The ventilatory response to modified rebreathing is unchanged by hyperoxic severity: implications for the hyperoxic hyperventilation paradox
Abstract
We measured the ventilatory response to hyperoxic CO2 rebreathing with O2 tension clamped at increasingly higher pressures. We hypothesized that the V̇E versus PCO2 relationship is fixed and independent of PO2. Twenty participants (10 females; mean± SD age: 24±4 years) performed three repetitions of modified rebreathing in 4, randomized, isoxic-hyperoxic conditions: mild: PO2=150 mmHg; moderate: PO2=200 mmHg; high: PO2=300 mmHg; and extreme: PO2≈700 mmHg. For each rebreathing trial, the PETCO2 at which V̇E rose was identified as the ventilatory recruitment threshold (VRT, mmHg), data before VRT provided baseline V̇E (V̇EBSL, L∙min-1) and the slope of the response above VRT gave the central chemoreflex sensitivity (V̇ES, L∙min-1∙mmHg-1 There were no effects of PETO2 on V̇EBSL(mild: 7.4±4.2 L∙min-1; moderate: 6.9±4.2 L∙min-1; high: 6.5±3.7 L∙min-1; extreme: 7.5±2.7 L∙min-1; p=0.24), VRT (mild: 42.8±3.2 mmHg; moderate: 42.5±2.7 mmHg; high: 42.3±2.7 mmHg; extreme: 41.8±2.7 mmHg; p=0.07), or V̇ES (mild: 4.88±2.6 L∙min-1∙mmHg-1; moderate: 4.76±2.2 L∙min-1∙mmHg-1; high: 4.81±2.3 L∙min-1∙mmHg-1; extreme: 4.39±1.9 L∙min-1∙mmHg-1; p=0.41). Hyperoxia does not independently stimulate breathing, nor does it affect central chemoreflex sensitivity.