"Barotrauma is
not only the
most common
diving medical
problem, it is
the most
common cause
of death in a
diving accident
(arterial gas
embolism)
(AGE)"

Lung Squeeze

I was recently asked by several divers who had suffered from decompression illness whether they could dive again and if so, what restrictions they should place on their diving. All of them have commented that there is very little information available about what to do in this crucially important situation. The approach that I use is to try to understand (as much as possible) what actually happened inside the diver's body when they were injured, match that with what we know about the physiological stresses of diving, add in the available research data and the experience of previous divers and finally consider the specific type of diving the person wants to do. The last step is to try to offer some useful advice to the diver.

The first step is to try to understand what happens inside our body when we suffer decompression illness. But before we can do that, we must know what is meant by the term decompression illness (DCI). Historically divers and diving medical experts talked about decompression sickness (Types I and II) (DCS) and arterial gas embolism (AGE). As we have slowly learned more about decompression sickness, it has become apparent that there is significant overlap between DCS and AGE and it is sometimes virtually impossible to tell them apart. In addition, over the years the treatment of DCS and AGE has become more similar with the current recommendation that treatment table six be used for almost all divers who have suffered one of these conditions. Therefore, because of the difficulty in determining what happened to the diver and because the treatment is now the same, the term decompression illness was developed to include all of the problems that divers can experience that are due to decompression.

I have written several columns on decompression sickness (The Decompression Stress/Sickness Continuum-March 94, DCS Name and Introduction-June 94, DCS One Disease or Many?-August 94, What Happens When We Get Bent?-September 94, and Risk Factors and DCS-November 94) but I have not yet written in DIVER Magazine on the types of injury that we can suffer because of pressure changes on the lungs (including AGE). Therefore, in this column we will look at the problem of lung squeeze and in the next few columns we will continue the discussion of lung problems. When we have completed the discussion on pulmonary barotrauma, we will look at diving after DCI.

Barotrauma
Barotrauma is the most common diving medical problem and the most frequent reason for divers to seek medical assistance. It is defined as the tissue damage that results from expansion or contraction of gas in the fixed volume spaces that are found within, or adjacent to the body. All gases react in accordance with Boyle's Law, (P1V1 = P2V2) and therefore, as the pressure increases during descent the volume of all gas in the body is reduced proportionally and, as the pressure is reduced during ascent the volume of all gas is proportionally increased. If gas is not allowed to enter or leave a space with constant volume, the pressure in that space will become different than the surrounding pressure (a pressure differential will develop) and tissue damage may result. It takes only 46 mm Hg of pressure (2 feet of sea water) to cause damage in some tissues and the alveoli in the lung will rupture when exposed to a pressure differential of 90 mm Hg (a change in depth of four feet of sea water or a change in altitude from sea level to 3000 feet above sea level). In diving the largest volume changes occur near the surface of the water (first 10 feet) while in aviation, volume changes increase with increasing altitude.

Barotrauma is not only the most common diving medical problem (ear squeezes) and extremely common in aviation, it is the most common cause of death in a diving accident (arterial gas embolism) (AGE). Another frequent cause of death in a diving accident and one that may now be more common than AGE is Myocardial Infarction (heart attack). As diving has expanded to include older and less fit students and as long time divers have become older and less active, the frequency of divers suffering a heart attack while diving has increased. There is no question that floating in warm, crystal clear, current free water taking pictures requires a low level of fitness. However, getting into and out of our dive gear, getting into and out of the water, swimming against the current to get back to the boat, and numerous other situations frequently encountered in diving demand a very high level of fitness. Many divers would never consider playing a serious game of basketball because they know they are not fit enough. What they often fail to realize is that diving frequently demands the same level of fitness. It is the exercise of diving that triggers the heart attack and the requirement to be fit to dive safely can not be over emphasized. However, before I get carried away on one of my pet topics, we should return to the topic at hand!

Lung Squeeze
One of the earliest forms of diving was breath-hold diving. It was commonly practised by the Ama divers of Japan and is still widely used today. In any form of activity there is always competition and so depth records for breath-hold dives were quickly and inevitably established. In the 1950s the depth record was just over 100 feet sea water (fsw). At the same time there were several diving accidents where the diver descended very rapidly (e.g. fell off the diving stage while hard hat diving) and lung damage was noted on autopsy. The diving medical specialists decided that the problem was "lung squeeze" and used the following logic: The rib cage can only contract so far and the diaphragm can only come up so high, therefore any further reduction in the volume of the lungs must result in damage. This minimal or residual volume (RV) is the lung volume after a full exhalation and if RV and Total Lung Capacity (TLC) are measured, the theoretical maximum depth for breath-hold diving for that individual can be calculated. For example, if TLC is 6 litres and RV 1.5 litres, this person could do a breath-hold dive to the depth where the pressure was increased four times (6/1.5 = 4) over the starting pressure. Therefore, the maximum breath-hold depth for this person would be 99 fsw (4 ATA). This calculated limit was in close agreement with the record at the time but that soon changed.

Depth (fsw)	Pressure (ATA)	Volume (litres)
0	1	6.0
33	2	3.0
66	3	2.0
99	4	1.5
132	5	1.2
165	6	1.0
198	7	0.86
231	8	0.75
264	9	0.67
297	10	0.60
330	11	0.55
363	12	0.50

A US Navy diver (Robert Croft) did a breath-hold dive to 240 fsw even though his TLC/RV ratio was 9.1/1.3 = 7.0 and his depth limit should have been 198 fsw. Jacques Mayol, whose TLC/RV ratio was 7.22/1.88 = 3.7 and whose theoretical depth limit was 90 fsw, dived to over 200 fsw and ultimately did a breath-hold dive to 345 fsw (105 meters)! Several divers have subsequently reached breath-hold depths in the same range and in 1991 the record was set at 361 fsw (110 meters). These dives caused the physiologists to question the belief that a persons maximum breath-hold depth limit was established by his pre-dive TLC / RV.

Further research has revealed that there is a major shift of blood into the lungs during compression (shifts of 750-1000 ml have been measured). This blood takes up volume that would normally be occupied by air and allows the residual volume of the air in the lungs to become much smaller than would be possible on the surface. Therefore, an average person with a TLC of 6.0 litres and a RV of 1.5 litres, should theoretically be able to dive to a depth of [6.0 / (1.5 - 1.0) = 6.0 / 0.5] 12 ATA or 363 fsw (110 meters). Lung squeeze is possible if the diver's heart has already stopped beating (no circulation and therefore no shift of blood into the chest) and in surface supplied diving. In surface supplied diving, the breathing gas is pumped down to the diver from the surface through a hose to the divers helmet. If the non-return valve on the hose fails or is not present, and the pressure in the hose is removed (hose torn, etc.), the diver is exposed to surface pressure. The water pressure will attempt to push the diver up the hose and lung squeeze will occur. Also in surface supplied diving, if the diver descends very quickly (falls off the stage), the breathing gas pressure will not keep up with the divers rate of descent and once again, lung squeeze might occur. However, in breath-hold diving lung squeeze should not occur.

Now that you understand the concept of lung squeeze and when it can occur, you might be tempted to rush out to try to set a personal breath-hold diving record (assuming you are escaping the Canadian winter by flying to some place warm). However, before you do that I must warn you about the possibility of death from shallow water blackout while breath-hold diving. Therefore, before you try to push your breath-hold diving limits, wait until you get your next issue of Diver Magazine in which I will discuss this problem.