Halloween is almost upon us, so it seems like a good time to talk about some seriously scary stuff: Neurological DCS and inner ear DCS – the really bad cases of the “bends”- that can sometimes result in permanent paralysis, deafness, even death.
You may be aware that many cases of neurological DCS are the result of a PFO (Patent Foramen Ovale) which is, essentially, a hole in the septum of the heart that divides the right side (which receives blood from the veins) from the left side (which pumps out oxygenated blood to the body). Venous blood passing through a PFO into the arterial system is not in itself a problem, except in the case of “medically significant” ( i.e. large) PFOs. PFOs are not uncommon in otherwise healthy individuals and usually go unnoticed. These individuals may also naturally develop small bubbles in their circulatory system, some of which may occasionally pass through even small PFO’s.
Even when bubbles do pass through the PFO, these AGEs, or Arterial Gas Emboli (which is what they are known as once they reach the arterial system) are usually harmless – until we consider diving. This is because AGEs coming through PFOs are generally small, which makes them thermodynamically unstable. Larger AGEs, if they were to occur, would be more stable, as both they and arterial blood would be pretty much equally saturated with gas. But with a small AGE, the surface tension increases the pressure on the gas within it, which causes it to dissolve rapidly. No more bubble, no problem.
By changing just a few features of an essentially harmless situation, compression and decompression during diving turn it into a potentially dangerous one. These are the changes that matter: 1) During compression, body tissues accumulate increasing amounts of nitrogen so that, during decompression, they become super-saturated with nitrogen; 2) During decompression, the number and size of bubbles in the veins increase; 3) The size of bubbles decreases during compression.
Here’s what happens then.
The first result of these changes during a dive is a much greater probability that a bubble will pass through the PFO, simply because there are so many more of them. (While the increase in number of bubbles occurs during decompression, remember that decompression includes, roughly speaking, all time spent after ascending from the deepest point of the dive.)
The second result is that a bubble with more gas in it could get in at depth than could happen when not diving. How can this be, when the size of the PFO hasn’t changed? When the size of bubbles decreases during compression, there are actually two separate things happening: One is that gas is escaping from bubbles under pressure – less gas in the bubble makes it smaller and some bubbles will disappear. The other thing that happens is that bubbles under pressure become smaller even without losing any gas. The radius of the bubble gets smaller, but it contains the same amount of gas. The effect of this is that a bubble small enough to pass through a small PFO will contain more gas at depth than will be in a bubble with the same radius at the surface. Or, to put it another way, a bubble that might be too big to pass through a PFO under normal (surface) conditions might be compressed enough at, for example, 100 fsw. to pass through.
So, we’re more likely to have a bubble pass through a PFO, and that bubble, now an AGE, will be slower to dissolve than an AGE of the same radius formed at the surface. This makes it more likely that the AGE will survive long enough to exit the artery into the capillaries of tissues that are supersaturated.
If the AGE does reach supersaturated tissues, it will grow, taking on nitrogen from the supersaturated tissues, and, if it gets large enough, can damage the tissues by blocking blood from reaching them (producing what’s known as ischemia) and/or by directly damaging some very sensitive tissues, like those in the inner ear, as just the pressure exerted by the growing bubble may be enough to cause them to tear. When this damage takes place in brain, spinal column or inner ear, the damage is often permanent.
While PFOs are the most common route for bubbles to become AGEs, they can also access the arteries through AVAs (Arterio-Venous Anastomoses, which are remnants of fetal pulmonary shunts bypassing the lungs that didn’t fully close at birth), or through the lungs themselves, when alveoli of the lungs fail to completely filter out the bubbles from the venous blood.
I have a scientific interest in AGEs, and have recently published a paper on “The lifetimes of small arterial gas emboli, and their possible connection to Inner Ear Decompression Sickness” which looks at the time required for an AGE of a particular size to dissolve and the time required for it to reach the inner ear. (I used the inner ear, because the arterial route to it is more amenable to calculation.) I’ve posted it under Articles, just in case anyone is interested.
But, like all of you, I have a personal interest, as a diver, in avoiding DCS in general and these particularly nasty manifestations of it in particular. Here are some precautions you may want to consider:
1. If you have a specific reason to suspect an PFO, you definitely should see a doctor, preferably one experienced in diving medicine, for further investigation. One example: you should suspect a PFO if you have had undeserved skin bends on more than one occasion. (And, by “undeserved” I mean, not just that you were technically within the limits set out by whatever NDL or dive computer you were using, but that you were far away from any such limits.) In some cases where a PFO is found to exist but is not “medically significant” a doctor may suggest that, if you continue to dive you should dive “conservatively”.
2. Unless you know otherwise, it’s safest to assume that you do have a PFO, an AVA or that the alveoli in your lungs miss filtering some bubbles.
Let’s suppose, then, that either you’ve been diagnosed with a PFO (but still permitted to dive), or that you’re being prudent by assuming you may have one. What does diving “conservatively” mean in this context?
To start with, it means doing essentially the same things you already do to avoid any form of DCS. (Once SAUL is available in computers, I would also suggest setting it for a lower acceptable probability of DCS than you might otherwise be inclined to do.) Beyond, this, to dive conservatively, you might particularly want to:
a) Allow a long surface interval between dives. Never do a second dive less than an hour after the first. Waiting more than an hour, if feasible, is even better.
b) If you’re a smoker, consider stopping. Smoking damages the lungs, which means it may increase the likelihood of the alveoli letting more and/or larger bubbles into arterial circulation.
c) Where feasible, do all your diving on Nitrox. If you aren’t already certified for Nitrox, get certified.
d) Wait at least 24 hours after diving before flying (or sightseeing, etc. that involves altitudes over 6,000 ft.)
e) Remain well hydrated.