No effect of elevated operating lung volumes on airway function during variable workrate exercise in asthmatic humans

We show that deliberate elevations in operating lung volumes during interval exercise do not prevent bronchoconstriction when workrate is reduced in asthmatic adults. In our subjects, exercise airway caliber fluctuated with changing workrate both when subjects ventilated spontaneously and when tidal volume and end-inspiratory lung volume were voluntarily and substantially increased at the lower of two exercise workloads. Thus exercise airway function behaved independently from lung stretch and oscillations in lung stretch.

Abstract

In asthmatic adults, airway caliber fluctuates during variable intensity exercise such that bronchodilation (BD) occurs with increased workrate whereas bronchoconstriction (BC) occurs with decreased workrate. We hypothesized that increased lung mechanical stretch would prevent BC during such variable workrate exercise. Ten asthmatic and ten nonasthmatic subjects completed two exercise trials on a cycle ergometer. Both trials included a 28-min exercise bout consisting of alternating four min periods at workloads equal to 40 % (Low) and 70% (High) peak power output. During one trial, subjects breathed spontaneously throughout exercise (SVT), such that tidal volume (V_T) and end-inspiratory lung volume (EILV) were increased by 0.5 and 0.6 liters during the high compared with the low workload in nonasthmatic and asthmatic subjects, respectively. During the second trial (MVT), V_T and EILV were maintained constant when transitioning from the high to the low workload. Forced exhalations from total lung capacity were performed during each exercise workload. In asthmatic subjects, forced expiratory volume 1.0 s (FEV_1.0) increased and decreased with the increases and decreases in workrate during both SVT (Low, 3.3 ± 0.3 liters; High, 3.6 ± 0.2 liters; P < 0.05) and MVT (Low, 3.3 ± 0.3 liters; High, 3.5 ± 0.2 liters; P < 0.05). Thus increased lung stretch during MVT did not prevent decreases in airway caliber when workload was reduced. We conclude that neural factors controlling airway smooth muscle (ASM) contractile activity during whole body exercise are more robust determinants of airway caliber than the ability of lung stretch to alter ASM actin-myosin binding and contraction.

NEW & NOTEWORTHY

We show that deliberate elevations in operating lung volumes during interval exercise do not prevent bronchoconstriction when workrate is reduced in asthmatic adults. In our subjects, exercise airway caliber fluctuated with changing workrate both when subjects ventilated spontaneously and when tidal volume and end-inspiratory lung volume were voluntarily and substantially increased at the lower of two exercise workloads. Thus exercise airway function behaved independently from lung stretch and oscillations in lung stretch.

bronchodilation (BD) occurs during whole body exercise in both nonasthmatic and asthmatic persons, although the magnitude of this BD is considerably greater in the asthmatic with baseline airway obstruction (10, 16, 20, 22, 30, 46). In the asthmatic with baseline airway narrowing, the exercise BD is a particularly critical response that allows for increased airflow during the exercise (10, 16, 37). Following the initial exercise-induced BD in the asthmatic, bronchoconstriction (BC) has been shown to occur during exercise when workrate is reduced (5, 20). Furthermore, subsequent increases in workrate once again cause BD. Thus BD is the initial default response during exercise in the asthmatic, but the airways may narrow or dilate thereafter depending on exercise intensity. The physiological mechanisms accounting for these complicated airway responses to exercise in the asthmatic are not known.

Based on the known coupling between lung volume (V_L) and airway diameter (8, 26), it is reasonable to suggest that changes in operating V_L might regulate, at least in part, airway caliber during exercise in the asthmatic. The airways are embedded within the lung parenchymal tissues, which impart an outward acting force, referred to as radial traction or tethering, on the airways. This tethering is a key component of the sum of forces that determine airway caliber at any given instant (29). Indeed, recent imaging studies and application of the forced oscillation technique have shown a clear influence of V_L on airway lumen diameter in both nonasthmatic and asthmatic adults (6, 8, 25). Furthermore, in nonasthmatic and asthmatic subjects with pharmacologically induced airway narrowing (e.g., methacholine), both a single deep inhalation to total lung capacity or several such inhalations dilate the airways in both subject groups (18, 38).

During exercise, tidal volume (V_T) and end-inspiratory lung volume (EILV) increase in proportion to exercise workload. These increases in operating V_L and the corresponding increase in lung stretch should dilate the airways due to the augmented radial traction. Likewise, reductions in exercise intensity are met with decreases in both V_T and EILV, which should theoretically decrease airway caliber due to a reduced mechanical load applied to the airways. Therefore, changes in operating V_L, and the consequent changes in lung stretch, during variable intensity exercise might contribute to both the BD and BC that occur during such exercise in the asthmatic.

The purpose of this study was to determine whether changes in V_T and EILV (i.e., variations in lung stretch) are responsible for variations in airway caliber during interval exercise in the mild-to-moderate asthmatic adult. We hypothesized that, due to the forces of interdependence between the airways and parenchyma, increased V_T and EILV will prevent BC during variable intensity exercise when workrate is reduced. To test this hypothesis, asthmatic and nonasthmatic subjects completed two variable workrate exercise studies. During one study, subjects breathed spontaneously throughout the interval exercise. During the second study, breathing was deliberately controlled such that V_T and EILV were increased at the low-intensity workloads.

METHODS

Subjects

Ten adult male and female asthmatic subjects and ten nonasthmatic subjects were recruited through advertisements in local newspapers and by flyers posted on the Johnson State College campus and in the community. Before participation, subjects were informed of the procedures, risks, and benefits of the study. Written informed consent was subsequently obtained. The experiments described in this manuscript were approved by the Johnson State College Institutional Review Board for research involving human subjects.

All participants were nonsmokers between the ages of 18 and 45 yr with a negative history for cardiovascular, metabolic, and pulmonary disease, other than asthma. All subjects had an absence of respiratory infection during the 4 wk before participation. All asthmatic subjects had a previous clinician diagnosis of asthma. Based on a clinical history, responses to Juniper's Asthma Control Questionnaire (21), and pulmonary function values via spirometry, disease severity in the asthmatic subjects represented a continuum between “intermittent” and “moderate-persistent” disease that was either “well-controlled” or “not well-controlled” at the time of study (45). Several asthmatic subjects had a prescriptive history for antihistamines, inhaled steroids, or nasal steroids; however, none were using these medications at the time of their participation in the studies. A total of 9/10 asthmatic subjects used a short-acting β₂-agonist on an as-needed basis; these subjects were instructed to refrain from their use for 12 h before each study. All nonasthmatic and asthmatic subjects were instructed to refrain from ingesting any caffeinated food or beverages for 8 h before each study.

Screening Studies

All subjects completed three separate screening studies on different days to determine eligibility for participation as an asthmatic or nonasthmatic subject.

Bronchodilator reversibility.

Following baseline pulmonary function tests (PFT), subjects inhaled four actuations of a fast-acting β₂-agonist. For each actuation, subjects exhaled to residual volume, depressed the actuator, inhaled slowly through a holding chamber (Aerochamber) to total lung capacity, and held their breath for 5 s before exhaling. Pulmonary function was assessed at several time points following β₂-agonist inhalation.

Eucapnic voluntary hyperpnea challenge.

Following baseline PFTs, subjects completed a eucapnic voluntary hyperpnea (EVH) challenge according to previously published methods (1). Briefly, subjects ventilated dry gas from a compressed gas tank (21% O₂, 5% CO₂, balance N) for 6 min at a target ventilation rate equal to forced expiratory volume 1.0 s × 30. Pulmonary function was assessed serially following hyperpnea, and the lowest value was used to determine the maximal airway narrowing response to the hyperpnea.

Maximal exercise test.

Subjects completed a maximal incremental exercise test on a cycle ergometer. The test consisted of 2-min stages with the initial workload set at 60 watts for females and 65 watts for males; workload was increased by 30 and 35 watts per stage for females and males, respectively. The purposes of this study were twofold: 1) provide familiarization with exercise on the cycle ergometer and with the other apparati used during the experimental trials; and 2) determine each subject's exercise workloads for the two experimental trials.

To qualify as an asthmatic, subjects were required to meet one or both of the following two criteria: 1) >12% increase in forced expiratory volume 1.0 s (FEV_1.0) after inhalation of the fast-acting β₂-agonist; and 2) >25% decrease in FEV_1.0 after the EVH challenge. Those not meeting these criteria completed the studies as nonasthmatic subjects.

Experimental Design

All nonasthmatic and asthmatic subjects completed two experimental exercise studies separated by at least 48 h. Both experimental studies included the same 28-min exercise protocol on a cycle ergometer. The protocol consisted of alternating four min exercise periods at 40 (Low) and 70% (Hi) of the peak workload obtained during the maximal exercise test. The entire protocol consisted of four stages at the low workload interspersed with three stages at the high workload (i.e., Low-1, Hi-1, Low-2, Hi-2, Low-3, Hi-3, and Low-4). As described below, the two trials differed only in how subjects ventilated during the exercise.

Experimental Exercise Studies

All subjects completed two experimental exercise studies. During the first study, subjects breathed spontaneously throughout the exercise protocol [spontaneous ventilation trial (SVT)]. During the second study, subjects maintained V_T and breathing frequency (fb) constant while exercising at the low exercise workloads following each high workload period [maintained ventilation trial (MVT)]. To control V_T, a computer monitor with a real-time volume trace was placed in front of the subject with the goal V_T clearly marked by a piece of tape; subjects voluntarily controlled inspiration to achieve the required volume. Breathing frequency was controlled by a computerized metronome with the duty cycle set at 0.5. To prevent hypocapnia during the MVT trial, 5 or 10% CO₂ (21% O₂, balance N) was added to the inspiratory tubing at a rate that maintained end-tidal CO₂ constant (33). To prevent airway drying and its potential for provoking BC, the compressed gas was saturated with water by first passing through a water bath before entering the inspiratory tubing.

Following the first and third minutes of each 4-min exercise workload, subjects performed a maximal inhalation to total lung capacity (TLC) followed immediately by a maximal forced exhalation. Subjects were instructed to exhale as forcefully as possible for at least 3 s and to resume spontaneous or controlled breathing immediately thereafter. Subjects were regularly reminded to fill their lungs completely and to exhale as forcefully as possible during each maneuver.

Exercise Apparatus and Measurements

Exercise was completed on a magnetically braked cycle ergometer (Velotron). Subjects breathed through a two-way, nonrebreathing valve (Hans-Rudolph) with nose clips in place. Separate Fleisch pneumotachographs (Hans Rudolph) were used to determine inspiratory and expiratory flow-rates. Separate oxygen and carbon dioxide gas analyzers (AEI Technologies) were used to measure expired gases. An additional CO₂ analyzer (VacuMed) was used to continuously monitor end-tidal CO₂ (ETCO₂) during the MVT trial. A Powerlab 16-channel data acquisition system (ADinstruments) interfaced with a laptop computer was used to collect exercise data.

Inspired and expired airflow rates and expired gases were used to calculate metabolic oxygen consumption (V̇o₂) and carbon dioxide production (V̇co₂), minute ventilation (V̇e), V_T, fb, total breathing cycle time (T_tot), and inspiratory duty cycle (T_I/T_Tot). Inspiratory capacity (IC), FEV_1.0, and peak expiratory flow (PEF) were determined from each forced exhalation maneuver completed after the first and third minutes of each exercise workload. EILV was calculated as [vital capacity-(IC-V_T)]. A Polar heart rate monitor (POLAR) was used to record heart rate during exercise.

Pulmonary Function

Resting pulmonary function (Medgraphics) was completed in the seated, upright position according to recommendations by the American Thoracic Society and European Respiratory Society (31). During each measurement, subjects completed maximal volitional flow-volume maneuvers for determination of forced vital capacity (FVC), FEV_1.0, PEF, and forced expiratory flow at 50% of FVC (FEF_50%). Predicted values are from Quanjer (36).

Statistical Analysis

A three-factor [GROUP (asthmatic vs. nonasthmatic) × TRIAL (SVT vs. MVT) × WORKLOAD (Low-1, Hi-1, Low-2, Hi-2, etc.)] repeated-measures ANOVA was used to analyze the exercise responses. When significant main effects were found, Bonferroni-corrected multiple pairwise comparisons were employed for post hoc analysis. Least squares linear regression was used to analyze relationships between selected variables. Tabular data are presented as means ± SD. Data in figures are presented as means ± SE. Significance was set at α < 0.05. The statistical software program SYSTAT (version 12) was used to analyze all data.

RESULTS

Descriptive characteristics, selected results from the maximal exercise test, and results from the two inclusion criteria studies assessing airways responsiveness are shown in Table 1. The groups were similarly matched for age, height, and exercise capacity. Baseline pulmonary function was lower in asthmatic compared with nonasthmatic subjects (FEV_1.0/FVC P = 0.003; FEF_50% P = 0.015). The increase in FEV_1.0 following inhalation of β₂-agonist (P = 0.004) and the decrease in FEV_1.0 following the EVH challenge were greater (P = 0.004) in asthmatic compared with nonasthmatic subjects. Thus baseline pulmonary function was compromised in the asthmatic subjects and they exhibited significantly greater airways responsiveness than the nonasthmatic subjects.

Table 1.

Descriptive characteristics and inclusion criteria results for nonasthmatic and asthmatic subjects

	Nonasthmatic Group	Asthmatic Group
Male/female	7/3	9/1
Age, yr	24.8 ± 7.4	28.7 ± 7.3
Height, m	1.72 ± 0.11	1.75 ± 0.07
Weight, kg	70.3 ± 14.2	84.0 ± 19.1
V̇o2peak, ml·kg−1·min−1	46.9 ± 8.2 (116 ± 17%)	47.9 ± 13.2 (106 ± 28%)
Peak power, watts	217 ± 43	255 ± 33*
Peak power, watts/kg	3.1 ± 0.5	3.2 ± 0.8
FVC, liters	4.8 ± 1.1 (104 ± 11)	5.3 ± 1.0 (109 ± 14)
FEV1.0, liters	3.9 ± 0.7 (100 ± 11)	3.7 ± 0.7 (90 ± 12)
FEV1.0/FVC	82.3 ± 7.5	70.1 ± 8.6*
FEF50%, l/s	4.4 ± 1.3 (87 ± 28)	3.1 ± 1.0 (58 ± 16)*
PEF, l/s	9.1 ± 2.0 (103 ± 21)	8.9 ± 1.5 (94 ± 10)
Asthma inclusion criteria
β2-Agonist reversibility, FEV1.0 %	+5.8 ± 4.6	+16.6 ± 8.9*
EVH, FEV1.0 %change	−9.0 ± 6.9	−32.1 ± 19.1*

Values are means ± SD. EVH, eucapnic voluntary hyperpnea challenge; FEF_50%, forced expiratory flow at 50% FVC; FEV_1.0, forced expiratory volume 1.0 s; FVC, forced vital capacity; PEF, peak expiratory flow. Percent predicted values in parentheses.

^*P < 0.05, significant difference between nonasthmatic and asthmatic subjects.

Figure 1 depicts airflow, expiratory V_T, and tidal CO₂ at rest and throughout the exercise protocol for the SVT and MVT in one asthmatic subject. Note the expected fluctuations in airflow and V_T with changing workrate during SVT. Conversely, during MVT, airflow and V_T remained constant at the low workloads following each high-intensity workload, as per the study design. Also note the maintained ETCO₂ while exercise was performed at the low workload during MVT. Finally, the spikes in airflow and V_T indicate the forced exhalation maneuvers used to determine FEV_1.0 and PEF.

Fig. 1.

Breath-by-breath airflow, expiratory tidal volume (VTexp), and tidal CO₂ during spontaneous (SVT) and maintained (MVT) ventilation trial in 1 asthmatic subject. Vertical dashed lines indicate changes in exercise workrate. Spikes in airflow and VTexp indicate the forced exhalations at the beginning and end of each 4-min exercise workload. During SVT, airflow and V_T increased and decreased (as expected) with increases and decreases in workrate. During MVT, airflow and V_T were stable despite the changing workrate. End-tidal CO₂ was maintained constant at the low-intensity exercise periods during MVT by adding 5% CO₂ to the inspirate; note the concomitant increases in inspiratory CO₂ at the low-intensity workloads during the MVT trial.

Ventilation and Breathing Pattern

During SVT, V̇e, V_T and fb fluctuated similarly with exercise workload in both subject groups (Fig. 2). The average difference in V̇e between the low and high workload was 24 ± 7 (SD) and 27 ± 9 l/min in nonasthmatic and asthmatic subjects, respectively (P < 0.05 for both groups). Similarly, the average difference in V_T between the workloads was 0.46 ± 0.24 (SD) and 0.58 ± 0.27 liters in nonasthmatic and asthmatic subjects (P < 0.05 for both groups). During MVT, all ventilatory variables were maintained nearly constant during the transitions from the high to the low workload in both subject groups. During low-intensity exercise, V_T was 131 and 126% higher during MVT relative to SVT in nonasthmatic and asthmatic subjects [+0.6 ± 0.3 (SD) and 0.6 ± 0.4 liters], respectively (P < 0.05).

Fig. 2.

Minute ventilation (V̇e), tidal volume (V_T), and breathing frequency (fb) in nonasthmatic (A, C, and E) and asthmatic subjects (B, D, and F) during spontaneous (SVT) and maintained (MVT) ventilation trials. During SVT, V̇e, V_T, and fb fluctuated with the changing exercise workload in both subject groups. During MVT, V_T and fb, and thus V̇e, were maintained constant during the final 3 low-intensity workloads (i.e., Low-2, Low-3, Low-4). There were no differences between nonasthmatic and asthmatic subjects. *P < 0.05 vs. SVT trial.

Operating Lung Volumes

In nonasthmatic subjects, IC increased with exercise during SVT and MVT (P < 0.05 vs. rest), demonstrating similar variation with changes in workload during both trials (Fig. 3A and Table 2). In asthmatic subjects, there was a significant time main effect for the increase in IC during SVT and MVT (P < 0.05); however, post hoc pairwise comparisons were not significant (Fig. 3B and Table 3). During SVT, IC fluctuated with exercise workload in asthmatic subjects (not significant). During MVT, however, IC did not vary with workload; rather, it remained at a nearly constant value throughout the exercise protocol.

Fig. 3.

Ratio of inspiratory capacity to vital capacity (IC/VC), ratio of tidal volume to inspiratory capacity (V_T/IC), and end-inspiratory lung volume (EILV) in nonasthmatic (A, C, and E) and asthmatic (B, D, and F) subjects during spontaneous (SVT) and maintained (MVT) ventilation trials. In nonasthmatic subjects, IC/VC responded similarly during SVT and MVT. In asthmatic subjects, IC/VC fluctuated with exercise workload during SVT whereas it remained constant throughout exercise during MVT. In both subject groups, the higher V_T/IC and EILV at the low-intensity workloads during MVT compared with SVT indicates increased operating lung volumes and greater lung stretch at the low workload during MVT. *P < 0.05 vs. SVT; †P < 0.05 vs. rest; #P < 0.05 vs. Low-1; ‡P < 0.05 vs. Hi-1; §P < 0.05 vs. Hi-2.

Table 2.

Exercise responses during spontaneous and controlled ventilation exercise in control subjects

	Rest	Low-1	Hi-1	Low-2	Hi-2	Low-3	Hi-3	Low-4
V̇o2, ml·kg−1·min−1
SVT	5.5 ± 1.3	21.5 ± 4.2	33.2 ± 6.0	22.5 ± 3.9	32.0 ± 5.5	23.5 ± 2.9	33.6 ± 5.1	23.7 ± 3.5
MVT	6.3 ± 1.7	21.7 ± 3.1	32.0 ± 4.7	—	31.1 ± 4.7	—	31.4 ± 5.2	—
V̇co2, ml·kg−1·min−1
SVT	5.1 ± 1.5	19.1 ± 4.7	34.4 ± 6.5	21.2 ± 4.2	32.0 ± 5.8	21.5 ± 3.2	33.2 ± 5.6	22.1 ± 3.8
MVT	5.6 ± 1.5	18.9 ± 4.1	33.8 ± 6.3	—	31.6 ± 5.8	—	32.5 ± 4.5	—
RER
SVT	0.95 ± 0.3	0.89 ± 0.1	1.04 ± 0.1	0.94 ± 0.1	1.00 ± 0.1	0.92 ± 0.1	0.99 ± 0.1	0.93 ± 0.04
MVT	0.91 ± 0.3	0.87 ± 0.1	1.05 ± 0.1	—	1.02 ± 0.1	—	1.05 ± 0.2	—
V̇e/V̇o2
SVT	35.5 ± 14.0	24.2 ± 3.2	29.2 ± 3.9	28.5 ± 4.5	30.3 ± 4.6	28.9 ± 4.7	30.4 ± 4.4	29.7 ± 3.9
MVT	33.8 ± 10.2	24.2 ± 2.9	29.0 ± 3.1	—	30.5 ± 4.2	—	32.0 ± 4.3	—
V̇e/V̇co2
SVT	36.7 ± 6.5	27.4 ± 3.2	28.2 ± 3.1	30.2 ± 3.6	30.1 ± 3.9	31.6 ± 4.3	30.7 ± 3.7	31.9 ± 4.4
MVT	36.8 ± 4.6	28.1 ± 2.4	27.6 ± 3.0	—	30.1 ± 3.8	—	30.8 ± 3.9	—
PEF, l/s
SVT	8.6 ± 1.9	8.8 ± 2.1	9.4 ± 2.2*	9.4 ± 2.4	9.3 ± 2.1	8.9 ± 2.1	9.6 ± 2.4	9.3 ± 2.1
MVT	8.8 ± 2.0	9.3 ± 2.1	9.3 ± 2.0	9.4 ± 2.2	9.1 ± 2.1	9.0 ± 2.0	9.3 ± 1.9	9.1 ± 1.9
IC, liters
SVT	2.6 ± 0.6	2.9 ± 0.7	3.1 ± 0.6*	3.0 ± 0.6	3.1 ± 0.7	3.1 ± 0.7	3.1 ± 0.7	2.9 ± 0.6
MVT	2.8 ± 0.7	3.1 ± 0.8	3.2 ± 0.7*	3.1 ± 0.8	3.2 ± 0.9	3.0 ± 0.8	3.1 ± 0.9	3.0 ± 0.9
ETCO2, %
SVT	4.9 ± 0.8	5.6 ± 0.7	5.2 ± 0.6	5.1 ± 0.6	5.0 ± 0.6	5.0 ± 0.6	4.9 ± 0.6	4.9 ± 0.6
MVT	4.9 ± 0.7	5.6 ± 0.7	5.2 ± 0.7	5.2 ± 0.8	5.0 ± 0.8	5.0 ± 0.7	4.8 ± 0.7	4.8 ± 0.7
TTot, s
SVT	6.2 ± 1.8	3.2 ± 0.8	2.1 ± 0.4	2.6 ± 0.5	2.0 ± 0.5	2.4 ± 0.4	2.0 ± 0.4	2.3 ± 0.5
MVT	5.8 ± 1.6	3.0 ± 0.8	2.3 ± 0.5	2.3 ± 0.6	2.2 ± 0.6	2.0 ± 0.4	2.0 ± 0.4	1.9 ± 0.4
TI/TTot
SVT	0.46 ± 0.07	0.46 ± 0.03	0.49 ± 0.03	0.47 ± 0.04	0.49 ± 0.02	0.47 ± 0.04	0.49 ± 0.03	0.46 ± 0.03
MVT	0.47 ± 0.05	0.46 ± 0.04	0.48 ± 0.03	0.47 ± 0.03	0.48 ± 0.02	0.48 ± 0.02	0.49 ± 0.02	0.48 ± 0.04
HR, beats/min
SVT	76 ± 4.8	121 ± 9.3	159 ± 13.1	135 ± 16.6	167 ± 13.0	141 ± 15.9	170 ± 14.0	145 ± 17.3
MVT	75 ± 9.2	120 ± 15.0	157 ± 16.7	132 ± 17.9	165 ± 16.1	140 ± 17.4	168 ± 14.6	144 ± 17.7

Values are means ± SD. ETCO₂, end-tidal CO₂; HR, heart rate; IC, inspiratory capacity; MVT, maintained ventilation trial; PEF; peak expiratory flow; RER, respiratory exchange ratio; SVT, spontaneous ventilation trial; TI/T_Tot, inspiratory to total breath cycle time; T_Tot, total breath cycle time; V̇e/V̇o₂, ventilatory equivalent for oxygen consumption; V̇e/V̇co₂, ventilatory equivalent for carbon dioxide production; V̇co₂, carbon dioxide production; V̇o₂, oxygen uptake.

^*P < 0.05 vs. rest.

Table 3.

Exercise responses during spontaneous and controlled ventilation exercise in asthmatic subjects

	Rest	Low-1	Hi-1	Low-2	Hi-2	Low-3	Hi-3	Low-4
V̇o2, ml·kg−1·min−1
SVT	5.9 ± 1.2	21.9 ± 4.2	32.8 ± 8.8	23.4 ± 5.9	32.7 ± 8.2	23.9 ± 6.6	34.2 ± 8.7	23.8 ± 5.8
MVT	5.9 ± 2.3	22.1 ± 4.2	30.8 ± 7.5	—	31.0 ± 7.2	—	32.1 ± 7.6	—
V̇co2, ml·kg−1·min−1
SVT	4.6 ± 1.1	19.9 ± 3.8	34.0 ± 9.1	22.0 ± 5.3	32.9 ± 8.7	22.3 ± 6.1	34.7 ± 8.8	22.2 ± 5.6
MVT	5.2 ± 1.7	19.1 ± 4.3	32.4 ± 7.2	—	32.2 ± 6.3	—	32.7 ± 6.7	—
RER
SVT	0.79 ± 0.1	0.91 ± 0.1	1.04 ± 0.1	0.94 ± 0.05	1.01 ± 0.04	0.93 ± 0.05	1.02 ± 0.04	0.93 ± 0.04
MVT	0.86 ± 0.2	0.87 ± 0.1	1.07 ± 0.2	—	1.05 ± 0.1	—	1.03 ± 0.1	—
V̇e/V̇o2
SVT	26.4 ± 5.3	24.5 ± 1.8	28.0 ± 3.1	27.8 ± 2.5	28.5 ± 2.8	28.9 ± 2.4	30.2 ± 2.7	29.6 ± 2.7
MVT	31.8 ± 6.2	25.2 ± 2.3	28.9 ± 5.3	—	28.7 ± 2.4	—	29.0 ± 2.7	—
V̇e/V̇co2
SVT	33.6 ± 4.3	27.0 ± 1.9	27.0 ± 2.6	29.5 ± 2.6	28.4 ± 3.0	31.0 ± 2.8	29.8 ± 3.1	31.8 ± 2.9
MVT	36.9 ± 3.2	28.3 ± 2.2	26.7 ± 2.4	—	28.7 ± 2.4	—	29.0 ± 2.7	—
PEF, l/s
SVT	8.2 ± 1.5	9.0 ± 1.3	9.1 ± 1.1	8.6 ± 1.9	9.0 ± 1.4	8.2 ± 1.6†	9.1 ± 1.6	8.4 ± 1.8
MVT	8.2 ± 2.2	8.7 ± 1.8	9.0 ± 1.7	8.8 ± 1.6	8.6 ± 1.9	8.2 ± 1.9	8.8 ± 2.3	8.3 ± 1.8
IC, liters
SVT	3.2 ± 0.7	3.6 ± 0.8	3.6 ± 0.7	3.5 ± 0.9	3.6 ± 0.8	3.4 ± 0.8	3.6 ± 0.8	3.4 ± 0.8
MVT	3.1 ± 0.6	3.6 ± 0.9	3.5 ± 0.9	3.5 ± 1.0	3.4 ± 1.0	3.4 ± 1.0	3.4 ± 0.9	3.3 ± 0.9
ETCO2, %
SVT	5.1 ± 0.3	5.7 ± 0.6	5.5 ± 0.6	5.4 ± 0.5	5.2 ± 0.5	5.2 ± 0.5	5.0 ± 0.6	5.0 ± 0.5
MVT	5.0 ± 0.2	5.7 ± 0.4	5.6 ± 0.5	5.6 ± 0.5	5.3 ± 0.5	5.3 ± 0.5	5.2 ± 0.5	5.1 ± 0.5
TTot, s
SVT	6.4 ± 2.2	3.1 ± 0.8	2.4 ± 0.4	2.6 ± 0.5	2.3 ± 0.5	2.4 ± 0.5	2.0 ± 0.4	2.3 ± 0.6
MVT	6.4 ± 3.5	3.1 ± 0.5	2.5 ± 0.5	2.6 ± 0.6	2.3 ± 0.5	2.2 ± 0.5	2.1 ± 0.5	2.1 ± 0.4
Ti/TTot
SVT	0.44 ± 0.05	0.45 ± 0.02	0.45 ± 0.03	0.44 ± 0.02	0.45 ± 0.02	0.45 ± 0.02	0.47 ± 0.02	0.45 ± 0.01
MVT	0.41 ± 0.05	0.45 ± 0.02	0.47 ± 0.03	0.48 ± 0.03*	0.47 ± 0.02	0.48 ± 0.03	0.48 ± 0.03	0.49 ± 0.03*
HR, beats/min
SVT	81 ± 13.9	118 ± 7.2	152 ± 8.1	134 ± 8.3	161 ± 9.8	142 ± 11.6	168 ± 10.5	148 ± 10.7
MVT	82 ± 17.7	124 ± 12.4	149 ± 9.6	140 ± 14.0	159 ± 11.0	145 ± 14.6	164 ± 10.0	146 ± 13.2

Values are means ± SD. ETCO₂, end-tidal CO₂; HR, heart rate; IC, inspiratory capacity; MVT, maintained ventilation trial; PEF; peak expiratory flow; RER, respiratory exchange ratio; SVT, spontaneous ventilation trial; TI/T_Tot, inspiratory to total breath cycle time; T_Tot, total breath cycle time; V̇e/V̇o₂, ventilatory equivalent for oxygen consumption; V̇e/V̇co₂, ventilatory equivalent for carbon dioxide production; V̇co₂, carbon dioxide production; V̇o₂, oxygen uptake.

^*P < 0.05 vs. spontaneous; †P < 0.05 vs. Hi-1.

In nonasthmatic and asthmatic subjects, V_T/IC and EILV increased with exercise during SVT and varied regularly with workload throughout the trial (Fig. 3, C–F). During MVT, V_T/IC and EILV did not fluctuate with exercise workload in either subject group; both variables were higher at the low workload during MVT compared with SVT in both subject groups (P < 0.05). Thus EILV was +0.53 ± 0.14 (SD) and +0.6 ± 0.37 liters higher at low-intensity exercise during MVT compared with SVT in nonasthmatic and asthmatic subjects, respectively.

Additional Exercise Responses

All variables responded to exercise as expected (Tables 2 and Table 3). Due to the variable addition of CO₂ to the inspirate while exercise was performed at Low-2, Low-3, and Low-4 during MVT, several of the parameters requiring known inhaled gas fractions were not obtainable at these workloads during MVT. There were no significant differences between nonasthmatic and asthmatic subjects for any of the variables.

Exercise Airway Function

The results for FEV_1.0 shown in Fig. 4 are from the forced exhalation maneuver performed during the fourth minute of exercise at each workload. In both subject groups, FEV_1.0 increased during exercise; however, the magnitude of the increase and fluctuations due to changing workload were different between asthmatic and nonasthmatic subjects. In nonasthmatic subjects, FEV_1.0 increased by a maximum of 8.4 and 8.5% during SVT and MVT [0.31 ± 0.17 (SD) and 0.30 ± 0.16 liters], respectively; there was a significant main effect for time during both trials, but Bonferroni-corrected comparisons were not significant. Following the initial increases in FEV_1.0 during Low-1 and Hi-1, it remained relatively constant throughout exercise during both trials. In asthmatic subjects, FEV_1.0 initially increased by 12.2 and 16.0% throughout Low-1 and Hi-1 during SVT and MVT [0.41 ± 0.15 (SD) and 0.49 ± 0.19 liters, P < 0.05]. Following Hi-1, FEV_1.0 fluctuated with exercise workload during both SVT and MVT, such that there was an average difference of 0.25 ± 0.2 (SD) and 0.19 ± 0.1 liters between low- and high-intensity exercise during SVT and MVT, respectively (P < 0.05). Additionally, FEV_1.0 decreased progressively throughout the 28-min exercise protocol during both trials. There were no significant differences between SVT and MVT in either the nonasthmatic or asthmatic group. In general, PEF responded similar to FEV_1.0 during the exercise trials in both subject groups (Tables 2 and 3).

Fig. 4.

Forced expiratory volume 1.0 s (FEV_1.0) in liters and as a percent change from baseline (BL) in nonasthmatic (A and C) and asthmatic (B and D) subjects during spontaneous (SVT) and maintained (MVT) ventilation trials. The FEV_1.0 is from the forced exhalation maneuver completed during the fourth min of exercise at each workload. In both subject groups, a progressive increase in FEV_1.0, signifying bronchodilation (BD), occurred during the 1st 2 exercise workloads during SVT and MVT; however, the magnitude of BD was greater in asthmatic subjects. Following Hi-1, airway caliber fluctuated with exercise workload in the asthmatic subjects whereas it remained constant in the nonasthmatic subjects. Note that FEV_1.0 fluctuated with exercise workrate similarly during SVT and MVT in asthmatic subjects. SVT trial: ^aP < 0.05 vs. rest; ^bP < 0.05 vs. Hi-1; ^cP < 0.05 vs. Hi-2; ^dP < 0.05 vs. Low-3; ^eP < 0.05 vs. Hi-3. MVT trial: ^a′P < 0.05 vs. rest; ^b′P < 0.05 vs. Hi-1; ^c′P < 0.05 vs. Hi-2.

Figure 5 shows preexercise maximal flow-volume loops and exercise maximal flow-volume loops at both exercise intensities during SVT and MVT for one asthmatic subject. In this subject, preexercise expiratory airflow was higher during the SVT; this reveals the typical daily variation in airway function in asthmatic patients. During both SVT and MVT, the exercise maximal flow-volume loops were markedly larger during the high-intensity exercise than the low-intensity exercise.

Fig. 5.

Preexercise maximal volitional flow-volume loops (MFVL) and exercise maximal flow-volume loops (FVL) during spontaneous (SVT) and maintained (MVT) ventilation trials in one asthmatic subject. A: low workload; B: high workload. During exercise at the low workload, maximal airflow was decreased, indicating bronchoconstriction. During exercise at the high workload, maximal airflow was increased to preexercise levels, indicating bronchodilation. Note that the bronchoconstriction occurred at the low workload during both exercise trials.

In Fig. 6, individual subject and group mean FEV_1.0 values are shown for both of the forced exhalation maneuvers completed during each 4-min exercise stage. Between- and within-subject variability was more pronounced in the asthmatic subjects. In both subject groups, FEV_1.0 increased progressively over time throughout the first low- and high-intensity stages. Thereafter, FEV_1.0 remained stable in nonasthmatic subjects. In asthmatic subjects, FEV_1.0 changed gradually over time during each 4-min workload during SVT and MVT. At the three low workloads following Hi-1 (i.e., Low-2, Low-3, Low-4), FEV_1.0 decreased by an average of 0.15 ± 0.1 and 0.13 ± 0.1 liters [4.9 ± 3.7 (SD) and 4.0 ± 3.7%] between the first and fourth minutes of exercise during SVT and MVT, respectively. Conversely, at the high workloads, FEV_1.0 increased by an average of 0.13 ± 0.1 and 0.14 ± 0.1 liters [4.1 ± 3.2 (SD) and 4.6 ± 3.6%] between the first and fourth minutes of exercise during SVT and MVT, respectively. Finally, the pattern of significant differences in asthmatic subjects demonstrates a progressive BC over time during the exercise bouts. There were no significant differences between SVT and MVT for nonasthmatic or asthmatic subjects.

Fig. 6.

Individual subject and group mean forced expiratory volume 1.0 s (FEV_1.0) expressed as a percent change from baseline in nonasthmatic (A and C) and asthmatic (B and D) subjects during the spontaneous and maintained ventilation trials. The FEV_1.0 values from the beginning and end of each 4-min workload are shown. The fluctuations in FEV_1.0 during exercise and the between-subject variability were greater in the asthmatic compared with nonasthmatic subjects. These figures also show that FEV_1.0 changed gradually over time at each workload, particularly in asthmatic subjects. There were no significant differences between SVT and MVT in nonasthmatic or asthmatic subjects. *P < 0.05 vs. rest and Low-1A; †P < 0.05 vs. Hi-2B; §P < 0.05 vs. Hi-3A; ‡P < 0.05 vs. Low-4B.

DISCUSSION

In asthmatic adults, airway caliber fluctuates during variable workrate exercise; BD occurs with increased workrate whereas BC occurs with decreased workrate. Due to the forces of interdependence between the airways and parenchyma, we hypothesized that increased V_T and EILV would prevent BC during variable workrate exercise in asthmatic adults. The main finding of this study is that increased V_T and EILV do not prevent a decrease in airway caliber during exercise when workload is reduced. Indeed, we found that FEV_1.0 fluctuated similarly with changing exercise workload when subjects breathed spontaneously and when V_T and EILV were increased at the low workload. These findings suggest that changes in operating V_L, and accompanying changes in lung stretch, during variable intensity exercise do not contribute to the fluctuating airway caliber seen during such exercise. Rather, physiological inputs other than lung stretch and oscillations in lung stretch must be more potent regulators of airway function during exercise in the mild-to-moderate asthmatic.

Exercise Model

There are several reasons we chose interval exercise to investigate the influence of lung mechanical stretch on airway function during exercise. Firstly, as shown previously (5, 20) and as replicated in this study, interval exercise causes alternating periods of BC and BD during a single exercise session. This fluctuating airway caliber provides a robust physiological signal for assessing the effects of our exercise intervention, namely, altered operating V_L. Secondly, the protocol consisted of multiple repeats of each low and high workload, which allowed us to analyze the effects of exercise duration on the relationships between lung mechanical strain and airway caliber. Finally, forced exhalations performed at the beginning and at the end of each 4-min workload allowed us to examine the rate of change in airway caliber when intensity was changed.

Rationale for Hypothesis

Our hypothesis that increased operating V_L would prevent decreases in airway caliber during exercise was founded on basic principles of lung mechanics and informed by several findings in the literature. Fundamental pulmonary mechanics dictate that airway diameter increases as a function of V_L (26). This coupling derives from the increased elastic recoil and consequent increase in tethering forces between the lung parenchyma and the intraparenchymal airways at larger V_L. Recent studies using high-resolution computed tomography and the forced oscillation technique show that increased V_L distends the airways in asthmatic humans (6–8, 25), although the degree of airway dilation may be less than in healthy persons.

In addition to the in vivo demonstrations of in-phase interdependence between V_L and airway diameter, a burgeoning number of studies at reduced biological scales support the postulate that increased V_L will attenuate airway narrowing during exercise. In vitro studies of airway smooth muscle (ASM) strips and investigations of airway sections from healthy animals have demonstrated a powerful capacity of increased mechanical stress and strain to reduce ASM force (3, 11, 15, 27, 34). Recent investigations of airway sections from both asthmatic mice and humans provide evidence that increased lung mechanical strain inhibits force generation in the asthmatic, as well (19, 32). Moreover, the reduction in ASM force is, in general, proportional to the magnitude of oscillatory strain (3, 15, 27, 34). These strain-induced declines in ASM force are thought to reflect perturbed myosin-actin cross-bridge binding due to stretching of the airway wall and ASM. The principle limitation of these reduced preparations is that they do not replicate the functional interdependence between the airways and surrounding parenchyma in the whole organism.

Accordingly, our hypothesis linking V_L with exercise airway caliber was based on the following reasoning. During exercise, increased V_T and EILV will distend the airways in proportion to the increased V_L. The distended airways should stretch the ASM, and if the degree of stretch is sufficient, myosin-actin binding will be perturbed. If the reapplication of force by ASM activation is slow relative to the rate of airway distention dictated by fb, airway caliber should remain increased (13). To the best of our knowledge, the current study represents the first in vivo attempt at manipulating lung stretch as a method to uncover the lung mechanical mechanisms accounting for variable airway caliber during exercise in the asthmatic.

Exercise Airway Function

In contrast to our hypothesis, we found that increased lung stretch did not prevent decreases in airway caliber during exercise when workrate was reduced in mild-to-moderate asthmatic adults. At the onset of exercise, however, airway caliber increased in nearly all of our asthmatic subjects (see Figs. 4 and 6). Airway caliber also increased during exercise in the nonasthmatic subjects, although the magnitude of dilation was less than in the asthmatics. Others have also shown that BD occurs during exercise in asthmatic and nonasthmatic adults (10, 16, 30, 46). In fact, Crimi et al. (10) showed that the bronchodilatory stimulus of exercise is powerful enough to increase airway caliber markedly in asthmatics that are in the midst of a spontaneous asthma attack. Similarly, another study reported a rapid BD in asthmatics that began exercise when their airways were significantly narrowed from a previous exercise bout (16). Both the rapidity and the magnitude of BD with exercise support the notion that this BD is at least partly due to increased V_L and airway tethering. Indeed, a modest BD also occurs during seated isocapnic hyperpnea in asthmatic subjects (41, 43).

Following the initial BD at exercise onset, airway caliber fluctuated with changing workrate throughout both exercise trials. Thus, during the MVT trial, exercise airway caliber behaved independently from the nearly constant V_T and EILV. Presumably, ASM contractile force varied in concert with workload throughout exercise whether or not the degree of lung stretch was maintained. Our findings thus imply that the influence of lung stretch (at least at the levels seen in this study) on ASM contractile activity is overwhelmed by coexisting influences operating during exercise that oppose the ability of lung stretch to maintain airway caliber.

One possible explanation for not only the initial exercise BD but also for the workload related fluctuations in airway caliber is altered cholinergic tone. Studies have shown that the initial exercise BD is likely due, at least in part, to the immediate withdrawal of cholinergic tone that accompanies whole body exercise (22, 28, 46). Also, parasympathetic activity decreases progressively with increases in workrate (28); presumably, its activity increases similarly when workrate is reduced. Studies in anesthetized dogs demonstrate that a neurogenic reflex originating in contracting skeletal muscle is probably responsible for the exercise-induced withdrawal of cholinergic tone (24). An additional input that might modulate cholinergic tone is lung inflation. In situ experiments in dogs have shown reflex withdrawal of cholinergic tone with increases in both fb and V_T, an effect likely mediated by pulmonary stretch receptors (40, 47). However, our findings of decreased airway caliber (i.e., increased cholinergic tone) at the low workload despite maintained V_T and V_L suggest that cholinergic tone is more sensitive to other reflexes active during exercise. Given all of this, we speculate that increased cholinergic tone was at least partly responsible for the airway narrowing that occurred when exercise workload was reduced in this study. If this is the case, it implies that the skeletal muscle reflex control of cholinergic tone and hence, ASM contractile activity, exerts a more potent influence on ASM cross-bridge behavior than the combative effect of increased lung stretch to disrupt cross-bridge binding and reduce ASM contraction.

Another novel finding in this study is that the workload-induced changes in airway caliber occurred gradually over the 4-min exercise stages (Fig. 6). The immediate increase in heart rate at the onset of exercise is due to decreased cholinergic tone, which responds within seconds of altering workrate (28). Similarly, the altered airway caliber seen during the first minute of exercise at each stage in this study may be due to altered cholinergic activity. The mechanism(s) for the further changes in airway caliber seen over time within each exercise stage are not known. Based on the findings from SVT, one could reasonably postulate that the time-dependent changes in airway caliber were due to cumulative effects of repetitive oscillations in lung stretch acting on ASM contraction. Similarly, gradual release of bronchoactive or bronchoprotective biological mediators during each exercise stage might also be expected to result in progressive changes in airway caliber. However, the finding that increased V_L during MVT failed to prevent reduced airway caliber at the lower exercise workload makes these possibilities unlikely. Sympathetic nervous system activity and circulating epinephrine increase progressively with both workload and exercise time (12). It is tempting to speculate that the progressive increases in airway caliber over time at the high workload were due to increased epinephrine binding to ASM β₂-receptors. However, β₂-receptor blockade does not prevent BD during exercise (46). Furthermore, BD occurs during isocapnic hyperpnea in asthmatic subjects (41, 43); a condition in which sympathetic activation is minimal. Collectively, we can only state that the balance between bronchodilatory and bronchoconstricting influences changes not only with altered exercise intensity but also over time within a given intensity. There is much room for future work directed at determining the control of airway function during exercise in asthma.

A final observation in this study providing insight into the control of airway function during exercise in the asthmatic is the gradual reduction in FEV_1.0 over time during both of the 28-min exercise bouts. Suman et al. (43) compared pulmonary function during a 20-min exercise bout with a 20-min period of isocapnic hyperpnea in asthmatic subjects. Following an initial BD, pulmonary function declined slowly throughout both trials; however, the airway narrowing during the 20-min exercise bout was of a greater magnitude than during the 20-min hyperpnea bout. In combination with these findings, our current results suggest that exercise-induced alterations in neuromuscular reflexes, circulating chemicals, or airway-derived mediators have a powerful influence on ASM tone; physiological inputs that supersede the interdependence between lung stretch and airway caliber.

The gradual airway narrowing is intuitively compatible with the slow release of one or more BC mediators during the exercise. A variety of chemical mediators released from resident or nonresident lung cells have been proposed as regulators of airway function during exercise in the asthmatic (4). Indeed, the airway narrowing that occurs after exercise in the asthmatic (i.e., exercise-induced bronchospasm) is principally due to the effects of proinflammatory mediators released from mast cells and potentially other cells during or after exercise (2, 14). However, due to the logistical difficulties inherent in obtaining biological samples during an exercise bout (as opposed to before and after exercise), we are not aware of any data addressing interactions between exercise time and inflammatory mediators in health or in the asthmatic. Any biological mediators released during exercise would also necessarily interact with concurrent neural and lung mechanical factors that control airway function. Finally, nitric oxide and epithelial cell-derived prostaglandin E₂ are both known to relax ASM, but their roles in modulating exercise airway function are not clear (42, 44). An improved understanding of airway inflammatory mediator release during exercise and the influences of such molecules on airway function will be necessary to understand the complicated control of exercise airway caliber in asthma.

Relationship to Deep Inspiration Literature

How do our findings relate to studies investigating the influence of deep inhalations (DI) on airway function? In nonasthmatic subjects, airway responsiveness is significantly increased when DIs are prohibited before inhalation of a direct-acting spasmogen (9, 23, 38, 39). In the asthmatic, however, this bronchoprotective effect of DI is variably impaired, exposing a defect in the relationship between V_L and airway behavior (23, 38). In contrast, in both nonasthmatic and asthmatic subjects with methacholine-induced airway narrowing, a single DI causes an immediate increase in airway caliber that returns to pre-DI levels within ∼60 s (18, 35, 38). The design and question in the current study are actually an amalgam of the two types of DI literature: 1) does increased lung strain prevent BC during exercise in the asthmatic (i.e., bronchoprotective effect of DI); or alternatively, 2) does increased lung strain maintain BD during exercise in the asthmatic (i.e., bronchdilatory effect of DI)?

Collectively, the DI literature limits the stimulus of increased lung mechanical stress and strain to between one and five perturbations to TLC. In the current study, exercise V_T and EILV were increased continuously by ∼500–600 ml for 4 min at a time. Given the differences in both the amplitude and number of perturbations in lung strain, we suggest that the nature of the stimulus in the current study does not compare well with that applied in the DI literature. However, our finding that repetitive increases in V_L did not prevent decreases in airway caliber when exercise intensity was reduced complements previous findings demonstrating a reduced ability for lung stretch to prevent ASM contraction in response to inhaled agonists in asthmatic subjects (23, 38). Creative manipulations of airway function, V_L, and exercise workload will be necessary to further clarify the role of lung stretch in determining exercise airway caliber in the asthmatic.

Conclusions

The airway responses to exercise in the asthmatic are dynamic and complicated. We investigated the effect of increased lung mechanical stretch on airway caliber during variable workrate exercise in nonasthmatic and mild-to-moderate asthmatic adults. Following an initial BD with exercise in asthmatic subjects, airway caliber subsequently fluctuated with decreased and increased workload throughout the exercise. In contrast to our hypothesis, increased lung mechanical stretch did not prevent reduced airway caliber when workload was decreased. We conclude that workrate-induced changes in skeletal muscle metabolic or mechanical reflexes are more robust determinants of ASM contractile activity than the influence of lung stretch on ASM myosin-actin binding and its subsequent force generation.

GRANTS

Research reported in this publication was supported by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences (NIGMS) of the National Institutes of Health (NIH) under Grant P20-GM-103449. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS or NIH.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

A.K., A.M.-T., S.A., D.L., and H.C.H. performed experiments; A.K., A.M.-T., S.A., D.L., and H.C.H. analyzed data; A.K., C.G.I., and H.C.H. interpreted results of experiments; A.K. and H.C.H. drafted manuscript; A.K., C.G.I., A.M.-T., S.A., D.L., and H.C.H. edited and revised manuscript; A.K., C.G.I., A.M.-T., S.A., D.L., and H.C.H. approved final version of manuscript; C.G.I. and H.C.H. conception and design of research; H.C.H. prepared figures.