Freediving - Think Small | part 2 (Lung Squeeze)
As discussed in think small part 1, it is important to keep your upper body and especially your chest area as small as possible to allow you to equalize and top up your mouthfill deeper. In this Blog we will discuss importance of maintaining a small chest once we start to dive below the residual volume using mouthfill.
During a dive, our lungs are compressed by the pressure of the water above us. As we enter our residual volume (the smallest we can compress our chest/lungs on the surface), the air in our lungs will continue to compress while our chest area stays the same size. To compensate for the negative pressure this creates, the capillaries in our lungs swell with blood. This means that the size of our lungs is not reduced in proportion to the size of the air in our lungs.
This mechanism is known as blood shift, it helps to reduce the amount of negative pressure within our chest. It is an adaptation which can be trained and varies in strength from person to person.
Lung squeeze is an injury which can happen to a diver who is diving below their residual volume. It can happen when the blood shift cannot compensate for the increase in negative pressure in the lungs, and blood passes into the alveoli. Once the diver surfaces he/she will show all the symptoms of having fluid in their lungs. In my opinion this is the most dangerous injury that can happen to a freediver and it should be regarded with the utmost caution.
Some people are more prone to lung squeeze than others, but we should all use every precaution to limit our chance of lung squeeze. Following are some recommendations which relate to the topic of this blog but there are many other factors to consider. Research regarding prevention is next to none so we must use experience and common sense to find our way forward.
To understand how Lung squeeze relates to keeping your body small you have to understand the mechanism of blood shift.
Imagine a diver has a total lung capacity of 6l and a residual volume of 1.5l. The diver will reach RV at 30m, from that depth onward the body will be compensating for the reduction of air volume in the lungs with blood shift. So at 60m the blood shift will be compensating for 0.64l (RV - (TLC/Bar)) of space in the lungs left by compression of the air. The blood shift won't fully compensate for all of the reduced space in the lungs, meaning there will be a slight negative pressure, the stronger the blood shift the longer it takes for the negative pressure to build up, this is one of the reasons some divers can Frenzel very deep.
As discussed in think small part 1, your body position can increase the size your chest can compress too. If this happened to the diver in the previous example then let's say the divers position only allowed his chest to compress to 2l (0.5l extra). Now at 60m the blood shift will have to compensate for 1.14l of reduced space in the chest, that's almost double! This is going to make it much harder for your blood shift to compensate for the negative pressure in the lungs. Making the chance of a lung squeeze much higher.
Most of the time, injuries occur when divers suddenly do something at depth that increases the size of the chest. Using the above two examples imagine a diver freefalling to 60m. The diver is relaxed and allows his diaphragm to move up into his chest thus becoming as small as possible. Then at 60m he turns, but does it by arching himself backwards and therefore expanding his rib cage by 0.5l. In a moment he has almost doubled the amount of space that needs to be compensated for and since your blood shift will not be able to increase so suddenly, you have a higher chance of lung squeeze.
How can we reduce our risks?
Not feeling any uncomfortable pressure in the lungs is very important, in fact so long as you have relaxed properly and you have given your body chance to acclimatize to deep diving you should not feel any uncomfortable pressure in your lungs. If you are feeling pressure despite the following advice, then the problem is your blood shift is not strong enough yet to equalize the pressure in your chest. This is dangerous and could lead to lung squeeze. So take 5m off the depth that is uncomfortable and don't dive deeper until the pressure is gone.
Once you have done the last top up of your mouthfill you must now relax your diaphragm and stomach muscles and allow the diaphragm to come up into your chest. This reduces the volume of your chest and as a result you will feel less pressure because the blood shift will not have to compensate for as much space as the air in the lungs is compressed.
Be aware of tension creeping back into your stomach or diaphragm during your freefall, this is what is known as pressure contractions. It is your body trying to resist the increasing pressure. If you think about it, tensing the diaphragm is a natural thing for you to do. For your whole life, whenever your lungs are small, you simply bring down your diaphragm and air will poor in. During the freefall your lungs have been compressed smaller than they ever could be on the surface. So of course there is an impulse for the diaphragm to contract, that is its job. Like so many things in freediving, the instinctive reaction is the wrong one, and we need strength of mind and training to replace this instinctive reaction with the appropriate one.
During the freefall you have kept your chest at a constant size, and your blood shift has compensated for the difference between the size the air in your lungs has been compressed to, and the size your chest is. I believe most lung squeezes happen at the turn and the first pull because a diver expands their chest volume at the deepest part of the dive, either by arching backward, tensing the diaphragm/stomach, or stretching and pulling their arm forcefully which will in turn expand the rib cage. This creates extra negative pressure at the deepest part of the dive and can lead to lung squeeze. You must train your turn at the bottom of the line to be smooth and not increase the size of your chest.
If you are diving in Free Immersion, then the assent is also a time you need to consider keeping yourself small until your body can handle bigger movements. If you stretch your arm out above your head fully, then unless you are incredibly flexible, the muscles from your chest and back will expand your chest. Therefore after the turn it is not wise to do big pulls. Unfortunately there are no given amounts of time you should not do full pulls for, what I do is start with small pulls and slowly increase the size, the whole time I am being aware of how my pulls are effecting my chest and how my chest feels. Another thing to think about in Free immersion is where you put the strength into your pulls at depth. Each pull should not be with consistent force, the start of the pull should be gentle until your elbow is close to your body, and the final straightening of the arm can be done with more power as that will not compromise the size of your chest.
If you dive with your arms above your head, then you should be sure that it is not effecting the size of your chest. If it is then you should not bring your arms up until you are closer to the surface. You can find some advice on how to check if the above head position affects your chest in freediving - think small part 1.
There are other factors which contribute to squeezes, such as genetics, fatigue, training, temperature and previous dives, but this blog is focusing on the body position.
Today's topics have been subtle ones, but they are very important once you start to progress into deep diving. They are all skills that should be mastered before increasing your depth and no amount of time spent training them is too much.
Have a good day and feel free to start a discussion below in the comments box.