Normalized resistance and normalized flow to describe air way resistance
TNO Voeding Instituut CIVO-Toxicologie en Voeding TNO
The effect of flow and flow patterns on the resistance of the airways was reinvestigated. A new way to describe pressure-flow relationships is introduced. Normalized resistance, Rn, is defined as resistance (P/φ) divided by the viscosity of the inspired gas (μ) relative to air (Rn = Rμ(u)/μ). Normalized flow φn is defined as flow times relative density (ρ/ρ(u)) and divided by relative viscosity (μ/μ(u)), yielding: φn = φ x ρ/ρ(u) x μ(u)/μ (ρ(u) and μ(u), density and viscosity of air). When Rn is plotted as a function of φn, then Rn is a unique function for low values of φn. Single-breath inhalation of foreign gases at a flow rate of 0.1 l · s-1 showed a good correspondence with the Rohrer equation. At higher flow rates, effects of pressure drop were detected comparing the data of ethane and air at the same φn, due to a pressure difference linearly related to (μ/μ(u))2(ρ(u)/ρ) that of air. Thus, the unique relationship between Rn and high φn is lost, due to distension of the airways. Discontinuities in Rn occur if the flow rate is increased and passes the critical Reynolds number in the trachea. In this paper we discuss under what measuring conditions the effect of airway distension can be distinguished from changes in flow pattern, and show how to use the Rn - φn diagram to determine viscosity of gas mixtures.
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Lung function test
European Respiratory Review, 4, 210-214