The difference between the two types of manoeuvres was thought to be caused by differences in the relative contributions of several types of ventilation inhomogeneity. For SBWvc manoeuvres, a dominant mechanism of nonuniform gas distribution was convective dependent, topographical ventilation inhomogeneity due to asynchronous emptying of large regions with unequal specific ventilations . In contrast with SBWSM manoeuvre, the major origin of the ventilation inhomogeneity was in the lung periphery due to either intraregional convective dependent gradients among closely adjacent regions or the interaction of diffusion and convection at peripheral branch points within the acinus . A characteristic feature of inhomogeneity due to the interaction of convection and diffusion at peripheral branch points, and to a lesser extent to inhomogeneity due to intrare-gional convective dependent gradients, was that the tracer gas concentration gradients caused by this type of inhomogeneity were markedly diminished with short periods of breath holding. This is as a result of cardiogenic and diffusive gas mixing within the lung periphery . Hence, the steeper Sn in smokers for SBWSM manoeuvres at 0 s of breath holding and the greater change in Sn in smokers over short periods of breath holding indicated a superior ability of the SBWSM manoeuvre to identify an abnormality in peripheral ventilation inhomogeneity in smokers compared with the SBWvc manoeuvre .