Sensitivity of the human ventilatory response to muscle metaboreflex activation during concurrent mild hypercapnia

New Findings

What is the central question of this study?

What is the relationship between level of systemic hypercapnia and magnitude of the additional hyperpnoea produced in response to a standardised level of muscle metaboreflex activation?

What is the main finding and its importance?

When a standardised activation of the muscle metaboreflex was combined with exposure to increasing levels of hypercapnia, the hyperpnoea this caused increased linearly. The concept of a synergistic interaction between muscle metaboreflex and central chemoreflex in humans is supported by this finding.

Abstract

Ventilation increases during muscle metaboreflex activation when post exercise circulatory occlusion (PECO) traps metabolites in resting human muscle but only under conditions of concurrent systemic hypercapnia. We hypothesise that a linear relationship exists between level of hypercapnia and magnitude of the additional hyperpnoea produced in response to a standardised level of muscle metaboreflex activation. 15, male subjects performed 4 trials in which end tidal pCO₂ (PET_CO2) was elevated by 1, 3, 7 or 10 mmHg above resting values using a dynamic end tidal forcing system. In each trial subjects were seated in an isometric dynamometer designed to measure ankle plantar flexor force. Rest for 2 minutes in room air was followed by 15 minutes of exposure to one of the four levels of hypercapnia, where 5 minutes further rest was followed by 2 minutes of sustained isometric calf muscle contraction, at 50% of predetermined maximal voluntary strength. Just prior to cessation of exercise, a cuff around the upper leg was inflated to suprasystolic pressure to cause PECO for 3 more minutes, before its deflation and a further 5 minutes of rest, concluding exposure to hypercapnia. PECO consistently elevated mean arterial blood pressure by ∼10 mmHg in all trials indicating similar levels of metaboreflex activation. Increased ventilation during PECO was related to PET_CO2 as described by the linear regression equation, ∆V̇_E (L.min⁻¹) = 0.85 X PET_CO2(mmHg) + 0.80 (L.min⁻¹)

This finding supports our hypothesis and furthers the idea of a synergistic interaction between muscle metaboreflex activation and central chemoreflex stimulation.

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