AUTHOR: DR PERSIA BOWATER
When practised safely, scuba diving can be an incredible experience, and is even sometimes a necessary activity in some occupations and for access to certain environments. However, it is a high-risk sport in which a lack of care can result in dangerous consequences. Decompression illness is one of these, and is the most reported diving related incident by the British Sub-Aqua Club (BSAC) (1).
What is a decompression illness ?
Decompression illness (DCI) is split into two conditions: decompression sickness (DCS) and arterial gas embolism (AGE). Decompression sickness or ‘the bends’ is where a rapid reduction in pressure of the patient’s environment results in bubbles of gas that are normally dissolved (usually nitrogen) forming in body tissues and/or blood. This can result in a wide range of non-specific presentations. AGE occurs when this gas enters the arterial circulation. Whilst these are often reported in scuba divers, they can also be seen in flying and space travel, or in caisson or mine workers.
Decompression illness (DCI) is caused as a result of ‘evolved’ gas. The basis of this is Henry’s law, where the concentration of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the solution. As divers descend, pressures increase and each inhaled breath contains a greater concentration of gases. Therefore, the concentration of dissolved gas in the blood increases. This leads to more nitrogen gas being absorbed into tissues than at normal pressures.
When divers begin to ascend, the tissue-based gas begins to move back into the bloodstream. This is known as ‘off-gassing’. Ideally this gas remains dissolved, however if a diver ascends too quickly, resulting in a rapid change in pressure, or if the level of absorbed gas is too high as too long has been spent at too greater depth, bubbles can form in the blood and tissues.
Figure 1 – Visual representation of Henry’s law (6).
Arterial gas embolism
Bubbles can find themselves in the arterial circulation by one of three processes.
Pulmonary barotrauma occurs when the volume that an amount of gas occupies increases as pressure decreases; this is known as ‘Boyle’s law’. If divers ascend while holding their breath, or have trapped air in their lungs from pulmonary disease such as COPD, the volume of inhaled gas in their lungs over-expands relative to lung volume, potentially resulting in pneumothorax, pneumomediastinum or alveolar rupture. This can result in an air embolism entering the arterial circulation.
If the venous gas bubble volume overwhelms the pulmonary filter that usually removes them, some will remain and pass into the arterial circulation. Additionally, having a right to left shunt such as a patent foramen ovale can cause bubbles to pass through this into the arterial circulation.
Bubbles can occlude arteries, resulting in ischaemia wherever the blockage occurs. Even after this resolves, there can be a rebound ischaemia caused by emboli as a result of vessel damage. This often results in neurological symptoms – one study found that 88.5% of divers treated for DCI had some form of neurological symptom at presentation.
Figure 2 – Visual representation of Boyle’s law (7).
Presentation of decompression illness
Decompression Illness (DCI) can occur anytime up to 72 hours post-depressurisation, however, the likelihood that this is the diagnosis increases the earlier after depressurisation that the symptoms occur – 90% of cases present in the first 6 hours after ascent.
Decompression Sickness (DCS) can lead to a wide range of presentations, as bubbles can form almost anywhere in the body, and therefore history and timing of presentation are important in diagnosis. Subclinical venous bubbles are common and cause no symptoms, and are usually filtered out by the pulmonary circulation.
In type 1 (mild) DCS, bubbles in the muscles, joints and tendons cause an aching pain and inflammation. Symptoms can be vague including malaise, fatigue and headache. An erythematous, mottled rash with areas of pallor (livedo reticularis / cutis marmorata) can develop briefly. Bubbles in the lymphatic system are rare but can cause localised pain around lymph nodes.
Type 2 DCS is more severe and may have life-threatening consequences affecting the inner ear, cardiorespiratory and neurological systems related to high venous bubble load or AGE. Inner ear symptoms include hearing loss, vertigo, nausea or impaired balance. Bubbles in the lungs can cause ‘the chokes’ – a dry cough and chest pain behind the sternum with associated dyspnoea. Arterial gas emboli may lodge in the coronary vessels causing chest pain and sometimes cardiopulmonary arrest. Sudden stroke-like symptoms (e.g. weakness, paraesthesia, visual changes, facial droop or altered speech), seizures, confusion or sudden loss of consciousness may be caused by a cerebral arterial gas embolus (CAGE). Spinal cord involvement can also lead to weakness and paraesthesia with loss of bowel or bladder control.
Figure 3 – Pulmonary barotrauma resulting in CAGE (5).
Patients should be assessed using an ABCDE format, with particular attention paid to chest examination, looking for signs of pneumothorax and immersion pulmonary oedema. Observations may reveal low oxygen saturations, tachycardia, and tachypnoea. Full neurological examination should also be carried out.
A chest X-ray and/or CT chest can be helpful in determining if any pulmonary barotrauma resulting in pneumothorax is present. If stroke-like symptoms are present, a CT head should be performed. However, decompression illness (DCI) is primarily a clinical diagnosis – a history of recent repressurisation in combination with wide-ranging and otherwise unexplained symptoms.
Prevention is always better than cure. The risk of decompression illness can be reduced by:
- Making sure that all divers have up to date and appropriate qualifications for the activity they are undertaking.
- Completing the recommended decompression stops during ascent including a mandatory 3-5 minute safety stop at 5-6 metres depth.
- Not exceeding the recommended time at depth.
- Leaving enough time between dives for tissue bubbles to resolve, remembering the added risks of travelling to altitude or flying after diving.
Recommended time at depth and between dives is calculated using a dive planner, prior to commencing the dive.
Figure 4 – An example of a recreational dive planner
Primary management of DCI is high-flow oxygen. Given at 15L per minute via a non-rebreather mask, this is the most effective treatment for DCI and will help to relieve any associated pain. Any other pain relief is usually not required. Nitrous oxide (Entonox) in particular is contraindicated for pain relief, as this will increase the quantity of dissolved nitrogen gas in body tissues.
Definitive treatment is via recompression therapy at a dedicated facility. This involves using a hyperbaric chamber to simulate a high pressure environment (2.5 to 3 atmospheres). 100% oxygen is given to the patient for up to 300 minutes at a time to help increase tissue and blood oxygen levels, and decrease gas bubble size and concentrations of nitrogen. Due to the use of 100% oxygen, opiate analgesia should be avoided as this can increase propensity to oxygen toxicity.
It is important to confirm if the patient has a barotrauma-associated pneumothorax prior to recompression therapy, so that a chest drain can be inserted before the patient is moved into the hyperbaric chamber.
Oral and/or IV fluids are recommended to promote increased tissue perfusion.
Differentials to consider
As the symptoms of decompression illness (DCI) can be so varied and vague, it can mimic a wide variety of conditions, from musculoskeletal pain to stroke. As such, it is important to take a detailed history to consider the context of the presentation and the symptoms having occurred in the first 72 hours since depressurisation.
Diving specific differentials to consider include inner ear barotrauma, middle ear/maxillary sinus over-inflation, contaminated diving gas/oxygen toxicity and immersion pulmonary oedema.
If there is chest pain, an MI or PE should be ruled out. If neurological symptoms such as retention/incontinence occur, spinal cord compression/injury should be considered.
Take home messages
- Decompression illness is where bubbles of inert gas form in body tissues as a result of rapid environmental pressure change.
- It comprises decompression sickness and arterial gas embolism.
- Symptoms are very varied due to the possibility of bubbles forming in almost any body tissue, but pain is the most common symptom.
- Patients with suspected DCI should be started on high flow oxygen regardless of their oxygen saturations.
- High flow oxygen usually provides sufficient analgesia. DO NOT give nitrous oxide (Entonox) for pain relief, as this will increase the quantity of dissolved nitrogen gas in body tissues. Avoid opiates as they can increase propensity to oxygen toxicity.
- Definitive treatment is via recompression therapy through use of a hyperbaric chamber.
- BSAC. Annual Diving Incident Report. 2021
- J Stephenson, Pathophysiology, treatment and aeromedical retrieval of SCUBA – related DCI, Original Research & Articles, Volume 17 No. 3, Doi No 11.2021-82717932
- S Clayton, C Walklett, Decompression Illness, Royal College of Emergency Medicine Learning, published 14/06/2022 https://www.rcemlearning.co.uk/reference/decompression-illness/#1567502925950-0cb9805c-1a89
- E Campbell, Arterial Gas Embolism, SCUBADOC – Diving Medicine Online (scuba-doc.com)
- R Vann, F Butler, S Mitchell, R Moon, Decompression illness, The Lancet, 2010; 377: 153–64.
- Diver’s Alert Network, Health & Medicine, Health Resources, Decompression Sickness, Chapter 3: Diagnosing Decompression Sickness, https://dan.org/health-medicine/health-resource/dive-medical-reference-books/decompression-sickness/diagnosing-dcs/
- Figure 1 – Colligative Properties | Scholars Online Chemistry (dorthonion.com)
- Figure 2 – 22.3 The Process of Breathing – Anatomy & Physiology (oregonstate.education)