Work of breathing
Work of breathing 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, 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.
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. 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 is breathing that 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 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, inertial resistance due to density, and to provide non-elastic components of movement of the airway tissues to accommodate pulmonary volume change.Dynamic airway compression
Dynamic airway compression occurs when intrapleural pressure equals or exceeds alveolar pressure, which causes dynamic collapsing of the lung airways. It is termed dynamic given the transpulmonary pressure varies based on factors including lung volume, compliance, resistance, existing pathologies, etc. It occurs during forced expiration when intrapleural pressure is greater than atmospheric pressure, and not during passive expiration when intrapleural pressure remains at subatmospheric pressures. Clinically, dynamic compression is most commonly associated to the wheezing sound during forced expiration such as in individuals with chronic obstructive pulmonary disorder. The density of the gas also influences the pressure reduction in the airways, and a higher density causes a greater drop in pressure for a given volumetric flow rate, which has consequences in ambient pressure diving, and can limit ventilation at densities over 6g/litre. It can be exacerbated by a negative static lung load. The effect is modeled by the Starling resistorMechanics
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
The term "work of breathing" should be more accurately referred to as the "power of breathing," unless it is in reference to the work associated with a specific number of breaths or a given interval of time. It is important to differentiate between the terms "breathing rate" and "breathing frequency." Although the two are frequently used interchangeably, "breathing rate" refers to the respiratory rate and is described in breaths per minute. On the other hand, "breathing frequency" refers to the frequency composition of a single breath and is described in hertz.
Clinical signs of increased work of breathing
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.Work of breathing in ambient pressure diving
Work of breathing is affected by several factors in underwater diving at ambient pressure. There are physiological effects of immersion, physical effects of ambient pressure and breathing gas mixture, and mechanical effects of the gas supply system.Immersion effects
The properties of the lung can vary if a pressure differential exists between the breathing gas supply and the ambient pressure on the chest. The relaxed internal pressure in the lungs is equal to the pressure at the mouth, and in the immersed diver, the pressure on the chest may vary from the pressure at the mouth depending on the attitude of the diver in the water. This pressure difference is the static lung load or hydrostatic imbalance.A negative static lung load occurs when the gas supply pressure is lower than the ambient pressure at the chest, and the diver needs to apply more effort to inhale. The small negative pressure differential inside the air passages induces blood engorgement of the distensible lung blood vessels, reducing the compliance of the lung tissue and making the lung stiffer than normal, therefore requiring more muscular effort to move a given volume of gas through the airways. This effect can occur in an upright open-circuit diver, where the chest is deeper than the regulator, and in a rebreather diver if the chest is deeper than the counterlung and will increase the work of breathing and in extreme cases lead to dynamic airway compression. The effects of positive static lung load in these circumstances have not been clearly demonstrated, but may delay this effect.
Effects of pressure and gas composition
Density of a given gas mixture is proportional to absolute pressure at a constant temperature throughout the range of respirable pressures, and resistance to flow is a function of flow velocity, density and viscosity.As density increases, the amount of pressure difference required to drive a given flow rate increases. When the density exceeds about 6g/litre the exercise tolerance of the diver becomes significantly reduced, and by 10 g/litre it is marginal. At this stage even moderate exertion may cause a carbon dioxide buildup that cannot be reversed by increased ventilation, as the work required to increase ventilation produces more carbon dioxide than is eliminated by the increased ventilation, and flow may be choked by the effects of dynamic airway compression. In some cases the person may resort to coughing exhalation to try to increase flow. This effect can be delayed by using lower density gas such as helium in the breathing mix to keep the combined density below 6 g/litre.
On air or nitrox, maximum ventilation drops to about half at 30 m, equivalent to 4 bar absolute and gas density of about 5.2 g/litre. The 6 g/litre recommended soft limit occurs at about 36 m and by the recommended recreational diving depth limit of 40 m, air and nitrox density reaches 6.5 g/litre
The maximum voluntary ventilation and breathing capacity are approximately inversely proportional to the square root of gas density, which for a given gas is proportional to absolute pressure. Use of a low density gas like helium or hydrogen to replace nitrogen in the mixture helps not only to reduce the narcotic effects, but also the density and thereby the work of breathing. To be non-combustible, there must be less than 4% by volume of oxygen in a hydrogen rich mixture. The presence and concentration of other diluents such as nitrogen or helium does not affect the flammability limit in a hydrogen rich mixture.
Underwater breathing apparatus
In the diving industry the performance of breathing apparatus is often referred to as work of breathing. In this context it generally means the external 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. Helium also has the effect of reducing the work of breathing by reducing density of the mixture, though helium's viscosity is fractionally greater than nitrogen's. Standards for these conditions exist and to make useful comparisons between breathing apparatus they must be tested to the same standard.Free-flow systems; In a free-flow breathing apparatus, the user breathes from the volume of ambient pressure gas in front of the face. If the supply is adequate, exhaled gas is flushed away by fresh gas flow, and only fresh gas is inhaled – there is no dead space. Work of breathing is affected by gas density due to pressure and gas composition, and there may be positive or negative static lung loading, but there is no additional external work of breathing due to airflow through the breathing apparatus. Surface-supplied divers who will be working hard underwater often use free-flow systems for this reason.
Demand systems:
Recirculating systems: Work of breathing of a rebreather has two main components: Resistive work of breathing is due to the flow restriction of the gas passages causing resistance to flow of the breathing gas, and exists in all applications where there is no externally powered ventilation. Hydrostatic work of breathing is only applicable to diving applications, and is due to difference in pressure between the lungs of the diver and the counterlungs of the rebreather. This pressure difference is generally due to a difference in hydrostatic pressure caused by a difference in depth between lung and counterlung, but can be modified by ballasting the moving side of a bellows counterlung.
Resistive work of breathing is the sum of all the restrictions to flow due to bends, corrugations, changes of flow direction, valve cracking pressures, flow through scrubber media, etc., and the resistance to flow of the gas, due to inertia and viscosity, which are influenced by density, which is a function of molecular weight and pressure. Rebreather design can limit the mechanical aspects of flow resistance, particularly by the design of the scrubber, counterlungs and breathing hoses. Diving rebreathers are influenced by the variations of work of breathing due to gas mixture choice and depth. Helium content reduces work of breathing, and increased depth increases work of breathing. Work of breathing can also be increased by excessive wetness of the scrubber media, usually a consequence of a leak in the breathing loop, or by using a grain size of absorbent that is too small. Both of these factors cause restrictions to the gas flow.
The semi-closed rebreather systems developed by Drägerwerk in the early 20th century as a scuba gas supply for Standard diving dress, using oxygen or nitrox, and the US Navy Mark V Heliox helmet developed in the 1930s for deep diving, circulated the breathing gas through the helmet and scrubber by using an injector system where the added gas entrained the loop gas and produced a stream of scrubbed gas past the diver inside the helmet, which eliminated external dead space and resistive work of breathing, but was not suitable for high breathing rates.