Short-term modulation of the exercise ventilatory response in older men

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

External dead space explains sex-differences in the ventilatory response to submaximal exercise in children with and without obesity

We compared the exercise ventilatory response (slope of the ventilation, V̇_E and carbon dioxide production, V̇CO₂ relationship) in boys and girls with and without obesity. 46 children with obesity (BMI percentile: 97.7 ± 1.4) and 27 children without obesity (BMI percentile: 55.1 ± 22.2) were included and divided into groups by sex (with obesity: 17 girls and 29 boys; without obesity: 13 girls and 14 boys). A 6 min constant load cycling test at 45 % of peak work rate was performed. The V̇_E/V̇CO₂ slope was similar (p = 0.67) between children with (32.7 ± 4.3) and without (32.2 ± 6.1) obesity; however, it was higher (p = 0.02) in girls (35.4 ± 5.6) than boys (32.6 ± 4.9). We also examined a corrected V̇_E/V̇CO₂ slope for the effects of mechanical dead space (V_DM), by subtracting V̇_DM from V̇_E (V̇_Ecorr/V̇CO₂ slope). The V̇_Ecorr/V̇CO₂ slope remained similar (p = 0.37) between children with (26.8 ± 3.2) and without obesity (26.1 ± 3.1); however, no sex differences were observed (p = 0.13). Therefore, V_DM should be accounted for before evaluating the V̇_E/V̇CO₂ slope, particularly when making between-sex comparisons.

Short-term modulation of the ventilatory response to exercise is preserved in obstructive sleep apnea

BACKGROUND

The ventilatory response to exercise can be transiently adjusted in response to environmentally (e.g., breathing apparatus) or physiologically altered conditions (e.g., respiratory disease), maintaining constant relative arterial P_CO2 regulation from rest to exercise (Mitchell and Babb, 2006); this augmentation is called short-term modulation (STM) of the exercise ventilatory response. Obesity and/or obstructive sleep apnea could affect the exercise ventilatory response and the capacity for STM due to chronically increased mechanical and/or ventilatory loads on the respiratory system, and/or recurrent (chronic) intermittent hypoxia experienced during sleep. We hypothesized that: (1) the exercise ventilatory response is augmented in obese OSA patients compared with obese non-OSA adults, and (2) the capacity for STM with added dead space is diminished in obese OSA patients.

METHODS

Nine obese adults with OSA (age: 39±6 yr, BMI: 40±5kg/m², AHI: 25±24 events/h [range 6-73], mean±SD) and 8 obese adults without OSA (age: 38±10 yr, BMI: 37±6kg/m², AHI: 1±2) completed three, 20-min bouts of constant-load submaximal cycling exercise (8min rest, 6min at 10 and 30W) with or without added external dead space (200 or 400mL; 20min rest between bouts). Steady-state measurements were made of ventilation (V˙_E)_, oxygen consumption V˙_O2), carbon dioxide production (V˙_CO2), and end-tidal P_CO2 (PET_CO2). The exercise ventilatory response was defined as the slope of the V˙E-V˙_CO2 relationship (ΔV˙E/ΔV˙_CO2).

RESULTS

In control (i.e. no added dead space), the exercise ventilatory response was not significantly different between non-OSA and OSA groups (ΔV˙E/ΔV˙_CO2 slope: 30.5±4.2 vs 30.5±3.8, p>0.05); PET_CO2 regulation from rest to exercise did not differ between groups (p>0.05). In trials with added external dead space, ΔV˙E/ΔV˙_CO2 increased with increased dead space (p < 0.05) and the PET_CO2 change from rest to exercise remained small (<2mmHg) in both groups, demonstrating STM. There were no significant differences between groups.

CONCLUSIONS

Contrary to our hypotheses: (1) the exercise ventilatory response is not increased in obese OSA patients compared with obese non-OSA adults, and (2) the capacity for STM with added dead space is preserved in obese OSA and non-OSA adults.

Effects of gas exchange on acid-base balance

This paper describes the interactions between ventilation and acid-base balance under a variety of conditions including rest, exercise, altitude, pregnancy, and various muscle, respiratory, cardiac, and renal pathologies. We introduce the physicochemical approach to assessing acid-base status and demonstrate how this approach can be used to quantify the origins of acid-base disorders using examples from the literature. The relationships between chemoreceptor and metaboreceptor control of ventilation and acid-base balance summarized here for adults, youth, and in various pathological conditions. There is a dynamic interplay between disturbances in acid-base balance, that is, exercise, that affect ventilation as well as imposed or pathological disturbances of ventilation that affect acid-base balance. Interactions between ventilation and acid-base balance are highlighted for moderate- to high-intensity exercise, altitude, induced acidosis and alkalosis, pregnancy, obesity, and some pathological conditions. In many situations, complete acid-base data are lacking, indicating a need for further research aimed at elucidating mechanistic bases for relationships between alterations in acid-base state and the ventilatory responses.

Obesity: challenges to ventilatory control during exercise--a brief review

Obesity is a national health issue in the US. Among the many physiological changes induced by obesity, it also presents a unique challenge to ventilatory control during exercise due to increased metabolic demand of moving larger limbs, increased work of breathing due to extra weight on the chest wall, and changes in breathing mechanics. These challenges to ventilatory control in obesity can be inconspicuous or overt among obese adults but for the most part adaptation of ventilatory control during exercise in obesity appears remarkably unnoticed in the majority of obese people. In this brief review, the changes to ventilatory control required for maintaining normal ventilation during exercise will be examined, especially the interaction between respiratory neural drive and ventilation. Also, gaps in our current knowledge will be discussed.

Short-term modulation of the exercise ventilatory response in younger and older women

The exercise ventilatory response (EVR; defined as the slope of the relationship between ventilation and CO(2) production) is reversibly augmented within a single exercise trial with increased respiratory dead space (DS) in both younger (Wood, H.E., Mitchell, G.S., Babb, T.G., 2008. Short-term modulation of the exercise ventilatory response in young men. J. Appl. Physiol. 104, 244-252) and older (Wood, H.E., Mitchell, G.S., Babb, T.G., 2010. Short-term modulation of the exercise ventilatory response in older men. Respir. Physiol. Neurobiol. 173, 37-46) men. The neural mechanism accounting for this augmentation is known as short-term modulation (STM) of the EVR. Since the effects of female sex hormones on STM are unknown, we examined the capacity for STM in healthy adult women of two age groups; nine younger (29±3 yrs, eumenorrheic) and seven older (69±3 yrs, postmenopausal) women were studied at rest and during cycle exercise (10 W, 30 W; not randomized) in control conditions and with added external DS (200 mL, 400 mL; randomized). Within groups, the main effects of DS and work rate on EVR were analyzed with a two-way repeated measures ANOVA; EVR comparisons between groups were made with unpaired t-tests. In both groups, EVR increased progressively with increasing DS volume (e.g. at 10 W 31±4 and 35±6 in control, 40±11 and 40±6 with 200 mL, 48±12 and 49±11 with 400 mL DS in younger and older women, respectively). In younger women, the effects of DS on EVR differed between work rates (significant interaction, p<0.05), although this was not the case for older women. In both groups, [Formula: see text] regulation was similar between DS and control; hence, increased EVR was not due to altered chemoreceptor feedback from rest to exercise. EVR with and without added DS did not differ between age groups. We conclude that the capacity for STM of the EVR with added DS is similar in healthy younger and older women.