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

Reducing Pulmonary Capillary Wedge Pressure During Exercise Exacerbates Exertional Dyspnea in Patients With Heart Failure With Preserved Ejection Fraction: Implications for V˙/Q˙ Mismatch

BACKGROUND

The primary cause of dyspnea on exertion in heart failure with preserved ejection fraction (HFpEF) is presumed to be the marked rise in pulmonary capillary wedge pressure during exercise; however, this hypothesis has never been tested directly. Therefore, we evaluated invasive exercise hemodynamics and dyspnea on exertion in patients with HFpEF before and after acute nitroglycerin (NTG) treatment to lower pulmonary capillary wedge pressure.

RESEARCH QUESTION

Does reducing pulmonary capillary wedge pressure during exercise with NTG improve dyspnea on exertion in HFpEF?

STUDY DESIGN AND METHODS

Thirty patients with HFpEF performed two invasive 6-min constant-load cycling tests (20 W): one with placebo (PLC) and one with NTG. Ratings of perceived breathlessness (0-10 scale), pulmonary capillary wedge pressure (right side of heart catheter), and arterial blood gases (radial artery catheter) were measured. Measurements of V˙/Q˙ matching, including alveolar dead space (Vdalv; Enghoff modification of the Bohr equation) and the alveolar-arterial Po2 difference (A-aDO2; alveolar gas equation), were also derived. The ventilation (V˙e)/CO2 elimination (V˙co2) slope was also calculated as the slope of the V˙e and V˙co2 relationship, which reflects ventilatory efficiency.

RESULTS

Ratings of perceived breathlessness increased (PLC: 3.43 ± 1.94 vs NTG: 4.03 ± 2.18; P = .009) despite a clear decrease in pulmonary capillary wedge pressure at 20 W (PLC: 19.7 ± 8.2 vs NTG: 15.9 ± 7.4 mm Hg; P < .001). Moreover, Vdalv (PLC: 0.28 ± 0.07 vs NTG: 0.31 ± 0.08 L/breath; P = .01), A-aDO2 (PLC: 19.6 ± 6.7 vs NTG: 21.1 ± 6.7; P = .04), and V˙e/V˙co2 slope (PLC: 37.6 ± 5.7 vs NTG: 40.2 ± 6.5; P < .001) all increased at 20 W after a decrease in pulmonary capillary wedge pressure.

INTERPRETATION

These findings have important clinical implications and indicate that lowering pulmonary capillary wedge pressure does not decrease dyspnea on exertion in patients with HFpEF; rather, lowering pulmonary capillary wedge pressure exacerbates dyspnea on exertion, increases V˙/Q˙ mismatch, and worsens ventilatory efficiency during exercise in these patients. This study provides compelling evidence that high pulmonary capillary wedge pressure is likely a secondary phenomenon rather than a primary cause of dyspnea on exertion in patients with HFpEF, and a new therapeutic paradigm is needed to improve symptoms of dyspnea on exertion in these patients.

Alveolar Dead Space Is Augmented During Exercise in Patients With Heart Failure With Preserved Ejection Fraction

BACKGROUND

Patients with heart failure with preserved ejection fraction (HFpEF) exhibit many cardiopulmonary abnormalities that could result in V˙/Q˙ mismatch, manifesting as an increase in alveolar dead space (VDalveolar) during exercise. Therefore, we tested the hypothesis that VDalveolar would increase during exercise to a greater extent in patients with HFpEF compared with control participants.

RESEARCH QUESTION

Do patients with HFpEF develop VDalveolar during exercise?

STUDY DESIGN AND METHODS

Twenty-three patients with HFpEF and 12 control participants were studied. Gas exchange (ventilation [V˙E], oxygen uptake [V˙o2], and CO2 elimination [V˙co2]) and arterial blood gases were analyzed at rest, twenty watts (20W), and peak exercise. Ventilatory efficiency (evaluated as the V˙E/V˙co2 slope) also was measured from rest to 20W in patients with HFpEF. The physiologic dead space (VDphysiologic) to tidal volume (VT) ratio (VD/VT) was calculated using the Enghoff modification of the Bohr equation. VDalveolar was calculated as: (VD / VT × VT) - anatomic dead space. Data were analyzed between groups (patients with HFpEF vs control participants) across conditions (rest, 20W, and peak exercise) using a two-way repeated measures analysis of variance and relationships were analyzed using Pearson correlation coefficient.

RESULTS

VDalveolar increased from rest (0.12 ± 0.07 L/breath) to 20W (0.22 ± 0.08 L/breath) in patients with HFpEF (P < .01), whereas VDalveolar did not change from rest (0.01 ± 0.06 L/breath) to 20W (0.06 ± 0.13 L/breath) in control participants (P = .19). Thereafter, VDalveolar increased from 20W to peak exercise in patients with HFpEF (0.37 ± 0.16 L/breath; P < .01 vs 20W) and control participants (0.19 ± 0.17 L/breath; P = .03 vs 20W). VDalveolar was greater in patients with HFpEF compared with control participants at rest, 20W, and peak exercise (main effect for group, P < .01). Moreover, the increase in VDalveolar correlated with the V˙E/V˙co2 slope (r = 0.69; P < .01), which was correlated with peak V˙o2peak (r = 0.46; P < .01) in patients with HFpEF.

INTERPRETATION

These data suggest that the increase in V˙/Q˙ mismatch may be explained by increases in VDalveolar and that increases in VDalveolar worsens ventilatory efficiency, which seems to be a key contributor to exercise intolerance in patients with HFpEF.

CO2-Enriched Air Inhalation Modulates the Ventilatory and Metabolic Responses of Endurance Runners During Incremental Running in Hypobaric Hypoxia

Cao, Yinhang, Naoto Fujii, Tomomi Fujimoto, Yin-Feng Lai, Takeshi Ogawa, Tsutomu Hiroyama, Yasushi Enomoto, and Takeshi Nishiyasu. CO₂-enriched air inhalation modulates the ventilatory and metabolic responses of endurance runners during incremental running in hypobaric hypoxia. High Alt Med Biol. 23:125-134, 2022. Aim: We measured the effects of breathing CO₂-enriched air on ventilatory and metabolic responses during incremental running exercise under moderately hypobairc hypoxic (HH) conditions. Materials and Methods: Ten young male endurance runners [61.4 ± 6.0 ml/(min·kg)] performed incremental running tests under three conditions: (1) normobaric normoxia (NN), (2) HH (2,500 m), and (3) HH with 5% CO₂ inhalation (HH+CO₂). The test under NN was always performed first, and then, the two remaining tests were completed in random and counterbalanced order. Results: End-tidal CO₂ partial pressure (55 ± 3 vs. 35 ± 1 mmHg), peak ventilation (163 ± 14 vs. 152 ± 12 l/min), and peak oxygen uptake [52.3 ± 5.5 vs. 50.5 ± 4.9 ml/(min·kg)] were all higher in the HH+CO₂ than HH trial (all p < 0.01), respectively. However, the duration of the incremental test did not differ between HH+CO₂ and HH trials. Conclusion: These data suggest that chemoreflex activation by breathing CO₂-enriched air stimulates breathing and aerobic metabolism during maximal intensity exercise without affecting exercise performance in male endurance runners under a moderately hypobaric hypoxic environment.

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

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.

Comparison between a facemask and mouthpiece on breathing mechanics and gas exchange variables during high-intensity exercise

Gas-collection masks are used as a comfortable alternative to the traditional mouthpiece and noseclip during exercise testing protocols in human performance laboratories. However, these masks may introduce potential problems which could affect metabolic and ventilatory parameters, including gas leaks and added dead space. Therefore, the purpose of this study was to compare breathing mechanics, gas exchange variables and ratings of perceived breathlessness (RPB) during high-intensity exercise between a mouthpiece and face mask. Fourteen men [⩒O₂peak = 55.3 ± 7.3 ml·kg^-1·min^-1] were recruited to perform 6 min of cycle ergometry (Velotron Pro, RacerMate, Inc., Seattle, WA) at a work rate corresponding to 90% of ⩒O₂peak while breathing on either (1) a mouthpiece (Hans Rudolph, KC, KS) with nose clip, or (2) a face mask (7450, Hans Rudolph, KC, KS). The difference in ⩒_E between the mouthpiece (156.8 ± 23.3 L/min) and face mask (153.3 ± 21.8 L/min) was not significant (p = 0.534). Similarly, there were no significant differences in breathing mechanics, gas exchange variables or RPB. These data suggest that the facemask can continue to be used interchangeably with the mouthpiece and may even be a more comfortable alternative during high-intensity exercise.

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.

OPTIMAL RESPIRATORY CONTROL SIMULATION AND COMPARATIVE STUDY OF HYPERCAPNIC VENTILATORY RESPONSES TO EXTERNAL DEAD SPACE LOADING

There has been considerable research effort regarding ventilatory responses to breathing with an imposed external dead space, and inhalation of fixed levels of CO 2 by human subjects. A human respiratory control model incorporating the optimality hypothesis can successfully demonstrate ventilatory responses to both chemical stimuli and muscular exercise. In this study, to verify the model behavior of the optimal chemical–mechanical respiratory control model, we simulated the ventilatory control under dead space loading and CO 2 inhalation. The simulation was provided by a LabVIEW® based human respiratory control simulator and signal monitoring system. The dead space measurement was described with two distinct models, derived from Gray and Coon, and predicted behaviors with corresponding ventilatory responses were investigated and compared with experimental findings. While both dead space models produced satisfactory predictions on simulated optimal [Formula: see text] versus Pa CO 2, [Formula: see text] versus Pa CO 2, F versus PI CO 2, VT versus PI CO 2, VD-total versus VT, VD- total /VT versus VT, [Formula: see text] versus VT and [Formula: see text] versus VT relationships, Gray's model provided better correlation and more consistent results throughout most of the ventilatory responses. The study of relative behavior of respiratory signals and comparative relationship of the ventilator responses between dead space loading during rest and CO 2 inhalation will certainly provide valuable understanding of increases in central respiratory motor command output of human respiratory control, which is also associated with Dyspnea on exertion, and give potential clinical perspective to realize the impaired ability to excrete CO 2 in patients diagnosed with acute respiratory distress syndrome.

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.

Short- and long-term modulation of the exercise ventilatory response

The importance of adaptive control strategies (modulation and plasticity) in the control of breathing during exercise has become recognized only in recent years. In this review, we discuss new evidence for modulation of the exercise ventilatory response in humans, specifically, short- and long-term modulation. Short-term modulation is proposed to be an important regulatory mechanism that helps maintain blood gas homeostasis during exercise.

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

During exercise with added dead space (DS), the exercise ventilatory response (DeltaV(E)/ DeltaV(CO(2))) is augmented in younger men, via short-term modulation (STM) of the exercise ventilatory response. We hypothesized that STM would be diminished or absent in older men due to age-related changes in respiratory function and ventilatory control. Men were studied at rest and during cycle exercise with and without added DS. DeltaV(E)/ DeltaV(CO(2)) increased progressively with increasing DS volume (p<0.01), such that CO(2) was not retained with added DS versus without. Hence, the increase in DeltaV(E)/ DeltaV(CO(2)) was not due to increased chemoreceptor feedback from rest to exercise. Increasing exercise intensity diminished the DeltaV(E)/ DeltaV(CO(2)) (p<0.01), and the size of this effect varied by DS volume (p<0.05). We conclude that STM of the exercise ventilatory response is robust in older men; hence, despite age-related changes in lung function and ventilatory control, the exercise ventilatory response can still adapt to increased DS, in order to maintain isocapnia during exercise relative to rest.

Spinal serotonin receptor activation modulates the exercise ventilatory response with increased dead space in goats

Small increases in respiratory dead space (VD) augment the exercise ventilatory response by a serotonin-dependent mechanism known as short-term modulation (STM). We tested the hypotheses that the relevant serotonin receptors for STM are in the spinal cord, and are of the 5-HT2-receptor subtype. After preparing adult female goats with a mid-thoracic (T6-T8) subarachnoid catheter, ventilation and arterial blood gases were measured at rest and during treadmill exercise (4.8 km/h; 5% grade) with and without an increased VD (0.2-0.3 L). Measurements were made before and after spinal or intravenous administration of a broad-spectrum serotonin receptor antagonist (methysergide, 1-2mg total) and a selective 5-HT2-receptor antagonist (ketanserin, 5-12 mg total). Although spinal methysergide had no effect on the exercise ventilatory response in control conditions, the augmented response with increased VD was impaired, allowing Pa(CO)(2) to increase from rest to exercise. Spinal methysergide diminished both mean inspiratory flow and frequency responses to exercise with increased VD. Spinal ketanserin impaired Pa(CO)(2) regulation with increased VD, although its ventilatory effects were less clear. Intrathecal dye injections indicated CSF drug distribution was caudal to the upper cervical spinal cord and intravenous drugs at the same total dose did not affect STM. We conclude that spinal 5-HT2 receptors modulate the exercise ventilatory response with increased VD in goats.