A Simplified Approach to the Measurement of Specific Airway Resistance

Abstract

Extract: A simple algebraic manipulation of known formulas leads to a direct expression for the specific airway resistance (SR_aw) which precludes separate measurements of the airway resistance (R_aw) and the thoracic gas volume (TGV). The equation is: SR_aw = tg β (P Bar - P H 2 O ) e₂, in which tg β stands for the relation between the plethysmographic box volume and pneumotachograph flow fluctuations; (P Bar - P H 2 O ) is the barometric pressure corrected for water vapor pressure at body temperature, and e₂ is a calibration for graphical units. Thanks to this new formula SR_aw can now be easily measured with great precision and quickly calculated, even in the case of those children (Table 1) with whom this was not previously possible. No further cooperation is needed than breathing at a normal rate through mouthpiece into a whole-body plethysmograph. The intraindividual coefficient of variation (CV) for SR_aw is about 8% and never exceeds 15% (Table 1). Speculation: Because of our proposed simplification of the measurement of SR_aw, the accumulation of data on bronchial obstruction and hence statistical analysis will be greatly facilitated in pediatrics. Further developments of this method should permit a comparison with previous studies in sedated children and answer the question of whether alteration in specific airway resistance is the consequence either of sedation or of some change associated with growth during the first 5 years life.