Work of breathing

Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not particularly useful without a reference to volume or time. It can be calculated in terms of the pulmonary pressure multiplied by the change in pulmonary volume, or in terms of the oxygen consumption attributable to breathing.^[1][2] In a normal resting state the work of breathing constitutes about 5% of the total body oxygen consumption. It can increase considerably due to illness^[3] or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.

Mechanism of breathing

The normal relaxed state of the lung and chest is partially empty. Further exhalation requires muscular work. Inhalation is an active process requiring work.^[4] Some of this work is to overcome frictional resistance to flow, and part is used to deform elastic tissues, and is stored as potential energy, which is recovered during the passive process of exhalation, Tidal breathing does not require active muscle contraction during exhalation. The required energy is provided by the stored elastic energy.

When there is increased gas flow resistance, the optimal respiratory rate decreases.

Work against elastic recoil

This work (generally during the inhalation phase) is stored as potential energy which is recovered during exhalation.

Work against non-elastic resistance

A pressure difference is required to overcome the frictional resistance to gas flow due to viscosity, and to provide non-elastic components of movement of the airway tissues to accommodate pulmonary volume change.

Mechanics

Work is defined as a force applied over a distance. The SI unit of work is the Joule, equivalent to a force of 1 Newton exerted along a distance of 1 metre. In gas flow across a constant section this equates to a volume flowing against a pressure:^{[note 1]}

Work = Pressure x Volume

and Power = Work / time

with SI units for Power: Watts = Joules per second

Work of breathing should more accurately be called power of breathing unless referring to the work associated with a specific number of breaths, or a given interval of time.

Underwater breathing apparatus

Graph of the breathing resistance of an open-circuit demand regulator. The area of the graph (green) is proportional to the net work of breathing for a single breathing cycle

In the diving industry the performance of breathing apparatus is often referred to as work of breathing. In this context it generally means the work of an average single breath taken through the specified apparatus for given conditions of ambient pressure, underwater environment, flow rate during the breathing cycle, and gas mixture - underwater divers may breathe oxygen-rich Breathing gas to reduce the risk of decompression sickness, or gases containing helium to reduce narcotic effects. Standards for these conditions exist and to make useful comparisons between breathing apparatus they must be tested to the same standard.

Standards for testing underwater breathing apparatus

BS EN 250:2014. Respiratory equipment – Open-circuit self-contained compressed air diving apparatus – Requirements, testing, marking. BS EN 14143:2013. Respiratory equipment. Self-contained re-breathing diving apparatus I.S.EN 15333 –1: 2008 COR 2009 – Respiratory Equipment – Open-Circuit Umbilical Supplied Compressed Gas Diving Apparatus – Part 1: Demand Apparatus.

Variations in work of breathing

Factors which influence the work of breathing of an underwater breathing apparatus include density and viscosity of the gas, flow rates, cracking pressure (the pressure differential required to open the demand valve), and back pressure over exhaust valves.

Notes

References