Gas, tubes and flow

An understanding of the principles of fluid mechanics is essential when considering the physical concepts of gas flow in relation to the airway. Gases and liquids act as fluids. Their behaviour is governed by the gas laws, and is described in terms of pressure, volume and temperature. Flow is the amount of fluid moving per unit time and can be laminar or turbulent. Laminar flow is governed by Poiseuille’s law, is a function of the viscosity of a fluid, and has a linear relationship with pressure. Flow becomes turbulent once a fluid is above critical velocity. The onset of turbulent flow can be predicted using Reynolds’s number. The rate of turbulent flow is a function of the density of a fluid. Flow through an orifice is always turbulent and is inversely proportional to the square root of the density of the fluid. The Bernoulli effect describes the pressure drop when a fluid accelerates through a constriction and is used in Venturi devices, which have wide clinical applications. Although small-diameter tracheal tubes theoretically increase the resistance to airflow and the work of breathing, there is no evidence of respiratory compromise when tubes with an internal diameter of 6.0 mm or 6.5 mm are used in adult anaesthesia.