Effects of exhaustive endurance exercise on pulmonary gas exchange and airway function in women

Oxygen Saturation Behavior by Pulse Oximetry in Female Athletes: Breaking Myths

The myths surrounding women’s participation in sport have been reflected in respiratory physiology. This study aims to demonstrate that continuous monitoring of blood oxygen saturation during a maximal exercise test in female athletes is highly correlated with the determination of the second ventilatory threshold (VT2) or anaerobic threshold (AnT). The measurements were performed using a pulse oximeter during a maximum effort test on a treadmill on a population of 27 healthy female athletes. A common behavior of the oxygen saturation evolution during the incremental exercise test characterized by a decrease in saturation before the aerobic threshold (AeT) followed by a second significant drop was observed. Decreases in peripheral oxygen saturation during physical exertion have been related to the athlete’s physical fitness condition. However, this drop should not be a limiting factor in women’s physical performance. We found statistically significant correlations between the maximum oxygen uptake and the appearance of the ventilatory thresholds (VT1 and VT2), the desaturation time, the total test time, and between the desaturation time and the VT2. We observed a relationship between the desaturation time and the VT2 appearance. Indeed, a linear regression model between the desaturation time and the VT2 appearance can predict 80% of the values in our sample. Besides, we suggest that pulse oximetry is a simple, fairly accurate, and non-invasive technique for studying the physical condition of athletes who perform physical exertion.

Is the healthy respiratory system built just right, overbuilt, or underbuilt to meet the demands imposed by exercise?

In the healthy, untrained young adult, a case is made for a respiratory system (airways, pulmonary vasculature, lung parenchyma, respiratory muscles, and neural ventilatory control system) that is near ideally designed to ensure a highly efficient, homeostatic response to exercise of varying intensities and durations. Our aim was then to consider circumstances in which the intra/extrathoracic airways, pulmonary vasculature, respiratory muscles, and/or blood-gas distribution are underbuilt or inadequately regulated relative to the demands imposed by the cardiovascular system. In these instances, the respiratory system presents a significant limitation to O₂ transport and contributes to the occurrence of locomotor muscle fatigue, inhibition of central locomotor output, and exercise performance. Most prominent in these examples of an "underbuilt" respiratory system are highly trained endurance athletes, with additional influences of sex, aging, hypoxic environments, and the highly inbred equine. We summarize by evaluating the relative influences of these respiratory system limitations on exercise performance and their impact on pathophysiology and provide recommendations for future investigation.

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.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Physical Activity and Lung Function Growth in a Cohort of Chinese School Children: A Prospective Study

BACKGROUNDS/OBJECTIVES

Evidence on the association between physical activity and lung function in children is sparse. The aim of this study was to evaluate children's lung function growth in relation to their physical activity level in Chinese children.

METHODS

A total of 1713 school children aged 9.89±0.86 years who were asthma-free at baseline were followed-up for 18 months from 2006 to 2008 in Guangzhou, China. Information on physical activity and other socio-economic status were obtained from self-administered questionnaires. Lung function tests were performed with a standard procedure.

RESULTS

At the baseline survey, physically active girls had significantly higher forced vital capacity (FVC) than inactive girls (1.79 l vs. 1.75 l, p<0.05). The growth rates for lung function indices were significantly higher for girls who were physically active at either or both follow-up surveys than those inactive at both surveys during the follow-up period forced expiratory flows at 25% (FEF25) difference per year (dpy) (0.20 l/s vs. 0.15 l/s), forced expiratory flows at 75% (FEF75) dpy (0.57 l/s vs. 0.45 l/s) and forced expiratory flows between 25% and 75% (FEF25-75) dpy (0.36 l/s vs. 0.28 l/s) (all p<0.05).

CONCLUSIONS

Physical activity is positively associated with lung function growth among Chinese school-aged girls. Promotion of physical activity among children is of great importance.

Exercise-induced arterial hypoxaemia and the mechanics of breathing in healthy young women

The purpose of this study was to characterize exercise-induced arterial hypoxaemia (EIAH), pulmonary gas exchange and respiratory mechanics during exercise, in young healthy women. We defined EIAH as a >10 mmHg decrease in arterial oxygen tension ( ) during exercise compared to rest. We used a heliox inspirate to test the hypothesis that mechanical constraints contribute to EIAH. Subjects with a spectrum of aerobic capacities (n = 30; maximal oxygen consumption ( ) = 49 ± 1, range 28-62 ml kg(-1) min(-1)) completed a stepwise treadmill test and a subset (n = 18 with EIAH) completed a constant load test (~85% ) with heliox gas. Throughout exercise arterial blood gases, oxyhaemoglobin saturation ( ), the work of breathing (WOB) and expiratory flow limitation (EFL) were assessed. Twenty of the 30 women developed EIAH with a nadir and ranging from 58 to 88 mmHg and 87 to 96%, respectively. At maximal exercise, was inversely related to (r = -0.57, P < 0.05) with notable exceptions where some subjects with low aerobic fitness levels demonstrated EIAH. Subjects with EIAH had a greater (51 ± 1 vs. 43 ± 2 ml kg(-1) min(-1)), lower end-exercise (93.2 ± 0.5 vs. 96.1 ± 0.3%) and a greater maximal energetic WOB (324 ± 19 vs. 247 ± 23 J min(-1)), but had similar resting pulmonary function compared to those without EIAH. Most subjects developed EIAH at submaximal exercise intensities, with distinct patterns of hypoxaemia. In some subjects with varying aerobic fitness levels, mechanical ventilatory constraints (i.e. EFL) were the primary mechanism associated with the hypoxaemia during the maximal test. Mechanical ventilatory constraints also prevented adequate compensatory alveolar hyperventilation in most EIAH subjects. Minimizing mechanical ventilatory constraints with heliox inspiration partially reversed EIAH in subjects who developed EFL. In conclusion, healthy women of all aerobic fitness levels can develop EIAH and begin to do so at submaximal intensities. Mechanical ventilatory constraints are a primary mechanism for EIAH in some healthy women and prevent reversal of hypoxaemia in women for whom it is not the primary mechanism.

Acidosis, hypoxia and bone

Bone homeostasis is profoundly affected by local pH and oxygen tension. It has long been recognised that the skeleton contains a large reserve of alkaline mineral (hydroxyapatite), which is ultimately available to neutralise metabolic H(+) if acid-base balance is not maintained within narrow limits. Bone cells are extremely sensitive to the direct effects of pH: acidosis inhibits mineral deposition by osteoblasts but it activates osteoclasts to resorb bone and other mineralised tissues. These reciprocal responses act to maximise the availability of OH(-) ions from hydroxyapatite in solution, where they can buffer excess H(+). The mechanisms by which bone cells sense small pH changes are likely to be complex, involving ion channels and receptors in the cell membrane, as well as direct intracellular effects. The importance of oxygen tension in the skeleton has also long been known. Recent work shows that hypoxia blocks the growth and differentiation of osteoblasts (and thus bone formation), whilst strongly stimulating osteoclast formation (and thus bone resorption). Surprisingly, the resorptive function of osteoclasts is unimpaired in hypoxia. In vivo, tissue hypoxia is usually accompanied by acidosis due to reduced vascular perfusion and increased glycolytic metabolism. Thus, disruption of the blood supply can engender a multiple negative impact on bone via the direct actions of reduced pO(2) and pH on bone cells. These observations may contribute to our understanding of the bone disturbances that occur in numerous settings, including ageing, inflammation, fractures, tumours, anaemias, kidney disease, diabetes, respiratory disease and smoking.

Exercise-induced hypoxemia: fact or fallacy?

Whereas the prevalence of exercise-induced hypoxemia (EIH) in endurance athletes is commonly reported as approximately 50%, most previous studies have not corrected PaO2 for exercise-induced hyperthermia. Furthermore, although a detrimental effect on aerobic performance has been assumed, no study has measured arterial oxygen content (CaO2) in this context.

PURPOSE

To determine the effect of temperature-correcting PaO2 values for rectal, arterial blood, esophageal, and exercising muscle temperatures during exercise on CaO2 and the prevalence of EIH.

METHODS

Twenty-three trained males (age 26 +/- 5 yr; VO2peak 65.2 +/- 1.6 mL x kg-1 x min-1) performed incremental treadmill exercise to exhaustion with PaO2 corrected for simultaneous temperature measurements at all four sites. EIH was defined as DeltaPaO2 >or= 10 mm Hg.

RESULTS

: With no temperature correction, DeltaPaO2 was -20.8 +/- 5.0 mm Hg and prevalence was 96% (n = 23), but when corrected for rectal temperature, DeltaPaO2 was -14.7 +/- 7.8 mm Hg and prevalence was 73% (n = 20); for arterial blood temperature, DeltaPaO2 was -7.7 +/- 6.5 mm Hg and prevalence was 35% (n = 20); and for esophageal temperature, DeltaPaO2 was -8.1 +/- 7.7 mm Hg and prevalence was 48% (n = 23), although when corrected for active muscle temperature, DeltaPaO2 was +8.2 +/- 7.8 mm Hg and prevalence was 0% (n = 10). There were no significant changes in CaO2 except for uncorrected values, and there was no correlation between DeltaPaO2 and VO2peak.

CONCLUSIONS

Although the prevalence of EIH depends on the temperature correction applied to PaO2 values, in no case is there a significant change in CaO2 or any relationship with maximal aerobic power.

Sex differences in pulmonary function during exercise

Structural and hormonal sex differences are known to exist that may influence the pulmonary system's response to exercise. Specifically, women tend to show reduced lung size, decreased maximal expiratory flow rates, reduced airway diameter, and a smaller diffusion surface than age- and height-matched men. Additionally, ovarian hormones, namely progesterone and estrogen, are known to modify and influence the pulmonary system. These differences may have an effect on airway responsiveness, ventilation, respiratory muscle work, and pulmonary gas exchange during exercise. Recent evidence suggests that during exercise, women demonstrate greater airway hyperresponsiveness and expiratory flow limitation, increased work of breathing, and, perhaps, greater exercise-induced arterial hypoxemia compared with men. The consequence of these pulmonary effects may influence exercise capacity.

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.

Treatment of airway inflammation improves exercise pulmonary gas exchange and performance in asthmatic subjects

BACKGROUND

Asthma is an inflammatory disease of the airways that can lead to impaired arterial blood oxygenation during exercise.

OBJECTIVE

We asked whether treatment of airway inflammation in asthmatic subjects would improve arterial blood gases during whole-body exercise.

METHODS

By using a double-blind parallel-group design, 19 asthmatic subjects completed treadmill exercise to exhaustion on 2 occasions: (1) before and (2) after 6 weeks' treatment with an inhaled corticosteroid (ICS; n = 9) or placebo (n = 10).

RESULTS

The ICS group had improved resting pulmonary function, decreased exercise-induced bronchospasm, and decreased postexercise sputum histamine during the posttreatment study compared with that during the pretreatment study. In the ICS group exercise Pao(2) was significantly increased after treatment (84.8 to 93.8 mm Hg). Increased alveolar ventilation (arterial Pco(2) decreased from 36.9 to 34.1 mm Hg) accounted for 37% of the increased Pao(2) and improved gas exchange efficiency (alveolar-to-arterial Po(2) difference decreased from 22.5 to 16.3 mm Hg) accounted for the remaining 63% of the increased Pao(2) after treatment. In the ICS group exercise time to exhaustion was increased from 9.9 minutes during the pretreatment study to 14.8 minutes during the posttreatment study.

CONCLUSION

Treatment of airway inflammation in asthmatic subjects can improve arterial blood oxygenation during exercise by (1) improving airway function, thereby allowing increased alveolar ventilation during exercise, and (2) improving the efficiency of alveolar-to-arterial blood O(2) exchange.

CLINICAL IMPLICATIONS

In asthmatic patients ICSs not only attenuate exercise-induced bronchospasm but also improve arterial blood oxygenation during exercise.

Pulmonary gas exchange does not worsen during repeat exercise in women

The purposes were to determine (1) if repeat exercise worsens pulmonary gas exchange in women, and, (2) if the level of pulmonary edema obtained in these same women is related to the gas exchange impairment during exercise. Fourteen women (27 +/- 4 yrs; maximal oxygen uptake = 3.12 +/- 0.42 L/min) with minimal arterial PO2 (PaO2) ranging from 76 to 104 mmHg with a maximal alveolar-arterial PO2 difference (AaDO2) ranging from 7 to 35 mmHg performed three bouts of near-maximal exercise on a cycle ergometer (236 +/- 27 W) for 5 min each with 10 min of rest between sets. Cardiorespiratory parameters and oxygenation were measured at rest, throughout exercise and recovery. Chest radiographs were obtained before and 30 min after the interval training session (see Respir Physiol Neurobiol, 153 (2006) 181-190). Repeat exercise did not affect pulmonary gas exchange between sets 1 and 3 (change in PaO2 = 3 +/- 2 mmHg; change in AaDO2 = 1 +/- 2 mmHg P > 0.05). Arterial PCO2 decreased by 4 +/- 2 mmHg (P < 0.05) between sets 1 and 2, which did not reduce further in set 3. The level of PaO2 or AaDO2 was not related to the change in edema score or the post-exercise edema score (P > 0.05). In conclusion, pulmonary gas exchange is not worsened in women during interval training despite the mild edema triggered by exercise.

Effect of prolonged exercise on arterial oxygen saturation in athletes susceptible to exercise-induced hypoxemia

This study examined the effect of prolonged endurance exercise on the development of exercise-induced hypoxemia (EIH) in athletes who had previously displayed EIH during an incremental maximal exercise test. Five male and three female endurance-trained athletes participated. Susceptibility to EIH was confirmed through a maximal incremental exercise test and defined as a reduction in the saturation of arterial oxygen (SpO(2)) of >/=4% from rest. Sixty minutes of running was conducted, on a separate day, at an oxygen consumption corresponding to 95% of ventilatory threshold. Immediately following the 60 min exercise bout, athletes commenced a time trial to exhaustion at 95% maximal oxygen consumption (VO(2max)). The reduction in SpO(2) was significantly greater during the maximal incremental test, than during the 60 min, or time trial to exhaustion (-8.8+/-1.4%, -3.3+/-1.1%, and -4.1+/-2.3%, P<0.05, respectively). The degree of desaturation during the 60 min was significantly related to the relative intensity of exercise at 95% ventilatory threshold (adjusted r(2)=0.54, P=0.02). In conclusion, athletes who did not exercise at greater than 73% VO(2max) during 60 min of endurance exercise did not display EIH, despite being previously susceptible during an incremental maximal test.

Effects of exercise-induced arterial hypoxaemia on limb muscle fatigue and performance

1. Reductions in arterial O(2) saturation (-5% to -10%S(a)o(2) below rest) occur over time during sustained heavy-intensity exercise in a normoxic environment, caused primarily by the effects of acid pH and increased temperature on the position of the HbO(2) dissociation curve. 2. We prevented the desaturation incurred during exercise at approximately 90% VO(2 MAX) via increased fraction of inspired O(2) (F(i)o(2)) (0.23 to 0.29) and showed that exercise time to exhaustion was increased. 3. We used supramaximal magnetic stimulation (1-100 Hz) of the femoral nerve to test for quadriceps fatigue. We used mildly hyperoxic inspirates (F(i)o(2) 0.23 to 0.29) to prevent O(2) desaturation. We then compared the amount of quadriceps fatigue incurred following cycling exercise at S(a)o(2) 91% vs 98% with each trial carried out at identical work rates and for equal durations. 4. Preventing the normal exercise-induced O(2) desaturation prevented about one-half the amount of exercise-induced quadriceps fatigue; plasma lactate and effort perception were also reduced. In a subset of less fit subjects who showed only minimal arterial hypoxaemia during sustained exercise (S(a)o(2) approximately 95%), breathing a mildly hypoxic inspirate (F(i)o(2) 0.17; S(a)o(2) approximately 88%) exacerbated the quadriceps fatigue. 5. We conclude that the normal exercise-induced O(2) desaturation during heavy-intensity endurance exercise contributes significantly to exercise performance limitation in part because of its effect on locomotor muscle fatigue.

Exhaled nitric oxide during normoxic and hypoxic exercise in endurance athletes

AIM

Endogenous nitric oxide (NO) through its relaxing effect on smooth muscle cells may be involved in pulmonary gas exchange as well as in the modulation of the hypoxic pulmonary vasoconstriction. As athletes with exercise-induced hypoxaemia (EIH) present pulmonary gas exchange abnormalities in normoxia that could be even greater in hypoxia, we hypothesized that pulmonary NO may be lower in such athletes with EIH.

METHODS

Eleven athletes with EIH [decrease in arterial oxygen blood partial pressure (PaO2) > 12 mmHg] and 9 without EIH (NEIH) exercised at 40%, 60% (10 min) and 90% (5 min) of normoxic maximal power output (Pmax) in normoxia, and at 40% and 60% (10 min) of Pmax in hypoxia (FiO2 = 15%). Exhaled NO concentration during a constant flow exhalation (FENO(0.170)) and arterialized blood gases were measured at every power output.

RESULTS

FENO(0.170) decreased from rest to exercise both in normoxia (-27.8 +/- 22.8% at 90% Pmax, P < 0.001) and hypoxia (-23.8 +/- 17.5% at 60% Pmax, P < 0.001). At 90% Pmax in normoxia, EIH athletes showed lower PaO2 (76.7 +/- 5.4 vs. 82.8 +/- 4.4 mmHg, P = 0.013) and greater FENO(0.170) decrement (-37.0 +/- 24.7% vs. -16.6 +/- 14.6%, P = 0.042) than NEIH athletes. During hypoxic exercise, P(a)O(2) and FENO(0.170) decreases were similar in both groups (P > 0.05).

CONCLUSION

The present study shows lower pulmonary NO in athletes with gas exchange abnormalities during intense exercise in normoxia, while EIH and NEIH athletes have similar decreases in blood gases and pulmonary NO during hypoxic exercise. Decreased pulmonary NO in such conditions may contribute to ventilation-perfusion inequality and/or increase pulmonary vascular tone in athletes.

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.

Exercise-induced arterial hypoxemia: consequences for locomotor muscle fatigue

Reductions in arterial O2 saturation (-5 to -10% SaO2 < rest) occur over time during sustained heavy intensity exercise in a normoxic environment, due primarily to the effects of acid pH and increased temperature on the position of the HbO2 dissociation curve. We prevented the desaturation via increased F1O2 (.23 to .29) and showed that exercise time to exhaustion was increased. We used supramaximal magnetic stimulation (1 - 100 Hz) of the femoral nerve to test for quadriceps fatigue. We used mildly hyperoxic inspirates (F1O2 .23 to .29) to prevent O2 desaturation. We then compared the amount of quadriceps fatigue incurred following cycling exercise at SaO2 98% vs. 91% with each trial carried out at equal exercise intensities (90% Max) and for equal durations. Preventing the normal exercise-induced O2 desaturation prevented about one-half the amount of exercise-induced quadriceps fatigue; plasma lactate and effort perception were also reduced. We conclude that the normal exercise-induced O2 desaturation during heavy intensity endurance exercise contributes significantly to exercise performance limitation in part because of its effect on locomotor muscle fatigue. These effects of EIAH were confirmed in mild environmental hypoxia (FIO2 .17, SaO2 88%) which significantly augmented the magnitude of exercise-induced quadriceps fatigue observed in normoxia.

Acidosis inhibits bone formation by osteoblasts in vitro by preventing mineralization

The negative effect of acidosis on the skeleton has been known for almost a century. Bone mineral serves an important pathophysiologic role as a reserve of hydroxyl ions to buffer systemic protons if the kidneys and lungs are unable to maintain acid-base balance within narrow physiologic limits. Extracellular hydrogen ions are now thought to be the primary activation signal for osteoclastic bone resorption, and osteoclasts are very sensitive to small changes in pH within the pathophysiologic range. Herein, we investigated the effects of acidosis on osteoblast function by using mineralized bone nodule-forming primary osteoblast cultures. Osteoblasts harvested from neonatal rat calvariae were cultured up to 21 days in serum-containing medium, with ascorbate, beta-glycerophosphate and dexamethasone. pH was manipulated by addition of 5 to 30 mmol/L HCl and monitored by blood gas analyzer. Abundant, matrix-containing mineralized nodules formed in osteoblast cultures at pH 7.4, but acidification progressively reduced mineralization of bone nodules, with complete abolition at pH 6.9. Osteoblast proliferation and collagen synthesis, assessed by 3H-thymidine and 3H-proline incorporation, respectively, were unaffected by pH in the range 7.4 to 6.9; no effect of acidification on collagen ultrastructure and organization was evident. The apoptosis rate of osteoblasts, assessed by the enrichment of nucleosomes in cell lysates, was also unaffected by pH within this range. However, osteoblast alkaline phosphatase activity, which peaked strongly near pH 7.4, was reduced eight-fold at pH 6.9. Reducing pH to 6.9 also downregulated messenger ribonucleic acid (mRNA) for alkaline phosphatase, but upregulated mRNA for matrix Gla protein, an inhibitor of mineralization. The same pH reduction is associated with two-and four-fold increases in Ca2+ and PO4(3-) solubility for hydroxyapatite, respectively. Our results show that acidosis exerts a selective, inhibitory action on matrix mineralization that is reciprocal with the osteoclast activation response. Thus, in uncorrected acidosis, the deposition of alkaline mineral in bone by osteoblasts is reduced, and osteoclast resorptive activity is increased in order to maximize the availability of hydroxyl ions in solution to buffer protons.

Gas exchange during exercise in habitually active asthmatic subjects

We determined the relations among gas exchange, breathing mechanics, and airway inflammation during moderate- to maximum-intensity exercise in asthmatic subjects. Twenty-one habitually active (48.2 +/- 7.0 ml.kg(-1).min(-1) maximal O2 uptake) mildly to moderately asthmatic subjects (94 +/- 13% predicted forced expiratory volume in 1.0 s) performed treadmill exercise to exhaustion (11.2 +/- 0.15 min) at approximately 90% of maximal O2 uptake. Arterial O2 saturation decreased to < or =94% during the exercise in 8 of 21 subjects, in large part as a result of a decrease in arterial Po2 (PaO2): from 93.0 +/- 7.7 to 79.7 +/- 4.0 Torr. A widened alveolar-to-arterial Po2 difference and the magnitude of the ventilatory response contributed approximately equally to the decrease in PaO2 during exercise. Airflow limitation and airway inflammation at baseline did not correlate with exercise gas exchange, but an exercise-induced increase in sputum histamine levels correlated with exercise Pa(O2) (negatively) and alveolar-to-arterial Po2 difference (positively). Mean pulmonary resistance was high during exercise (3.4 +/- 1.2 cmH2O.l(-1).s) and did not increase throughout exercise. Expiratory flow limitation occurred in 19 of 21 subjects, averaging 43 +/- 35% of tidal volume near end exercise, and end-expiratory lung volume rose progressively to 0.25 +/- 0.47 liter greater than resting end-expiratory lung volume at exhaustion. These mechanical constraints to ventilation contributed to a heterogeneous and frequently insufficient ventilatory response; arterial Pco2 was 30-47 Torr at end exercise. Thus pulmonary gas exchange is impaired during high-intensity exercise in a significant number of habitually active asthmatic subjects because of high airway resistance and, possibly, a deleterious effect of exercise-induced airway inflammation on gas exchange efficiency.

Heterogeneous Airway Versus Tissue Mechanics and Their Relation to Gas Exchange Function During Mechanical Ventilation

We have advanced a commercially available ventilator (NPB840, Puritan Bennett/Tyco Healthcare, Pleasanton, CA) to deliver an Enhanced Ventilation Waveform (EVW). This EVW delivers a broadband waveform that contains discrete frequencies blended to provide a tidal breath, followed by passive exhalation. The EVW allows breath-by-breath estimates of frequency dependence of lung and total respiratory resistance (R) and elastance (E) from 0.2 to 8 Hz. We hypothesized that the EVW approach could provide continuous ventilation simultaneously with an advanced evaluation of mechanical heterogeneities under heterogeneous airway and tissue disease conditions. We applied the EVW in five sheep before and after a bronchial challenge and an oleic acid (OA) acute lung injury model. In all sheep, the EVW maintained gas exchange during and after bronchoconstriction, as well as during OA injury. Data revealed a range of disease conditions from mild to severe with heterogeneities and airway closures. Correlations were found between the arterial partial pressure of oxygen (PaO2) and the levels and frequency-dependent features of R and E that are indicative of mechanical heterogeneity and tissue disease. Lumped parameter models provided additional insight on heterogeneous airway and tissue disease. In summary, information obtained from EVW analysis can provide enhanced guidance on the efficiency of ventilator settings and on patient status during mechanical ventilation.

Échanges gazeux pulmonaires pendant l’exercice musculaire chez le sujet sain

INTRODUCTION

The modifications of gas exchange on exercise reflect the consequences of the control and limits of adaptation of the respiratory apparatus to the mechanical loads imposed on the muscles and the oxygen requirements of the organism. In the majority of cases, even if the thoraco-pulmonary apparatus is perfectly adapted to the increase in these requirements, the balance between the metabolic demands of the tissues and the pulmonary supply appears difficult to satisfy beyond certain limits without hypoxaemia, particularly in those subjects with a low ventilatory response to exercise. Based on the populations reported in the literature the functional limits of the control of the thoraco-pulmonary system and the possible modifications of the structures of the lung are discussed for each of these mechanisms.

STATE OF KNOWLEDGE

At certain levels of duration and intensity of exercise there is an increase in the alveolar-arterial oxygen gradient [P(A-a)O2] associated inconsistently with a fall in PaO2. It is mainly the use of inert gas techniques that has established over many years the respective roles of the different possible patho-physiological mechanisms: shunt, unequal distribution of VA/Q ratios, limitation of alveolar-capillary diffusion and its components. The inequalities of VA/Q increase at low levels of exercise but beyond certain levels of VO2 limitation of oxygen diffusion may develop. In effect, particularly in subjects capable of high levels of exercise, the interaction between diminished transit time of the red cells in the pulmonary capillaries and possible delay in equilibration of partial pressures between the blood and gas phases may create a limitation of diffusion. This added to the inequalities of distribution of VA/Q and reduction in PVO2 leads, in certain subjects, to a transitory exercise induced hypoxaemia.

VIEWPOINTS AND CONCLUSIONS

New techniques of investigation seem to be necessary to clarify the sources of the observed changes and the development of modifications of pulmonary structure that establish the functional limits of the lungs on exercise. It remains to demonstrate the true impact of these anomalies on the limitation of human performance.

Acute hypervolemia does not improve arterial oxygenation in maximally exercising thoroughbred horses

Recently, it was reported that acute hypervolemia improves arterial oxygen tension in human athletes known to experience exercise-induced arterial hypoxemia. Since exercise-induced arterial hypoxemia is routinely observed in racehorses and is known to limit performance, we examined whether pre-exercise induction of acute hypervolemia would similarly benefit arterial oxygenation in maximally exercising thoroughbred horses. Two sets of experiments, namely, placebo [intravenous (IV) physiological saline] and acute hypervolemia (IV 7.2% NaCl, causing an 18.2% expansion of plasma volume) studies were carried out in random order on 13 healthy, exercise-trained thoroughbred horses, 7 days apart. An incremental exercise protocol leading to 120 s of galloping at 14 m s(-1) on a 3.5% uphill incline was used. Galloping at this workload elicited maximal heart rate and induced pulmonary hemorrhage in all horses in both treatments. In the placebo study, arterial oxygen tension decreased to 76.1 (2) mmHg (P<0.0001) at 30 s of maximal exertion, but further significant changes did not occur as exercise duration increased to 120 s [arterial oxygen tension 72.4 (2) mmHg]. A significant arterial hypoxemia also developed in galloping horses in the acute hypervolemia study [arterial oxygen tension at 30 and 120 s was 76.7 (1.7) and 71.9 (1.6) mmHg, respectively], but significant differences between treatments could not be demonstrated. In both treatments, a similar desaturation of arterial hemoglobin was also observed at 30 s of maximal exercise, which intensified with increasing exercise duration as hyperthermia, acidosis and hypercapnia intensified. Thus, acute expansion of plasma volume did not benefit arterial oxygenation in maximally exercising thoroughbred horses.

Effects of prolonged exercise to exhaustion on left-ventricular function and pulmonary gas exchange

The purpose of this study was to simultaneously examine left-ventricular (LV) function and pulmonary gas exchange during prolonged constant-rate cycling in an attempt to explain the exercise-induced impairment in gas exchange. Eleven competitive cyclists rode their racing bicycles on a computerized cycle trainer at 25 W below the lactate threshold until exhaustion (exercise time = 2.51 +/- 0.86 h). LV systolic function was evaluated with two-dimensional echocardiography while arterial blood gases were used to assess pulmonary gas exchange. All variables were assessed concurrently before, during, and after exercise. LV function and cardiac output increased at the onset of exercise and were maintained until exhaustion. The alveolar-arterial P(O(2)) difference (A-a D(O(2))) increased within 15 min of the onset of exercise, was unchanged through to exhaustion, and returned to baseline 5 min post-exercise. Gas exchange was not related to cardiovascular function at the onset, or at end exercise. The results indicate that the widening A-aD(O(2)) during exercise is due to a readily reversible change in gas exchange function.

Exercise-induced intrapulmonary arteriovenous shunting in healthy humans

We hypothesized that increasing exercise intensity recruits dormant arteriovenous intrapulmonary shunts, which may contribute to the widened alveolar-arterial oxygen difference seen with exercise. Twenty-three healthy volunteers (13 men and 10 women, aged 23-48 yr) with normal lung function and a wide range of fitness (mean maximal oxygen uptake = 126% predicted; range = 78-200% predicted) were studied by agitated saline contrast echocardiography (4-chamber apical view). All 23 subjects had normal resting contrast echocardiograms without evidence of intracardiac or intrapulmonary shunting. However, with cycle ergometer exercise, 21 of 23 (91%) of the subjects showed a delayed (>3 cardiac cycles) appearance of contrast bubbles in the left heart. This pattern is consistent with passage of contrast bubbles through the pulmonary circulation. Because the contrast bubbles are known to be significantly larger than pulmonary capillaries, we propose that they are traveling through direct arteriovenous intrapulmonary shunts. In all cases, the intrapulmonary shunting developed at submaximal oxygen uptakes [%maximal oxygen uptake = 59 +/- 20 (SD)] and once evident persisted at all subsequent work rates. Within 3 min of exercise termination, the contrast echocardiograms with bubble injection showed no evidence of intrapulmonary shunting. These dynamic shunts will contribute significantly to the widened alveolar-arterial oxygen difference seen with exercise. They may also act as a protective parallel vascular network limiting the rise in regional pulmonary vascular pressure while preserving cardiac output during exercise.

Evidence of exercise-induced arterial hypoxemia in prepubescent trained children

Exercise-induced arterial hypoxemia (EIAH) is a recognized phenomenon in highly trained adults. Like adult athletes, prepubescent trained children may develop high-level metabolic demand but with a limited lung capacity in comparison with adults. The purpose of this investigation was to search for evidence of EIAH in prepubescent trained children. Twenty-four prepubescent (age: 10.3 +/- 0.2 y) trained children (10.0 +/- 0.7 h of weekly physical activity) performed pulmonary function tests and a graded maximal exercise test on a cycle ergometer. EIAH was defined as a drop of at least 4% from resting level arterial oxygen saturation (Sao(2)) measured by pulse oximetry. EIAH was observed in seven children. Forced vital capacity (FVC), ventilatory response to exercise (Delta(E)/Deltaco(2)), and breathing reserve at maximal exercise were significantly lower, whereas tidal volume relative to FVC was higher in hypoxemic children than in nonhypoxemic children; weekly physical activity and maximal oxygen uptake were similar. Moreover, positive relationships were found between Sao(2) at maximal exercise and breathing reserve (r = 0.56; p < 0.05) or volume relative to FVC (r = 0.70; p < 0.01). EIAH may occur in prepubescent trained children with a relatively low maximal oxygen uptake (42 mL. min(-1). kg(-1)); however, the mechanisms remain unclear and need to be investigated more accurately.

NaHCO3 does not affect arterial O2 tension but attenuates desaturation of hemoglobin in maximally exercising Thoroughbreds

The objective of the present study was to examine the effects of preexercise NaHCO3 administration to induce metabolic alkalosis on the arterial oxygenation in racehorses performing maximal exercise. Two sets of experiments, intravenous physiological saline and NaHCO3 (250 mg/kg iv), were carried out on 13 healthy, sound Thoroughbred horses in random order, 7 days apart. Blood-gas variables were examined at rest and during incremental exercise, leading to 120 s of galloping at 14 m/s on a 3.5% uphill grade, which elicited maximal heart rate and induced pulmonary hemorrhage in all horses in both treatments. NaHCO3 administration caused alkalosis and hemodilution in standing horses, but arterial O2 tension and hemoglobin-O2 saturation were unaffected. Thus NaHCO3 administration caused a reduction in arterial O2 content at rest, although the arterial-to-mixed venous blood O2 content gradient was unaffected. During maximal exercise in both treatments, arterial hypoxemia, desaturation, hypercapnia, acidosis, hyperthermia, and hemoconcentration developed. Although the extent of exercise-induced arterial hypoxemia was similar, there was an attenuation of the desaturation of arterial hemoglobin in the NaHCO3-treated horses, which had higher arterial pH. Despite these observations, the arterial blood O2 content of exercising horses was less in the NaHCO3 experiments because of the hemodilution, and an attenuation of the exercise-induced expansion of the arterial-to-mixed venous blood O2 content gradient was observed. It was concluded that preexercise NaHCO3 administration does not affect the development and/or severity of arterial hypoxemia in Thoroughbreds performing short-term, high-intensity exercise.

[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.

Strenuous Exercise Increases Plasmatic and Urinary Leukotriene E4 in Cyclists

The purpose of this study was to investigate plasma and urinary levels of leukotriene (LT) and the changes in pulmonary function induced by strenuous exercise in highly trained cyclists (HT) with mild exercised-induced hypoxemia (EIH). Method: Nine HT and five untrained subjects (UT) performed a 30-min exercise at 78% of their [Formula: see text] Leukotriene E4 (LTE4) was assayed in plasma and urine. Pulmonary function tests and pulmonary diffusion capacity (DLCO) were examined before and after exercising. Ear arterialized blood gases were assessed at rest and during exercise. Results: The mean drop in partial oxygen pressure was 15 mmHg in HT during exercise; and the DLCO decreased by 7.5% following exercise. No significant changes were found in forced vital capacity or forced expiratory flows. LTE4 levels increased significantly in HT following exercise: urinary LTE4 was 42.9 ± 6.3 ng•mmol−1 creatinine at rest and 66.3 ± 11.9 ng•mmol−1 creatinine 2 hrs after exercise, and plasma LTE4 rose from 528 ± 91 pg•mL−1 at rest to 897 ± 123 pg•mL−1 after exercise. By contrast, urinary LTE4 level was unchanged in the UT group. Among the HT group, there was no significant correlation between urinary LTE4 changes and PO2, air flow rates, or DLCO changes. Conclusion: These results suggest that strenuous exercise induces an increase of LTE4 release in highly trained cyclists with mild EIH. These changes in LTE4 levels were not related to significant impairment of lung function. Key words: athletes, hypoxemia, pulmonary function, leukotrienes

Preexercise hypervolemia does not affect arterial hypoxemia in Thoroughbreds performing short-term high-intensity exercise

It is reported that preexercise hyperhydration caused arterial O(2) tension of horses performing submaximal exercise to decrease further by 15 Torr (Sosa-Leon L, Hodgson DR, Evans DL, Ray SP, Carlson GP, and Rose RJ. Equine Vet J Suppl 34: 425-429, 2002). Because hydration status is important to optimal athletic performance and thermoregulation during exercise, the present study examined whether preexercise induction of hypervolemia would similarly accentuate the arterial hypoxemia in Thoroughbreds performing short-term high-intensity exercise. Two sets of experiments (namely, control and hypervolemia studies) were carried out on seven healthy, exercise-trained Thoroughbred horses in random order, 7 days apart. In resting horses, an 18.0 +/- 1.8% increase in plasma volume was induced with NaCl (0.30-0.45 g/kg dissolved in 1,500 ml H(2)O) administered via a nasogastric tube, 285-290 min preexercise. Blood-gas and pH measurements as well as concentrations of plasma protein, hemoglobin, and blood lactate were determined at rest and during incremental exercise leading to maximal exertion (14 m/s on a 3.5% uphill grade) that induced pulmonary hemorrhage in all horses in both treatments. In both treatments, significant arterial hypoxemia, desaturation of hemoglobin, hypercapnia, acidosis, and hyperthermia developed during maximal exercise, but statistically significant differences between treatments were not found. Thus preexercise 18% expansion of plasma volume failed to significantly affect the development and/or severity of arterial hypoxemia in Thoroughbreds performing maximal exercise. Although blood lactate concentration and arterial pH were unaffected, hemodilution caused in this manner resulted in a significant (P < 0.01) attenuation of the exercise-induced expansion of the arterial-to-mixed venous blood O(2) content gradient.

Arterial hypoxemia in exercising thoroughbreds is not affected by pre-exercise nedocromil sodium inhalation

It has been reported that pulmonary injury (i.e. capillary stress failure) evoked histamine release from airway inflammatory/mast cells contributes to exercise-induced arterial hypoxemia (EIAH) and that pre-exercise inhalation of nedocromil sodium mitigated EIAH in human subjects 'Med. Sci. Sports Exercise 29, (1997) 10-16'. Because exercise-induced pulmonary hemorrhage due to capillary stress failure is routinely observed in racehorses, we examined whether nedocromil inhalation would similarly benefit EIAH and desaturation of hemoglobin in horses. Two sets of experiments, namely, placebo studies followed in 7 days by pre-exercise nedocromil sodium (30 puffs=60 mg) inhalation experiments were carried out on 7 healthy, sound, exercise-trained thoroughbred horses. In both treatments, arterial and mixed-venous blood-gas/pH measurements were made at rest pre- and post-placebo/drug inhalation, as well as during incremental exercise leading to galloping at 14 m/sec on a 3.5% uphill grade-a workload that elicited maximal heart rate and caused pulmonary hemorrhage in all horses in both treatments, thereby indicating capillary stress failure had occurred. In both treatments, significant (P<0.0001) EIAH of a similar magnitude had developed by 30 sec of maximal exertion, and further significant changes in arterial O(2) tension did not occur as exercise duration progressed to 120 sec. Thus, pre-exercise inhalation of nedocromil sodium was ineffective in modifying the development and/or severity of EIAH in the present study. These findings argue against the airway inflammatory mediator(s) release hypothesis for causing arterial hypoxemia in racehorses.

Anti-inflammatory agent, dexamethasone, does not affect exercise-induced arterial hypoxemia in Thoroughbreds

In view of the suggestion that pulmonary injury-induced release of histamine and/or other chemical mediators from airway inflammatory and mast cells contribute to the exercise-induced arterial hypoxemia (EIAH) in human athletes, we examined the effects of pretreatment with a potent anti-inflammatory agent, dexamethasone, on EIAH and desaturation of hemoglobin in horses. Seven healthy, sound, exercise-trained Thoroughbreds were studied in the control (no medications) experiments, followed in 7 days by intravenous dexamethasone (0.11 mg.kg(-1).day(-1) for 3 consecutive days) studies. Blood-gas measurements were made at rest and during incremental exercise leading to maximal exertion at 14 m/s on a 3.5% uphill grade. Galloping at this workload induced pulmonary hemorrhage in all horses in both treatments, thereby indicating that stress failure of pulmonary capillaries had occurred. In both treatments, significant EIAH, desaturation of hemoglobin, hypercapnia, acidosis, and hyperthermia developed during maximal exercise, but significant differences between the control and dexamethasone treatments were not discerned. The failure of pretreatment with dexamethasone to significantly affect EIAH suggests that pulmonary injury-evoked airway inflammatory response may not play a major role in EIAH in racehorses. However, our observations in both treatments that EIAH developed quickly (being evident at 30 s of exertion) and that its severity remained unaffected by increasing exercise duration (to 120 s) suggest that EIAH has a functional basis, probably related to significant shortening of the transit time for blood in the pulmonary capillaries as cardiac output increases dramatically.

Role of lung inflammatory mediators as a cause of exercise-induced arterial hypoxemia in young athletes

We examined whether lung inflammatory mediators are increased during exercise and whether pharmacological blockade can prevent exercise-induced arterial hypoxemia (EIAH) in young athletes. Seventeen healthy athletes (9 men, 8 women; age 23 +/- 3 yr) with varying degrees of EIAH completed maximal incremental treadmill exercise tests after administration of fexofenadine, zileuton, and nedocromil sodium or placebo in a randomized double-blind crossover study. Lung function, arterial blood gases, and inflammatory metabolites in plasma, urine, and induced sputum were assessed. Drug administration did not improve EIAH or gas exchange during exercise. At maximal exercise, oxygen saturation fell to 91.4 +/- 2.6% (drug trial) and 91.9 +/- 2.1% (placebo trial) and alveolar-arterial oxygen difference widened to 28.1 +/- 6.3 Torr (drug trial) and 29.3 +/- 5.7 Torr (placebo trial). Oxygen consumption, ventilation, and other exercise variables were similarly unaffected by drug treatment. Although plasma histamine increased with exercise, values did not differ between trials, and urinary leukotriene E(4) and 11beta-prostaglandin F(2alpha) levels were unchanged after exercise. Postexercise sputum revealed no significant changes in markers of inflammation. These results demonstrate that EIAH in young athletes is not attenuated with acute administration of drugs targeting histamine and bioactive lipids. We conclude that airway inflammation is of insufficient magnitude to cause impairments in gas exchange and does not appear to be linked to EIAH in healthy young athletes.

Early onset of pulmonary gas exchange disturbance during progressive exercise in healthy active men

Some recent studies of competitive athletes have shown exercise-induced hypoxemia to begin in submaximal exercise. We examined the role of ventilatory factors in the submaximal exercise gas exchange disturbance (GED) of healthy men involved in regular work-related exercise but not in competitive activities. From the 38 national mountain rescue workers evaluated (36 +/- 1 yr), 14 were classified as GED and were compared with 14 subjects matched for age, height, weight, and maximal oxygen uptake (VO2 max; 3.61 +/- 0.12 l/min) and showing a normal response (N). Mean arterial PO2 was already lower than N (P = 0.05) at 40% VO2 max and continued to fall until VO2 max (GED: 80.2 +/- 1.6 vs. N: 91.7 +/- 1.3 Torr). A parallel upward shift in the alveolar-arterial oxygen difference vs. %VO2 max relationship was observed in GED compared with N from the onset throughout the incremental protocol. At submaximal intensities, ideal alveolar PO2, tidal volume, respiratory frequency, and dead space-to-tidal volume ratio were identical between groups. As per the higher arterial PCO2 of GED at VO2 max, subjects with an exaggerated submaximal alveolar-arterial oxygen difference also showed a relative maximal hypoventilation. Results thus suggest the existence of a common denominator that contributes to the GED of submaximal exercise and affects the maximal ventilatory response.

H1-receptor antagonist, tripelennamine, does not affect arterial hypoxemia in exercising Thoroughbreds

It has been suggested that pulmonary injury and inflammation-induced histamine release from airway mast cells may contribute to exercise-induced arterial hypoxemia (EIAH). Because stress failure of pulmonary capillaries and EIAH are routinely observed in exercising horses, we examined whether preexercise administration of an H1-receptor antagonist may mitigate EIAH. Two sets of experiments, placebo (saline) and antihistaminic (tripelennamine HCl at 1.10 mg/kg iv, 15 min preexercise) studies, were carried out on seven healthy, exercise-trained Thoroughbred horses in random order 7 days apart. Arterial and mixed venous blood-gas and pH measurements were made at rest before and after saline or drug administration and during incremental exercise leading to maximal exertion at 14 m/s on 3.5% uphill grade for 120 s. Galloping at this workload elicited maximal heart rate and induced exercise-induced pulmonary hemorrhage in all horses in both treatments, thereby indicating that capillary stress failure-related pulmonary injury had occurred. In both treatments, EIAH, desaturation of hemoglobin, hypercapnia, and acidosis of a similar magnitude developed during maximal exertion, and statistically significant differences between the placebo and antihistaminic studies could not be demonstrated. The failure of the H1-receptor antagonist to modify EIAH significantly suggests that pulmonary injury-induced histamine release may not play a major role in bringing about EIAH in Thoroughbred horses.