The Upper Airway: How New Technologies are Changing what We Thought we Understood

The equine upper airway is a fascinating area in which knowledge has grown significantly over the last few years. We now understand the importance of upper airway diameter for patency of airflow. Many equine researchers have indicated that small changes in laryngeal diameter cause large changes in impedance, using the Hagen-Poiseuille resistance (R) formula for a cylindrical tube with a length of L and radius of r (Bird et al., 2007) to calculate resistance. While conventional calculations of impedance presume that resistance is proportional to the fourth power of the radius, this equation is, in fact, accurate only for laminar flow. Computer modeling places the range of the Reynolds numbers of the horse upper airway at exercise at 130,000 to 141,000 (Rakesh et al., 2008). With the threshold for laminar flow being below a Reynolds number of 2300, we should, instead, be using a turbulent flow equation. However, the equation for turbulent flow holds true only for a circular tube of uniform radius r and a fully developed steady flow, i.e., the flow does not change with time; also the tube section does not include the entrance region. These conditions do not apply to the equine upper airway, which has a variable cross-sectional area. Therefore, none of these correlations is reliable for determining impedance. […]