From Air to Trimix at depth

[John C. Ratliff]

Okay, let me be more specific. Here's a definition of oxygen debt (not hypoxia, which is different).

debt [det]something owed.
oxygen debt the extra oxygen that must be used in the oxidative energy processes after a period of strenuous exercise toreconvert lactic acid to glucose and decomposed ATP and creatine phosphate to their original states.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
oxygen debt - definition of oxygen debt by Medical dictionary

Note that this is happening on the cellular level because of exercise. It is using oxygen on a cellular level faster than the body can replenish it, causing the diver to need to stop, take deep breaths, and let his/her body "catch up" with the exercise that has just happened. This can happen with higher pO2 at depth, and is simply using the body's oxygen faster than the body can replenish it on a cellular basis. Here is an example:

...Once Bunton and Mazzone made it back to the surface, Bob Barth and Scott Carpenter went down to retrieve the camera, a valuable piece of equipment that Bunton, a former Army paratrooper turned civilian specialist in underwater photography, would need while living on Sealab II. Carpenter spotted it and swam at top speed for the botom, leaving a great trail of bubbles in his wake. Barth tried t slow him down, knowing he dangers of breathing hard when in the realm of the narcotic haze, but coulhdn't stop Carpenter before he got to the camera. As the two paused for a decompression stop, holding on to the line they followed between the surface and the bottom, a dizying array of synaptic bells and whistles went off in Carpenter's head. Disoriented, he hung upside down on the line like a drunken trapeze artist, eager for his disconcerting first bout with nitrogen narcosis to pass...
Hellwarth, Ben, Sealab, America's Forgotten Quest to Live and Work on the Ocean Floor, Simon & Schuster, New York, 2012, page 129-130.

But he was on decompression prior to the Sealab saturation dives on a 200 foot plus bounce dive to recover a camera, presumable at fairly shallow depths (within 50 feet of the surface). Could this be both a buildup of CO2 and an oxygen debt playing out? That is my question. Nitrogen narcosis usually clears quickly upon ascent. Note also that these two divers were almost certainly using double hose DA Aquamaster regulators, which due to regulator positioning may have higher inhalation resistance than today's current single hose regulators. This dive happened in August of 1965.

SeaRat

 

[Omisson]

A diver breathing gas at depth should normally be getting enough Oxygen. The partial pressure of oxygen at depth is high (hopefully below 1.4 or 1.6) but each inhalation is delivering MUCH more oxygen that you get at the surface.

So the issue is not so much low oxygen, but rather high CO2 that is the issue here.


With regard to too little oxygen: An individual who is getting too little oxygen, but no build up in CO2 may not feel much discomfort at all and can lose the ability to function before they realize they have a problem.

But for divers, the issue is CO2 build up not hypoxia. That does cause stress, discomfort and confusion.

This video is very cool, it shows the effects of low oxygen, when the guy does not have the benefit of a warning that there is a problem because he IS venting CO2. He is significantly impaired. This video is probably most useful to freedivers who do expose themselves to hypoxia (as well as high CO2 levels)


[video=youtube;WTNX6mr753w]https://www.youtube.com/watch?v=WTNX6mr753w[/video]

The effects of respiratory acidosis are somewhat different than hypoxia or narcosis... But the point was the oxygen/CO2 relationship is different then the oxygen-nitrogen relationship in diving.

There is - absent overwork or inhalation/exhalation issues, no CO2 change at depth in Open Circuit diving, per se. In closed circuit co2 is always an added concern.

 

[Michael Guerrero]

I think hypoxia and narcosis are different things- though they share some sensation similarities.

They are, but I think he's describing Hypercapnia (a.k.a. Hypercarbia), the build up of CO2 in the body, not Hypoxia, which is insufficient delivery of O2 to the cells of the body.

---------- Post added October 23rd, 2014 at 02:37 PM ----------

There is - absent overwork or inhalation/exhalation issues, no CO2 change at depth in Open Circuit diving, per se. In closed circuit co2 is always an added concern.

From a practical perspective, you're probably right that CO2 is not a significant issue strictly from a depth perspective because before you're likely to encounter the gas density issues of air/Nitrox you should be on trimix, at which point gas density really isn't an issue.

But it is important to remember that work of breathing matters, and that what you think is not significant work can actually have an impact at depth due to CO2 retention. And like John described, sometimes the effort to get yourself and your rig to the spot to descend is enough to cause issues initially in the water. It's almost always a good idea to rest for a bit at the surface before descending if conditions allow so you can catch your breath, collect your thoughts, and slow your heart rate.

 

[Peter69_56]

Is anyone here not worried about isobaric counterdiffusion at depth? Swapping from helium to air to helium would expose a diver to this at depth, however no one seems concerned???.

Isobaric counterdiffusion - Wikipedia, the free encyclopedia

 

[Dr. Lecter]

Is anyone here not worried about isobaric counterdiffusion at depth? Swapping from helium to air to helium would expose a diver to this at depth, however no one seems concerned???.

Isobaric counterdiffusion - Wikipedia, the free encyclopedia

In theory, I think one tries to avoid swings of more than 0.5 ATA. In practice, it doesn't seem to be an issue for switches above 100m or so with most 50-60% or lower helium blends.

 

[Michael Guerrero]

The original post pertained to switching from air to trimix at a depth of 40m (130ft), and if CO2 was a contributory factor to narcosis. So I would say no, ICD is not a significant concern.

 

[TONY CHANEY]

Having had gone through flight crew training to include the "chamber" I was rather suprised at how members subccumbed to the effects faster than others. In the video the person was only effected by lack of O2. Now add the effects of CO2 buildup and you have a different animal altogether. To make matters worse add in nitrogen narcosis. Please do not mix up hypoxia with hypoxemia.

 

[TSandM]

Oxygen debt occurs in highly metabolic tissues with heavy exertional loads -- eg. Muscle beds. Muscles are uniquely capable of switching smoothly from aerobic to anaerobic metabolism, because even in the face of superb fitness and maximal cardiac output, the oxygen supply to muscle can be inadequate to meet the demands. In these cases, blood returning to the lungs is very desaturated, having had the maximum amount of oxygen extracted from it in the tissue beds. There IS evidence that arterial oxygen tensions can drop a little in this situation, but only when it is extreme. In the ordinary person, the lungs are more than capable of transferring enough oxygen to return oxygen tension to what would normally be expected in arterial blood. This type of person will be limited by fatigue or muscle pain long before they've desaturated the arterial side, so the blood going to the brain would be expected to be carrying a normal oxygen load.

For this reason, I do not think oxygen debt plays any role in narcosis.

 

[DiverRtNY]

I hear mentioning of hypercapnea being related to hypoxia (hypoxemia). But the other twist would be hyperoxemia... getting too much oxygen. Outside of talking about oxygen toxemia, let's talk about individuals that are more prone to being hypercapneic: COPD emphysema patients, patients with hypoventilatory syndrome (morbid obesity, diaharagmatic paralysis, etc...)
Indivduals who fall in these catagories are also prone to having lower levels of PaO2 in their blood... oxygen. These individuals have sensors in their noggins that are prone to trigger based on oxygen levels as opposed to co2 levels. If these individuals are abosrbing a higher level of oxygen, hyperoxemia, there central system will depress their systems to ventilate less to compenste for the high levels of oxygen. This resulting in a build up of CO2.
As a respiratory therapist, we see patients in the hospital that go into hypercapneic respiratory failure due to being exposed to too much oxygen in the inpatient setting. Hyperoxemia, not hypoxemia, will lead to hypercapnea and thus leading to lethargy, altered mental status and impending respiratory failure. Hypoxic patients, initially, will increase their ventilation and blow off too much CO2 and will be Alkalotic... not acidotic.

Don't let this confuse you with oxygen toxicity... that is a whole other ball of wax as we know due to very high oxygen levels and absorption due to increased positive pressure that can't realistically be obtained on the surface by oxygen alone. Hence why diving it IS realistic.

Someone that is a co2 retainer (COPDers) have a lot to consider with deep diving. Coming from my pulmonary background, best option would be to get serial Arterial Blood Gasses on individual divers at different depths to specifically tune what each individuals should be breathing to keep themselves in a nice safety zone when pertaining to oxygen requirements, and lack there of.

This post went off the OP but rolling with it.:meeting:



An individual can be hypercapneic and hypoxic but that's a chicken or the egg question. There were events that led to hypercapnea that led to respiratory failure and THEN led to hypoxemia (cause you aren't breathing!). Not the other way around. Unless they were hypoxic and exhausted ones respiratory drive by breathing very heavy and fatiguing themselves into respiratory failure

 

[TSandM]

The ideas regarding hyperoxia and loss of hypoxic ventilatory drive come up here from time to time, but really aren't relevant to scuba. Patients who are dependent on hypoxic drive for ventilation are not usually in any kind of physical condition to consider diving, or much of any other exertion.

I have looked at the data on CO2 retention with hyperoxia and wondered if we all operate just a little bit on hypoxic drive, and that's why we see a bit of hypercapnia when normal people are exposed to high ppO2s. That's not what is generally taught, but the data make one wonder.