Intense hypoxic cycle exercise does not alter lung density in competitive male cyclists

Effects of exercise on thoracic blood volumes, lung fluid accumulation, and pulmonary diffusing capacity in heart failure with preserved ejection fraction

Patients with heart failure with preserved ejection fraction (HFpEF) experience symptoms of exertional dyspnea that may be related to lung fluid accumulation during exercise. A computed tomography (CT)-based method was used to measure exercise-induced changes in extravascular lung fluid content and thoracic blood volumes and to determine the effect of lung fluid on lung diffusing capacity for carbon monoxide (DL_CO) in stable subjects with HFpEF and healthy controls. Nine subjects with HFpEF (age = 68 ± 8 yr; body mass index = 32.1 ± 2.6 kg/m²) and eight healthy controls (62 ± 9 yr, 23.8 ± 2.4 kg/m²) performed triplicate rebreathe DL_CO/DL_NO (lung diffusing capacity for nitric oxide) tests in a supine position at rest and duplicate measurements during two 5-min submaximal exercise stages (15W and 35W) and recovery. Subjects subsequently performed a 5-min exercise bout (35W) inside a CT scanner, and extravascular lung fluid content and thoracic blood volumes were quantified at rest and immediately following exercise from thoracic and contrast perfusion scans, respectively. Subjects with HFpEF had a higher lung fluid content at rest compared with controls (means ± SD, HFpEF: 14.4 ± 1.7%, control: 12.8 ± 1.7%, P = 0.043) and a higher lung fluid content following exercise (15.2 ± 2.0% vs. 12.6 ± 1.5%, P = 0.009). Higher lung fluid content was associated with a lower DL_CO and alveolar-capillary membrane conductance (D_m) in subjects with HFpEF (DL_CO: R = -0.57, P = 0.022, D_m: R = -0.61, P = 0.012) but not in controls. Pulmonary blood volume was not altered by exercise and was similar between groups. Submaximal exercise elicited a greater accumulation of lung fluid in subjects with HFpEF compared with in controls, and lung fluid content was negatively correlated with lung diffusing capacity and alveolar-capillary membrane conductance in subjects with HFpEF.

Heavy upright exercise increases ventilation-perfusion mismatch in the basal lung: indirect evidence for interstitial pulmonary edema

Ventilation-perfusion (V̇a/Q̇) mismatch during exercise may result from interstitial pulmonary edema if increased pulmonary vascular pressure causes fluid efflux into the interstitium. If present, the increased fluid may compress small airways or blood vessels, disrupting V̇a/Q̇ matching, but this is unproven. We hypothesized that V̇a/Q̇ mismatch would be greatest in basal lung following heavy upright exercise, consistent with hydrostatic forces favoring edema accumulation in the gravitationally dependent lung. We applied new tools to reanalyze previously published magnetic resonance imaging data to determine regional V̇a/Q̇ mismatch following 45 min of heavy upright exercise in six athletes (V̇o2max = 61 ± 7 mL·kg-1·min-1). In the supine posture, regional alveolar ventilation and local perfusion were quantified from specific ventilation imaging, proton density, and arterial spin labeling data in a single sagittal slice of the right lung before exercise (PRE), 15 min after exercise (POST), and in recovery 60 min after exercise (REC). Indices of V̇a/Q̇ mismatch [second moments (log scale) of ventilation (LogSDV) and perfusion (LogSDQ) vs. V̇a/Q̇ distributions] were calculated for apical, middle, and basal lung thirds, which represent gravitationally nondependent, middle, and dependent regions, respectively, during upright exercise. LogSDV increased after exercise only in the basal lung (PRE 0.46 ± 0.06, POST 0.57 ± 0.14, REC 0.55 ±0.14, P = 0.01). Similarly, LogSDQ increased only in the basal lung (PRE 0.40 ± 0.06, POST 0.51 ± 0.10, REC 0.44 ± 0.09, P = 0.04). Increased V̇a/Q̇ mismatch in the basal lung after exercise is potentially consistent with interstitial pulmonary edema accumulating in gravitationally dependent lung during exercise.NEW & NOTEWORTHY We reanalyzed previously published MRI data with new tools and found increased ventilation-perfusion mismatch only in the basal lung of athletes following 45 min of cycling exercise. This is consistent with the development of interstitial edema in the gravitationally dependent lung during heavy exercise.

Maximal exercise does not increase ventilation heterogeneity in healthy trained adults

The effect of exercise on ventilation heterogeneity has not been investigated. We hypothesized that a maximal exercise bout would increase ventilation heterogeneity. We also hypothesized that increased ventilation heterogeneity would be associated with exercise‐induced arterial hypoxemia (EIAH). Healthy trained adult males were prospectively assessed for ventilation heterogeneity using lung clearance index (LCI), S_cond, and S_acin at baseline, postexercise and at recovery, using the multiple breath nitrogen washout technique. The maximal exercise bout consisted of a maximal, incremental cardiopulmonary exercise test at 25 watt increments. Eighteen subjects were recruited with mean ± SD age of 35 ± 9 years. There were no significant changes in LCI, S_cond, or S_acin following exercise or at recovery. While there was an overall reduction in SpO₂ with exercise (99.3 ± 1 to 93.7 ± 3%, P < 0.0001), the reduction in SpO₂ was not associated with changes in LCI, S_cond or S_acin. Ventilation heterogeneity is not increased following a maximal exercise bout in healthy trained adults. Furthermore, EIAH is not associated with changes in ventilation heterogeneity in healthy trained adults.

Cycling performance decrement is greater in hypobaric versus normobaric hypoxia

BACKGROUND

The purpose of this study was to determine whether cycling time trial (TT) performance differs between hypobaric hypoxia (HH) and normobaric hypoxia (NH) at the same ambient PO2 (93 mmHg, 4,300-m altitude equivalent).

METHODS

Two groups of healthy fit men were matched on physical performance and demographic characteristics and completed a 720-kJ time trial on a cycle ergometer at sea level (SL) and following approximately 2 h of resting exposure to either HH (n = 6, 20 ± 2 years, 75.2 ± 11.8 kg, mean ± SD) or NH (n = 6, 21 ± 3 years, 77.4 ± 8.8 kg). Volunteers were free to manually increase or decrease the work rate on the cycle ergometer. Heart rate (HR), arterial oxygen saturation (SaO2), and rating of perceived exertion (RPE) were collected every 5 min during the TT, and the mean was calculated.

RESULTS

Both groups exhibited similar TT performance (min) at SL (73.9 ± 7.6 vs. 73.2 ± 8.2), but TT performance was longer (P < 0.05) in HH (121.0 ± 12.1) compared to NH (99.5 ± 18.1). The percent decrement in TT performance from SL to HH (65.1 ± 23.6%) was greater (P < 0.05) than that from SL to NH (35.5 ± 13.7%). The mean exercise SaO2, HR, and RPE during the TT were not different in HH compared to NH.

CONCLUSION

Cycling time trial performance is impaired to a greater degree in HH versus NH at the same ambient PO2 equivalent to 4,300 m despite similar cardiorespiratory responses.

Pulmonary gas exchange and acid-base balance during exercise

As the first step in the oxygen-transport chain, the lung has a critical task: optimizing the exchange of respiratory gases to maintain delivery of oxygen and the elimination of carbon dioxide. In healthy subjects, gas exchange, as evaluated by the alveolar-to-arterial PO2 difference (A-aDO2), worsens with incremental exercise, and typically reaches an A-aDO2 of approximately 25 mmHg at peak exercise. While there is great individual variability, A-aDO2 is generally largest at peak exercise in subjects with the highest peak oxygen consumption. Inert gas data has shown that the increase in A-aDO2 is explained by decreased ventilation-perfusion matching, and the development of a diffusion limitation for oxygen. Gas exchange data does not indicate the presence of right-to-left intrapulmonary shunt developing with exercise, despite recent data suggesting that large-diameter arteriovenous shunt vessels may be recruited with exercise. At the same time, multisystem mechanisms regulate systemic acid-base balance in integrative processes that involve gas exchange between tissues and the environment and simultaneous net changes in the concentrations of strong and weak ions within, and transfer between, extracellular and intracellular fluids. The physicochemical approach to acid-base balance is used to understand the contributions from independent acid-base variables to measured acid-base disturbances within contracting skeletal muscle, erythrocytes and noncontracting tissues. In muscle, the magnitude of the disturbance is proportional to the concentrations of dissociated weak acids, the rate at which acid equivalents (strong acid) accumulate and the rate at which strong base cations are added to or removed from muscle.

Hypoxia induced changes in lung fluid balance in humans is associated with beta-2 adrenergic receptor density on lymphocytes

BACKGROUND

Previous studies have demonstrated an important role for beta-2 adrenergic receptors (β(2)AR) in lung fluid clearance. The purpose of this investigation was to examine the relationship between β(2)AR density on lymphocytes and indices of lung water in healthy humans exposed to ≈ 17 h of hypoxia (FIO2 = 12.5% in a hypoxia tent).

METHODS

Thirteen adults (mean ± SEM; age=31 ± 3 years, BMI=24 ± 1 kg/m(2), VO2 Peak = 40 ± 2 ml/kg/min ) participated. Pulmonary function, CT derived lung tissue volume (V(tis)-tissue, blood and water), lung diffusing capacity for carbon monoxide (D(CO)) and nitric oxide (D(NO)), alveolar-capillary conductance (D(M)), pulmonary capillary blood volume (V(c)) and lung water (CT V(tis)-V(c)) were assessed before and after ≈ 17 h normobaric hypoxia (FIO2 = 12.5%). β(2)AR density on lymphocytes was measured via radioligand binding. Arterial oxygen saturation (SaO2), cardiac output (Q), right ventricular systolic pressure (RVSP) and blood pressure (BP) were also assessed.

RESULTS

After 17 h hypoxia, SaO2 decreased from 97 ± 1 (normoxia) to 82 ± 4% and RVSP increased from 14 ± 3 (normoxia) to 29 ± 2 mmHg (p<0.05) with little change in Q or BP. V(c) and D(M) both increased with hypoxia with a small increase in D(M)/V(c) ratio (p>0.05). CT V(tis) decreased and lung water was estimated to decline 7 ± 13%, respectively. β(2)AR density averaged 1497 ± 187 receptors/lymphocyte and increased 21 ± 34% with hypoxia (range -31 to +86%). The post-hypoxia increase in β(2)AR density was significantly related to the reduction in lung water (r=-0.64, p<0.05), with the subjects with the greatest increase in density demonstrating the largest decline in lung water.

CONCLUSIONS

Lung water decreases with 17 h normobaric hypoxia are associated with changes in beta adrenergic receptor density on lymphocytes in healthy adults.

Genetic variation of αENaC influences lung diffusion during exercise in humans

Exercise, decompensated heart failure, and exposure to high altitude have been shown to cause symptoms of pulmonary edema in some, but not all, subjects, suggesting a genetic component to this response. Epithelial Na(+) Channels (ENaC) regulate Na(+) and fluid reabsorption in the alveolar airspace in the lung. An increase in number and/or activity of ENaC has been shown to increase lung fluid clearance. Previous work has demonstrated common functional genetic variants of the α-subunit of ENaC, including an A→T substitution at amino acid 663 (αA663T). We sought to determine the influence of the T663 variant of αENaC on lung diffusion at rest and at peak exercise in healthy humans. Thirty healthy subjects were recruited for study and grouped according to their SCNN1A genotype [n=17 vs. 13, age=25±7 years vs. 30±10 years, BMI=23±4 kg/m(2) vs. 25±4 kg/m(2), V(O2 peak) = 95±30%pred. vs. 100±31%pred., mean±SD, for AA (homozygous for αA663) vs. AT/TT groups (at least one αT663), respectively]. Measures of the diffusing capacity of the lungs for carbon monoxide (DL(CO)), the diffusing capacity of the lungs for nitric oxide (DL(NO)), alveolar volume (V(A)), and alveolar-capillary membrane conductance (D(M)) were taken at rest and at peak exercise. Subjects expressing the AA polymorphism of ENaC showed a significantly greater percent increase in DL(CO) and DL(NO), and a significantly greater decrease in systemic vascular resistance from rest to peak exercise than those with the AT/TT variant (DL(CO)=51±12% vs. 36±17%, DL(NO)=51±24% vs. 32±25%, SVR=-67±3 vs. -50±8%, p<0.05). The AA ENaC group also tended to have a greater percent increase in DL(CO)/VA from rest to peak exercise, although this did not reach statistical significance (49±26% vs. 33±26%, p=0.08). These results demonstrate that genetic variation of the α-subunit of ENaC at amino acid 663 influences lung diffusion at peak exercise in healthy humans, suggesting differences in alveolar Na(+) and, therefore, fluid handling. These findings could be important in determining who may be susceptible to pulmonary edema in response to various clinical or environmental conditions.

Influence of sildenafil on lung diffusion during exposure to acute hypoxia at rest and during exercise in healthy humans

We sought to determine the influence of sildenafil on the diffusing capacity of the lungs for carbon monoxide (DLCO) and the components of DLCO (pulmonary capillary blood volume VC, and alveolar-capillary membrane conductance DM) at rest and following exercise with normoxia and hypoxia. This double-blind placebo-controlled, cross-over study included 14 healthy subjects (age = 33 +/- 11 years, ht = 181 +/- 8 cm, weight = 85 +/- 14 kg, BMI = 26 +/- 3 kg/m2, peak normoxic VO2 = 36 +/- 6 ml/kg, mean +/- SD). Subjects were randomized to placebo or 100 mg sildenafil 1 h prior to entering a hypoxic tent with an FiO2 of 12.5% for 90 min. DLCO, VC, and DM were assessed at rest, every 3 min during exercise, at peak exercise, and 10 and 30 min post exercise. Sildenafil attenuated the elevation in PAP at rest and during recovery with exposure to hypoxia, but pulmonary arterial pressure immediately post exercise was not different between sildenafil and placebo. Systemic 02 saturation and VO2peak did not differ between the two conditions. DLCO was not different between groups at any time point. VC was higher with exercise in the placebo group, and the difference in DM between sildenafil and placebo was significant only when corrected for changes in VC (DM/VC = 0.57 +/- 0.29 vs. 0.41 +/- 0.16, P = 0.04). These results suggest no effect of sildenafil on DLCO, but an improvement in DM when corrected for changes in VC during short-term hypoxic exposure with exercise.

Exercise-induced arterial hypoxaemia in active young women

Studies examining pulmonary gas exchange during exercise have primarily focused on young healthy men, whereas the female response to exercise has received limited attention. Evidence is accumulating that the response of the lungs, airways, and (or) respiratory muscles to exercise is less than ideal and this may significantly compromise oxygen transport in certain groups of otherwise healthy, fit, active, male subjects. Women may be even more susceptible to exercise-induced pulmonary limitations than height-matched men, by virtue of their smaller lung volumes, lower maximal expiratory flow rates, and smaller diffusion surface areas. We have recently shown that exercise-induced arterial hypoxaemia (EIAH) is more prevalent and occurs at relatively lower fitness levels in females than in males. Despite this finding, few physiologically based mechanisms have been identified to explain why women may be more susceptible to EIAH than men. Potential mechanisms of EIAH include relative alveolar hypoventilation, ventilation–perfusion inequality, and diffusion limitation. Whether these mechanisms are different between sexes remains controversial. The primary purpose of this review is to summarize the available data on EIAH in women and to discuss potential sex-based mechanisms for gas exchange impairment. Furthermore, we discuss unresolved questions dealing with pulmonary system limitations during exercise in women.

Lung density is not altered following intense normobaric hypoxic interval training in competitive female cyclists

Noninvasive imaging techniques have been used to assess pulmonary edema following exercise but results remain equivocal. Most studies examining this phenomenon have used male subjects while the female response has received little attention. Some suggest that women, by virtue of their smaller lungs, airways, and diffusion surface areas may be more susceptible to pulmonary limitations during exercise. Accordingly, the purpose of this study was to determine if intense normobaric hypoxic exercise could induce pulmonary edema in women. Baseline lung density was obtained in eight highly trained female cyclists (mean +/- SD: age = 26 +/- 7 yr; height = 172.2 +/- 6.7 cm; mass = 64.1 +/- 6.7 kg; Vo(2max) = 52.2 +/- 2.2 ml.kg(-1).min(-1)) using computed tomography (CT). CT scans were obtained at the level of the aortic arch, the tracheal carina, and the superior end plate of the tenth thoracic vertebra. While breathing 15% O(2), subjects then performed five 2.5-km cycling intervals [mean power = 212 +/- 31 W; heart rate (HR) = 94.5 +/- 2.2%HRmax] separated by 5 min of recovery. Throughout the intervals, subjects desaturated to 82 +/- 4%, which was 13 +/- 2% below resting hypoxic levels. Scans were repeated 44 +/- 8 min following exercise. Mean lung density did not change from pre (0.138 +/- 0.014 g/ml)- to postexercise (0.137 +/- 0.011 g/ml). These findings suggest that pulmonary edema does not occur in highly trained females following intense normobaric hypoxic exercise.

Human lung density is not altered following normoxic and hypoxic moderate-intensity exercise: implications for transient edema

The purpose of this study was to examine the effects of exercise on extravascular lung water as it may relate to pulmonary gas exchange. Ten male humans underwent measures of maximal oxygen uptake (Vo2 max) in two conditions: normoxia (N) and normobaric hypoxia of 15% O2 (H). Lung density was measured by quantified MRI before and 48.0 +/- 7.4 and 100.7 +/- 15.1 min following 60 min of cycling exercise in N (intensity = 61.6 +/- 9.5% Vo2 max) and 55.5 +/- 9.8 and 104.3 +/- 9.1 min following 60 min cycling exercise in H (intensity = 65.4 +/- 7.1% hypoxic Vo2 max), where Vo2 max = 65.0 +/- 7.5 ml x kg(-1) x min(-1) (N) and 54.1 +/- 7.0 ml x kg(-1) x min(-1) (H). Two subjects demonstrated mild exercise-induced arterial hypoxemia (EIAH) [minimum arterial oxygen saturation (SaO2 min) = 94.5% and 93.8%], and seven subjects demonstrated moderate EIAH (SaO2 min = 91.4 +/- 1.1%) as measured noninvasively during the Vo2 max test in N. Mean lung densities, measured once preexercise and twice postexercise, were 0.177 +/- 0.019, 0.181 +/- 0.019, and 0.173 +/- 0.019 g/ml (N) and 0.178 +/- 0.021, 0.174 +/- 0.022, and 0.176 +/- 0.019 g/ml (H), respectively. No significant differences (P > 0.05) were found in lung density following exercise in either condition or between conditions. Transient interstitial pulmonary edema did not occur following sustained steady-state cycling exercise in N or H, indicating that transient edema does not result from pulmonary capillary leakage during sustained submaximal exercise.