Gas, tubes and flow

Gases behave as ‘fluids’ under flow conditions. There are two main flow patterns: laminar and turbulent. Here, we review the flow characteristics of gases and how they relate to the airway and endotracheal tubes. An understanding of these characteristics can be manipulated to improve flow in clinical situations; for example, using a gas with a lower density than air such as heliox reduces turbulent flow and may be helpful in patients with airway obstruction. The Bernoulli principle and Venturi effect have been used to develop fixed-performance masks, jet ventilators and suction devices.