Physiology of Rebreathers Part 3

By Dr. Sawatzky

The first is the risk of infection. The breathing loop is warm, moist and dark, ideal breeding grounds for fungi, bacteria and viruses. Whatever bugs are in the divers' mouth, throat and lungs will end up inside the rebreather. Many of us have very nasty bugs in our throats all the time, even when we are not sick. Therefore, you must disinfect the rebreather thoroughly at the end of every dive trip or before anyone else uses it. To dive a rebreather that has been used by someone else without disinfecting it thoroughly is worse than extended, 'deep throat' kissing that person!

The second negative point of rebreathers is that they require significantly more care and maintenance than OC. The gas cylinders have to be filled, although far less often than OC. The chemical used to remove the CO2 has to be changed and disposed of. The O2 sensors have to be dried after every dive (open up that part of the rebreather to the air) because too much water on the sensors will interfere with their function. The batteries and O2 sensors have to be changed periodically, and of course the breathing loop has to be cleaned. All of this maintenance makes rebreather diving more expensive than diving air. However, it is less expensive than diving nitrox and much less expensive when diving trimix or heliox. This is especially true in remote locations where the cost of diving gases can be extremely high. In fact, an active trimix diver in Japan will save the cost of their CC rebreather in only two years!

How can a rebreather kill you? The three major problems are hypoxia, hyperoxia and hypercarbia. In English, not enough O2, too much O2, and too much CO2. Remember from your previous reading/training, or by referring back to my columns on Oxygen Toxicity in the April and May 1995 issues of Diver Magazine, that we can only tolerate partial pressures of O2 between 0.16 and 1.6 ATA. Also remember that any PO2 greater than 0.5 ATA will be toxic if breathed long enough. Therefore, we need some way to control the PO2 within this fairly narrow range. In the two previous columns we explored several ways to do this. Unfortunately, there are complications that we have not yet discussed. One of these is that the PO2 in the counterlung can change dramatically when the diver changes depth. As the diver descends, the PO2 in the counterlung rises due to the increase in pressure, and as they ascend, the PO2 drops.

For example, if the diver starts on the surface with a PO2 of 0.7 ATA (70% O2, the default low setting on the Inspiration) and rapidly descends to 100 fsw, the PO2 in the breathing bag will be 4 * 0.7 ATA = 2.8 ATA, a very dangerous level indeed! To make life even more complex, the volume of gas in the breathing bag will be reduced to of its original volume (the diver will have very little gas to breathe) so the diver has to add gas to the bag during descent. What gas does he add? That depends on the way the rebreather is designed. On the Inspiration, for this kind of dive, the diver would most likely have air in his diluent bottle. As he adds air during the descent, the PO2 will be reduced (at 100 fsw air has a PO2 of 4 * 0.21 ATA = 0.84 ATA). In addition, the diver will be absorbing O2 during the descent and this will also cause the PO2 to drop. Therefore, a dangerously high PO2 spike is only a problem during rapid descents, or if the diver adds too much pure O2 by mistake at any time during the dive. The last way to have too much O2 is if the rebreather malfunctions and adds too much O2. The exact way in which this can happen depends on the rebreather. On the Inspiration, the O2 injector could fail in the open position, continuously adding O2 (the response is to go on OC bailout gas, turn off the O2 bottle and flush the counterlung with diluent until the PO2 is back in the safe range). There are many ways to try to control this problem. The Inspiration has three O2 monitors and two independent controllers. When the average PO2 of the two O2 sensors that read closest to the same reading, is higher than 1.6 ATA, there is an audible alarm that goes off. The diver is also trained to check the PO2 readings every minute of the dive and to listen for the O2 injector (easy to hear as the set is totally quiet, no bubbles!). If it is injecting too often, check the PO2 even more frequently. The Inspiration also monitors the amount by which the third O2 sensor differs from the other two. If the reading is more than 10 percent different from the average reading of the other two O2 sensors, the alarm goes off.

The second problem is hypoxia or not enough O2. If the diver is at 100 fsw with a PO2 of 0.7 ATA in the counterlung and they rapidly ascend to the surface, the PO2 will drop to 0.7/4 = 0.175 ATA or 17.5 percent O2 on the surface. That is OK but remember, the diver is also absorbing O2 during the ascent and the real PO2 will be lower. How do we solve this problem? One way is to do slow ascents. The rebreather will be adding gas on the way up, maintaining the PO2. The diver can also add gas/O2 during the ascent. The one thing you do NOT want to do is to ascend if the PO2 becomes too low (you will pass out and probably die). Another way in which the PO2 can become too low is if the rebreather is not adding gas/O2 as required. The diver is rebreathing the same gas over and over and the PO2 will drop until they pass out. Remember that the CO2 is being absorbed by the chemical in the scrubber so the diver will have absolutely no warning. The most common cause of this problem is the diver forgetting to turn on the O2 bottle or turning it off during the dive. In addition, on sets like the Inspiration, the O2 injector could fail closed, not adding O2. The diver always has the option of manually adding O2 but they have to be paying attention to the PO2 readings to realize that they have a problem before they pass out. The Inspiration also sets off the audible alarm if the PO2 is lower than 0.5 ATA.

Finally, we have the problem of hypercapnia or too much CO2. Rebreathers all contain a chemical to absorb the CO2 produced by the diver. If the diver leaves the chemical in the rebreather too long, it will become saturated with CO2 and no longer be able to remove the CO2 from the gas. Therefore, the diver will be rebreathing gas with a rising level of CO2. On the surface, rising PCO2 is relatively easy to spot because the increasing PCO2 causes us to breathe deeper and faster (the body is trying to get rid of the extra CO2). At depth however, we may be distracted by other things, the naturally increased difficulty of breathing through a regulator and breathing set may cause us to not notice the increased respiration, or the increased PO2 may dampen the normal respiratory drive caused by CO2 so that the first thing we notice is paranoia, anxiety, panic, or loss of consciousness. The only practical way to prevent this problem is to change the CO2 absorbent as often as recommended by the rebreather manufacturer.

Several people are working on a reasonably priced, functional PCO2 monitor for rebreathers but so far, there are none available.

The preceding discussion has shown that rebreathers can be awesomely more efficient than open circuit but they can also kill you quicker and in more ways than you can imagine. One senior rebreather instructor describes them as "Hideous Insidious Killing Machines". The efficiency is roughly proportional to the price with less expensive (a few thousand dollars) semi-closed circuit rebreathers being five to 10 times as efficient as OC.

Fully closed circuit rebreathers like the Inspiration are expensive $10,000 to $30,000 but come very close to 'perfect' efficiency. Semi-closed rebreathers have less problems with hypoxia and hyperoxia, but people still are dying on them.

In conclusion, rebreathers are fabulous toys. They produce few or no bubbles, allowing you to take better pictures and to get much closer to marine life. They are potentially awesomely efficient but they are also potentially extremely dangerous. It is vital that you obtain thorough instruction from an excellent instructor who has lots of time on the specific rebreather you are learning, and that you remember the following fact. When you start diving a rebreather, you are a complete novice. Your thousands of open circuit dives have simply taught you a lot of habits that can kill you in an instant, on a rebreather.