1 Division of Biostatistics, School of Public Health, University of Minnesota, Minneapolis, MN.
2 Department of Community Health Sciences, University of Manitoba, Winnipeg, Manitoba, Canada.
3 Division of Pulmonary and Critical Care Medicine, Oregon Health Sciences University, Portland, OR.
4 School of Medicine, Johns Hopkins University, Baltimore, MD.
5 Pulmonary Division, University of Alabama at Birmingham, Birmingham, AL.
6 School of Medicine, University of Pittsburgh, Pittsburgh, PA.
Received for publication August 7, 2002; accepted for publication December 18, 2002.
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ABSTRACT |
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clinical trials; lung diseases, obstructive; sex; smoking cessation; spirometry
Abbreviations: Abbreviations: COPD, chronic obstructive pulmonary disease; FEV1, forced expiratory volume in 1 second; FEV1%, forced expiratory volume in 1 second expressed as a percentage of the predicted value of FEV1 given the persons age, height, gender, and race.
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INTRODUCTION |
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The Lung Health Study was a randomized 10-center (nine in the United States, one in Canada) intervention trial conducted from October 1986 to April 1994 and was designed to test whether intervention with a bronchodilator and a smoking cessation program could reduce the rate of decline in pulmonary function among middle-aged smokers with mild to moderate airflow limitation. The primary endpoint was the rate of decline in postbronchodilator forced expiratory volume in 1 second (FEV1) over a 5-year follow-up period.
The intention-to-treat analysis of Lung Health Study data has been reported previously (8). Smoking cessation was found to have important effects on pulmonary function, particularly in the first year of the study, with a substantial improvement in FEV1 and forced expiratory volume in 1 second expressed as a percentage of the predicted value of FEV1 given the persons age, height, gender, and race (FEV1%) of predicted. In subsequent years, FEV1 declined considerably less in the quitters than in those who continued to smoke. This paper focuses on differences between men and women in pulmonary function changes as they relate to randomized group assignment in the intention-to-treat analysis and also in a retrospective analysis as they relate to quitting smoking or continuing to smoke.
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MATERIALS AND METHODS |
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We used mean change in postbronchodilator FEV1 between baseline and year 5 to indicate overall change in lung function. Because quitters typically showed an initial increase in FEV1 followed by a linear decline, we also analyzed changes between years 1 and 5. In the intention-to-treat analysis, participants were categorized by their randomized assignments: 1) smoking intervention with either ipratropium bromide (an anticholinergic bronchodilator) or placebo inhaler or 2) usual care. In the analysis, participants were categorized as continuous smokers, intermittent quitters, or sustained quitters on the basis of their annual, biochemically confirmed smoking status (15). They were classified as smokers if their salivary cotinine levels were higher than 20 ng/ml. In those who used nicotine gum or other nicotine substitutes that invalidated cotinine measurement, compliance with smoking cessation was checked by measuring exhaled carbon monoxide concentrations; values of less than 10 parts per million defined successful compliance. Participants who did not attend follow-up visits for validation of smoking status were assumed to be smokers. Continuous smokers were defined as those biochemically verified as smokers at each annual clinic visit. Intermittent quitters were biochemically verified as smoking at between one and four annual follow-up visits, and sustained quitters were biochemically verified as nonsmokers at all annual clinic visits. Because ipratropium bromide had only a small and transient effect on lung function (8), in this analysis we combined both intervention groups as "smoking intervention" to focus on the effects of smoking changes and gender.
Statistical methods included the following: Descriptive statistics are presented in this paper as counts for categorical data and as means and standard deviations for continuous variables. Bivariate comparisons were made by using chi-square or t-test statistics, as appropriate. Graphic data are presented as mean ±2 standard errors. Linear regression and analysis of covariance were used to adjust changes in FEV1 (or percent predicted) for baseline values, body mass index, number of cigarettes smoked per day, and methacholine reactivity. Terms for interaction of randomized treatment assignment and of smoking status with gender were used to evaluate gender differences in lung-function response to smoking intervention. Statistical significance was inferred when p < 0.05 (two sided).
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RESULTS |
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Intention-to-treat analysis
The rate of change per year in postbronchodilator FEV1% of predicted is shown by gender and by treatment group in figure 1 and in table 2. In the unadjusted analysis, there was a greater spread between treatment groups in womens FEV1% predicted change data (0.486 vs. 1.081) than in those for men (0.555 vs. 0.922). We found significant effects for gender, treatment group, and the interaction of gender with treatment group in this analysis.
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The mean FEV1% predicted values for smoking intervention participants increased notably in the first year: approximately 1.5 percent predicted for women and 0.5 percent predicted for men. Thereafter, the values declined and converged to approximately the same value for both genders at year 5. The values for men and women in the usual care group decreased steadily during the 5 years, with mean lung function loss for women being greater than that for men. The net effect of intervention after 5 years of follow-up was clearly greater for women than for men.
Retrospective analysis of smoking history
Men in the Lung Health Study were more likely than women to achieve sustained smoking cessation (18.3 percent vs. 15.7 percent, p = 0.016). However, the genders did not differ significantly regarding the proportions of intermittent quitters or continuing smokers.
The rates of change per year in postbronchodilator FEV1% of predicted are shown in table 3 by gender and by categories of smoking history. In the unadjusted analysis, women sustained quitters actually gained lung function on average across the study years compared with men, whose status remained essentially the same across the 5 study years. Even women intermittent quitters lost less lung function than men did. Women intermittent quitters averaged slightly more years of smoking in the trial than did men intermittent quitters: 2.81 (standard error, 1.18) years for women versus 2.75 (standard error, 1.18) years for men. Men and women who were verified at each annual visit as continuing to smoke were approximately equivalent in their percent predicted lung function loss. Although this analysis was unadjusted, FEV1% of predicted effectively adjusts for gender, height, race, and age so that results for men and women can be compared.
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Figure 2 presents longitudinal data on FEV1% predicted for men and women verified as being either sustained quitters or continuing smokers during follow-up. Although much of the difference between smoking categories occurred during the first year, differences for both genders continued to increase across subsequent annual visits.
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Female sustained quitters showed about a 2.3-fold larger improvement in FEV1% predicted in the first year, increasing from 78.3 to 82.0, than was noted in men (p < 0.001). This increase was sustained through the second year of the trial and then decreased slightly. At year 5, pulmonary function in female sustained quitters was still better than that noted at baseline: a net increase of 1.9 percent (standard error, 0.4 percent) in FEV1% predicted versus a net increase of 0.4 percent (standard error, 0.3 percent) in men.
The difference in response to smoking cessation among men and women was significant and was due largely to differing increases in the first year of smoking cessation. After year 1, no difference was found in mean change in FEV1% predicted between men and women who quit smoking. Among continuing smokers, the mean pattern of decline was quite similar for both genders, with women losing slightly more FEV1% predicted by year 5 than men did (5.4 percent predicted vs. 5.0 percent predicted).
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DISCUSSION |
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The Lung Health Study had a number of strengths that enabled us to detect the gender/smoking intervention interaction described here, including a large sample size, a relatively high proportion of women, an aggressive smoking cessation program with high rates of initial quitting and low relapse rates (16), a 5-year period of follow-up with follow-up rates at the fifth annual visit in excess of 94 percent, and high-quality measurement of pulmonary function. The study focused on smokers of both genders who already had mild to moderate COPD, for whom relatively high rates of decline in FEV1 could be expected.
Note that participants recruited for the study were self-selected and were not a random sample of middle-aged smokers. As in most clinical trials, this factor might limit generalizability of our findings.
Many Lung Health Study participants were found to be reactive to methacholine (11, 14). Reactivity was closely related to FEV1 and FEV1% predicted and was substantially more common in women (87 percent) than in men (63 percent) (17). Airways reactivity, as suggested by the "Dutch hypothesis" (18), was a strong determinant of lung function decline, particularly in continuing smokers. However, our multivariate analyses indicated that the greater response of Lung Health Study women to quitting smoking was only partially explained by gender differences in airways reactivity.
Both differences and similarities exist between the Lung Health Study and other longitudinal studies of pulmonary function and smoking cessation. Lange et al. (19), who followed a cohort of 7,764 men and women aged 20 years or older in the Copenhagen, Denmark, population, noted that, among women smokers older than age 55 years, the rate of decline in FEV1 was greater for heavy smokers than for light smokers. No such relation was seen among men. Xu et al. (20), in a study of 3,287 men and women aged 4069 years in China, found that women smokers had a greater decrease in pulmonary function than men did. The Chen et al. (21) study of 1,149 adults aged 2559 years in Saskatchewan, Canada, noted that gender and smoking intensity affected FEV1 and maximal midexpiratory flow and that women were more affected than men. The observed interaction of gender and smoking intensity was independent of age, height, and weight.
Langhammer et al. (22), in a survey of more than 65,000 men and women in Norway, noted that, of current and previous smokers, women were significantly more likely than men to report respiratory symptoms and asthma, even after the authors controlled for pack-years of smoking and current levels of cigarette smoking per day. Their conclusion that "women seemed to be more susceptible to the effect of tobacco smoking than men" (22, p. 917) parallels our findings on lung function.
The explanation for an apparent heightened sensitivity to cigarette smoke in women is unknown. It is possible that, since womens lungs tend to be smaller than mens, cigarette smoke is more concentrated in the airways. Thus, pack-years or reported number of cigarettes smoked per day could underrepresent the effective smoke exposure to the lungs of women. If so, women could be expected to have higher levels of cotinine than men do. However, mean levels of salivary cotinine in our study were lower in women than in men, consistent with the baseline difference in number of cigarettes smoked per day.
A second possible explanation might be that, because the FEV1 of women is lower than that of men, the effect might be related to airways caliber. Our analyses of covariance suggest that that is not the complete explanation. After we controlled for covariates including baseline FEV1, gender remained a powerful predictor of response to smoking cessation. In the present analysis, FEV1 and FEV1% predicted were postbronchodilator values and were presumably less subject to day-to-day environmental exposures that may differ by gender.
To summarize, our three most important findings were the following:
1. Among participants who quit smoking in year 1, women showed greater improvements in both FEV1 and FEV1% predicted. Among sustained quitters, 5-year losses in FEV1 and FEV1% predicted were smaller in women than in men.
2. Among participants who continued to smoke, declines in FEV1% predicted were about equivalent in women and in men.
3. Evidence of an interaction exists between gender and smoking behavior as determinants of changes in lung function.
The observed gender effects persisted even after adjustment for such potentially confounding factors as baseline lung function, airways reactivity, smoking level, age, and body mass index. Our results should serve as an impetus for more emphasis on smoking cessation among women, especially as the number of women with COPD continues to increase. For women at risk of COPD, there is a clear health-promotion message here.
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ACKNOWLEDGMENTS |
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The initial primary author of this manuscript was Dr. Gregory R. Owens, who died in September 1999.
The principal investigators and senior staff of the clinical and coordinating centers and of the National Heart, Lung, and Blood Institute, as well as members of the Safety and Data Monitoring Board and the Morbidity and Mortality Review Board, are as follows: Case Western Reserve University, Cleveland, OhioDr. M. D. Altose (Principal Investigator), Dr. A. F. Connors (Co-Principal Investigator), Dr. S. Redline (Co-Principal Investigator), Dr. C. D. Deitz, and Dr. R. F. Rakos; Henry Ford Hospital, Detroit, MichiganDr. W. A. Conway, Jr. (Principal Investigator), Dr. M. Eichenhorn (Principal Investigator), Dr. A. DeHorn (Co-Principal Investigator), Dr. J. C. Ward (former Co-Principal Investigator), C. S. Hoppe-Ryan, R. L. Jentons, J. A. Reddick, and C. Sawicki; Johns Hopkins University School of Medicine, Baltimore, MarylandDr. R. A. Wise (Principal Investigator), Dr. S. Permutt (Co-Principal Investigator), and Dr. C. S. Rand (Co-Principal Investigator); Mayo Clinic, Rochester, MinnesotaDr. P. D. Scanlon (Principal Investigator), Dr. L. J. Davis (Co-Principal Investigator), Dr. R. D. Hurt (Co-Principal Investigator), Dr. R. D. Miller (Co-Principal Investigator), Dr. D. E. Williams (Co-Principal Investigator), G. M. Caron, G. G. Lauger, and S. M. Toogood (Pulmonary Function Quality Control Manager); Oregon Health Sciences University, Portland, OregonDr. A. S. Buist (Principal Investigator), W. M. Bjornson (Co-Principal Investigator), and Dr. L. R. Johnson (Lung Health Study Pulmonary Function Coordinator); University of Alabama at Birmingham, Birmingham, AlabamaDr. W. C. Bailey (Principal Investigator and Associate Chief of Staff for Education, Department of Veterans Affairs Medical Center, Birmingham), Dr. C. M. Brooks (Co-Principal Investigator), Dr. J. J. Dolce, D. M. Higgins, M. A. Johnson, and B. A. Martin; University of California, Los Angeles, Los Angeles, CaliforniaDr. D. P. Tashkin (Principal Investigator), Dr. A. H. Coulson (Co-Principal Investigator), Dr. H. Gong (former Co-Principal Investigator), Dr. P. I. Harber (Co-Principal Investigator), Dr. V. C. Li, (Co-Principal Investigator), Dr. M. A. Nides, M. S. Simmons, and I. P. Zuniga; University of Manitoba, Winnipeg, Manitoba, CanadaDr. N. R. Anthonisen (Principal Investigator, Steering Committee Chair), Dr. J. Manfreda (Co-Principal Investigator), Dr. R. P. Murray (Co-Principal Investigator), S. C. Rempel-Rossum, and J. M. Stoyko; University of Minnesota Coordinating Center, Minneapolis, MinnesotaDr. J. E. Connett (Principal Investigator), Dr. M. O. Kjelsberg (Co-Principal Investigator), Dr. M. K. Cowles, D. A. Durkin, Dr. P. L. Enright, K. J. Kurnow, W. W. Lee, P. G. Lindgren, S. Mongin, Dr. P. OHara (Lung Health Study Intervention Coordinator), H. T. Voelker, and Dr. L. Waller; University of Pittsburgh, Pittsburgh, PennsylvaniaDr. G. R. Owens (Principal Investigator (deceased)), Dr. R. M. Rogers (Principal Investigator), Dr. J. J. Johnston, F. P. Pope, and F. M. Vitale; University of Utah, Salt Lake City, UtahDr. R. E. Kanner (Principal Investigator), Dr. M. A. Rigdon (Co-Principal Investigator), K. C. Benton, and P. M. Grant; The Salt Lake City Center has been assisted by the Clinical Research Center, Public Health Research Grant M01-RR00064 from the National Center for Research Resources; Safety and Data Monitoring BoardDr. M. Becklake, Dr. B. Burrows (deceased), Dr. P. Cleary, Dr. P. Kimbel (Chairperson (deceased)), L. Nett (former member), Dr. J. K. Ockene, Dr. R. M. Senior (Chairperson), Dr. G. L. Snider, W. Spitzer (former member), and Dr. O. D. Williams; National Heart, Lung, and Blood Institute staff, Bethesda, MarylandDr. S. S. Hurd (Former Director, Division of Lung Diseases), Dr. J. P. Kiley (Former Project Officer and Director, Division of Lung Diseases), and Dr. M. C. Wu (Division of Epidemiology and Clinical Applications); Mortality and Morbidity Review BoardDr. S. M. Ayres, Dr. R. E. Hyatt, and Dr. B. A. Mason.
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NOTES |
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REFERENCES |
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