Respiratory frequency response to progressive isocapnic hypoxia.

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Abstract
Ventilatory, tidal volume and frequency responses to progressive isocapnic hypoxia were measured in 29 healthy subjects by a rebreathing technique. A strong correlation was found between ventilatory response to hypoxia (.DELTA..ovrhdot.VI/.DELTA.SAO2) and frequency response to hypoxia (.DELTA.f/.DELTA.SaO2) (r = 0.82, P < 0.001). There was a lesser correlation between .DELTA..ovrhdot.VI/.DELTA.SaO2 and tidal volume response (.DELTA.VT/.DELTA.SaO2) (r = 0.50, P < 0.01). The wide range of ventilatory response to hypoxia among subjects was apparently mainly determined by differences in frequency response and contrasted with previous findings in studies of the response to progressive hypercapnia. The breathing pattern during progressive hypoxia and hypercapnia was compared in 10 subjects. Ventilation/tidal volume plots were constructed and patterns of response were further analyzed in terms of inspiratory duration (TI), expiratory duration (TE) and mean inspiratory flow rate (.ovrhdot.VI). Increments in ventilation during hypoxia were achieved with a greater respiratory frequency and a smaller tidal volume than during hypercapnia in 8 of the 10 subjects studied. In 2 subjects, no difference in breathing pattern during hypoxia and hypercapnia was observed. Changes in respiratory frequency during progressive hypoxia were achieved in all subjects by a progressive shortening of TI and TE. By contrast, TI remained constant during hypercapnia until VT had increased to 3-5 times the eupneic value; during hypercapnia the increase in frequency was achieved mainly by a progressive shortening of TE. Different mechanisms may be involved in altering respiratory frequency when ventilation was driven progressively by these different chemical stimuli.