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Thesis Format

Integrated Article

Degree

Master of Science

Program

Kinesiology

Supervisor

Keir, Daniel A

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.

Summary for Lay Audience

It is known that high-oxygen breathing causes people to breathe more than normal. The higher the oxygen level above normal, the harder one breathes, but we do not know the reason for why this happens. Two possible reasons are that high oxygen turns of breathing on its own or that high oxygen causes the body to hold on to carbon dioxide. When carbon dioxide levels rise in the brain, special gas sensors called central chemoreceptors become excited and tell other areas of the brain that are in charge of breathing to increase breathing more. Some people think that high oxygen makes these sensors more excitable. If this is the case, then breathing responses to carbon dioxide should become more vigorous with higher levels of high-oxygen breathing. In this study, we tested this idea by asking 10 males and 10 females to breathe carbon dioxide at 4 levels of higher-than-normal oxygen breathing. Before carbon dioxide levels were raised, we had these volunteers breathe these 4 levels of oxygen when carbon dioxide levels were low and the central chemoreceptors were “turned off”. We found that how hard people breathed did not change regardless of high oxygen level and this was the same when carbon dioxide levels were low and high. Our findings indicate that oxygen on its own or through a sensitization of central chemoreceptors does not explain why high-oxygen breathing causes people to breathe more than normal.

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