The Mechanisms of Decompression Illness Part 1 of a 3-part article by - Part 1 Dr. Simon Mitchell Bubbles can form in tissues and blood Few subjects are more central from dissolved gas… to diving medicine than decompression illness, the most The first source is bubble formation important diving medical from nitrogen that has been disorder. We are all taught about dissolved in tissues during the dive. the disorder and its potentially As you have been taught in your most basic training course, during a serious consequences during dive, air is breathed at increased diver training courses but the pressure, and therefore more of the treatment given to the subject by nitrogen molecules from the air can the diver training organisations is dissolve into the blood. Take a few relatively superficial. Recognising moments to study the very simplified its importance, many divers want human circulation diagram (Figure 1) and read the explanatory caption. to know more; those few extra A simple definition…. details that might help them The nitrogen enters the blood in the lung capillary bed (top of diagram), improve their diving practice, Decompression illness is a disorder and after passing through the left make things a little safer, or caused by bubbles forming in body tissues heart, is distributed to the tissues via simply satisfy their curiosity. To or in the blood; in other words, in places the arteries. In the capillary beds of help with all of these, the where they shouldn’t be. These bubbles the head and body, nitrogen leaves following series of three articles cause damage by a variety of the blood and diffuses into tissues. in the next issues of Alert Diver mechanisms that I will return to in The deeper the dive, the faster the the next issue. How do these bubbles nitrogen is taken up from the air we SEAP will consider the topic of form? decompression illness in more breathe, and the longer the dive, the more time it has to accumulate in the detail than you will usually see. There are two potential sources tissues. In this first article, I will explain of bubbles… the sources of the bubbles that During the subsequent ascent, are the central cause of this There are two potential sources of pressure falls and less nitrogen can puzzling disorder. In my next bubbles, and even though these remain dissolved in the tissues. As article, we will examine how sources are quite distinct (see pressure falls the nitrogen diffuses below), we refer to the adverse out of tissues and into the venous these bubbles cause damage, and effects of bubbles from either source what symptoms result. Finally, in blood in the capillary beds of the under the “umbrella” term head and body (see the circulation the third article we will discuss “decompression illness” (DCI). This diagram). It is carried in the veins the various potential risk factors is because, not surprisingly, it is often back to the lungs for elimination. for decompression illness, and difficult to tell which source of Ideally, this elimination process how the risk can be minimised. bubbles has caused a patient’s occurs fast enough to dissipate the problem. So what are these two nitrogen molecules without bubble sources of bubbles? Alert Diver, SEAP 6 Figure 1. The human circulation. Oxygenated blood (“arterial blood”) (red vessels) leaves the lungs and is carried to the be eliminated as described earlier. left side of the heart. It enters the left atrium (LA), then the left ventricle (LV), which They enter the blood in the capillary pumps the blood through the arteries to all beds of the head and body and seem the various tissues of the head and body. In prone to forming bubbles there. these tissues the blood flows through thin These bubbles are then carried back walled capillaries, and across these thin walls, to the right side of the heart in the oxygen in the blood is exchanged for carbon dioxide in the tissue. De-oxygenated blood veins (follow it on the diagram) and (“venous blood”) (blue vessels) passes back then to the capillary bed of the lungs. to the right heart in the veins and is then The lung capillary bed is the first pumped to the lungs. Once again, in the lungs network of small blood vessels the deoxygenated blood flows through thin where these bubbles are likely to trap. walled capillaries where carbon dioxide in the blood is exchanged for oxygen in the tiny Indeed the lung appears to be a good alveoli (air sacs) of the lung tissue. See text filter for venous bubbles, and seems for explanation of points 1 and 2. able to tolerate a significant proportion of its capillaries being obstructed by bubbles without formation. However, in tissues that the medium – fast tissues, such as adverse effects. This is fortunate cannot get rid of their nitrogen very spinal cord white matter, that often since without this “lung capillary quickly, the pressure of dissolved cause the problem in typical short filter” many of these bubbles might nitrogen will exceed the ambient duration sport dives. They can find their way into the arterial (surrounding) pressure at some point accumulate a large nitrogen load circulation (the red side of the during the ascent (a process reasonably quickly, but might not lose diagram) where they can do more commonly referred to as it quickly enough during the ascent, harm (see later). The fact that blood “supersaturation”), and the especially if the bottom time was bypasses the lungs in the circulation molecules of dissolved nitrogen may long and the ascent a little fast. of a foetus (and is therefore not not be able to “resist” the physical “Slower” tissues such as tendons are filtered in this way), is one of the stimulus to form a bubble. often not a problem after a short concerns over diving during dive, even if it is deep, because they pregnancy. It is common to hear mention of don’t have enough time to “fast” and “slow” tissues in accumulate significant quantities of As an aside, when various references discussions of these processes. nitrogen. However, they become mention “Doppler bubble These terms refer to the speed with more important during long dives, or monitoring”, it is the detection of which a particular tissue takes up and repetitive dives, when nitrogen can these venous nitrogen bubbles using eliminates nitrogen. This is build up over a long time. Doppler ultrasound to which they determined by the amount of blood refer. From the above discussion it the tissue receives (more blood flow Bubble formation from dissolved gas should be obvious why bubble usually means that nitrogen uptake can occur both within the tissues counts made using this technique do and elimination is faster) and other themselves and in the blood. Here’s not always correlate with the risk of factors such as the relative solubility an important point: bubbles forming DCI. As I have mentioned, most of of nitrogen in the tissue and blood. in the blood do so in the capillaries these venous bubbles will be filtered As a rule, very fast tissues are quite on the venous side of the head and out in the lung capillary bed and may resistant to bubble formation body circulation (see point 1 on the do no harm. In contrast, the real because although they take up diagram). If you think about it villains might be the bubbles forming nitrogen quickly, they usually get rid carefully, this is not surprising, since in the tissues themselves, and of it quickly enough to prevent during ascent all the nitrogen Doppler techniques cannot count bubble formation, so long as the molecules from the tissues are these bubbles. ascent is made at the correct rate. It’s “trying” to get back to the lungs to 7 Bubbles can be introduced to the blood by damage to the lungs… The second potential source of Figure 2. The human circulation showing bubbles in DCI is the introduction a persistent patent foramen ovale (PFO). of air bubbles to the arterial Compare this to Figure 1. In this situation a circulation because of lung over- communication exists between the right and expansion. This has nothing to do left atria of the heart, and blood may with dissolved nitrogen, or time and potentially move from the venous side of the circulation to the arterial side (blue arrow). depth for that matter. Indeed, this This “shunting” blood may carry venous problem can arise during ascent from bubbles that would normally be filtered out depths as shallow as 1 – 2 metres. by the capillary bed of the lungs into the Open water trainees are always arterial circulation where they are potentially taught that the most important rule more dangerous. In reality, most PFOs are small and may shunt very little, if at all. Small in scuba diving is “to breathe PFOs are of doubtful significance in diving, normally at all times; never hold your but there is a clear association between large breath”. This is because any air shunting PFOs and DCI. trapped in the lungs during an ascent (by holding the breath for example), will expand as pressure decreases. If Considering that the lung filters there is sufficient over-expansion of is the capillary beds of various venous bubbles while arterial bubbles the lung it may rupture some of the tissues in the head and body. Some can block blood vessels in vital small airways and the associated of the organs potentially affected in structures such as the brain, it might blood vessels. Such damage is this way, such as the brain, are much seem fair to conclude that the referred to as pulmonary barotrauma.