INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

SOME HIGHLY TRAINED MALE ENDURANCE athletes experience decreases in arterial PO₂ (Pa_O2) and arterial oxyhemoglobin saturation (Sa_O2) and a widened alveolar-arterial difference for O₂ (A-aDO₂) during heavy exercise (5, 13). The cause and significance of exercise-induced arterial hypoxemia (EIH) has been the topic of a considerable research effort; however, the mechanism(s) responsible remains controversial. Four potential factors have been identified: 1) venoarterial shunt, 2) relative alveolar hypoventilation, 3) ventilation-perfusion (A/) inequality, and 4) diffusion limitation. Venoarterial shunt has been identified as a minor contributor, and relative alveolar hypoventilation has a controversial role in the pathophysiology of EIH (6). A/ relationships have been shown to worsen with exercise (10), and, during maximal exercise, ~60% of the widened A-aDO₂ can be explained by A/ mismatch (14). Pulmonary interstitial edema may explain both A/ inequality and diffusion limitations (11, 26). This would be expected to negatively influence gas exchange in the lung by lowering the compliance of the alveoli and by compressing small blood vessels, resulting in nonuniform airflow and blood flow distribution in the lungs (11). However, present techniques have failed to provide precise quantification of extravascular lung water or identification of the specific mechanisms responsible for the development of interstitial edema (6).

Inhaled nitric oxide (NO) is a potent and selective pulmonary vasodilator (8, 9). Inhalation of NO has been used effectively to treat several respiratory disorders in humans (1, 15, 16) but has been shown to be detrimental in chronic obstructive pulmonary disease (1). The positive effects of NO are explained by a preferential distribution of inhaled NO to well-ventilated alveolar units, a reduction in the dispersion of ventilation distribution, lowering of pulmonary vascular pressures, and improved gas exchange. Inhalation of NO has also been utilized to exert a beneficial effect on arterial oxygenation in mountaineers with high-altitude pulmonary edema (27). Although the mechanisms of EIH remain debatable, there is some indirect evidence to support the development of transient interstitial edema as a mechanism for a widened A-aDO₂ (3, 23). We sought to test the hypothesis that inhaled NO would improve oxygenation during exercise in athletes with EIH by reducing A-aDO₂.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Subjects. Highly trained male cyclists were recruited to participate in this study (n = 8). One subject was forced to withdraw due to difficulties with placement of the arterial catheter; therefore, all data are reported for n = 7. This investigation was divided into two parts. Subjects who met the inclusion criteria in part 1 participated in part 2. Inclusion criteria were 1) normal spirometry, that is, no history of asthma or cardiorespiratory disease, 2) maximal O₂ consumption (O_2 max) 60 ml · kg¹ · min¹ and/or 5 l/min, 3) development of EIH (maximal exercise Sa_O2 91.0%), and 4) between the ages of 18 and 40 yr. Before testing was started, subjects received a verbal description of the experiment and completed a written, informed consent form. This study was approved by the Clinical Screening Committee for Research and Other Studies Involving Human Subjects of the University of British Columbia.

Preliminary screening: part 1. Subjects reported to the Applied Physiology Laboratory in the Allan McGavin Sports Medicine Center (Univ. British Columbia), having refrained from exhaustive exercise for 24 h, abstained from ingestion of food or fluid for 4 h, except for water, and abstained from alcohol and caffeine for 12 h. Subjects' weight and height were measured and recorded. Both spirometry and pulmonary diffusion measurements for carbon monoxide (DL_CO) were collected using the same commercial apparatus (Collins DS/PLUS II, Braintree, MA). DL_CO was determined using the single-breath method. Before DL_CO and spirometry measurements were made, subjects sat and rested for 30 min to ensure a resting heart rate and pulmonary capillary blood volume.

Inhaled NO: part 2. After completion of both O_2 max tests (normoxic and hypoxic), subjects who met the inclusion criteria returned on a separate day at least 72 h later. A time line describing the experimental protocol is shown in Fig. 1. Subjects were randomly assigned and blinded to each of the four following conditions: 1) normoxia (N), 2) normoxia + NO (N/NO), 3) hypoxia (H), and 4) hypoxia + NO (H/NO). Participants performed a 10- to 15-min cycling warm-up at a self-selected workload and then sat quietly on the cycle ergometer for 5 min, at which point resting data were obtained. Cycling intensity was then manually increased over 1 min to 100% of their respective maximum normoxic or hypoxic workload as determined in part 1. Subjects cycled at this intensity for 5 min, and cardiorespiratory variables were recorded at each minute in the same fashion as for part 1. Arterial blood samples were drawn at rest and at each min of the 5-min test. After each test condition, subjects cycled easily (30-50 W) for 10 min and then rested for 50 min before commencing the next test condition. A physician was in attendance at all times and was responsible for the safety of the subjects during the study.

View larger version (10K): [in this window] [in a new window]   Fig. 1.   Overview of experimental protocol, repeated for each gas mixture condition of normoxia (N), normoxia + 20 parts per million (ppm) nitric oxide (N/NO), hypoxia (H), and hypoxia + 20 ppm nitric oxide (H/NO). S, sample for blood and cardiorespiratory variables taken after breathing test gas mixture at rest for 5 min and during each minute of 5-min maximal cycling; O2 max, maximal O2 consumption.

Arterial blood sampling. A 20-gauge arterial catheter was inserted in the radial artery of the nondominant hand by percutaneous cannulation using 1% local anesthesia (lidocaine) and sterile technique and was then secured to the skin. Adequate collateral circulation via the ulnar artery (Allen's test) was estimated before the cannula was inserted. A minimum volume extension tube, connected in series with two three-way stopcocks arranged at right angles, was flushed with a saline-heparin solution. A rapid response (<0.01 s) thermistor (18T, Physitemp Instruments, Clifton, NJ) used to measure peak arterial blood temperature was inserted through a Touhy-Borsch heparin lock (Abbott Hospitals, North Chicago, IL). Catheter patency was maintained with a continuous heparin infusion (1 ml 1:1,000 units in 500 ml normal saline at 3 ml/h). At the onset of sampling, 12 ml of blood was withdrawn, and the final 3 ml was collected in preheparinized plastic syringes. The remaining 9 ml were then slowly reinfused. Samples were withdrawn at rest and at 1-min intervals for the duration of each test (4 conditions × 6 samples per condition = 24 samples/subject). Blood samples were placed on ice until analyzed for H⁺ concentration, PO₂, PCO₂, base excess, and HCO₃ (CIBA-Corning 278 blood-gas system, CIBA-Corning Diagnostics, Medfield, MA). Pa_O2 was corrected for temperature and H⁺ concentration. Temperature increased 0.9 ± 0.1°C (mean ± SD) from rest to 5 min of exercise across all trials. Sa_O2 levels were calculated based on corrected Pa_O2. The alveolar gas equation was used to calculated alveolar PO₂ and A-aDO₂ (20).

Statistical analyses. Mean values and measures of variability were determined for descriptive, anthropometric, and lung function variables obtained during preliminary screening. Maximal cycle ergometry data from part 1 were compared using t-tests for dependent samples (normoxia vs. hypoxia). Experimental data were analyzed using a four (condition) by six (time) two-way factorial ANOVA with repeated measures on both factors. When sphericity was not assumed, Greenhouse-Geisser P values were utilized. When significant F ratios were observed, Scheffé's test was applied post hoc to determine where the differences occurred. The level of significance was set at P < 0.05. Statistical power calculations were performed a priori to estimate an appropriate minimum sample size of five. A sample size of seven was utilized to ensure sufficient statistical power (1   = 0.8).

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Physical and maximal exercise data. Descriptive data and resting pulmonary function data are presented in Table 1. Lung parameters were within normal values predicted for men of similar age, height, and weight except for DL_CO, which was significantly elevated. Data from normoxic and hypoxic maximal cycle ergometry tests are presented in Table 2. Significant differences were observed between normoxic and hypoxic conditions for O_2 max, RER, maximal heart rate, and power output, whereas no significant differences were detected for E. From rest to maximal exercise, mean values for Sa_O2 dropped significantly under both normoxic (97.7 to 90.2) and hypoxic (97.0 to 75.5) conditions.

Metabolic and power output during 5-min cycling. Metabolic and power output data are shown in Tables 3 and 4. No significant differences were detected between N and N/NO or between H and H/NO for O₂, E, RER, heart rate, or power output.

Arterial blood variables during 5-min cycling. All data are reported in Tables 5 and 6 and Figs. 2 and 3. Across all time points, there were no significant differences for Pa_O2 between N and N/NO or between H and H/NO (see Fig. 2). Both hypoxic conditions were significantly lower than both normoxic conditions at all measurement periods. Pa_O2 values were significantly lower at 1, 2, 3, 4, and 5 min of exercise compared with rest for all inspired gas conditions. Similar results were observed for Sa_O2, except that values at 1 and 2 min were not significantly different from rest under conditions of N and N/NO (see Fig. 3). A-aDO₂ was significantly different at all time periods compared with rest for all inspired gas mixtures, and significant differences were detected between N/NO and H at rest and during all exercise measurements. Arterial PCO₂ (Pa_CO2) was not significantly different between gas conditions but was lower compared with rest throughout all exercise for H and H/NO and at minutes 3, 4, and 5 for both N and N/NO. No statistically significant differences were detected between N vs. N/NO or between H vs. H/NO for pH, HCO₃, or base excess (Tables 5 and 6).

View larger version (12K): [in this window] [in a new window]   Fig. 2.   Arterial PO2 (PaO2) during rest and at each minute of 5-min cycle ergometry tests under condition N (), N/NO (), H (), and H/NO (). Values are means ± SD. *N and N/NO are significantly different from resting condition (rest) (P < 0.05). H and H/NO significantly different from rest (P < 0.05).

View larger version (12K): [in this window] [in a new window]   Fig. 3.   Percent arterial oxyhemoglobin saturation (SaO2) during rest and at each minute of 5-min cycle ergometry tests under condition N (), N/NO (), H (), and H/NO (). Values are means ± SD. *N and N/NO are significantly different from rest (P < 0.05). H and H/NO significantly different from rest (P < 0.05).

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The present study is the first to systematically examine the effects of inhaled NO during normoxic and hypoxic exercise in athletes with EIH. Unique to this investigation was the observation that inhalation of 20 ppm NO during normoxic and hypoxic high-intensity, short-duration exercise did not significantly affect gas exchange, O₂, or cycling power in highly trained athletes with EIH.

The mechanism(s) of EIH remains controversial. However, A/ inequality and diffusion limitations may be causative (6, 11, 30). The mechanisms explaining the increases in A/ inequality and diffusion limitation are unknown but may be related to the development of transient pulmonary edema (6). Some authors have suggested that, in elite athletes, the integrity of the pulmonary blood-gas barrier is altered at extreme levels of exercise (29). However, identification of pulmonary edema and pulmonary stress failure has been difficult to achieve in exercising humans and remains controversial. Although the specific mechanism for A/ inequality and diffusion limitations is debatable, the end result is a widened A-aDO₂. Inhaled NO, a selective pulmonary vasodilator (8, 9), is used in the treatment of diseases characterized by pulmonary hypertension and hypoxemia (1, 15, 16). The rationale is based on the fact that NO, given by inhalation, only dilates those pulmonary vessels that are well ventilated. As a result, pulmonary gas exchange is improved. We hypothesized that inhaled NO would improved gas exchange in athletes with EIH.

Inhaled NO during normoxia. The present investigation demonstrated no differences in gas exchange, O₂, power, or heart rate between N and N/NO at rest or during exercise. These findings do not support our original hypothesis but do agree with observations in resting sheep (17) and the fact that Pa_O2 was not altered in normal individuals who inhaled NO at rest (8). In addition, our findings are consistent with data that reported no effects of inhaled NO on or DL_CO in humans (2) or Pa_O2 and A/ in dogs (12) under normoxic conditions.

Inhaled NO during hypoxia. Unique to this investigation was the delivery of NO to individuals with EIH during hypoxic exercise. As expected, during H and H/NO, Pa_O2 and Sa_O2 were significantly lower than N and N/NO at all time points (Figs. 2 and 3). The drop in oxygenation during hypoxic exercise was consistent with that observed in previous EIH studies (22). Similar to the normoxic trials, inhaled NO did not alter gas exchange, O₂, or cycling power during hypoxia. Pison et al. (21) showed that addition of 20 ppm NO to a hypoxic (FI_O2 = 12%) gas mixture returned pulmonary gas-exchange measures to baseline values in mechanically ventilated sheep. These are in contrast to our results in which no statistical difference was observed for gas-exchange variables between H and H/NO. This was an unexpected result; we expected that HPV would be induced by using a hypoxic inspiratory gas mixture at rest and during exercise.

Limitations of the study. A debatable point is how much of the inhaled NO actually reached the lower airways? Although it was not possible to measure the amount of inhaled NO that reaches the alveoli, we assume that subjects in the present study did indeed inhale 20 ppm NO. We are confident that NO was delivered to the respiratory system, as it was measured 5 cm from the point of inspiration. The experimental approach and NO concentration employed in our study were similar to those employed by others (7). It is possible that the concentration of NO used in this study was not sufficient to induce vasodilation; however, this seems unlikely given that similar concentrations have been used previously to show significant alterations in gas exchange and pulmonary pressures (19, 21, 25). Our choice of 20 ppm was based on the above-mentioned studies and other clinical investigations.