Study group: Twenty-one smokers and 21 nonsmokers were recruited from the community and hospital personnel. In both groups as previously reported , subjects denied recent respiratory symptoms. Spirometric testing revealed that the forced vital capacity (FVC), the forced expired volume in 1 s (FEV1) and the maximal mid-expiratory flows (FEF25-75) were within the 95% confidence limits of normal using previously reported regressions . Smokers had a current cigarette consumption of at least 10 cigarettes per day and a cumulative exposure of at least four pack-years. Nonsmokers had no current exposure and had smoked less than 100 cigarettes in their lifetime.
Apparatus and equipment: Using equipment previously described , seated subjects, at rest, breathed test gas containing 0.3% carbon monoxide, 10% helium, 21% oxygen and the balance nitrogen through a low dead space twoway valve. Flow and volume were measured continuously with a pneumotach mounted in the wall of a bag-in-box system, helium concentration with a mass spectrometer and carbon monoxide concentration with a rapidly responding infrared analyzer. Throughout each SBW, including two to three preceding tidal breaths and a standardized deep breath, flow, volume, carbon monoxide and helium signals were stored digitally (50 Hz) for later computer analysis. Protocol: The protocol used in this study has been previously reported . All SBWs were preceded by a standardized deep breath of room air and were performed singly in random order on one study day. An individualized volume versus time ‘template’ of the prescribed manoeuvre was displayed on a monitor to assist subjects in performing the following SBWs: SBWsm manoeuvres consisting of slow inhalation from FRC to one-half IC, with either 0 s or 10 s of breath holding and slow exhalation to RV; and conventional SBWVC manoeuvres consisting of slow inhalation from RV to total lung capacity, and without breath holding, slow exhalation to RV. All inspired and expired flows for the preceding deep breath and for all SBW manoeuvres were maintained at 0.5 L/s.