The ESTxENDS Trial: Pulmonary Function Substudy

--\> This is a substudy of the main ESTxENDS trial (NCT03589989). Pulmonary function outcomes should be considered secondary outcomes of the main smoking cessation outcome formulated in NCT03589989. Cigarette smoking is the leading cause of preventable death in Switzerland and still more than a quarter of the Swiss population smokes cigarettes. Recently, electronic nicotine delivery systems (ENDS; also called vaporizer, e-cigarette or electronic cigarette) have become popular with smokers who want to stop smoking or reduce their exposure to inhaled chemicals since ENDS use appears to be safer than tobacco smoking. The inhaled ENDS aerosol appears safe in laboratory conditions, its long-term effects on pulmonary function remains unknown. This study will therefore investigate the impact of ENDS on lung function in smokers, who attempt to quit using ENDS compared with smokers, receiving only smoking cessation counseling with nicotine replacement therapy (NRT). Multiple breath washout (MBW) is an established technique for assessment of peripheral airway function in pediatrics and its use is emerging in adult medicine, as it offers a better sensitivity to assess small airway disease before large airway involvement can be detected using conventional pulmonary function tests (PFT - spirometry, body plethysmography and diffusing capacity test). Preliminary evidence from limited studies suggest MBW parameters (LCI, Scond, Sacin) can be impaired in smokers with normal lung function and improve after smoking abstinence. Primary objective of this substudy is to assess changes in LCI from baseline to 6 months post target quit date (TQD) in cigarette smokers randomized to the intervention group compared to the control group. For the main ESTxENDS trial (NCT03589989), cigarette smokers motivated to quit smoking cigarettes will be included. Participants in the intervention group will receive an ENDS and nicotine-containing e-liquids, which they will be allowed to use ad libitum