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The reduced pressure in aircraft cabins may cause severe hypoxemia and respiratory distress in patients with chronic obstructive pulmonary disease (COPD). The primary objective of this study was to determine the prevalence of in-flight symptoms in COPD patients and non-COPD subjects, and evaluate associations between these symptoms and pre-flight variables.
Methods
In a cross-sectional study of 391 COPD patients and 184 non-COPD subjects, we recorded lung function, blood gas values, exercise capacity, air travel habits and in-flight symptoms.
Results
Fifty-four percent of the COPD patients had travelled by air the last two years. Hypoxia-related symptoms during air travel were experienced in 25% of the COPD patients and 9% of the non-COPD subjects (p < 0.001). After adjusting for smoking status, age and gender, the odds ratio for COPD patients to experience dyspnea or air hunger was 6.6 (95% CI 2.5–17.3, p < 0.001) compared to non-COPD subjects. In the COPD patients, in-flight dyspnea or air hunger was strongly associated with pre-flight score on the Medical Research Council (MRC) Dyspnea scale (p < 0.001).
Conclusion
COPD patients had significantly increased risk of in-flight dyspnea or air hunger compared to non-COPD subjects. In COPD patients these symptoms were strongly associated with pre-flight MRC Dyspnea score.
At maximal cruising altitude, the cabin pressure is allowed to decrease to the equivalent of 2438 m altitude. This may cause a significant decrease in arterial oxygen tension (PaO2) in patients with respiratory disease, such as chronic obstructive pulmonary disease (COPD).
These studies lack comparison with healthy subjects. Moreover, it is not known whether the occurrence of in-flight symptoms can be predicted on the basis of pre-flight examination of the patient.
Here we report a study on unselected, well characterized COPD patients and a group of subjects without COPD. The primary objectives were to determine the prevalence and kind of symptoms during air travel in COPD patients and in a community sample, and to assess whether sea-level values of lung function, arterial blood gases, exercise dyspnea, walking distance or desaturation during exercise were related to in-flight symptoms. Secondary aim was to determine air travel habits.
Methods
Study design
The present cross-sectional survey included 433 COPD patients and 233 subjects without COPD from the Bergen COPD Cohort Study (BCCS). The patients were recruited through outpatient clinics from several hospitals in Western Norway, and from three private specialist practices in Bergen (Norway).
All COPD patients had a smoking history of at least 10 pack-years, post-bronchodilator FEV1/FVC<0.7, and FEV1<80% predicted. The BCCS baseline visit in 2006 included clinical examination, Medical Research Council (MRC) Dyspnea scale
At the one-year follow-up visit, we collected questionnaire data on air travel habits and symptoms experienced during air travel within the previous two years. Of the eligible subjects, 575 (86%) completed the questionnaire and were included in the further analyses.
Written informed consent was obtained from all participants. The study was approved by The Regional Committee for Medical Research Ethics.
Questionnaire
The questionnaire included questions on air travel habits, reasons for not flying (if applicable), number and duration of flights within the last two years, pre-flight physician consultation, and in-flight symptoms. Unscheduled use of in-flight oxygen and healthcare within 48 h post-flight were registered. Symptoms were classified as hypoxia related (dyspnea, dizziness, headache, chest pain, air hunger, cough, fainting, palpitations) or hypoxia unrelated (ear pressure, sinus pressure, swollen legs). Wording of the questionnaire and alternatives for answering are given in the Online supplement.
Pulmonary function testing, blood gas measurement and functional walking test
Methods for spirometry and arterial blood gas measurements were performed as previously described.
Diffusing capacity of the lung (DL,CO) and total lung volumes were measured according to standardised criteria (SensorMedics Vmax Encore, VIASYS Healthcare Respiratory Technologies, Yorba Linda, USA).
Lung volumes and forced ventilatory flows. Report Working Party Standardization of Lung Function Tests, European Community for Steel and Coal. Official Statement of the European Respiratory Society.
Data for DL,CO, blood gases, MRC Dyspnea scale, and 6MWT were missing in 123 (21%), 40 (7%), 33 (6%), 37 (9%) subjects, respectively.
Statistical analysis
Chi-squared tests, two sample t-tests and Mann Whitney tests were applied as appropriate. For identifying factors associated with prevalence of symptoms, a logistic regression model was used. Adjustment was made for smoking, age, and gender. A significance level of 5% was considered as statistically significant. The analysis was performed with SPSS version 16 (SPSS Inc., Chicago, IL, USA).
Results
Population characteristics, entire study group
Of the 575 subjects who completed the flight outcome questionnaire, there were 391 COPD patients and 184 non-COPD subjects (Fig. 1). The COPD group included significantly more men, and had a higher mean age (Table 1). According to the GOLD classification, 189 (48%), 141 (36%), and 61 (16%) of the COPD patients were in GOLD stages II, III, and IV, respectively.
Data are presented as n (%), mean (SD). FEV1%predicted: post-bronchodilator forced expiratory volume in 1 s in percent of predicted; FVC: forced vital capacity; DL,CO: diffusing capacity of the lung for carbon monoxide; TLC: total lung capacity; RV: residual volume; Pa,O2: arterial partial pressure of oxygen; Pa,CO2: arterial partial pressure of carbon dioxide; Sp,O2: arterial oxygen saturation by pulse oximetry.
COPD patients who did not fly the previous two years
Forty-six percent of the COPD patients did not travel by air, compared to 13.6% of those without COPD (p < 0.001). The COPD patients who did not fly were older, had more reduced lung function, lower PaO2, a more pronounced exercise desaturation and a shorter 6-min walking distance than those who flew (Table 2).
Table 2Comparison of COPD patients who flew and did not fly and subjects without COPD who flew.
COPD patients n = 391
Subjects without COPD n = 184
Flew n = 211
Did not fly n = 180
P1)
Flew n = 159
P2)
Sex, M/F
124/87
113/67
0.419
80/79
0.106
Age, yrs
61.9 (6.7)
63.9 (6.7)
<0.01
53.6 (8.6)
<0.001
FEV1,% of predicted
51.6 (12.6)
45.7 (15.2)
<0.01
103.0 (9.2)
<0.001
DL,CO, % of predicted
61.4 (17.9)
54.7 (18.6)
0.001
84.3 (11.0)
<0.001
RV/TLC, %
43.1 (9.2)
47.8 (11.0)
<0.001
27.6 (6.8)
<0.001
Blood gases and pulse oximetry
Pa,O2, kPa
9.5 (1.0)
9.1 (1.3)
<0.001
10.8 (1.2)
<0.001
Sp,O2, %
95.5 (2.3)
94.3 (3.0)
<0.001
97.5 (1.2)
<0.001
Six-minute walk test
Distance, m
459 (99)
401 (106)
<0.001
–
End Sp,O2, %
92.0 (4.7)
89.9 (6.6)
<0.001
–
MRC Dyspnea scale
n = 194
n = 167
<0.001
n = 157
<0.001
Stage 0
35 (18)
24 (14)
132 (84)
Stage 1
87 (45)
42 (25)
24 (15)
Stage 2
52 (27)
58 (35)
0
Stage 3
14 (7)
22 (13)
1 (1)
Stage 4
6 (3)
2 (1)
0
Data are presented as n (%) or mean (SD). FEV1%predicted: forced expiratory volume in 1 s in percent of predicted; DL,CO: diffusing capacity of the lung for carbon monoxide; TLC: total lung capacity; RV: residual volume; Pa,O2: arterial partial pressure of oxygen; Sp,O2: arterial oxygen saturation by pulse oximetry; MRC: modified Medical Research Council. -:test not performed. p1) = between COPD patients who flew and did not fly; p2) = between COPD patients who flew and subjects without COPD who flew.
Of the 180 COPD patients who did not fly, 143 (79.4%) had no reason to travel by air during the previous two years, 16 (8.9%) did not dare to fly due to their lung disease, 16 (8.9%) stated other reasons (general fear of flying, economy, and hypersensitivity to perfume), and 5 (2.8%) were advised by a physician or other health professionals not to fly. As for the subjects without COPD, one patient (4.0%) did not fly due to fear of flying, and 24 (96.0%) reported no reason to fly.
Characteristics of COPD and non-COPD subjects who flew
Two-hundred eleven (54.0%) of the COPD patients and 159 (86.4%) of those without COPD flew during the previous two years (p < 0.001) (Fig. 2). During this period, 82.5% of the COPD patients had two or more flights, with a most common duration of 3–6 h (Fig. 2). The COPD patients travelled less frequently than those without COPD (median number 2–4 flights vs. more than 4 flights, respectively, p < 0.001).
Figure 2Frequency and duration of flights in COPD patients and subjects without COPD.□: COPD patients, ■: subjects without COPD. *:p < 0.001.
The COPD group had higher mean age and pre-flight MRC Dyspnea score, and significantly lower FEV1% predicted, DL,CO% predicted, Pa,O2 and Sp,O2 than the non-COPD subjects (Table 2).
Symptoms
Symptoms during air travel were more frequently experienced in the COPD group (28.4%) than the non-COPD group (16.4%) (OR = 2.0, 95% CI 1.2–3.4, p < 0.001) (Fig. 3). One or more hypoxia related symptoms were reported by 52 (24.6%) of the COPD patients and by 14 (8.8%) of the non-COPD subjects (OR = 3.4, 95% CI 1.8–6.4, p < 0.001)(Fig. 3). The most frequent hypoxia related symptoms in the COPD group were dyspnea and air hunger, which were significantly higher in the COPD than in the group without COPD (p < 0.001) (Table 3). There was no significant difference between the groups with regard to symptoms that were not hypoxia related; ear pressure, sinus pressure, and swollen legs (OR = 0.7, 95% CI 0.3–1.6) (Fig. 3).
Figure 3Reported symptoms in COPD patients and subjects without COPD. Values are given in percent of each group. Overall symptoms: all symptoms reported. Hypoxia related symptoms: dyspnea, dizziness, headache, chest pain, air hunger, cough, fainting, and palpitations. Other symptoms (the most frequent; ear pressure, sinus pressure, swollen legs). □: COPD patients, ■: subjects without COPD. *:p < 0.001.
After adjustment for confounders (smoking status, age, and gender), patients with COPD had a more than 3-fold higher risk of experiencing hypoxia related symptoms than those without COPD (OR = 3.3, 95% CI 1.6–6.7). For the respiratory symptoms, dyspnea or air hunger, the risk was nearly 7-fold higher (OR = 6.6, 95% CI 2.5–17.3).
Associations between pre-flight parameters and in-flight symptoms
Only the MRC Dyspnea score and exercise Sp,O2 showed a significant relationship to in-flight dyspnea and air hunger in COPD patients (Table 4). As for DL,CO and walking distance, there was a non-significant tendency towards a relationship. A logistic regression model including age, gender, MRC Dyspnea score, exercise desaturation, walking distance, and DL,CO was used to study associations between pre-flight variables and symptoms during air travel in patients with COPD. The risk for experiencing dyspnea and air hunger during flight was significantly related to the MRC Dyspnea score. Level 2 or higher on the MRC Dyspnea scale gave an OR 4.8 (95% CI 1.2–19.3) for in-flight dyspnea and air hunger compared to MRC Dyspnea score 0. The OR for experiencing in-flight dyspnea and air hunger was 0.93 (95% CI 0.87–0.99) per year increase in age. No other statistically significant associations were found.
Table 4Characteristics of COPD patients who had or did not have dyspnea and air hunger, n = 211.
n
Had dyspnea and air hunger n = 44
n
Did not have dyspnea and air hunger n = 167
p
Sex, M/F
44
22/22
167
102/65
0.228
Age, yrs
44
61.2 (6.8)
167
62.1 (6.7)
0.885
FEV1, % of predicted
44
46.2 (13.1)
167
48.9 (12.3)
0.197
DL,CO, % of predicted
38
56.8 (15.0)
151
62.5 (18.4)
0.077
RV/TLC, %
37
46.9 (7.5)
144
45.2 (7.9)
0.258
Blood gases and pulse oximetry
Pa,O2, kPa
42
9.4 (1.0)
162
9.5 (1.0)
0.746
Sp,O2, %
42
95.4 (2.4)
157
95.5 (2.3)
0.688
6 min walk test
41
147
Distance, m
435 (102)
466 (97)
0.077
End exercise Sp,O2, %
90.7 (5.9)
92.4 (4.3)
0.039
MRC Dyspnea scale
43
151
0.001
Stage 0
3 (7)
32 (21)
Stage 1
13 (30)
74 (49)
Stage 2
18 (42)
34 (23)
Stage 3
5 (12)
9 (6)
Stage 4
4 (9)
2 (1)
Data are presented as n (%), and mean (SD). FEV1% predicted: forced expiratory volume in 1 s in percent of predicted; DL,CO: diffusing capacity of the lung for carbon monoxide; TLC: total lung capacity; RV: residual volume; Pa,O2: partial pressure of oxygen; Sp,O2: arterial oxygen saturation; MRC: modified Medical Research Council.
Use of in-flight oxygen and healthcare before and after the flight
Before planning to travel by air, twenty-three (5.9%) of the COPD patients had consulted a physician, while two (1.1%) of those without COPD had a pre-flight physician consultation (p = 0.007). Fourteen of the twenty-three COPD patients were advised not to travel. Nine of those patients travelled despite the physicians’ advice, and five of them experienced hypoxia related symptoms. Eleven of the 391 COPD patients were on long-term oxygen therapy (LTOT). Two of them flew, both with supplementary oxygen, and none of them reported symptoms during air travel. Two of the 209 patients without LTOT needed unscheduled use of supplementary oxygen during flight. The pre-flight Pa,O2 in these patients were 9.6 kPa and 8.3 kPa, and their FEV1%pred were 52% and 30%, respectively.
In the time span of 48 h after air travel, nine (4.3%) COPD patients needed unscheduled healthcare, of these, four (1.9%) were hospitalized. Four of the nine patients had hypoxia related symptoms during flight. One of the subjects without COPD was hospitalized after air travel, but the subject in question did not report any symptoms during flight.
Discussion
More than fifty percent of an unselected, western COPD population had travelled by air during the previous two years. One fourth of them experienced hypoxia related symptoms during air travel, compared to nine percent of individuals without COPD. The risk of experiencing dyspnea or air hunger was almost seven times higher in the COPD group than in those without COPD. In patients with COPD, there was a strong association between in-flight dyspnea or air hunger and sea-level MRC Dyspnea score. Desaturation during 6MWT was also related to in-flight symptoms.
In 1991 and 1993 two studies from USA and Britain reported that 44% and 35% of the COPD patients had travelled by air.
In the present study, 54% of the COPD patients had travelled by air, most of them more than once during the previous two years. Taking into account the high and increasing prevalence of COPD, the number of flight passengers suffering from this disease is considerable and likely to increase further.
To our knowledge, this is the first flight outcome study that compares COPD patients with non-COPD subjects. Our data show a 3-fold increase in hypoxia related symptoms, and a near 7-fold increase in dyspnea and air hunger. Altogether, one fourth of the COPD patients experienced hypoxia related symptoms during flight. We acknowledge that symptoms classified as hypoxia related may have other causes than hypobaric hypoxia. However, the occurrence of other air travel related symptoms like ear pressure, sinus pressure, and swollen legs did not differ between the groups, indicating that the COPD patients were not more prone to report symptoms in general.
Although the COPD population in the present study had a milder disease than in the study by Coker et al, the prevalence of in-flight symptoms was higher. This discrepancy can probably be explained by difference in patient selection.
The patients in Coker’s study either used supplementary oxygen during flight or, according to a respiratory specialist assessment, were not expected to develop in-flight hypoxemia. Thus, it seems reasonable to assume that the prevalence of symptoms presented in the current study is more representative for an unselected population of flight passengers with COPD.
The difference in symptom prevalence between the COPD and the non-COPD group might have been influenced by difference in age, gender and smoking habits. Correcting for these parameters, however, did not influence the outcome variables significantly.
In previous studies, the majority of COPD patients reporting in-flight symptoms had severe hypoxemia during subsequent testing with Hypoxia-altitude simulation test (HAST).
Thus, it seems reasonable to assume that the symptomatic patients in the current study suffered from hypoxemia, and that pre-flight testing would have resulted in the use of supplementary oxygen. It should be noted, however, that patients may become severely hypoxemic during hypobaric and normobaric hypoxia without experiencing symptoms.
Nine patients needed healthcare after arrival, and almost half of those patients had symptoms during flight. It is worth noting that a large proportion of those who travelled against the advice of their physician experienced in-flight symptoms.
There are various methods for predicting in-flight hypoxemia, but as far as we know, prediction of in-flight symptoms has not previously been studied.
Whereas lung function is only weakly correlated with in-flight hypoxemia, exercise related variables may give useful information for pre-flight assessment.
We evaluated the association between these variables and the occurrence of in-flight symptoms. The MRC Dyspnea score at sea-level was strongly associated with in-flight dyspnea and air hunger. This is an interesting and not previously described observation, which may be clinically useful. Desaturation during a 6MWT also showed a significant relationship with in-flight dyspnea and air hunger and corroborates earlier observations of associations between exercise desaturation and in-flight hypoxemia.
Inclusion of both MRC Dyspnea score and exercise desaturation may possibly be valuable in pre-flight evaluation algorithms.
It would have been of interest to establish whether in-flight symptoms were associated with development of hypoxemia during HAST, but hypoxic challenge testing was not performed. In addition, the current study has some other limitations. The time between the measurements and air travel could have been up to one year, and possible worsening of the lung disease may have influenced the results. In addition, the severity of the symptoms was not recorded. Also, the design of the study may give recall bias, which might result in under-reporting of symptoms. On the other hand, a design where the participants are asked to record respiratory distress during actual flights might lead to increased symptom awareness, and thereby over-reporting of symptoms. Although age and gender differences between subjects with and without COPD were corrected for in the analyses, these differences could conceivably have influenced the results.
In conclusion, a large proportion of patients with moderate to severe COPD travel by air. One fourth of them reported hypoxia related symptoms during air travel. The COPD patients had a near 7-fold higher risk of experiencing dyspnea or air hunger than those without COPD. The symptoms were strongly associated with MRC Dyspnea score, and an association between exercise desaturation during a 6MWT was also observed. The high prevalence of symptoms seems to justify pre-flight evaluation of COPD patients. The optimal algorithm for this evaluation remains to be established, but our results indicate that a symptom-based approach in the pre-flight evaluation might be useful.
Acknowledgments
The authors would like to thank professor in Medical Statistics, L. Sandvik, University of Oslo, for statistical support.
Sources of support
The study was funded by grants from The Norwegian Heart and Lung Patient Organization, The Norwegian Foundation for Health and Rehabilitation, The Foundation for Respiratory Research, University of Bergen, Norway and by grants from Center for Clinical Research, Haukeland University Hospital, Norway.
Conflict of Interest Statement
None.
Appendix. Supplementary data
The following are the Supplementary data related to this article:
Lung volumes and forced ventilatory flows. Report Working Party Standardization of Lung Function Tests, European Community for Steel and Coal. Official Statement of the European Respiratory Society.
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