Dr Sarah Clayton, Dr Claire Walklett, Dr Becca Jancis
Sarah, Claire and Becca work at DDRC Healthcare, the hyperbaric medical facility in Plymouth. Combined they have a wealth of experience in diving across the world and in remote locations. Trained to a level of DMAC Level IID and with first-hand experience of treating decompression illness, they are keen to share some of their knowledge with the wider expedition medicine community to increase the confidence of medics who may be treating divers.
Imagine it. You are the medic on a dive site in Fiji. On a remote tropical island, you are with a research expedition team studying manta rays. You are twelve hours from the nearest decompression chamber and fast asleep. It’s been a long day. You’ve had a lovely couple of dives, a delicious dinner and are in your little beach hut dreaming about fish.
That is, until one of the marine biology students bursts into your room. “Doc, doc wake up. I can’t sleep. My elbow hurts so badly!” What are you going to do next?
Operating in the marine environment presents unique challenges: (1) pressure, immersion and temperature can significantly alter physiology, (2) divers usually have a limited supply of breathing gas, and (3) there are, of course, some potentially dangerous fauna and flora.
The idea of spending weeks diving in tropical waters in some remote and beautiful corner of the world is understandably enticing to a lot of people. As well as having a thoroughly enjoyable trip however, it is worth being familiar with some of the problems you may come across to ensure your patients are in safe hands. Here we will look at some of the basic science underpinning diving medicine, decompression illness and the management of sick divers.
Basic Physics and Physiology of Diving
Understanding the physics and physiology of diving is crucial to get a firm grasp of dive medicine. Although possibly daunting at first, once understood, it explains divers’ clinical presentations and why we treat them as we do. Knowledge of the basic science will also really help in deciding whether a problem in a diver is dive-related or not.
The most important concept to understand in diving medicine is pressure. At sea level, the pressure is 1 atmosphere (1 ATA). With every 10m of seawater, this pressure increases by 1 ATA: 0 m is 1 ATA, 10m is 2 ATA, 20m is 3 ATA etc.
The gas laws underpin diving physics and physiology. The most important are Boyle’s law and Henry’s law.
Boyle’s Law / The volume of gas is inversely proportional to the absolute pressure. This means that if you increase pressure (i.e. descend deeper), you reduce the volume of a gas filled space. On ascending, the volume of the space increases again.
Henry’s Law / The amount of gas that will dissolve in a liquid is proportional to the partial pressure of the gas over that liquid. In other words, the higher the pressure, the more gas will dissolve in a liquid. Equally, when the pressure reduces on an ascent, the dissolved gas will come out of the liquid again.
We can now look at how these gas laws explain decompression illness (DCI).
DCI is caused by a reduction in ambient pressure. In diving, this reduction in pressure happens when you ascend to the surface. DCI encompasses two separate disease processes:
- Decompression sickness (DCS): ‘evolved gas’
- Arterial gas embolism (AGE): ‘escaped gas’
Decompression Sickness / As a diver goes deeper underwater, the pressure will increase. According to Henry’s law, this means that nitrogen, the inert gas in their air supply, will dissolve in increasing quantities in their blood. However, when they ascend, the pressure will fall again, this gas will come out of solution, creating lots of tiny bubbles of nitrogen. This is often described as ‘evolved gas’. These bubbles may block vessels or collect in tissues triggering local inflammatory changes, leading to DCS. It can occur in almost any part of the body but is most frequently seen in the joints and spinal cord.
Arterial Gas Embolism / The pathophysiology of AGE also involves bubbles but this time in the arterial circulation. This typically occurs due to pulmonary barotrauma, which can be explained by Boyle’s law. As a diver ascends, the pressure decreases and so the volume of air in their lungs will increase. If the diver holds their breath, or has lung pathology causing gas trapping, this may cause an over-inflation injury. The alveoli burst, leading to a pneumothorax. The gas from the pneumothorax can then find its way in to the pulmonary circulation, and back to the left side of the heart, where it is pumped out in to the systemic (arterial) circulation. The gas embolism may go anywhere in the body, including the brain.
Some divers have a communication between their right and left circulations, such as a patent foramen ovale (PFO). In these divers, arterial gas embolism may occur without pulmonary barotrauma. This is because bubbles formed in the venous circulation (as per Henry’s law) may pass through the shunt into the arterial circulation. Unfortunately, little is known about the effects of pressure on PFO patency. It has been suggested that PFO patency may increase with age, meaning older divers may be more at risk. However, further research is still required in this area.
AGE is often described as ‘escaped gas’ as there does not need to be a big accumulation of inert gas in the blood for it to occur. A single bubble of sufficient size in the arterial system can have catastrophic effects.
How does DCI present?
As bubbles can reach almost any part of the body, DCI can present with almost any clinical picture.
The most common presentations, in order of frequency are:
- Neurological: weakness, numbness, unconsciousness
- Musculoskeletal: joint pain
- Constitutional: headache, malaise, loss of appetite etc
- Cutaneous: a blotchy, marbled rash
- Chest pain or breathing difficulties: this may be a sign of pulmonary barotrauma
90% of cases present within the first six hours following a dive. AGE tends to present more rapidly than DCS.
Management of DCI
In a remote area with limited access to healthcare, the initial first aid of divers is of paramount importance. If you suspect decompression illness, do the following:
100% oxygen at 15 L/min / If possible should be given to patients regardless of their saturations. Saturations are not a particularly helpful guide as the aim is not to increase the patient’s oxygenation, but instead to establish a pulmonary ventilation-perfusion gradient, so excess nitrogen diffuses out of the capillaries and is exhaled. A high partial pressure of oxygen establishes a diffusion gradient even at normal pressure. This means excess nitrogen can be exhaled more efficiently.
Fluids / Oral is usually ok, but consider IV if they are vomiting, dehydrated or unconscious. Due to immersion diuresis, alcohol or divers deliberately reducing their fluid intake (nobody wants to pee in a drysuit on a long dive!) most divers are dehydrated. Fluids help correct dehydration and manage third space losses due to oedema and inflammation resulting from DCI.
Get Help / In the UK, you can call the National Diving Accident Helpline on 07831 151 523. If you are abroad, ensure you have found out where your nearest chamber is in advance and call them for further guidance. The Divers Alert Network (DAN) can help put you in contact with a nearby chamber.
Arrange a casualty evacuation (CASEVAC) / The only definitive treatment for DCI is recompression therapy and so it is important to evacuate divers to the nearest hyperbaric chamber as soon as possible. In-water recompression (i.e. taking them underwater again) is not recommended due to safety concerns.
Key Learning Points
- Oxygen is the very best first aid you can give to a patient with suspected decompression illness. Give at the highest possible flow rate, regardless of oxygen saturations
- Know the location and phone number of your nearest hyperbaric chamber before embarking on a dive expedition
- Decompression illness can present in so many different ways, from just not feeling quite right, to a rash, joint pain, neurological symptoms, and even unconsciousness (and possibly death). Always have it in the back of your mind when assessing divers
- Cerebral arterial gas embolism can present very similarly to stroke. Both conditions are time critical so getting advice from a diving doctor early if a patient presents after diving can be very important to secure a speedy diagnosis.
If this brief introduction to dive medicine has whet your appetite, please take a look at DDRC’s Expedition Dive Medicine Course which is being run on the weekend of 6-7 April 2019. The course is aimed at doctors, nurses and paramedics planning on going on dive expeditions to give them the skills and confidence to safely manage divers in the field. It’s set to be a fascinating couple of days and will prepare you for any diving medicine opportunities which crop up in beautiful locations across the globe.
Photos: Tom Everett and Sarah Clayton