Gas, tubes and flow

Abstract

An understanding of the principles of fluid mechanics is essential when considering the physical concepts of gas flow in relation to the airway. Both gases and liquids are termed fluids, their behaviour is described in terms of pressure, volume and temperature and is governed by the gas laws. Flow is the amount of fluid moving per unit time and can be laminar or turbulent. Laminar flow is governed by Poiseuille’s law; it is a function of the viscosity of a fluid and has a linear relationship with pressure. Flow becomes turbulent above the critical velocity. The onset of turbulent flow can be predicted using the Reynolds 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 density. The Bernoulli effect describes the pressure drop when a fluid accelerates through a constriction and is utilized in Venturi devices, which have wide clinical applications. Though 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 or 6.5 mm are used in adult anaesthesia.