Respiratory Response to Exercise in Pediatrics: The Gordian Knot that is Dyspnea

What is it about?

Dyspnea is defined as “a subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity.” Our current understanding of regulation of breathing during exercise argues it is most likely via feed-forward mechanism, whereby signals of spontaneous respiratory motor drive from brainstem are conveyed to the cortex as corollary discharges, in addition to descending to activate respiratory muscles. When resultant stretch or volume displacement of the respiratory pump does not match “expectation” for a given respiratory motor drive, individuals perceive dyspnea. This sensation is intensified by stimuli that heighten respiratory drive: e.g. PCO2 ↑, PO2 ↓; or sometimes by mere awareness of [↑] level of neural respiratory drive. The COPD literature is rife with mechanistic studies of exertional dyspnea, which can be distilled down to development of dynamic hyperinflation to avert or minimize EFL, constraining tidal volume response to exercise, and increasing work of breathing for a given respiratory motor drive. Can one apply or extrapolate this model to pediatrics?

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Photo by Jaco Pretorius on Unsplash

Photo by Jaco Pretorius on Unsplash

Why is it important?

Cardiopulmonary exercise testing is often used to estimate degree of limitation and potential explanation(s) for exercise intolerance. Godfrey observed >50 years ago that younger children reached peak exercise very close to MVV, which conventional wisdom identifies as indicating ventilatory limitation. Expiratory flow limitation (EFL) documented by plotting expiratory flow volume loops within the maximum flow volume envelope is increasingly recognized as an alternative means to determine ventilatory limitation. The past decade has seen a surge of interest in this technique to study the ventilatory response to exercise in children and adolescents, and it turns out that the majority of pre-pubertal children manifest EFL but this tends to improve after puberty.

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