV appeared to be an insufficient explanation for discrepancies in derived TLC's in these two, suggesting that other as yet unidentified factors may influence the plethysmographic determination of TGV.

1. The effect of mental arithmetic tasks on ventilation, breathing pattern, oxygen intake and carbon dioxide output was studied during air breathing and carbon dioxide rebreathing in healthy subjects. 2. Ventilation and breathing frequency increased significantly on performance of the task during 4 min air breathing and 4 min rebreathing; tidal volume was unchanged. The slopes of the ventilatory, frequency and tidal volume responses to carbon dioxide changed little during task performance. 3. During 15 min air breathing, oxygen intake was unchanged with task performance. Carbon dioxide output increased significantly with task performance, as a result of wash-out of carbon dioxide from body stores by the increased ventilation. 4. Mental arithmetic had no effect on the coefficients of variation of the slope and position variables of the ventilatory, frequency and tidal volume responses to carbon dioxide. It is concluded that task performance does not improve the reproducibility of these responses.