The Role of CO₂ on Respiration and Metabolism During Hypercapnic and Normocapnic Recovery From Exercise

ABSTRACT

High intensity exercise can lead to depletion of CO₂ from the body (hypocapnia). This disturbance becomes more noticeable during recovery or between seasons of intermittent exercise, putting the subject in a neural fatigue state. Objectives: A possible hypothesis to address this condition would be to provide high CO₂ mixtures (hypercapnic) during the recovery period from exercise in order to relieve hypocapnia. Methods: Eight men (23.8 ± 1.2 yrs, VO_2max = 45 ± 1.9 ml▪kg-1▪min−1) performed cycling exercise at 80%VO_2max for 6–7 min. During recovery (23 min) they inhaled hypercapnic air (EXP–21%O2, 3%CO₂, and 76%N₂) or normal air (CON–21%O₂, 0.003%CO₂, and 79%N₂). Respiratory parameters were collected with open spirometry and heart rate was measured. Results: Exercise caused mild hypocapnia {9.9 mmHg drop of CO₂ end-expiratory partial pressure (P_ETCO₂)} in CON condition after exercise (p < .005). P_ETCO₂ elevated close to the rest values during the three hypercapnic phases in EXP condition (main effect of condition p < .001 between EXP and CON), but after hypercapnic breathing it returned to hypocapnia similarly with CON. The ventilatory response (V_E▪P_ETCO₂⁻¹) and the exhaled volume of CO₂ (VCO₂) progressively increased during and also after ventilatory manipulations in EXP compared to CON condition (V_E▪P_ETCO₂⁻¹: post hoc p < .001, VCO₂: pVCO₂: p < .05-.001), and VO₂ became lower after the end of second hypercapnic manipulation (p < .05 between EXP and CON). Conclusion: It seems that hypercapnic breathing after exercise is not a good strategy to reverse exercise hypocapnia, because of great hyperventilation caused by CO₂ and exercise mechanisms during the recovery period leading to increased CO₂ removal from body. This intervention may also decrease O₂ supply and muscles blood flow.

KEYWORDS:

• Carbon dioxide stores
• carbonic acid inhalation
• respiratory acidosis
• ventilatory response

Footnotes

What is already known on this topic

1. Intense exercise leads to hyperventilation and removal of CO₂ from the body that promotes the drop of arterial CO₂ and hypocapnia.

2. Severe hypocapnia affects neural function, cardiovascular system, and oxygen metabolism.

What this study adds

1. Exercise and post-exercise hypercapnic breathing results in extreme ventilatory sensitivity to CO₂ inspiration leading to pronounced CO₂ removal from the body after exercise.

2. The increased hypocapnia and hyperventilation with hypercapnic breathing does not favor P_ETCO₂ homeostasis after heavy exercise.