Witness to a Rebreather Fatality Copyright ©2008 by Mark Derrick, IANTD CCR Instructor Permission is granted to copy and/or distribute this document under the terms of the GNU Free Documentation License On a pleasant afternoon in March of 2008, off the coast of Hillsboro Beach Florida, I suddenly found myself at the back of a dive boat kneeling over a fellow rebreather diver and administering CPR. This article contains my opinions and speculation about what happened that day, and is offered to help other rebreather divers understand some of the factors that led to the accident. It assumes the reader is familiar with closed-circuit rebreathers. The boat was a commercial dive charter operation and the itinerary was a light technical dive on a wreck followed by a drift dive on a reef. Among the boat passengers were several divers with instructor level ratings, although none were acting in an instruction capacity. Of particular note, the boat captain was an open-circuit (OC) technical diving instructor, and an instructor for DAN’s Diving Emergency Management course. The dive master was a CPR instructor. Also aboard was an experienced technical closed-circuit rebreather (CCR) instructor (myself), along with a non-diving rider who was an OC technical diving instructor. The victim was an experienced OC instructor and semi-closed rebreather (SCR) instructor who had recently received a CCR instructor rating. The first dive was unremarkable except that at conclusion of the first dive, during exit from the water, one of the divers dropped a bailout cylinder. Once all the other divers were aboard, during the surface interval I did a bounce dive to find and recover the cylinder. The second dive of the day was a reef drift dive. The typical routine is that the boat carefully positions over the top of the reef then the dive master calls for a team of divers to quickly enter the water. A buddy team enters the water together and immediately swims to the bottom. One of the team will carry ‘the flag’, a marker buoy with a dive flag and line towed by the diver. The captain then repositions with some separation between flags and successively drops each of the teams along the reef line. One team elected not to perform the second dive. The other teams had made their entry; the victim and his buddy were the last team to enter the water. Also, my buddy and I had delayed our entry for the second dive in order to extend my surface interval. Shortly after the victim and his buddy entered the water, I was in the process of gearing up when I heard a call of “emergency” and then heard the engine power up to reposition the boat. I removed my equipment and went to the back of the boat at the dive platform to assist. The dive master entered the water and brought the unconscious victim to the dive platform. The victim was extremely blue, much more so than I had observed in other drownings, and there was a slight amount of foaming from his mouth but no blood. The victim was large as well as obese, and there was a struggle with several of us attempting to pull him into the boat. I was concerned the DSV mouthpiece might be open, causing the rebreather to flood and the victim to sink. I eventually got hold of the breathing hoses and found the DSV to be closed. Unable to bring the fully geared diver into the boat, we cut the rebreather free and it was left drifting in the water. Even free of equipment, there was additional delay as it required the combined efforts of several people to bring him on board. Once on board, CPR was begun and oxygen was administered using a bag valve mask. The color of the victim did rapidly improve, but there was no further response to CPR. A marine police boat came along side with the intention of transferring the victim to the police boat in order to more rapidly deliver him to shore based EMS, as is standard procedure. However, the dive boat was close to the inlet, thus the captain elected not to incur the delay of a transfer and drove the victim directly to the EMS waiting at the dock. (The other dive teams were all still in the water and were recovered by another dive boat). CPR was administered continuously until reaching the dock where the EMS personnel took over care of the victim. The victim was transported to hospital and later pronounced dead. The time from the beginning of the emergency to the time the victim was in the care of EMS at the dock was approximately 30 minutes. The rebreather was recovered and brought to the dock about an hour after the accident occurred. It was configured with cylinders containing normoxic trimix diluent and pure oxygen. The diluent cylinder valve was open about two turns, and the gauge indicated over half full. The oxygen valve was closed tight and the oxygen gauge indicated zero. The oxygen cylinder was also found to be over half full. The electronics indicated alarms for low level of oxygen (PO2) on their displays with a breathing loop PO2 of 0.07 ATA. The heads-up-display was configured to indicate the actual PO2 for each of three oxygen sensors. The electronics were set to automatically shutdown after two minutes when the unit was out of the water. The BC wing included a BC integrated alternate air source for on-board bailout to diluent. The off-board bailout cylinder was full, the valve closed, the hose and second stage were stowed in a tidy configuration. Further, the off-board bailout was found to be disabled because the DIN first stage was not fully screwed into the valve and the regulator was flooded. What follows contains some speculation because all the facts will never be known, but it’s a reasonable scenario that fits the information available. The victim was not observed to use a checklist and this suggests at least some pre-dive checks were omitted. With the electronics configured for an auto shutdown value of two minutes, it is unlikely he was routinely performing a three to five minute pre-breathe test. The victim performed his first dive without incident, but probably did not have a working off-board bailout. Upon conclusion of the first dive, he did something typical of SCR divers although unnecessary for CCR divers: he closed the on-board gas supply valves. While preparing for the second dive, he did not perform essential pre-dive checks or a pre-breathe test which would have alerted him to the closed valves. When told to jump for the second dive, like a sport open-circuit diver, he simply stuck the mouthpiece in his mouth and entered the water. Upon entering the water and starting the descent, unable to get a breath he would have discovered one of his two gases was off so he reached back and opened his diluent supply valve. For an SCR diver this would not be entirely out of character, and the victim had recently taught an SCR class. In order to conserve gas, some SCR divers turn their single oxygen rich supply gas on at the last possible moment and turn it off at the first opportunity. However, CCR’s have two supply gases (a low oxygen content diluent and pure oxygen) and he did not turn on his oxygen supply valve. During the descent, the increasing pressure plus residual oxygen in the supply hoses, would have served to delay a severe drop in the oxygen level of the breathing loop. His buddy, slowed in descent by the flag line, reached the bottom a moment later and the victim gave his buddy an “OK” hand signal. The buddy turned his attention to taking up the slack on the flag line. It would have been at this point the victim either checked his PO2 in the HUD, or received a PO2 alarm in the HUD, or looked at his handset PO2 display with the intention of raising the set point. In any case he became aware of a low PO2 and closed his mouthpiece to cease breathing from the CCR. The victim then began swimming toward his buddy. When the buddy looked up from the task of managing the flag line, the victim was swimming toward him with the mouthpiece out of his mouth and reaching for the bailout on the buddy. The buddy began to deploy the bailout, but the hypoxic victim became unconscious adjacent to his buddy. The buddy raised the victim from 50 feet to the surface and alerted the nearby dive boat. A breathing loop PO2 of 0.07 at the surface would have been increased by the effects of pressure at depth to a mildly hypoxic 0.18 at 50 feet. A PO2 of 0.16 is low (hypoxic) and will cause mild symptoms of fatigue and confusion. A PO2 of 0.10 and lower causes unconsciousness and will not sustain life. It is pure speculation why the victim took the actions he did to address a mildly hypoxic loop. He had several options for immediately getting a safe breathing gas. His training emphasizes going off a hypoxic loop and switching to OC bailout. His first choice was switching to his BC integrated alternate air source for on-board OC bailout. Because the off- board OC regulator was undisturbed, I don’t believe the victim attempted to use his second choice off-board bailout.