Experimental evaluation and model development for analysis of pressure drop in the lungs of children

  • Author / Creator
    Paxman, Tyler
  • Airway resistance describes the ratio between pressure drop and flow rate through the conducting respiratory airways. Analytical models of airway resistance for tracheobronchial airways have previously been developed and assessed without upper airways positioned upstream of the trachea. This work investigated pressure drop as a function of flow rate and gas properties for upper and central airway replicas of 10 child subjects, ages 4–8. Replica geometries were built based on computed tomography scan data and included airways from the nose through 3–5 distal branching airway generations. Pressure drop through the replicas was measured for constant inspiratory flows of air and heliox. For both the nose-throat and branching airways, the relationship between non-dimensional coefficient of friction, C_F, with Reynolds number, Re, was found to resemble the turbulent Blasius equation for pipe flow, where C_F ∝ Re^(-0.25). Additionally, pressure drop ratios between heliox and air were consistent with analytical predictions for turbulent flow. The presence of turbulence in the branching airways likely resulted from convection of turbulence produced upstream in the nose and throat. An airway resistance model based on the Blasius pipe friction correlation for turbulent flow was proposed for prediction of pressure drop through the branching bronchial airways downstream from the upper airway. The modified-Blasius model was then incorporated into a model for estimating pressure drop across a single path through the tracheobronchial airways of children of ages 4 to 8. Analysis of model sensitivity to airway dimensions (age-related), flow rates (exertion level- and age-related) and gas properties was performed. To capture the convection and then dissipation of turbulence generated upstream in the nose-throat, the modified-Blasius model was used for the first five tracheobronchial generations only, and the well-known Pedley model was used for more distal airway generations. Gas properties had moderately larger impact on pressure drop at higher flow rates. The age-averaged pressure drop percent changes at tidal breathing (14 L/min) and heavy exertion (60 L/min), respectively, were -35% and -46% when changing air to He-O2 (80/20), and 9.9% and 14% when changing air to N2O-O2 (50/50). He-O2 (80/20) is shown to reduce pressure drop significantly whereas N2O-O2 (50/50) has less impact on pressure drop across the range of flow rates studied. Single path tracheobronchial pressure drop results obtained using a combined model (with a transition generation of 6) were compared with usage of both pure models (modified-Blasius or Pedley). At typical tidal breathing flow rates (14 L/min), the combined model predicted higher values than either of the pure model cases when considering air and N2O-O2 (50/50), whereas similar values to those of the Pedley model were predicted for He-O2 (80/20). At flow rates typical of heavy exertion (30–65 L/min), the combined model results were closest to the pure modified-Blasius model for air or N2O-O2 (50/50). This combined pressure drop model incorporation of a modified-Blasius equation for analytical predictions in the first 5 generations of the conducting airways, in conjunction with the Pedley model in the more distal airways, provides an improvement for pressure drop prediction in the lungs of 4- to 8-year-old children.

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    Master of Science
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