Abstract
High-altitude pulmonary edema (HAPE) is a life-threatening condition occurring in predisposed subjects at altitudes above 2,500 m. It is not clear whether, in addition to hemodynamic factors and defective alveolar fluid clearance, inflammation plays a pathogenic role in HAPE. We therefore made serial measurements of exhaled pulmonary nitric oxide (NO), a marker of airway inflammation, in 28 HAPE-prone and 24 control subjects during high-altitude exposure (4,559 m). To examine the relationship between pulmonary NO synthesis and pulmonary vascular tone, we also measured systolic pulmonary artery pressure (Ppa). In the 13 subjects who developed HAPE, exhaled NO did not show any tendency to increase during the development of lung edema. Throughout the entire sojourn at high altitude, pulmonary exhaled NO was roughly 30% lower in HAPE-prone than in control subjects, and there existed an inverse relationship between Ppa and exhaled NO (r = − 0.51, p < 0.001). These findings suggest that HAPE is not preceded by airway inflammation. Reduced exhaled NO may be related to altered pulmonary NO synthesis and/or transport and clearance, and the data in our study could be consistent with the novel concept that in HAPE-prone subjects, a defect in pulmonary epithelial NO synthesis may contribute to exaggerated hypoxic pulmonary vasoconstriction and in turn to pulmonary edema.
High-altitude pulmonary edema (HAPE) is a life-threatening condition occurring in predisposed subjects at altitudes above 2,500 m (1-3). Although exaggerated hypoxic pulmonary vasoconstriction, possibly related to endothelial dysfunction and sympathetic overactivation (4-6), and a defect in transepithelial sodium and water transport (7), are thought to play important roles in HAPE, they may not be sufficient to trigger HAPE (8), suggesting that additional mechanisms play a role in this condition. On the basis of the presence of markers of inflammation in the bronchoalveolar lavage fluid (BALF) and urine of patients with HAPE, it has been postulated that inflammation may represent one of these additional mechanisms (9-11). Such findings, however, do not allow determination of whether inflammation is a primary causal event in HAPE or is merely the consequence of fluid accumulation in the lung, since the measurements of inflammatory markers were made after the onset of HAPE.
Nitric oxide (NO) is present in the exhaled air of many animal species and of humans (12), and represents a marker of inflammation in the lung (13). Pulmonary exhaled NO is augmented during the pulmonary inflammation associated with acute rejection after lung transplantation, involving vessels, airways, and interstitium (14). Exhaled NO is also augmented in inflammatory respiratory diseases such as bronchiectasis (15), lower respiratory tract infection (16, 17), and asthma (16, 18, 19). Furthermore, antiinflammatory treatment with systemic or topical corticosteroids (but not β2-agonists [20, 21]) decreases exhaled NO in asthma (22, 23).
We therefore studied the effects of exposure to a high altitude (4,559 m) on pulmonary exhaled NO in mountaineers susceptible to HAPE and those resistant to this condition. To gain insight into the relationship between pulmonary NO production and the regulation of pulmonary vascular tone, we also measured systolic pulmonary artery pressure (Ppa).
We studied 28 mountaineers (10 women and 18 men), aged 40.3 ± 10.9 yr (mean ± SD), who had had HAPE documented radiographically within the previous 4 yr. Another 24 mountaineers (nine women and 15 men), aged 38.3 ± 14.4 yr and with a history of repeated alpine-style climbing to peaks above 4,000 m and no symptoms of HAPE or acute mountain sickness, served as controls. After a baseline examination at low altitude (580 m; barometric pressure: 710 mm Hg [95,960 Pa]), the subjects ascended in groups of two to four members each from an altitude of 1,130 m to 4,559 m (barometric pressure: 440 mm Hg [59,310 Pa]) within a period of 22 h. The ascent consisted of transport by cable car to an altitude of 3,200 m; a 1.5-h climb to an altitude of 3,611 m, where the subjects stayed overnight; and, on the next day, a 4.5-h climb to a high-altitude research laboratory at Capanna Regina Margherita in the Italian Alps. The subjects then spent 2 d and two nights at this hut. The experimental protocol was approved by the review board on human investigation of our institution, and all subjects provided written informed consent.
At 12, 24, 36, and 48 h after the subjects' arrival at the high-altitude research laboratory, the NO content of their exhaled breath was measured with the subjects in the sitting position, using a chemiluminescence analyzer (Model 280 NOA; Sievers, Boulder, CO). This device is not affected by atmospheric pressure or oxygen content, since bottled oxygen is delivered to the NO analyzer at a constant pressure (6.2 psi) and the chemiluminescence cell is maintained at a constant pressure (16 mbar) through a vacuum pump. The analyzer was calibrated daily with a calibration gas containing 45 ppm NO in pure nitrogen, and the zero level was checked with an NO-free gas. For the measurement of exhaled NO, the subjects inspired to TLC and, without breathholding, exhaled into a sampling mouthpiece containing a needle resistor. During the exhalation, the subjects were given a visual display of the pressure output and were asked to maintain a constant positive airway pressure of 20 mm Hg. This procedure results in closure of the soft palate and exclusion of the nasal cavity from the airway, thereby preventing admixture of NO generated in the upper airways (14), with the result that the measured exhaled NO originates from the alveoli and lower airways (14, 24-26). The generated flows were 70 ml/s and 90 ml/s at low- and high-altitude, respectively. With this method, the exhaled NO profile shows a characteristic initial rise (corresponding to washout of the dead space) and then reaches a steady-state plateau. The NO concentration was measured during the last 7 s of the end-expiratory plateau, and was corrected for flow, expressed in pmol/s, through use of the Boyle-Mariott equation. The reported values are the mean of three determinations that varied from one another by less than 10%. The ambient NO concentration during the measurements was < 20 ppb. We also measured exhaled NO at low altitude in a subgroup of 13 HAPE-prone and nine control subjects.
To measure systolic Ppa, we obtained echocardiographic recordings from 26 HAPE-prone and 16 control subjects with a real-time, phased-array sector scanner (Model 2500; Hewlett-Packard, Andover, MA) having an integrated color Doppler system and a transducer containing crystal sets for imaging (2.5 MHz) and for continuous-wave Doppler recording (1.9 MHz). The recordings were stored on VHS videotape for analysis by an investigator who was unaware of the subject's clinical history. All reported values represent the mean of at least three measurements. Systolic Ppa was calculated from the pressure gradient between the right ventricle and right atrium, as recorded with continuous-wave Doppler echocardiography, and the clinically determined mean jugular venous pressure. Color Doppler echocardiography was used to locate the tricuspid regurgitation jet. The maximal tricuspid flow velocity was then determined by careful application of the continuous-wave sampler to the regurgitation jet. To calculate the transtricuspid pressure gradient, a modified Bernoulli equation was used, in which transtricuspid pressure equals four times the square of the tricuspid-jet velocity (4). At the high-altitude laboratory, systolic Ppa measurements obtained with echocardiography were found to be closely correlated with those obtained by pulmonary artery catheterization (8).
On the morning before descent, posteroanterior chest radiographs of the subjects were obtained with a mobile unit (TRS; Siemens, Stockholm, Sweden) with a fixed target-to-film distance of 140 cm, at 133 kV and 4 to 6 mA/s. Additional radiographs were obtained at the time that their symptoms first appeared from subjects in whom clinical evidence of HAPE developed. The radiographs were analyzed according to previously described criteria, by a radiologist who was unaware of the subject's clinical history (4).
Statistical analysis (JMP statistical software; SAS Institute, Cary, NC) was done with analysis of variance for between-group comparisons and with the two-tailed t test for single comparisons. Relations between variables were analyzed by calculating Pearson's product- moment correlation coefficient, r. A value of p < 0.05 was considered statistically significant. Unless otherwise indicated, data are expressed as mean ± SE.
At low altitude, the exhaled NO concentration was comparable in the two study groups, with values of 59 ± 11 pmol/s and 51 ± 8 pmol/s in the control and HAPE-susceptible subjects, respectively. After 24 to 48 h at 4,559 m, 13 of the 28 HAPE-prone subjects and one of the control subjects had radiographic evidence of pulmonary edema (radiographic score: 6 to 14; 9.3 ± 2.6 [mean ± SD]). Figure 1 shows that throughout the sojourn at high altitude, exhaled pulmonary NO was significantly lower (p < 0.001) in HAPE-susceptible than in control subjects. At 12, 24, 36, and 48 h after arrival of the subjects at high altitude, the values for exhaled NO in HAPE-susceptible subjects were 37 ± 4, 42 ± 4, 42 ± 4, and 44 ± 4 pmol/s, respectively, whereas those for control subjects were 47 ± 5, 51 ± 5, 61 ± 6, and 61 ± 6 pmol/s, respectively. Moreover, as shown in Figure 1, exhaled NO showed no tendency to increase at any moment in time in the subjects who developed HAPE.
Fig. 1. Line graphs showing effects of high-altitude exposure (4,559 m) on exhaled pulmonary NO (mean ± SE) in 13 HAPE-susceptible subjects who developed pulmonary edema (open squares), 15 HAPE-susceptible subjects who did not develop pulmonary edema (closed squares), and 24 control subjects (open circles). Throughout the sojourn at high altitude, exhaled pulmonary NO was significantly lower (p < 0.001) in HAPE-susceptible than in control subjects. In the HAPE-susceptible subjects who developed pulmonary edema, exhaled NO was lower than in those without edema (p < 0.01), and did not show any tendency to increase before the development of edema.
[More] [Minimize]As expected, systolic Ppa was higher in subjects prone to HAPE than in those resistant to this condition (62.4 ± 2.5 mm Hg versus 47.7 ± 2.2 mm Hg, p < 0.0001). Figure 2 shows that an inverse relationship existed between the amount of exhaled pulmonary NO and systolic Ppa (r = −0.51, p < 0.001). Moreover, HAPE-prone subjects who developed pulmonary edema had a higher pulmonary artery pressure (69.5 ± 3.4 mm Hg versus 57.6 ± 3.0 mm Hg, p = 0.02) and lower exhaled pulmonary NO concentration than did those without pulmonary edema (p < 0.01; Figure 1).
Fig. 2. Plot of relationship between exhaled pulmonary NO and systolic Ppa in 26 HAPE-susceptible (closed squares) and 16 control subjects (open circles). r = −0.51, p < 0.001.
[More] [Minimize]To examine the possibility of pulmonary inflammation preceding the development of HAPE we performed serial measurements of exhaled pulmonary NO in a group of subjects susceptible to this condition and in control subjects. We found that in subjects who subsequently developed pulmonary edema, the concentration of exhaled NO did not show any tendency to increase before the development of edema. These findings represent the first sequential monitoring of exhaled NO as a marker of pulmonary inflammation at high-altitude, and suggest that HAPE is not preceded by airway inflammation.
There is abundant evidence that exhaled NO represents a reliable marker of pulmonary inflammation (14, 27-29) in a wide variety of respiratory diseases (14, 16, 18, 19). In asthmatic patients, inducible nitric oxide synthase (iNOS) is upregulated in macrophages and in the respiratory epithelium, and correlates with exhaled NO (27). In patients with inflammatory lung disease, increased concentrations of exhaled NO were directly correlated with markers of inflammation and iNOS upregulation in macrophages in BALF (28, 29). The sequential measurements of exhaled NO, in the present study provide no evidence for airway inflammation during the development of HAPE. The lack of an increase in exhaled NO is not related to altered alveolar clearance of NO as a result of pulmonary edema, since in the subjects who developed HAPE, the first measurement of exhaled NO, and in all but four cases the second measurement as well, was obtained before the development of lung edema. Our findings suggest that the inflammatory markers found in the BALF and urine of patients with HAPE since at least 24 h (9-11) are a consequence rather than the cause of pulmonary edema. This interpretation is consistent with data from a study in which gallium-labeled pulmonary transferrin escape was used as a marker of inflammation (30). Taken together, these data suggest that augmented alveolar fluid flooding, related to exaggerated pulmonary vasoconstriction, with the possible conjunction of a defect in alveolar fluid clearance, are the major pathogenic factors in HAPE (7, 31).
In this regard, observation in the present study of roughly 30% lower values of exhaled pulmonary NO in HAPE-susceptible subjects could be important. Exhaled pulmonary NO does not represent a marker of vascular endothelial function in humans (32) because it is mainly of respiratory epithelial origin. It is possible, however, that the NO synthesized in the respiratory tract diffuses into the pulmonary vasculature, where it may exert hemodynamic effects, as evidenced by lower pulmonary vascular resistance during nasal than during oral breathing in humans (33), and by augmented hypoxic pulmonary vasoconstriction during selective inhibition of respiratory epithelial NO synthesis in the isolated perfused rabbit lung (34). The observation at high-altitude of an inverse relationship between the exhaled pulmonary NO concentration and systolic Ppa in the present study could be consistent with this concept. Reduced exhaled NO also has been reported in other forms of pulmonary hypertension (35, 36). Alternatively, in the present and earlier studies, differences between groups of subjects in exhaled NO may have reflected altered NO transport or clearance, rather than differences in NO production. We speculate that a defect in pulmonary NO production could be one of the factors contributing to the exaggerated hypoxia-induced pulmonary hypertension in HAPE-prone subjects.
The authors are indebted to the Sezione Varallo del Club Alpino Italiano for providing the locations in the Capanna Regina Margherita; to Franziska Keller for invaluable help with the studies reported here and for taking the chest radiographs at high altitude; to Drs. Ernst Lipp and Damian Hutter for measurements of pulmonary artery pressure at high altitude; to Dr. Marco Maggiorini for allowing them to study patients under his care; to their mountain guides, Andrea Enzio and Bruno Brand; to the Hewlett-Packard Corporation for providing the echocardiographic equipment used in the study; and to the Swiss Army for providing the radiographic equipment used in the study and for transporting part of the study material.
Supported by grants from the Swiss National Science Foundation, the International Olympic Committee, the Hamasil Foundation, and the Placide Nicod Foundation.
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Presented in part at the 1999 International Conference of the American Thoracic Society, San Diego, California, April 23–28, 1999.
