chbare Posted December 18, 2009 Author Posted December 18, 2009 No, I am going to tie the hypothermia in a little later on because something about this situation sets divers up for hypothermia. No worries, not forgetting about the hypothermia because it is absolutely relevant. Absolutely; failure of the scrubber can be a primary concern and something to rule out. Failure of the electronic monitoring and mixing systems is also another thing to rule out. This is also why the prudent rebreather diver carries an open circuit bailout system. For this scenario, lets say there were no failures of the equipment. The FiO2 is a tricky thing to understand; however, if we look at Dalton's law, it explains the sum of all the individual partial pressures equals the total pressure of the environment. So, at 1 ATM the total pressure is about 760 mm/Hg, and if we have an FiO2 of oxygen of 0.21, this means that about 21% of that 760 mm/Hg will be oxygen. I will leave vapor pressure and other concepts out for the sake of simplicity. So, this gives us a partial pressure of oxygen of about 160 mm/Hg at sea level. However, what does going underwater do? It increases pressure. To breath underwater, the pressure need approximate or be higher than the pressure in the environment. This explains why we cannot breath through a 30 foot snorkel. The gas laws can further explain pressure, temperature, and volume relationships in more depth; however, I will keep the math to a minimum. We know that about every 33 feet of water equals an atmosphere. So, lets say we are 100 feet under water. The pressure equals about 4 ATM or about 3,040 mm/Hg. So, an FiO2 of .21 at sea level will have a much different meaning than an FiO2 .21 at 100 feet under water. Assuming equal pressures to keep things simple, an FiO2 of .21 at 100 feet under water equals about 638 mm/hg. Therefore, we run the real risk of oxygen toxicity when we dive because the partial pressure of oxygen is so high, even if the FiO2 remains 0.21. So, it is common for divers to transition to "hypoxic" gas mixes at deeper depths. (Hypoxic at sea level, but just fine at depth.) Therefore, an FiO2 of 10% may be just fine at depth. In addition, divers will typically replace nitrogen with helium. Helium is a stable gas with a full octet and very low density making it an ideal choice for deep dives where nitrogen narcosis is a problem. However, do you think a simple 90% helium and 10% oxygen mix could have problems at deep depths? Is there a reason deep divers actually begin adding a little nitrogen back in the mix during really deep dives? AKA trimix (Helium, Oxygen, & Nitrogen). What advantages does trimx offer over heliox, and can this explain what occurred? Take care, chbare.
Richard B the EMT Posted December 19, 2009 Posted December 19, 2009 FSW is Feet of Sea Water? Is there a difference between depth in salt water as opposed to fresh water? Perhaps there is a difference between Atlantic and Pacific Ocean sea water, as I read somewhere, in grade school perhaps, that the saline density is different between the 2 oceans. Just asking. Besides, my area is an hour drive time from the main recompression chambers in this city, but no more than 10 minutes for a helicopter from any one point to any other point. Over 35 years, never had a Bends patient, plus, I'm BLS not ALS.
chbare Posted December 19, 2009 Author Posted December 19, 2009 Yes, there exists a small difference between fresh water and seawater, and even a difference between different areas of the ocean and the oceans and seas themselves. Take care, chbare.
rock_shoes Posted December 19, 2009 Posted December 19, 2009 However, do you think a simple 90% helium and 10% oxygen mix could have problems at deep depths? Is there a reason deep divers actually begin adding a little nitrogen back in the mix during really deep dives? AKA trimix (Helium, Oxygen, & Nitrogen). What advantages does trimx offer over heliox, and can this explain what occurred? Take care, chbare. I've taken enough Chemistry and Physics to understand the partial pressures concept but in all honesty I don't know enough about dive medicine to know the reasoning behind using tri-mix. The only public baro-chamber in BC is in Vancouver so the best available treatment for this patient is a quick flight to Vancouver General (flying as low as possible) patching in to give the team a chance to prepare. Excellent idea to take the guy's dive partner and dive computer along for further information. Truth be told the guy's dive partner is probably a way better source of information than the girlfriend both in terms of the dive history and medical history. I know for a fact my climbing partners know me better than any girlfriend I've ever had in that respect. As for the hypothermia, is there any research showing it to be beneficial in the event of barotrauma? If it has shown to be beneficial you might not even want to do any rewarming until the patient has been "returned to depth". Sounds like a trip to the barometric chamber did the trick for this fellow anyway. I would be very interested to know the cause of the patient's seizure like activity at depth. I suspect the mix this patient was breathing in from his re-breather was off since many of the symptoms cleared up when he switched to his rescue tank. Knowing the results of the post analysis of this patient's dive equipment would be interesting I suspect.
chbare Posted December 19, 2009 Author Posted December 19, 2009 Sounds good, and this is a tough, unconventional, and potentially unrealistic scenario given the scenario background. However, I tend to present scenarios is a somewhat unrealistic atmosphere to emphasize points that can be lost otherwise. So, the reason we replace nitrogen with helium is to prevent nitrogen narcosis at depth. The exact action of nitrogen narcosis is somewhat of a mystery; however, it is a well known and even documented concept. Here is a video of divers who are "narced," clearly this is a potentially deadly situation: Helium, being a non reactive gas because of a concept know as meeting the octet rule. Without going over the top, this essentially means that the valence shell, or outermost orbital of electrons in the elements atom have met what is known as the octet rule. This is a very stable configuration and it makes these elements very stable and non-reactive under "normal" situations. However, a very strange thing occurs in people who breath high levels of helium at extreme depths. The numbers are all over the place; however, I think the current guidelines state that you should not breath heliox under 400 FSW. This concept is known as high pressure nervous syndrome, HPNS. (Old school people may remember the term "helium tremors.") We are not sure of the exact physiological mechanisms that cause it; however, it occurs in heliox breathing divers at extreme depth and can include; confusion, erratic behavior, and even seizures. We have found that even adding very small amounts of nitrogen to the mix can prevent the onset of HPNS. Hence, the reason for using trimix at deeper depths. In some cases, people have even experimented and used exotic gas mixes such as hydrogen and oxygen. Clearly, sofe safety concerns exist with these mixes. So, the scenario went like this:patient screwed up his gas mix, developed helium tremors, transitioned to trimix, then terminated the dive. Even though he ascended according to dive tables and compute, the bends can still occur and breathing the unexpected heliox may have been another factor to consider. One last question, why would breathing helium enriched mixes lead to hypothermia faster than other modalities? I hope you guys enjoyed this rather unconventional scenario. In school, we rarey spend much time on this topic, so I hope people found this to be informative and fun. Take care, chbare.
WestSideBorderPatrol Posted December 19, 2009 Posted December 19, 2009 As a diver, this was an interesting scenario! Dive medicine is fascinating to me b/c strange and amazing things can happen at depth.
rock_shoes Posted December 19, 2009 Posted December 19, 2009 One last question, why would breathing helium enriched mixes lead to hypothermia faster than other modalities? From the following article 1)"Helium-oxygen gas mixtures have exceptionally high thermal conductivity." 2)"As the air from the scuba tank expands, it cools down tremendously, which leads to increased heat loss from the lungs." Investigating Recreational and Commercial Diving Accidents
rock_shoes Posted December 19, 2009 Posted December 19, 2009 Sounds like he should have gone by ground. You really need to weigh the pros and cons in this situation. 7 minutes to definitive care by air vs. 30 minutes by ground. Flying low to the deck will minimize the drop in barometric pressure so your options are 7 minutes at a slightly lower pressure than sea-level or 30 minutes at sea-level. Given the depth this patient was at The shorter transport time to a barometric chamber will benefit him more than keeping him at sea-level for the transport.
p3medic Posted December 19, 2009 Posted December 19, 2009 You really need to weigh the pros and cons in this situation. 7 minutes to definitive care by air vs. 30 minutes by ground. Flying low to the deck will minimize the drop in barometric pressure so your options are 7 minutes at a slightly lower pressure than sea-level or 30 minutes at sea-level. Given the depth this patient was at The shorter transport time to a barometric chamber will benefit him more than keeping him at sea-level for the transport. I respect your opinion however disagree that the time difference is worth it. You have a ground assett ready to transport, why go through the trouble of setting up an LZ and wait for a helicopter? I could see if the boat was still at sea and you could arrange the transport from the dock, I agree that a helicopter "could" be a safe mode of transport, I just think it isn't warranted here.
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