End Tidal CO2 Regulation During Rest versus Exercise Under Beginning Conditions of Eucapnia versus Hypocapnia

Abstract

PURPOSE: The exercise ventilatory response in mammals is feed forward with respect to arterial CO2 pressure (PaCO2) regulation, such that PaCO2 during exercise is regulated relative to the resting level, independent of sensory feedback. Arterial and alveolar PaCO2 are indirectly reflected by end tidal CO2 (EtCO2). These experiments employed EtCO2 measurements to examine possible differences in feed forward CO2 regulation from rest to exercise under beginning conditions of eucapnia (EC) and hypocapnia (HC). It was hypothesized that, compared to rest, normal increases in EtCO2 would be attenuated following hyperventilation induced HC to lower resting EtCO2 immediately prior to exercise, but lower EtCO2 would not persist to the same degree at rest following hyperventilation. METHODS: VO2 (ml/kg/min) and EtCO2 (mmHg) were collected on 20 subjects (n=10 males & 10 females, 20.9±0.6 y.o.) during the fifth minute of steady-state rest (baseline), and during each subsequent minute of either mild exercise (5 min. walking @ 1% grade, 4.83 k.p.h.) or further rest (5 min.) under conditions EC and HC (7 deep, rapid breaths immediately prior to exercise or further rest). Percent change from baseline EtCO2 values were determined for each minute of rest and exercise. All EH and HC conditions were compared for both rest and exercise using a one-way repeated measures ANOVA or paired t-test. Data were expressed as mean±S.E.M of % change from baseline EtCO2, and p<0.05 was considered significant. RESULTS: Hyperventilation significantly decreased EtCO2 prior to both exercise (-37±1.9%, p<0.001) and further rest (-34.2±2.1%, p<0.001). No differences in %EtCO2 change were observed between EC and HC conditions (EC %EtCO2 change - HV %EtCO2 change) within the resting or exercise measurements. EtCO2 differences between EC and HC conditions were significantly larger during exercise following hyperventilation, versus rest following hyperventilation, during the fourth minute (3.30±2.18% vs -0.91±1.52%, p=0.041) and fifth minute (4.67±2.66% vs -0.70±1.58%, p=0.034). CONCLUSIONS: EtCO2 data trends were consistent with our hypothesis. Mean EtCO2 % change remained consistently lower during five minutes of mild exercise following hyperventilation, when compared to five minutes of rest following hyperventilation, although statistical differences between rest and exercise HC and EC conditions were observed only during the fourth and fifth minutes.