Arterial hypoxemia and performance during intense exercise

Is the Lung Built for Exercise? Advances and Unresolved Questions

ABSTRACT

Nearly 40 yr ago, Professor Dempsey delivered the 1985 ACSM Joseph B. Wolffe Memorial Lecture titled: "Is the lung built for exercise?" Since then, much experimental work has been directed at enhancing our understanding of the functional capacity of the respiratory system by applying complex methodologies to the study of exercise. This review summarizes a symposium entitled: "Revisiting 'Is the lung built for exercise?'" presented at the 2022 American College of Sports Medicine annual meeting, highlighting the progress made in the last three-plus decades and acknowledging new research questions that have arisen. We have chosen to subdivide our topic into four areas of active study: (i) the adaptability of lung structure to exercise training, (ii) the utilization of airway imaging to better understand how airway anatomy relates to exercising lung mechanics, (iii) measurement techniques of pulmonary gas exchange and their importance, and (iv) the interactions of the respiratory and cardiovascular system during exercise. Each of the four sections highlights gaps in our knowledge of the exercising lung. Addressing these areas that would benefit from further study will help us comprehend the intricacies of the lung that allow it to meet and adapt to the acute and chronic demands of exercise in health, aging, and disease.

EXERCISE-INDUCED ARTERIAL HYPOXEMIA IN FEMALE MASTERS ATHTLETES

The purpose of the study was to characterize exercise induced arterial hypoxemia (EIAH) in female masters athletes (FMA). We hypothesized that FMA would experience EIAH during treadmill running. Eight FMA (48-57 years) completed pulmonary function testing and an incremental exercise test until exhaustion (V̇O₂max=45.7±6.5, range:35-54ml/kg/min). On a separate day, the participants were instrumented with a radial arterial catheter and an esophageal temperature probe. Participants performed three to four constant load exercise tests at 60-70, 75, 90, 95, and 100% of maximal oxygen uptake while sampling arterial blood and recording esophageal temperature. We found that FMA decrease their partial pressure of oxygen (86.0±7.6, range:73-108mmHg), arterial saturation (96.2±1.2, range:93-98%), and widen their alveolar to arterial oxygen difference (23.2±8.8, range:5-42mmHg) during all exercise intensities however, with variability in terms of severity and pattern. Our findings suggest that FMA experience EIAH however aerobic fitness appears unrelated to occurrence or severity (r=0.13, p=0.756).

Effects of Drinking Oxygenated Water on Blood Oxygen Saturation During Exercise Under Normobaric Hypoxic Conditions: A Randomized Placebo-controlled Single-blinded Trial

Objectives

This study aimed to investigate the effects of drinking oxygenated water on oxygen saturation during exercise under normobaric hypoxic conditions.

Materials

A randomized placebo-controlled single-blinded trial was performed. Twenty-two healthy adults (16 men and 6 women), with a mean age (standard deviation) of 22.4 (2.73) years, participated in the study. The participants were randomly assigned to one of two groups: an OX group (drinking oxygenated mineral water) and a control group (drinking normal mineral water). Both groups performed walking exercises under normobaric hypoxic conditions. Blood oxygen saturation (SpO2), pulse rate (PR), and walking distance were measured during exercise.

Results

SpO2 decreased and PR increased during exercise in both groups. The decrease in SpO2 was smaller and the increase in PR was greater in the OX group compared with those in the control group. No significant difference was found in walking distance between the two groups.

Conclusions

Drinking oxygenated water before exercise may inhibit SpO2 reduction under normobaric hypoxic conditions.

Peripheral chemoresponsiveness during exercise in male athletes with exercise-induced arterial hypoxaemia

NEW FINDINGS

What is the central question of this study? Do highly trained male endurance athletes who develop exercise-induced arterial hypoxaemia (EIAH) demonstrate reduced peripheral chemoresponsiveness during exercise? What is the main finding and its importance? Those with the lowest arterial saturation during exercise have a smaller ventilatory response to hypercapnia during exercise. There was no significant relationship between the hyperoxic ventilatory response and EIAH. The findings suggest that peripheral chemoresponsiveness to hypercapnia during exercise could play a role in the development of EIAH. The findings improve our understanding of the mechanisms that contribute to EIAH.

ABSTRACT

Exercise-induced arterial hypoxaemia (EIAH) is characterized by a decrease in arterial oxygen tension and/or saturation during whole-body exercise, which may in part result from inadequate alveolar ventilation. However, the role of peripheral chemoresponsiveness in the development of EIAH is not well established. We hypothesized that those with the most severe EIAH would have an attenuated ventilatory response to hyperoxia and hypercapnia during exercise. To evaluate this, on separate days, we measured ventilatory sensitivity to hyperoxia and separately hypercapnia at rest and during three different exercise intensities (25, 50% of V ̇ O 2 max and ventilatory threshold (∼67% of V ̇ O 2 max )) in 12 males cyclists ( V ̇ O 2 max  = 66.6 ± 4.7 ml kg^-1  min^-1 ). Subjects were divided into two groups based on their end-exercise arterial oxygen saturation (ear oximetry, S p O 2 ): a normal oxyhaemoglobin saturation group (NOS, S p O 2  = 93.4 ± 0.4%, n = 5) and a low oxyhaemoglobin saturation group (LOS, S p O 2  = 89.9 ± 0.9%, n = 7). There was no difference in V ̇ O 2 max (66.4 ± 2.9 vs. 66.8 ± 6.0 ml kg^-1  min^-1 , respectively, P = 0.9), peak ventilation during maximal exercise (182 ± 15 vs. 197 ± 32 l min^-1 , respectively, P = 0.36) or ventilatory response to hyperoxia (P = 0.98) at any exercise intensity between NOS and LOS groups. However, those in the LOS group had a significantly lower ventilatory response to hypercapnia (P = 0.004, (η²  = 0.18). There was also a significant relationship between the mean hypercapnic response and end-exercise S p O 2 (r = 0.75, P = 0.009) but not between the mean hyperoxic response and end-exercise S p O 2 (r = 0.21, P = 0.51). A blunted hypercapnic ventilatory response may contribute to EIAH in highly trained men due to a failure to increase ventilation sufficiently to offset exercise-induced gas exchange impairments.

Exercise-induced arterial hypoxemia; some answers, more questions

Exercise-induced arterial hypoxemia (EIAH) is characterized by the decrease in arterial oxygen tension and oxyhemoglobin saturation during dynamic aerobic exercise. Since the time of the initial observations, our knowledge and understanding of EIAH has grown, but many unknowns remain. The purpose of this review is to provide an update on recent findings, highlight areas of disagreement, and identify where information is lacking. Specifically, this review will place emphasis on (i) the occurrence of EIAH during submaximal exercise, (ii) whether there are sex differences in the development and severity of EIAH, and (iii) unresolved questions and future directions.

Regulation of exercise blood flow: Role of free radicals

During exercise, oxygen and nutrient rich blood must be delivered to the active skeletal muscle, heart, skin, and brain through the complex and highly regulated integration of central and peripheral hemodynamic factors. Indeed, even minor alterations in blood flow to these organs have profound consequences on exercise capacity by modifying the development of fatigue. Therefore, the fine-tuning of blood flow is critical for optimal physical performance. At the level of the peripheral circulation, blood flow is regulated by a balance between the mechanisms responsible for vasodilation and vasoconstriction. Once thought of as toxic by-products of in vivo chemistry, free radicals are now recognized as important signaling molecules that exert potent vasoactive responses that are dependent upon the underlying balance between oxidation-reduction reactions or redox balance. Under normal healthy conditions with low levels of oxidative stress, free radicals promote vasodilation, which is attenuated with exogenous antioxidant administration. Conversely, with advancing age and disease where background oxidative stress is elevated, an exercise-induced increase in free radicals can further shift the redox balance to a pro-oxidant state, impairing vasodilation and attenuating blood flow. Under these conditions, exogenous antioxidants improve vasodilatory capacity and augment blood flow by restoring an "optimal" redox balance. Interestingly, while the active skeletal muscle, heart, skin, and brain all have unique functions during exercise, the mechanisms by which free radicals contribute to the regulation of blood flow is remarkably preserved across each of these varied target organs.

Effect of end-tidal CO2 clamping on cerebrovascular function, oxygenation, and performance during 15-km time trial cycling in severe normobaric hypoxia: the role of cerebral O2 delivery

During heavy exercise, hyperventilation-induced hypocapnia leads to cerebral vasoconstriction, resulting in a reduction in cerebral blood flow (CBF). A reduction in CBF would impair cerebral O₂ delivery and potentially account for reduced exercise performance in hypoxia. We tested the hypothesis that end-tidal Pco₂ (PETCO₂) clamping in hypoxic exercise would prevent the hypocapnia-induced reduction in CBF during heavy exercise, thus improving exercise performance. We measured PETCO₂, middle cerebral artery velocity (MCAv; index of CBF), prefrontal cerebral cortex oxygenation (cerebral O₂Hb; index of cerebral oxygenation), cerebral O₂ delivery (DO₂), and leg muscle oxygenation (muscle O₂Hb) in 10 healthy men (age 27 ± 7 years; VO₂max 63.3 ± 6.6 mL/kg/min; mean ± SD) during simulated 15-km time trial cycling (TT) in normoxia and hypoxia (FIO₂ = 0.10) with and without CO₂ clamping. During exercise, hypoxia elevated MCAv and lowered cerebral O₂Hb, cerebral DO₂, and muscle O₂Hb (P < 0.001). CO₂ clamping elevated PETCO₂ and MCAv during exercise in both normoxic and hypoxic conditions (P < 0.001 and P = 0.024), but had no effect on either cerebral and muscle O₂Hb (P = 0.118 and P = 0.124). Nevertheless, CO₂ clamping elevated cerebral DO₂ during TT in both normoxic and hypoxic conditions (P < 0.001). CO₂ clamping restored cerebral DO₂ to normoxic values during TT in hypoxia and tended to have a greater effect on TT performance in hypoxia compared to normoxia (P = 0.097). However, post hoc analysis revealed no effect of CO₂ clamping on TT performance either in normoxia (P = 0.588) or in hypoxia (P = 0.108). Our findings confirm that the hyperventilation-induced hypocapnia and the subsequent drop in cerebral oxygenation are unlikely to be the cause of the reduced endurance exercise performance in hypoxia.

Alveolar-membrane diffusing capacity limits performance in Boston marathon qualifiers

Purpose

(1) to examine the relation between pulmonary diffusing capacity and marathon finishing time, and (2), to evaluate the accuracy of pulmonary diffusing capacity for nitric oxide (DLNO) in predicting marathon finishing time relative to that of pulmonary diffusing capacity for carbon monoxide (DLCO).

Methods

28 runners [18 males, age = 37 (SD 9) years, body mass = 70 (13) kg, height = 173 (9) cm, percent body fat = 17 (7) %] completed a test battery consisting of measurement of DLNO and DLCO at rest, and a graded exercise test to determine running economy and aerobic capacity prior to the 2011 Steamtown Marathon (Scranton, PA). One to three weeks later, all runners completed the marathon (range: 2∶22:38 to 4∶48:55). Linear regressions determined the relation between finishing time and a variety of anthropometric characteristics, resting lung function variables, and exercise parameters.

Results

In runners meeting Boston Marathon qualification standards, 74% of the variance in marathon finishing time was accounted for by differences in DLNO relative to body surface area (BSA) (SEE = 11.8 min, p<0.01); however, the relation between DLNO or DLCO to finishing time was non-significant in the non-qualifiers (p = 0.14 to 0.46). Whereas both DLCO and DLNO were predictive of finishing time for all finishers, DLNO showed a stronger relation (r² = 0.30, SEE = 33.4 min, p<0.01) compared to DLCO when considering BSA.

Conclusion

DLNO is a performance-limiting factor in only Boston qualifiers. This suggests that alveolar-capillary membrane conductance is a limitation to performance in faster marathoners. Additionally, DLNO/BSA predicts marathon finishing time and aerobic capacity more accurately than DLCO.

Carbon monoxide exposure during exercise performance: muscle and cerebral oxygenation

AIM

To investigate the effect of carbon monoxide (CO) in the inspired air as anticipated during peak hours of traffic in polluted megalopolises on cerebral, respiratory and leg muscle oxygenation during a constant-power test (CPT). In addition, since O(2) breathing is used to hasten elimination of CO from the blood, we examined the effect of breathing O(2) following exposure to CO on cerebral and muscle oxygenation during a subsequent exercise test under CO conditions.

METHODS

Nine men participated in three trials: (i) 3-h air exposure followed by a control CPT, (ii) 1-h air and 2-h CO (18.9 ppm) exposure succeeded by a CPT under CO conditions (CPT(COA)), and (iii) 2-h CO and 1-h 100% normobaric O(2) exposure followed by a CPT under CO conditions (CPT(COB)). All exercise tests were performed at 85% of peak power output to exhaustion. Oxygenated (Δ[O(2)Hb]), deoxygenated (Δ[HHb]) and total (Δ[tHb]) haemoglobin in cerebral, intercostal and vastus lateralis muscles were monitored with near-infrared spectroscopy throughout the CPTs.

RESULTS

Performance time did not vary between trials. However, the vastus lateralis and intercostal Δ[O(2)Hb] and Δ[tHb] were lower in CPT(COA) than in CPT. During the CPT(COB), the intercostal Δ[O(2) Hb] and Δ[tHb] were higher than in the CPT(COA). There were no differences in cerebral oxygenation between the trials.

CONCLUSION

Inspiration of 18.9 ppm CO decreases oxygenation in the vastus lateralis and serratus anterior muscles, but does not affect performance. Breathing normobaric O(2) moderates the CO-induced reductions in muscle oxygenation, mainly in the intercostals, but does not affect endurance.

Pulmonary system limitations to endurance exercise performance in humans

Accumulating evidence over the past 25 years depicts the healthy pulmonary system as a limiting factor of whole-body endurance exercise performance. This brief overview emphasizes three respiratory system-related mechanisms which impair O(2) transport to the locomotor musculature [arterial O(2) content (C(aO(2))) × leg blood flow (Q(L))], i.e. the key determinant of an individual's aerobic capacity and ability to resist fatigue. First, the respiratory system often fails to prevent arterial desaturation substantially below resting values and thus compromises C(aO(2)). Especially susceptible to this threat to convective O(2) transport are well-trained endurance athletes characterized by high metabolic and ventilatory demands and, probably due to anatomical and morphological gender differences, active women. Second, fatiguing respiratory muscle work (W(resp)) associated with strenuous exercise elicits sympathetically mediated vasoconstriction in limb-muscle vasculature, which compromises Q(L). This impact on limb O(2) transport is independent of fitness level and affects all individuals, but only during sustained, high-intensity endurance exercise performed above ∼85% maximal oxygen uptake. Third, excessive fluctuations in intrathoracic pressures accompanying W(resp) can limit cardiac output and therefore Q(L). Exposure to altitude exacerbates the respiratory system limitations observed at sea level, further reducing C(aO(2)) and substantially increasing exercise-induced W(resp). Taken together, the intact pulmonary system of healthy endurance athletes impairs locomotor muscle O(2) transport during strenuous exercise by failing to ensure optimal arterial oxygenation and compromising Q(L). This respiratory system-related impact exacerbates the exercise-induced development of fatigue and compromises endurance performance.

The effect of exercise-induced hypoxemia on blood redox status in well-trained rowers

Exercise-induced arterial hypoxemia (EIAH), characterized by decline in arterial oxyhemoglobin saturation (SaO(2)), is a common phenomenon in endurance athletes. Acute intensive exercise is associated with the generation of reactive species that may result in redox status disturbances and oxidation of cell macromolecules. The purpose of the present study was to investigate whether EIAH augments oxidative stress as determined in blood plasma and erythrocytes in well-trained male rowers after a 2,000-m rowing ergometer race. Initially, athletes were assigned into either the normoxemic (n = 9, SaO(2) >92%, [Formula: see text]: 62.0 ± 1.9 ml kg(-1) min(-1)) or hypoxemic (n = 12, SaO(2) <92%, [Formula: see text]: 60.5 ± 2.2 ml kg(-1 )min(-1), mean ± SEM) group, following an incremental [Formula: see text] test on a wind resistance braked rowing ergometer. On a separate day the rowers performed a 2,000-m all-out effort on the same rowing ergometer. Following an overnight fast, blood samples were drawn from an antecubital vein before and immediately after the termination of the 2,000-m all-out effort and analyzed for selective oxidative stress markers. In both the normoxemic (SaO(2): 94.1 ± 0.9%) and hypoxemic (SaO(2): 88.6 ± 2.4%) rowers similar and significant exercise increase in serum thiobarbituric acid-reactive substances, protein carbonyls, catalase and total antioxidant capacity concentration were observed post-2,000 m all-out effort. Exercise significantly increased the oxidized glutathione concentration and decreased the ratio of reduced (GSH)-to-oxidized (GSSG) glutathione in the normoxemic group only, whereas the reduced form of glutathione remained unaffected in either groups. The increased oxidation of GSH to GSSG in erythrocytes of normoxemic individuals suggest that erythrocyte redox status may be affected by the oxygen saturation degree of hemoglobin. Our findings indicate that exercise-induced hypoxemia did not further affect the increased blood oxidative damage of lipids and proteins observed after a 2,000-m rowing ergometer race in highly-trained male rowers. The present data do not support any potential link between exercise-induced hypoxemia, oxidative stress increase and exercise performance.

Two days of hypoxic exposure increased ventilation without affecting performance

The aim of this study was to test the short-term effects of using hypoxic rooms before a simulated running event. Thirteen subjects (29 +/- 4 years) lived in a hypoxic dormitory (1,800 m) for either 2 nights (n = 6) or 2 days + nights (n = 7) before performing a 1,500-m treadmill test. Performance, expired gases, and muscle electrical activity were recorded and compared with a control session performed 1 week before or after the altitude session (random order). Arterial blood samples were collected before and after altitude exposure. Arterial pH and hemoglobin concentration increased (p < 0.05) and PCO2 decreased (p < 0.05) upon exiting the room. However, these parameters returned (p < 0.05) to basal levels within a few hours. During exercise, mean ventilation (VE) was higher (p < 0.05) after 2 nights or days + nights of moderate altitude exposure (113.0 +/- 27.2 L.min) than in the control run (108.6 +/- 27.8 L.min), without any modification in performance (360 +/- 45 vs. 360 +/- 42 seconds, respectively) or muscle electrical activity. This elevated VE during the run after the hypoxic exposure was probably because of the subsistence effects of the hypoxic ventilatory response. However, from a practical point of view, although the use of a normobaric simulating altitude chamber exposure induced some hematological adaptations, these disappeared within a few hours and failed to provide any benefit during the subsequent 1,500-m run.

Control of ventilation in humans following intermittent hypoxia

Exposure to chronic or intermittent hypoxia produces alterations in the ventilatory response to hypoxia. These adaptations can differ depending on the severity of the hypoxic stimulus, its duration, its pattern, and the presence or absence of other chemical stimuli. As such, there are significant differences between the responses to intermittent versus continuous hypoxia. Intermittent hypoxia (IH) has been shown to elicit significant changes in the peripheral chemoresponse, but the functional implications of these changes for resting and exercise ventilation are not clear. We summarize the impact of IH on resting chemosensitivity and discuss the use of IH to better understand ventilatory control during exercise. We also suggest future directions for this relatively young field, including potential clinical applications of IH research.

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.

Arterial oxygenation influences central motor output and exercise performance via effects on peripheral locomotor muscle fatigue in humans

Changing arterial oxygen content (C(aO(2))) has a highly sensitive influence on the rate of peripheral locomotor muscle fatigue development. We examined the effects of C(aO(2)) on exercise performance and its interaction with peripheral quadriceps fatigue. Eight trained males performed four 5 km cycling time trials (power output voluntarily adjustable) at four levels of C(aO(2)) (17.6-24.4 ml O(2) dl(-1)), induced by variations in inspired O(2) fraction (0.15-1.0). Peripheral quadriceps fatigue was assessed via changes in force output pre- versus post-exercise in response to supra-maximal magnetic femoral nerve stimulation (DeltaQ(tw); 1-100 Hz). Central neural drive during the time trials was estimated via quadriceps electromyogram. Increased C(aO(2)) from hypoxia to hyperoxia resulted in parallel increases in central neural output (43%) and power output (30%) during cycling and improved time trial performance (12%); however, the magnitude of DeltaQ(tw) (-33 to -35%) induced by the exercise was not different among the four time trials (P > 0.2). These effects of C(aO(2)) on time trial performance and DeltaQ(tw) were reproducible (coefficient of variation = 1-6%) over repeated trials at each F(IO(2)) on separate days. In the same subjects, changing C(aO(2)) also affected performance time to exhaustion at a fixed work rate, but similarly there was no effect of Delta C(aO(2)) on peripheral fatigue. Based on these results, we hypothesize that the effect of C(aO(2)) on locomotor muscle power output and exercise performance time is determined to a significant extent by the regulation of central motor output to the working muscle in order that peripheral muscle fatigue does not exceed a critical threshold.

Effect of exercise-induced arterial hypoxemia on quadriceps muscle fatigue in healthy humans

The effect of exercise-induced arterial hypoxemia (EIAH) on quadriceps muscle fatigue was assessed in 11 male endurance-trained subjects [peak O2 uptake (V̇o2 peak) = 56.4 ± 2.8 ml·kg−1·min−1; mean ± SE]. Subjects exercised on a cycle ergometer at ≥90% V̇o2 peak to exhaustion (13.2 ± 0.8 min), during which time arterial O2 saturation (SaO2) fell from 97.7 ± 0.1% at rest to 91.9 ± 0.9% (range 84–94%) at end exercise, primarily because of changes in blood pH (7.183 ± 0.017) and body temperature (38.9 ± 0.2°C). On a separate occasion, subjects repeated the exercise, for the same duration and at the same power output as before, but breathed gas mixtures [inspired O2 fraction (FiO2) = 0.25–0.31] that prevented EIAH (SaO2 = 97–99%). Quadriceps muscle fatigue was assessed via supramaximal paired magnetic stimuli of the femoral nerve (1–100 Hz). Immediately after exercise at FiO2 0.21, the mean force response across 1–100 Hz decreased 33 ± 5% compared with only 15 ± 5% when EIAH was prevented ( P < 0.05). In a subgroup of four less fit subjects, who showed minimal EIAH at FiO2 0.21 (SaO2 = 95.3 ± 0.7%), the decrease in evoked force was exacerbated by 35% ( P < 0.05) in response to further desaturation induced via FiO2 0.17 (SaO2 = 87.8 ± 0.5%) for the same duration and intensity of exercise. We conclude that the arterial O2 desaturation that occurs in fit subjects during high-intensity exercise in normoxia (−6 ± 1% ΔSaO2 from rest) contributes significantly toward quadriceps muscle fatigue via a peripheral mechanism.

Effects of inhaled bronchodilators and corticosteroids on exercise induced arterial hypoxaemia in trained male athletes

OBJECTIVES

To determine the effect of prophylactic treatment with an inhaled bronchodilator and anti-inflammatory on arterial saturation (SaO2) in trained non-asthmatic male athletes with exercise induced arterial hypoxaemia (EIAH).

METHODS

Nine male athletes (mean (SD) age 26.3 (6.7) years, height 182.6 (7.9) cm, weight 79.3 (10.5) kg, VO2MAX 62.3 (6.3) ml/kg/min, SaO2MIN 92.5 (1.1)%) with no history of asthma were tested in two experimental conditions. A combination of a therapeutic dose of salbutamol and fluticasone or an inert placebo was administered in a randomised crossover design for seven days before maximal cycling exercise. Oxygen consumption (VO2), ventilation (VE), heart rate (HR), power output, and SaO2 were monitored during the exercise tests.

RESULTS

There were no significant differences between the drug (D) and placebo (P) conditions for minimal SaO2 (D = 93.6 (1.4), P = 93.0 (1.1)%; p = 0.93) VO2MAX (D = 61.5 (7.2), P = 61.9 (6.3) ml/kg/min; p = 0.91), peak power (D = 444.4 (48.3), P = 449.4 (43.9) W; p = 0.90), peak VE (D = 147.8 (19.1), P = 149.2 (15.5) litres/min; p = 0.82), or peak heart rate (D = 182.3 (10.0), P = 180.8 (5.5) beats/min; p = 0.76).

CONCLUSIONS

A therapeutic dose of salbutamol and fluticasone did not attenuate EIAH during maximal cycling in a group of trained male non-asthmatic athletes.

O2 arterial desaturation in endurance athletes increases muscle deoxygenation

PURPOSE

The aim of this study was to compare the muscle deoxygenation measured by near infrared spectroscopy in endurance athletes who presented or not with exercise-induced hypoxemia (EIH) during a maximal incremental test in normoxic conditions.

METHODS

Nineteen male endurance sportsmen performed an incremental test on a cycle ergometer to determine maximal oxygen consumption (VO2max) and the corresponding power output (P(max)). Arterial O2 saturation (SaO2) was measured noninvasively with a pulse oxymeter at the earlobe to detect EIH, which was defined as a drop in SaO2 > 4% between rest and the end of the exercise. Muscle deoxygenation of the right vastus lateralis was monitored by near infrared spectroscopy and was expressed in percentage according to the ischemia-hyperemia scale.

RESULTS

Ten athletes exhibited arterial hypoxemia (EIH group) and the nine others were nonhypoxemic (NEIH group). Training volume, competition level, VO2max, Pmax, and lactate concentration were similar in the two groups. Nevertheless, muscle deoxygenation at the end of the exercise was significantly greater in the EIH group (P < 0.05).

CONCLUSION

Greater muscle deoxygenation at maximal exercise in hypoxemic athletes seems to be due, at least in part, to reduced oxygen delivery--that is, exercise-induced hypoxemia--to working muscle added to the metabolic demand. In addition, our finding is also consistent with the hypothesis of greater muscle oxygen extraction in order to counteract reduced O2 availability.

The effect of sustained heavy exercise on the development of pulmonary edema in trained male cyclists

To determine whether intense, prolonged activity can induce transient pulmonary edema, eight highly trained male cyclists (mean +/- S.D.: age, 26.9 +/- 3.0 years; height, 179.9 +/- 5.7 cm; weight, 76.1 +/- 6.5 kg) performed a 45-min endurance cycle test (ECT). V(O2,max) was determined (4.84 +/- 0.4 L min(-1), 63.7 +/- 2.6 ml min(-1) g(-1)) and the intensity of exercise for the ECT was set at 10% below ventilatory threshold (approximately 76% V(O2, max) 300 +/- 25 W). Pre- and post-exercise pulmonary diffusion (DL(CO)) measurements and magnetic resonance imaging of the lung were made. DL(CO) and pulmonary capillary blood volume (VC) decreased 1h post-exercise by 12% (P = 0.004) and 21% (P = 0.017), respectively, but no significant change in membrane diffusing capacity (DM) was found. The magnetic resonance scans demonstrated a 9.4% increase (P = 0.043) in pulmonary extravascular water 90 min post-exercise. These data support the theory that high intensity, sustained exercise in well-trained athletes can result in transient pulmonary edema.

[The John Sutton Lecture: CSEP, 2002]. Pulmonary system limitations to exercise in health

It is commonly held that the structural capacity of the normal lung is "overbuilt" and exceeds the demand for pulmonary O2 and CO2 transport in the healthy, exercising human. On the other hand, the adaptability of pulmonary system structures to habitual physical training is substantially less than are other links in the O2 transport system. Accordingly, in some highly fit, and even in some not so fit habitually active individuals, the lung's diffusion surface, airways, and/or chest-wall musculature are underbuilt relative to the demand for maximal O2 transport. Two specific pulmonary limitations to exercise performance are proposed: (1) exercise-induced arterial hypoxemia secondary to excessive widening of the alveolar to arterial O2 difference, inadequate hyperventilation, and metabolic acidosis; and (2) highly fatiguing levels of respiratory muscle work which effectively steals blood flow from locomotor muscles via sympathetically mediated reflexes and heightens the perception of limb discomfort and dyspnea. In this brief review, we describe the characteristics and causes of each of these proposed pulmonary limitations and their consequences to maximal O2 uptake and exercise performance.

Arterial desaturation during exercise in man: implication for O2 uptake and work capacity

Exercise-induced arterial hypoxaemia is defined as a reduction in the arterial O2 pressure (PaO2) by more than 1 kPa and/or a haemoglobin O2 saturation (SaO2) below 95%. With blood gas analyses ideally reported at the actual body temperature, desaturation is a consistent finding during maximal ergometer rowing. Arterial desaturation is most pronounced at the end of a maximal exercise bout, whereas the reduction in PaO2 is established from the onset of exercise. Exercise-induced arterial hypoxaemia is multifactorial. The ability to maintain a high alveolar O2 pressure (PAO2) is critical for blood oxygenation and this appears to be difficult in large individuals. A large lung capacity and, in turn, diffusion capacity seem to protect PaO2. A widening of the PAO2-PaO2 difference does indicate that a diffusion limitation, a ventilation-perfusion mismatch and/or a shunt influence the transport of O2 from alveoli to the pulmonary capillaries. An inspired O2 fraction of 0.30 reduces the widened PAO2-PaO2 difference by 75% and prevents a reduction of PaO2 and SaO2. With a marked increase in cardiac output, diffusion limitation combined with a fast transit time dominates the O2 transport problem. Furthermore, a postexercise reduction in pulmonary diffusion capacity suggests that the alveolo-capillary membrane is affected. An antioxidant attenuates oxidative burst by neutrophilic granulocytes, but it does not affect PaO2, SaO2 or O2 uptake (VO2), and the ventilatory response to maximal exercise also remains the same. It is proposed, though, that increased concentration of certain cytokines correlates to exercise-induced hypoxaemia as cytokines stimulate mast cells and basophilic granulocytes to degranulate histamine. The basophil count increases during maximal rowing. Equally, histamine release is associated with hypoxaemia and when the release of histamine is prevented, the reduction in PaO2 is attenuated. During maximal exercise, an extreme lactate spill-over to blood allows pH decrease to below 7.1 and according to the O2 dissociation curve this is critical for SaO2. When infusion of sodium bicarbonate maintains a stable blood buffer capacity, acidosis is attenuated and SaO2 increases from 89% to 95%. This enables exercise capacity to increase, an effect also seen when O2 supplementation to inspired air restores arterial oxygenation. In that case, exercise capacity increases less than can be explained by VO2 and CaO2. Furthermore, the change in muscle oxygenation during maximal exercise is not affected when hyperoxia and sodium bicarbonate attenuate desaturation. It is proposed that other organs benefit from enhanced O2 availability, and especially the brain appears to increase its oxygenation during maximal exercise with hyperoxia.

Exercise-induced arterial hypoxaemia in athletes: a review

During exercise, healthy individuals are able to maintain arterial oxygenation, whereas highly-trained endurance athletes may exhibit an exercise-induced arterial hypoxaemia (EIAH) that seems to reflect a gas exchange abnormality. The effects of EIAH are currently debated, and different hypotheses have been proposed to explain its pathophysiology. For moderate exercise, it appears that a relative hypoventilation induced by endurance training is involved. For high-intensity exercise, ventilation/perfusion (V(A)/Q) mismatching and/or diffusion limitation are thought to occur. The causes of this diffusion limitation are still under debate, with hypotheses being capillary blood volume changes and interstitial pulmonary oedema. Moreover, histamine is released during exercise in individuals exhibiting EIAH, and questions persist as to its relationship with EIAH and its contribution to interstitial pulmonary oedema. Further investigations are needed to better understand the mechanisms involved and to determine the long term consequences of repetitive hypoxaemia in highly trained endurance athletes.

Training in hypoxia: modulation of metabolic and cardiovascular risk factors in men

PURPOSE

This study was designed to determine changes in metabolic and cardiovascular risk factors following normobaric hypoxic exercise training in healthy men.

METHODS

Following a randomized baseline maximal exercise test in hypoxia and/or normoxia, 34 physically active subjects were randomly assigned to either a normoxic (N = 14) or a hypoxic (N = 18) training group. Training involved 4 wk of cycling exercise inspiring either a normobaric normoxic (F(IO2) = approximately 20.9%) or a normobaric hypoxic (F(IO2) = approximately 16.0%) gas, respectively, in a double-blind manner. Cycling exercise was performed three times per week for 20-30 min at 70-85% of maximum heart rate determined either in normoxia or hypoxia. Resting plasma concentrations of blood lipids, lipoproteins, total homocysteine, and auscultatory arterial blood pressure responses at rest and in response to submaximal and maximal exercise were measured before and 4 d after physical training.

RESULTS

Total power output during the training period was identical in both normoxic and hypoxic groups. Lean body mass increased by 1.4 +/- 1.5 kg following hypoxic training only (P < 0.001). While dietary composition and nutrient intake did not change during the study, both normoxic and hypoxic training decreased resting plasma concentrations of nonesterified fatty acids, total cholesterol, high density lipoprotein (HDL), and low density lipoprotein (LDL) (P < 0.05 - < 0.001). Apolipoproteins AI and B decreased following normoxic training only (P < or = 0.001). Plasma concentrations of resting total homocysteine decreased by 11% following hypoxic training (P < or = 0.05) and increased by 10% (P < 0.05) following normoxic training. These changes were independent of changes in serum vitamin B12 and red cell folate which remained stable throughout. A decreased lactate concentration during submaximal exercise was observed in response to both normoxic and hypoxic training. Hypoxic training decreased maximal systolic blood pressure by 10 +/- 9 mm Hg (P < 0.001) and the rate pressure product by 14 +/- 23 mm Hg x beats x min(-1)/100 (P < or = 0.001) and increased maximal oxygen uptake by 0.47 +/- 0.77 L x min(-1) (P < 0.05).

CONCLUSION

Normoxic and hypoxic training was associated with significant improvements in selected risk factors and exercise capacity. The stimulus of intermittent normobaric hypoxia invoked an additive cardioprotective effect which may have important clinical implications.

Effect of exercise-induced arterial O2 desaturation on VO2max in women

PURPOSE

We have recently reported that many healthy habitually active women experience exercise induced arterial hypoxemia (EIAH). We questioned whether EIAH affected VO2max in this population and whether the effect was similar to that reported in men.

METHODS

Twenty-five healthy young women with widely varying fitness levels (VO2max, 56.7 +/- 1.5 mL x kg(-1) x min(-1); range: 41-70 mL x kg(-1) x min(-1)) and normal resting lung function performed two randomized incremental treadmill tests to VO2max (FIO2: 0.21 or 0.26) during the follicular phase of their menstrual cycle. Arterial blood samples were taken at rest and near the end of each workload during the normoxic test.

RESULTS

During room air breathing at VO2max, SaO2 decreased to 91.8 +/- 0.4% (range 87-95%). With 0.26 FIO2, SaO2, at VO2max remained near resting levels and averaged 96.8 +/- 0.1% (range 96-98%). When arterial O2 desaturation was prevented via increased FIO2, VO2max increased in 22 of the 25 subjects and in proportion to the degree of arterial O2 desaturation experienced in normoxia (r = 0.88). The improvement in VO2max when systemic normoxia was maintained averaged 6.3 +/- 0.3% (range 0 to +15%) and the slope of the relationship was approximately 2% increase in VO2max for every 1% decrement in the arterial oxygen saturation below resting values. About 75% of the increase in VO2max resulted from an increase in VO2 at a fixed maximal work rate and exercise duration, and the remainder resulted from an increase in maximal work rate.

CONCLUSIONS

These data demonstrate that even small amounts of EIAH (i.e., >3% delta SaO2 below rest) have a significant detrimental effect on VO2max in habitually active women with a wide range of VO2max. In combination with our previous findings documenting EIAH in females, we propose that inadequate pulmonary structure/function in many habitually active women serves as a primary limiting factor in maximal O2 transport and utilization during maximal exercise.

Exercise-induced arterial hypoxemia

Exercise-induced arterial hypoxemia (EIAH) at or near sea level is now recognized to occur in a significant number of fit, healthy subjects of both genders and of varying ages. Our review aims to define EIAH and to critically analyze what we currently understand, and do not understand, about its underlying mechanisms and its consequences to exercise performance. Based on the effects on maximal O(2) uptake of preventing EIAH, we suggest that mild EIAH be defined as an arterial O(2) saturation of 93-95% (or 3-4% <rest), moderate EIAH as 88-93%, and severe EIAH as <88%. Both an excessive alveolar-to-arterial PO(2) difference (A-a DO(2)) (>25-30 Torr) and inadequate compensatory hyperventilation (arterial PCO(2) >35 Torr) commonly contribute to EIAH, as do acid- and temperature-induced shifts in O(2) dissociation at any given arterial PO(2). In turn, expiratory flow limitation presents a significant mechanical constraint to exercise hyperpnea, whereas ventilation-perfusion ratio maldistribution and diffusion limitation contribute about equally to the excessive A-a DO(2). Exactly how diffusion limitation is incurred or how ventilation-perfusion ratio becomes maldistributed with heavy exercise remains unknown and controversial. Hypotheses linked to extravascular lung water accumulation or inflammatory changes in the "silent" zone of the lung's peripheral airways are in the early stages of exploration. Indirect evidence suggests that an inadequate hyperventilatory response is attributable to feedback inhibition triggered by mechanical constraints and/or reduced sensitivity to existing stimuli; but these mechanisms cannot be verified without a sensitive measure of central neural respiratory motor output. Finally, EIAH has detrimental effects on maximal O(2) uptake, but we have not yet determined the cause or even precisely identified which organ system, involved directly or indirectly with O(2) transport to muscle, is responsible for this limitation.

Erythropoietin concentration and arterial haemoglobin saturation with supramaximal exercise

The aim of this study was to determine if the hypoxaemic stimulus generated by intense exercise results in the physiological response of increased erythropoietin production. Twenty athletes exercised for 3 min at 109 +/- 2.8% (mean +/- s) maximal oxygen consumption. Estimated oxyhaemoglobin saturation was measured by reflective probe pulse oximetry (Nellcor N200) and was validated against arterial oxyhaemoglobin saturation by CO-oximetry in eight athletes. Serum erythropoietin concentrations-as measured using the INCSTAR Epo-Trac radioimmunoassay-increased significantly by 28 +/- 9% at 24 h post-exercise in 11 participants, who also had an arterial oxyhaemoglobin saturation < or = 91% (P < 0.05). Decreased ferritin levels and increased reticulocyte counts were observed at 96 h post-exercise. However, no significant changes in erythropoietin levels were observed in nine non-desaturating athletes and eight non-exercise controls. Good agreement was shown between arterial oxyhaemoglobin saturation and percent estimated oxyhaemoglobin saturation (limits of agreement = -3.9 to 3.7%). In conclusion, short supramaximal exercise can induce both hypoxaemia and increased erythropoietin levels in well-trained individuals. The decline of arterial hypoxaemia levels below 91% during exercise appears to be necessary for the exercise-induced elevation of serum erythropoietin levels. Furthermore, reflective probe pulse oximetry was found to be a valid predictor of percent arterial oxyhaemoglobin saturation during supramaximal exercise when percent estimated oxyhaemoglobin saturation > or = 86%.

The time course of pulmonary diffusing capacity for carbon monoxide following short duration high intensity exercise

We investigated the time course of changes in post-exercise pulmonary diffusing capacity for carbon monoxide (DLCO), membrane diffusing capacity (DM), and pulmonary capillary blood volume (VC) in highly trained (HT), moderately trained (MT) and untrained (UT) male subjects (n = 8/group). Subjects were assigned to groups based on their aerobic capacity from a preliminary VO2max test (HT > or = 65, MT = 50-60, UT < or = 50 ml x kg(-1) x min(-1)). Resting (BASE) DLCO, DM and VC were obtained, then subjects cycled to fatigue at the highest workrate attained during the preliminary tests. Diffusion measurements were then made at 1, 2, 4, 6 and 24 h. DLCO was depressed at 1 h, lowest at 6 h and approached BASE values at 24 h in all groups. The DLCO change was paralleled by a change in VC. Alterations to VC were similar between groups except at 24 h where MT and HT subjects had returned to BASE while UT did not. DM was significantly lower than BASE at 1, 2, 4, and 6 h, and was similar between groups. The changes in DLCO post-exercise appear to be primarily due to a decrease in VC. Comparable diffusion decrements were observed in all subjects. The results of this study suggest that post-exercise alterations in DLCO, DM and VC are not related to aerobic capacity.

The relationship between test protocol and the development of exercise-induced hypoxemia (EIH) in highly trained athletes

Healthy male endurance-trained cyclists [n = 11, age = 27.3 (3.9) years; mass = 73.0 (9.3) kg; height = 180.5 (6.9) cm; maximal oxygen consumption (VO2max) = 71.1 (5.8) ml.kg-1.min-1, mean +/- (SD)] were recruited to assess the relationship between test protocol and the development of desaturation of arterial hemoglobin with oxygen, during incremental exercise tests to maximal aerobic capacity (VO2max). All subjects demonstrated resting pulmonary function within normal limits [forced vital capacity (FVC) = 6.0 (0.9); forced expiratory volume (FEV1.0) = 4.9 (0.6); FEV1.0/FVC = 0.8 (0.1)] and completed three ramped VO2max tests (Mijnhardt KEM-3 electronically braked cycle ergometer) beginning at 0 W with increments of either 20,30 or 40 W.min-1. All periods of testing were separated by a minimum of 72 h. VO2max, peak minute ventilation (VEpeak) (Medical Graphics, CPX-D), peak heart rate (fcpeak), peak power output (Wpeak), and minimum percentage arterial oxyhemoglobin saturation (% SaO2min) (Omeda Biox 3740 pulse oximeter) were determined. There were no significant differences (p > 0.05) in VEpeak [191.5 (26.2), 196.0 (24.4), 194.3 (23.9) l.min-1] fcpeak [191.4 (7.0), 190.3 (5.5), 187.8 (5.9) beats.min-1], VO2max [5.0 (0.5), 5.1 (0.4), 5.1 (0.5) l.min-1] or %SaO2min [89.5 (1.5), 89.6 (1.3), 90.0 (2.3)] between protocols. The 20-W protocol [417 (27) W] demonstrated significantly lower Wpeak (P < 0.05) than the 30-W [434 (36) W] and 40-W [453 (38) W] protocols, indicating that peripheral fatigue may play an important factor in response to these tests. The results of this study demonstrate that arterial desaturation occurs as a result of intense exercise in highly trained athletes independent of the rate of attainment of VO2max.

[Hypoxemia and exhaustion time to maximal aerobic speed in long-distance runners]

A recent paper (Billat et al., 1994a) has shown the reproducibility but also the great variability between subelite long-distance runners in their time to exhaustion at the velocity which elicits VO2max, called the maximal aerobic speed (MAS). The present study delved further into the reasons for this large difference between runners having the same VO2max. The question addressed was whether the exercise-induced hypoxemia (EIH) was more important for athletes having the longest time to exhaustion at 90 (Tlim 90), 100 (Tlim 100), or 105% (Tlim 105) of MAS. The study was conducted on 16 elite male runners. EIH was observed, that is, arterial oxyhemoglobin saturation and arterial partial pressure of oxygen dropped significantly after all the Tlim tests. However, EIH was only correlated with Tlim 90 (r = -0.757; -0.531, respectively).