Work of breathing
[1][2] In a normal resting state the work of breathing constitutes about 5% of the total body oxygen consumption.
[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 is breathing that does not require active muscle contraction during exhalation.
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, inertial resistance due to density, and to provide non-elastic components of movement of the airway tissues to accommodate pulmonary volume change.
Clinically, dynamic compression is most commonly associated to the wheezing sound during forced expiration such as in individuals with chronic obstructive pulmonary disorder (COPD).
The effect is modeled by the Starling resistor[9] 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.
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.
[9] 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[9] 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.
The presence and concentration of other diluents such as nitrogen or helium does not affect the flammability limit in a hydrogen rich mixture.
Diving rebreathers are influenced by the variations of work of breathing due to gas mixture choice and depth.
Exceeding this maximum continuous exertion may lead to carbon dioxide buildup, which can cause accelerated breathing rate, with increased turbulence, leading to lower efficiency, reduced RMV and higher work of breathing in a positive feedback loop.
At extreme depths this can occur even at relatively low levels of exertion, and it may be difficult or impossible to break the cycle.
The resulting stress can be a cause of panic as the perception is of an insufficient gas supply due to carbon dioxide buildup though oxygenation may be adequate.
[12] Gas density at ambient pressure is a limiting factor on the ability of a diver to effectively eliminate carbon dioxide at depth for a given work of breathing.
Once this occurs further attempts to increase flow rate are actively counterproductive and contribute to further accumulation of carbon dioxide.
[21][9] To reduce risk of hypercapnia, divers may adopt a breathing pattern that is slower and deeper than normal rather than fast and shallow, as this gives maximum gas exchange per unit effort by minimising turbulence, friction, and dead space effects.
[24] Carbon dioxide is a product of cell metabolism which is eliminated by gas exchange in the lungs while breathing.
[24] Carbon dioxide retention as a consequence of excessively high work of breathing may cause direct symptoms of carbon dioxide toxicity, and synergistic effects with nitrogen narcosis and CNS oxygen toxicity which is aggravated by cerebral vasodilation due to high carbon dioxide levels causing increased dosage of oxygen to the brain.
