Airway resistance is a concept used in respiratory physiology to describe mechanical factors which limit the access of inspired air to the pulmonary alveoli, and thus determine airflow. Resistance is greatest at the bronchi of intermediate size, in between the fourth and eighth bifurcation.^[1] Because airway resistance is dictated by the diameter of the airways and by the density of the inspired gas, the low density of heliox reduces airway resistance, and makes it easier to ventilage the lungs. Resistance can be calculated using Ohm’s law^[1] or Poiseuille’s law.^[1]

<math>R = \frac {\Delta P}{V^*} = \frac {P_{mouth} – P_{alveoli}}{V^*}</math>

• R = resistance
• P = pressure
• V* = airflow (the asterisk should be read as a dot over the letter, which is used to denote rate in respiratory physiology.)

<math> R = \frac{8nl}{\pi r^{4}} </math>

• R = resistance
• n = viscosity
• l = length
• r = radius

Because of the fourth power in the denominator, resistance increases rapidly as diameter decreases.

• Turbulent flow
• Laminar flow
• Reynolds number
• Upper Airway Resistance Syndrome (UARS)

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