Factors affecting turbulent flow

Turbulent gas flow increases with increasing gas density and gas flow rate.

• Decreasing gas density with heliox (molecular weight 9.6) instead of air (molecular weight 29) to ventilate patients helps to relieve airway obstruction e.g. in tracheal stenosis.
• Decreasing gas flow rate, as in taking slower deep breaths, also helps to ventilate patients with airway obstruction in larger airways e.g. tracheal stenosis (remember that disease of peripheral airways such as emphysema or cystic fibrosis contribute little total airway resistance since the total cross-sectional area of small airways is very large and flow there is laminar).

In contrast, laminar flow (for fluid dynamics or airway resistance) depends on the viscosity of the blood as described by Poiseuille’s law [resistance = (8 * length * viscosity) / (pi * r^4)].