OVER 60-PLUS YEARS I’ve trained many divers, written training packages and 14 books on decompression and diving. So I thought I knew a thing or two about diving, but immersion pulmonary oedema (IPO) was a new one on me.
IPO signifies fluid build-up in the lungs. This leads to shortness of breath, coughing and reduced gas-exchange, which in turn lowers oxygen levels and increases the chance of loss of consciousness, cardiac arrest and death. It is drowning from the inside.
The condition can affect surface-swimmers but can also occur during a dive. I reckon few divers currently recognise IPO as a diving-related topic, but we need to understand as much as there is to know about it, and take precautions to minimise the risks.
The medical profession has no clear idea of the causes of IPO, because fatal cases are often mistaken for drowning. The lungs are waterlogged and heavy either way. Most of what we do know about IPO comes from survivors, some of whom have had recurrent episodes.
IPO is said to be a rare condition, but how can we know this, if some instances are identified simply as drowning?
In 2016 Dr Richard Moon, medical director of a US hyperbaric centre, stated that: “During immersion in water, particularly cold water, some people have an exaggerated degree of the normal redistribution of blood from the extremities to the chest area, causing increased pressure in the blood vessels of the lungs, and leakage of fluid into the lungs.”
If swimmers over-hydrate themselves, it seems, hydrostatic pressure can force fluid from their circulatory system through the alveoli walls into the lung
air-space. This process could be exacerbated by working hard under water and so breathing harder.
Dr Peter Wilmshurst put the case strongly when addressing last year’s BSAC conference: “IPO is a life-threatening condition and may be the commonest cause of death during diving.”
He and others had published the study Cold-Induced Pulmonary Oedema in Scuba Divers and Swimmers and Subsequent Development of Hypertension in the Lancet in 1989.
For the first time IPO-related deaths were reported in that year’s BSAC Diving Incidents Report, with two cases from overseas plus a further 13 incidents in which IPO was a suspected factor.
So what can we do about this vaguely defined threat? Be aware that certain behaviours increase the risk.
If you have a heart condition or hypertension (high blood pressure), get an examination and advice from a hyperbaric doctor (not a GP) before diving again.
You can lose a lot of body heat even in “warm” water, depending on the length of a dive. If you feel cold, you are cold. Cold water cause blood-vessels to constrict, so wear enough thermal protection.

ONE OF THE BIGGEST risk-factors, however, seems to be over-enthusiastic pre-hydration. Bodily hydration is not constant. Physical work and warm weather can cause sweating, and loss of bodily fluids.
In normal life such losses must be rebalanced to allow the body to function correctly. It’s critical that divers do the same, to permit normal blood viscosity and flow to off-gas excess nitrogen
during decompression, thus avoiding decompression illness. Thick, dehydrated blood restricts off-gassing.
Diving reduces the effects of gravity and this, added to breathing cold gas and a drop in ambient temperature, causes blood from the extremities and that which normally pools in the feet and legs to circulate in your trunk.
Nature doesn’t like such inequities, so it removes some of the excess by converting part of the blood into urine – the outcome of which equals dehydration!
Being hydrated before a dive is important in reducing the risk of DCI but, with IPO in mind, it’s also important to avoid overdoing it. Don’t over-hydrate before diving and during the diving day.

HOW CAN YOU GAUGE THIS? Instead of carrying superfluous pre-dive fluid, rehydrate as soon as possible after the dive, without going mad. Monitor the colour of your urine, which should be no darker than pale straw.
This visual method tracks dehydration but not over-hydration, though you can use a colour-changing Urinalysis Dipstick to test your specific gravity (SG).
SG measures the concentration of solutes in the urine and provides a ratio of urine density to water – the reference range is 1.002-1.030. A lower value could indicate over-hydration; just follow the simple instructions on the packet.
Elevated work of breathing that increases the risk of IPO can happen when we swim against tide, lift heavy objects, use a badly maintained regulator or rebreather or fail to open cylinder-valves fully.
If you find yourself over-breathing, reduce your workload, stop as soon as possible, relax and regain composure.
At a recent Society of Underwater Technology meeting it was reported that two divers who presented for IPO treatment had overly tight drysuit neck-seals that might have restricted returning blood. Drysuits and other clothing need to fit well but not be excessively tight.
Signs of IPO that should prompt action include coughing, difficulty breathing, cyanosis (blue face), coughing up froth/blood-stained sputum or shallow-water blackouts. If you feel unwell under water, terminate the dive immediately. Ascend with your buddy at the correct rate, making all appropriate deco and/or safety stops.
Failing that, if you’re on open circuit position yourself so that the diaphragm of your demand-valve is below the level of your lungs (head-down, feet-up). With a rebreather, the counter-lung should be lower than your lungs.

BOTH MANOUEVRES CAUSE positive gas-flow to your mouth, reducing the effects of gas viscosity caused by depth and the mechanical effort required to operate the breathing equipment. Just breathe – there will be more gas available than you need.
Once calm and collected, with your breathing controlled, terminate the dive and ascend in the same controlled way.
If another diver (or surface swimmer) becomes breathless, support them with an inflatable jacket until their breathing is under control, then remove them to boat/shore as quickly as possible.
This will move excess blood from the chest area to the lower limbs, reversing the effects of hydrostatic pressure.
First-aid treatment is the same as for DCI, and needs to be applied promptly. Once on deck or firm ground, sit the casualty up, keep them warm and administer 100% oxygen. But – this is vital – do not give them a drink!
Call the emergency services to get them to hospital as quickly as possible.
After a full recovery and before returning to diving, every IPO casualty must be cleared by a hyperbaric doctor.
A full cardiac work-up is required, because the IPO could be a first sign of underlying heart disease.
IPO is not confined to the unfit. It’s the well-informed and well-prepared diver who has the best chance of avoiding/ surviving it. My thanks to Dr John King and Dr Oli Firth for their input.