Effect of cigarette smoking on maternal airway function during pregnancy Tarun K. Das, PhD: Jean-Marie Moutquin, MD: and Jean-Guy Parent, MD b Quebec City, Quebec, Canada The effects of cigarette smoking on maternal airway function during pregnancy were investigated in a cross-sectional study of 97 smokers and 175 nonsmokers at different gestational ages. The groups were comparable in age, height, and weight. All subjects were healthy. Forced vital capacity, forced expiratory volume in 1 second, their ratio, the forced expiratory flow rates between 0.2 and 1.2 L, 25% and 75%, and 75% and 85%, and instantaneous flows at lung volumes of 25%, 50%, and 75% were measured. All spirometric tests were unaffected by gestational age. However, all parameters of spirometry were significantly less in smokers than in nonsmokers when cumulative data during pregnancy were compared. Forced vital capacity, forced expiratory volume in 1 second, and their ratio were minimally reduced (4%, p < 0.05; 8%, P < 0.001; and 4%, p < 0.001; respectively) in smokers as compared with nonsmokers. Larger reductions were noted in forced expiratory flow rates between 0.2 and 1.2 L (14%, P < 0.001) and between 25% and 75% (16%, P < 0.001), and in instantaneous maximum flows at lung volumes of 75% (11 %, P < 0.001) and 50% (13%, p < 0.001). Maximum reduction of forced expiratory flow rates between 75% and 85% (26%, p < 0.001) and in instantaneous flows at maximum lung volumes of 25% (23%, P < 0.001) suggests marked increases in small-airway resistance and early small-airway disease in smokers. The progression of small-airway disease is related to the level of cigarette exposure. The results of our study demonstrate that the bronchodilatory effect expected in pregnancy is not sufficient to overcome the deleterious effects of cigarette smoking. (AM J OBSTET GYNECOL 1991 ;165: 675-9.)

Key words: Cigarette smoke, pregnancy, airway function It has long been recognized that pregnancy can alter lung function and the natural history of certain pulmonary diseases. Although maximum flow rate and forced expiratory volume in 1 second (FEV,) are not measurably altered, I the reduced total pulmonary resistance and increased airway conductance" observed during pregnancy suggest a bronchodilatory effect. This improvement in respiratory function is attributed to an increased concentration of progesterone, which is known to have a marked smooth-muscle-relaxing effect. 3 Indeed, progesterone has been given therapeutically to patients with obesity hypoventilation syndrome,' asthma,' and chronic obstructive pulmonary disease. 6 It could be hypothesized that the deleterious effects of smoking on airway function may be decreased during pregnancy, especially during the third trimester when progesterone levels are highest. Although cigarette smoking can cause airflow limitation in nonpregnant women,' little information is available on its efFrom the Department of Obstetrics and Gynecology" and the Division of Respiratory Physiology, Department of Medicine,' Saint-Fran(ois D'Assise Hospital, Laval University. Supported by a Laval University postdoctoral fellowship (T.K.D.) and Club des Services Sociaux, Quebec. Received for publication June 28, 1990; revised March 11, 1991; accepted March 15, 1991. Reprint requests: Jean-M. Moutquin, MD, Saint-Fran(ois D'Assise Hospital, 10 Rue I'Espinay, Quebec City, Quebec, Canada GIL 3L5. 611 /29607

fects on maternal airway function during pregnancy. This study was designed to assess the effects of cigarette smoking on airway function in normal pregnant women and to determine whether these effects are related to gestational age.

Material and methods This was a cross-sectional study of women in each of the three trimesters of pregnancy who were recruited after written informed consent. The study was performed at the antenatal clinic of Saint-Fran