IPE during a rebreather-dive

In healthy divers, the occurrence of immersion pulmonary edema (IPE) is usually caused by factors such as exertion and cold water. A case report of IPE during rebreather diving illustrates that breathing against negative pressure also plays a significant role in the development of IPE during scuba diving.

Case report

A highly trained 26-year-old combat diver developed shortness of breath and coughing during a dive with a rebreather. The exercise consisted of returning to the surface from a depth of 7 m in an upright position within 10 minutes without releasing gas bubbles into the water. This had to be repeated five times within an hour. At the end of the fourth attempt, he experienced breathing difficulties and coughing. As soon as the counterlung was filled again and he assumed a horizontal position, the symptoms disappeared. On the last attempt, shortness of breath, foaming cough and chest tightness occurred. Pulmonary edema was diagnosed in the emergency ward. The symptoms subsided within a few hours with oxygen treatment.

Static lung load

It corresponds to the water column that rests on the lungs and must be overcome during inhalation or exhalation. This is determined by the relative positions of the regulator and the lungs. A SCUBA diver in a vertical position has a negative static lung load of approx. 20 cm H2O, as the lungs are at a depth of approx. 20 cm greater than the regulator. This means that the pressure of the supplied breathing gas is 0.02 bar (=20 cm H2O) lower than the lung pressure. This is referred to as a negative transrespiratory gradient. This must be overcome by breathing in order for air to flow at all. On land, it is negligible (20 cm air pressure). The opposite is true for a head-down maneuver when diving: The regulator comes to rest below the level of the lungs, the static lung load reverses to positive, i.e. the pressure of the supplied breathing gas is higher than the pressure in the lungs. As a result, inhalation is less strenuous. On the other hand, the pressure difference during exhalation must be overcome by breathing. The static lung load is lowest in the horizontal position, as the regulator and lungs are at the same level.

If the transrespiratory gradient becomes too negative, fluid can be “sucked” from the pulmonary capillaries into the alveoli, as their pressure falls too low below the perfusion pressure (blood flow pressure) of the capillaries. This results in pulmonary edema. Even small pressure differences are sufficient for this if they persist for a some time.

When diving with rebreathers, the static lung load depends on the position of the counterlung: if it is on the back, the transrespiratory gradient becomes negative and inhalation becomes somewhat more strenuous (in a horizontal diving position); if the counterlung is in front, inhalation becomes easier and exhalation somewhat more difficult.

PEEP

The concept is routinely used under the term PEEP (positive end-expiratory pressure) in the magnitude of a water column of 5 cm and more in emergency and intensive care medicine using breathing masks for the treatment of various respiratory disorders. This therapy corresponds to a positive transrespiratory gradient (exhalation against slightly increased pressure) and is used to keep small airways and the alveoli open, e.g. in the case of pulmonary edema.

With regard to IPE, counterlungs worn on the front are therefore advantageous when rebreather diving. The diver in question therefore used this configuration, which should have prevented an IPE (however only in the horizontal position and not during ascent in vertical position).

Military tactics

In order to compensate for a loss of buoyancy control due to an increase in buoyancy as a result of gas expansion in the system (Boyle-Marriotte) during ascent with a rebreather, gas would have to be vented from the counterlung. The associated bubble formation is undesirable under military circumstances. For this reason, combat divers reduce their ascent speed in order to reduce the volume of the counterlung by consuming oxygen. At the same time, they intentionally reduce their breathing volume in order to reduce buoyancy. To facilitate this, the automatic diluent valve (ADV) can be tightened so that gas from the diluent tank does not inadvertently flow into the system and negate the effect. However, tightening the ADV too much can cause the counterlung to empty to a large extent, which requires a very strong inhalation effort, i.e. results in a very negative inhalation pressure. To fill the counterlung, a diver can ascend about 20-40 cm (Byole-Marriotte).

Self-treatment with PEEP

In fact, reproducing the situation that led to pulmonary edema in the combat diver showed that considerable negative inhalation pressures arose: up to - 15 mbar (15 cm H2O) with an empty counterlung and open ADV, up to - 50 mbar (50 cm H2O) with an empty counterlung and maximum tightening of the ADV. As expected, there were no critical pressure differences when breathing through a full counterlung (max. - 5 mbar inhalation pressure). The symptoms disappeared after filling the counterlung because inhalation became easier (full counterlung) and PEEP developed during exhalation (horizontal position with counterlung slightly below the lungs), whereby the diver unconsciously applied the emergency therapy of pulmonary oedema and treated himself.

Conclusion

This case illustrates the importance of static lung load during diving and that excessive negative transrespiratory gradients play a significant role in the development of IPE during diving.