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. 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 or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.

The normal relaxed state of the lung and chest is partially empty. Further exhalation requires muscular work. Inhalation is an active process requiring work. 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.

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

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. Total work done against non elastic forces is 35% of total.

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:

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.

Because measuring the work of breathing requires complex instrumentation, measuring it in patients with acute serious illness is difficult and risky. Instead, physicians determine if the work of breathing is increased by gestalt or by examining the patient looking for signs of increased breathing effort. These signs include nasal flaring, the contraction of sternomastoid, and thoraco-abdominal paradox.

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