A Little Knowledge


Larry "Harris" Taylor, Ph.D.   


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After several hundred dives in the Port Huron region of the St. Clair River (one of North America's most intense navigable waterways ... current under the Blue Water Bridge can exceed 10 knots), we were looking for something a bit more challenging (thrilling). We decided to do a "fly" (never touch the bottom)  under the Blue Water Bridge starting from the mouth of the St Clair river to our favorite exit spot, the (now non-accessible) Bramble dock. Since we knew the current would be intense and this dive would require major physical exertion, we chose to do the dive using an oxygen enriched air breathing gas (NOAA I or 32 % oxygen). Our thinking was the added oxygen would facilitate the anticipated extreme aerobic exertion of the dive.

Our plan was to enter above the mouth of the St. Clair River,  drop down into the main channel (~60 ffw) and then to be rapidly hurled downstream. After 12 minutes, we would move to a posture at right angles to the current and vigorously swim until we reached a bottom contour. Then, we would use our "river sticks"  (see River diving lecture slides that illustrate our specialized river diving equipment)  to facilitate moving towards the exit point.

During the dive we flew over a white object that looked like a Blazer automobile, but we were moving too fast (in ~ 10 foot (3 Meter) visibility) for a positive identification. After 12 minutes, we went to right angles to the current and started swimming. Shortly after doing this, I felt enormously tired ... more tired than I have ever been underwater. This surprised (and concerned) me 'cause I considered myself to be in excellent physical condition (At this time, I was doing an hour a day of high-impact aerobics from national champion aerobic instructors at a studio known for its intense workouts.  I was even a token male on their aerobics exercise TV program). The experienced fatigue was extreme.  Once reaching the river bottom, we stopped several times with our "river sticks" embedded in the river bottom to hold position while we caught our breath. We reached our exit point (after covering a total of ~1.5 miles (2.4 km) ) in 14 minutes. 1.5 miles in 26 minutes means our average downstream speed was ~ 3.5 miles/hour ( 5.6 km/hr, 3.03 knots.)

After the dive, we wondered about the white object we saw and decided after a 2 hour surface interval (and lunch), to repeat the dive and confirm that what we saw was a Blazer. Turns out we had found a Blazer sitting upright and facing upstream that had apparently been used as a suicide vehicle the previous winter. (Someone had intentionally driven out on the ice. The ice could not support the vehicle and so the Blazer fell through the ice).

Once again, the profound fatigue (voluminous perspiration, shortness of breath (feeling air-starved), and an overwhelming sense of tiredness) was present.

I did not understand why we both had felt so fatigued (air starved) on the dive.**

A few years later, I ran across an article in Undersea Biomed Research where the Israeli military reported that the slightly increased density of oxygen enriched air exacerbated CO2 retention.  We then repeated this dive on compressed air and did not experience the sensation of massive air starvation associated with our earlier dives. So,  I now believe that the extreme fatigue we both experienced was a result of unanticipated CO2 retention.

We thought we were being clever in using an oxygen enriched air mix, but it turns out that our cleverness worked against us on that particular dive.

The points are:

1. A little (insufficient) knowledge can be a dangerous thing.

2. In diving, as in life, it is the unknown that can pose a significant risk

3. Divers doing physical tasks on oxygen enriched air (Nitrox) should pay particular attention to symptoms of CO2 build-up.

4. No matter how much is known, there is always more to learn.


The dive described above is not for the feint of heart, the physically unfit, improperly equipped, or limited swift water experienced diver. (Suggested experience is ~200 dives in extremely intense swift water). We labeled this dive the "push the envelope dive."

**Things we considered to not be the proximate cause of the oxygen enriched breathing gas air starvation experience:

1. Gas contamination: Divers obtained their oxygen enriched air from different dive shops. All cylinders were on-site analyzed with two different brand oxygen analyzers to be 32.0 % oxygen. No CO was detected. Similar contamination from different sources is unlikely.

2. Pre-dive anxiety: No observed signs / symptoms of CO2 buildup prior to dive was observed by either diver. Divers had more than 200 dives in this region of the river. The predominant emotion was eager anticipation of an awesome (later confirmed) thrill.

3. During dive separation anxiety: Divers were experienced as a team river divers; there was no "lost contact" during the dive.

4. Different paths: While no two river dives are exactly the same, our "under the bridge dives" had a similar trajectory based on recognition of bottom objects (like the white blazer and the stern of the Tremble wreck) which allowed a mental image of our location relative to surface landscape.

5. Different current flow: The St Clair river is one of North America's most intense navigable waterways (velocity in excess of 10 knots)  and even in the presence of some altered current velocity, the current speed would have been extreme.

6. Different work loads: Swimming for  greater than 10 minutes at right angles in extreme current would generate massive amounts of CO2. So, lots of metabolic CO2 was present on all dives as we moved from main channel to the shore.  

7. Poor Equipment: The regulators were top of the line (but each diver had a different brand) tuned for maximum performance. No air starvation issues were noticed by either diver on any other river dives.

No matter what was considered, the bottom line was the air starvation sensation (and other typical CO2 buildup symptoms) for both divers occurred  on two separate dives (4 total cylinders of EAN32) only while breathing the oxygen enriched air. Although the difference in density between air and NOAA I is extremely small, apparently this difference becomes significant when ventilating large amounts of metabolic CO2. Since this was a dive in extreme current requiring extremely vigorous aerobic activity, this dive should not be compared to typical recreational diving.



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 About The Author:  

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 200 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.

  Copyright 2001-2024 by Larry "Harris" Taylor

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