The ventilatory response to moderate hypocapnic exercise in human beings

Area of Science:

• Exercise Physiology
• Respiratory Control

Background:

• Exercise hyperpnoea, the increase in breathing during exercise, is complex.
• Voluntary hyperventilation (VHV) can induce hypocapnia (low CO2), but is affected by the afterdischarge (AD) phenomenon.
• A novel technique was needed to isolate the exercise hyperpnoea response from AD effects.

Purpose of the Study:

• To investigate the ventilatory response to exercise under hypocapnic conditions.
• To differentiate between neural and humoral control mechanisms influencing breathing during exercise.
• To characterize the distinct phases of ventilation during hypocapnic exercise.

Main Methods:

• Eight healthy subjects performed hyperoxic cycle ergometry at three exercise intensities.
• A new method was used to isolate the pure ventilatory (VE) response to hypocapnic exercise (VEsub) by subtracting resting recovery from exercise response.
• Ventilatory parameters and end-tidal partial pressure of carbon dioxide (PETCO2) were measured.

Main Results:

• VEsub showed an abrupt increase at exercise onset (phase 1) across all work rates.
• Phase 1 ventilation remained constant until a PETCO2 threshold was reached.
• Beyond the PETCO2 threshold, VEsub increased further to a second plateau (phase 2).

Conclusions:

• Phase 1 of VEsub suggests a neurally mediated control of breathing at exercise onset.
• Phase 2, linked to PETCO2 levels, indicates the involvement of a humorally mediated mechanism.
• These findings differentiate neural and humoral contributions to respiratory control during exercise under hypocapnia.