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Mildly Bent: Decompression Stress and Micro-Bubbles

The issue of decompression stress is badly understood by many divers. This article seeks to explain what it is, and how it links to microbubbles and sub-clinical DCS.

What is sub-clinical DCS?

Bubbles occur in our bodies on the ascent from most scuba dives, but those bubbles have to reach a sufficient size before they cause enough immediate harm to present diagnoseable symptoms of DCS.

Bubbles of insufficient size to cause clinically identifiable decompression sickness symptoms are named ‘micro-bubbles’. These bubbles may remain sub-clinically small in size but may occur in large numbers.  They may also, in sufficiently large numbers, still have a noticeable (if not immediately harmful) impact upon us.

It can be proposed that post-dive fatigue, malaise, reduced vitality and drowsiness are symptoms of the increased frequency of non-symptomatic micro-bubbles. This is called “sub-clinical” or ‘non-symptomatic’  mild DCS.

“… “mild DCI” is harder to define. This is largely because bubbles form, and can be detected in the venous blood, following a significant proportion of dives that do not result in symptoms that would normally be labelled “DCI”. It is therefore hardly surprising that there can be a poorly defined boundary between wellness and mild DCI following diving. […] It is not infrequent for divers to report highly non-specific and vague symptoms of variable latency, such as mild evanescent aches and pains, fatigue, demotivation and headache“.


“Inappropriate fatigue is a clear-cut symptom of subclinical decompression illness. I’m not a technical diver, Lord knows; but I see a lot of these guys. I send them out with decompression tables and they tell me what happens. They use their fatigue, their feeling of well-being – we called it constitution yesterday – as a major item in judging whether their decompression was adequate. The fact that they don’t have any joint pain or tingling or rash or anything is secondary. Their major thermometer as to whether they’ve been properly decompressed is how well they feel. And that has to do with the fatigue and inappropriate fatigue. So we have to keep this as a symptom, not as a side issue“.


Doppler monitoring has revealed the presence of gas phase bubbles in divers ascending even from relatively shallow dives. We have also all experienced symptoms such as headache and fatigue in the immediate post-dive phase which we have put down to overexertion on exiting the water etc. In more recent years, we have been told that these symptoms are those of subclinical DCI and we have lived with these, perhaps naively, thinking that because they are not associated with overt symptoms of DCI, they are not causing any damage and are nothing to worry about. However, enter the iceberg principle. Nine tenths of the damage may be invisible and cummulative”.

THE TROUBLE WITH BUBBLES.  Richard Heads PhD. 9-90 Magazine: UK Diving in Depth

What is decompression stress?

Decompression stress is defined as “the amount of inert gas dissolved in various tissues throughout the body” (Alert Diver Magazine, Ascent Rates). It is measured by counting a ‘bubble scores’ (via the Spencer Scale, Powell Grade or KM code) on the size and frequency of micro-bubbles in the body after a dive using Doppler ultrasound devices.

“…we know that bubbles are probably present after most dives, so we don’t have to exceed an M-value to produce bubbles in the absence of DCS. […] Although an M-value may not have been exceeded, symptoms of decompression stress such as fatigue, malaise, drowsiness etc. could well be the result”.

THE TROUBLE WITH BUBBLES.Richard Heads PhD. 9-90 Magazine: UK Diving in Depth

Decompression stress typically presents as post-dive fatigue. The diver will feel sleepy, lethargic and generally low in vitality. There are theories that these symptoms may be caused by micro-bubbles provoking an immuno-response in the body, and inflammatory damage to the endothilial tissues of the veinous system caused by bubble passage.

Read more about DCS and the Immune System.

What are micro-bubbles?

It is well proven in multiple studies that all divers develop small bubbles in their body after diving, but these micro-bubbles (also known as silent bubbles) are insufficiently large to cause us significant, immediate harm.  The frequency (number) of those bubbles is determined by the effectiveness of off-gassing nitrogen on the ascent. Poor off-gassing leads to ‘decompression stress’ and is represented by a larger frequency of micro-bubbles in the body.

Decompression models used in dive tables and computers assign a maximum value (m-value) to ascent supersaturation in order to prevent those micro-bubbles from growing to a size that causes clinically diagnoseable harm (symptomatic decompression sickness).

Some modern decompression models, known as ‘bubble-models’, also include calculations that reduce the number of fast tissue micro-bubbles by off-gassing at deeper depths. That alleviates post-dive symptoms of decompression stress, but the trade-off is that deeper stops empower more slow-tissue ongassing. On a like-for-like decompression time basis, deep stop decompression models cause more incidents of clinical DCS than traditional dissolved gas models, such as Buhlmann ZHL-16.

Factors known to influence off-gassing efficiency and micro-bubble elimination include; ascent speed, ascent stops, dive profile and ascent gas selection.

About the Author

andy davis technical diving philippines

Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia.  Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.

Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.

He is currently writing a series of books to be published on advanced diving topics.  Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.

Originally posted 2018-03-07 23:56:24.

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