Variability of the ventilatory response to Duffin's modified hyperoxic and hypoxic rebreathing procedure in healthy awake humans.

We quantified the magnitude of within- and between-day, within-subject variability of the ventilatory response to Duffin's modified rebreathing procedure in 20 healthy humans. The P(ETCO2) at which ventilation increased with progressive increases in P(ETCO2) during rebreathing was identified as the ventilatory recruitment threshold (VRT(CO2)); the ventilatory response below and above the VRT(CO2) was taken as an estimate of non-chemoreflex drives to breathe (V(EB)) and chemoreflex sensitivity (V(ES)), respectively. Within- and between-day intraclass correlation coefficients for each of these parameters were >0.60 (range: 0.62-0.93), indicating good-to-excellent test-retest reliability. Within- and between-day, within-subject coefficients of variation for hyperoxic and hypoxic V(EB) (range: 24.6-30.7%) and V(ES) (range: 18.5-32.7%) were relatively high but acceptable, while those for the VRT(CO2) were very small (range: 3.0-3.8%). In conclusion, Duffin's modified rebreathing procedure, in both its hyperoxic and hypoxic form, is a highly reliable tool for measurement of chemoreflex and non-chemoreflex ventilatory control characteristics over short and long periods of time in healthy humans.