Nasal airflow resistance at simulated altitude.

This issue includes an important paper by BARRY et al. [1] in which the changes in nasal peak inspiratory flow (NPIF), and oral peak inspiratory flow (OPIF), during acclimatized exposure to low barometric pressure (equivalent to a simulated altitude of 8000 m, about the height of Mount Everest) of subjects in a hypobaric chamber were analysed. The changes were presumably due to hypoxia and its chemical sequelae, and influenced by the processes of acclimatization. They show that NPIF increases by a mean of 16%, whereas OPIF increases by 47%, and conclude that these results are consistent with a decrease in upper airways luminal calibre at altitude, correlating with the symptomatic nasal blockage and impairment of mucociliary function seen in similar conditions. The changes must be due to chronic hypoxia rather than to inhalation of cold and dry air, since environmental temperature and humidity were maintained at values close to those found at sea level. The study is important because it may point to physiological mechanisms determining airway calibre at altitude, and to changes in the airways that will influence breathing at altitude. The authors must have confronted formidable problems in experimental design, since their subjects were confined to a hypobaric chamber for over a month, and the apparatus which could be used to assess their airway mechanics must inevitably have been limited. The authors fully justify their use of rather simple methods to assess airways9 resistances. However, while these limitations can be appreciated, there are important considerations about the methodology and interpretation of results. A clear distinction between airways9 conductance, airways9 resistance and airways9 calibre needs to be made. The former two are reciprocally related and depend, inter alia, on the viscosity and density of the transmitted gas, and other aerodynamic features; however calibre will affect both conductance and resistance, but is not directly affected by the physical properties of the gas. NPIF and OPIF primarily reflect airways9 conductance. NPIF and OPIF are rather imprecise indications of airways9 conductance. If, as the authors found, lower airways9 inspiratory conductance (OPIF) is increased at simulated altitude by a mean of 47%, lower airways9 inspiratory resistance would be decreased by a mean of 32%. If upper airway (nasal) resistance is approximately equal to that of the lower airways [2], as seems to be the case in the authors9 own experiments (their fig. 1), and if during nasal breathing the nose was to act as a constant series resistance to the lower airways, then a mechanical equivalent to Ohm9s Law for electricity can be applied. This shows that NPIF (involving both upper and lower airways9 conductances) should increase by 19% just because of the increase in lower airways9 conductance, a value close to that obtained by the authors (16%). It should be emphasized that this calculation is based on no change in upper airways9 resistance, assuming it to be a constant, and it assumes that the method used to assess peak flows was accurately volume/flowdependent and not viscosityor density-dependent. But the calculation suggests that, in the experimental conditions and subject to important reservations (see later), upper airways9 resistance may have remained unchanged (their fig. 1), and not have increased during the chronic hypoxia. Resistance changes, or the lack of them, need to be interpreted in terms of airway calibre when gas viscosity and density are changed. To take the authors9 own values (their fig. 1), NPIF was about one-half that of OPIF, so that upper and lower airways9 resistances were about equal. On the acclimatized step from simulated 0 to 8000 m altitude, if OPIF increased by 47% (resistance decreasing by 32%), and if NPIF increased by 16% (resistance decreasing by 14%), this could be explained by an increase in nasal resistance of 4%, small and probably statistically insignificant. However, although it is desirable that the study should have given more precise and physically acceptable assessments of upper and lower airways9 resistances, and that the statistical analysis should have been based on such values, the authors9 conclusions are valid. In this respect it is significant that on the step from 5000 to 8000 m altitude NPIF remained virtually constant or even decreased; if the nasal calibre had remained constant an increase in NPIF (as indeed happened to OPIF) due to the reduction in viscosity and density of the gas would have been expected. The conclusion must be that nasal calibre decreased. These results raise the question as to why hypobaric conditions reduce lower airways9 resistance, and appear to decrease upper airways calibre in such a way that nasal resistance (with rarified gases) GKT School of Biomedical Sciences, Human Physiology and Aerospace Medicine, Guy9s Campus, London, UK.