The effect of orientation to gas narcosis during descent?

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Subcooled

ScubaBoard Supporter
ScubaBoard Supporter
Scuba Instructor
Messages
2,526
Reaction score
1,693
Location
Finland
# of dives
200 - 499
On a normoxic trimix course that I recently took I was told that "descending feet first reduces both nitrogen narcosis and the effects of CO2 because the orientation is more natural and lungs work better" :confused: I find this very difficult to believe. I wonder how anybody could have coined such an idea. Is this related to extremely fast descents in a wetsuit, where people may forget to vent out CO2 efficiently enough or what?

I do not believe the claim because:

- If I descend horizontally and watch forward, as I should to maintain contact with the team, then my head is about in the same position as it would be while descending feet first. The orientation thus shouldn't affect balance.

- In a vertical position, all the gas in the drysuit rushes up to the neck region and causes extra squeeze around the neck. Hardly pleasant nor good for circulation I guess.

- If I lay flat, then my lungs are at the same depth as the pressure regulating first stage and I can bearthe gas at ambient pressure. My head is also tilted back, thus opening the airway. If I descend feet first, then my lungs are deeper, and on each breath I must to some work to overcome this pressure difference. Further, I would need to look down, which puts my chin against my chest, which obstructs the airway. Breathing in an upright position should thus be harder.

What's the truth?
 
Last edited:
  • Like
Reactions: Jax
I have never perceived any difference. Commercial divers often descend feet first on a down-line and I have always descended head first in Scuba. Temporary sinus-related equilibrium issues “might” be a little worse on head-down descents, but it may be that the rates are usually faster than when a tender is feeding out the umbilical.
 
The position you assume on descent has no practical physiologic significance except if you can't equalize your ears to match your descent rate. The easiest descent is done either upright or horizontal in terms of opening the Eustachian tube. But many can do head down without issue, as is typical of freedivers.

There is no practical physiologic difference in the depth of your feet to your head on descent, at most we are talking 5-6' difference that percentage wise, is rapidly insignificant as you go deeper. N2 narcosis is depth related but is rapidly worsened by exertion and colder water temperature. N2 narcosis can be accelerated thus by stress by tasks during descent; c02 retention can occur if the stress leads to over breathing a regulator by stress or constant attempts to equalize.

A dry suit should only be inflated to the minimum needed to allow movement and avoid squeeze so if the gas in your neck leads to squeeze while vertical, your suit has too much gas.
 
+1 on the "distance from head to foot just isn't significant". Same for degassing. I see everywhere that you should be horizontal in the water for the safety stop. I call bushwah. There just isn't enough pressure differential from head to foot to make a real difference. Someone gets their big idea (based on nothing) and blows smoke like it's scientific fact.
 
There is a lot of stuff taught in scuba that has no basis in physiology.
 
  • Like
Reactions: Jax
I prefer the rapid head down swimming descent -even for dives that will cause narcosis- but those are deep air. I descend around 150 feet per minute (or more).

However, unless you are extremely used to this type of rapid descent in a head down position, I think it is more likely to induce narcosis. The head up - feet first or even spread eagle descent is a more natural orientation for most people and I suspect that they feel less narcosis in that position.
 
difference in the depth of your feet to your head on descent, at most we are talking 5-6' difference that percentage wise, is rapidly insignificant as you go deeper.

You forgot to think about the pressure regulation at the demand valve.

I would approximate the distance between my mouth and the middle of my lungs/chest as 8-9''. The absolute pressure difference that one has to overcome on each breath is thus about 0.023 ATM.

How long is the longest humanly breathable snorkel? Two feet? Surely not three? If my measurement is correct then the 0.023 ATM difference is maybe a third of what is humanly possible. It may be a bit less and it may be insignificant during a descent as you and Akimbo say. I have descended feet first too, and I didn't suffocate :D

---------- Post added January 28th, 2015 at 10:12 AM ----------

Someone gets their big idea (based on nothing) and blows smoke like it's scientific fact.

I strongly agree.

However... The blood actually has a weight, and it needs to be pumped up and down around the body. This is obviously easier on the heart when horizontal... but this is hardly and issue as we spend most of our time in an upright position anyway. Maybe if you get a heart attack at depth, then you should lay down (joking) :D

A good reason for a horizontal position would be this: some wings or bcds may feel more comfortable in this position, and the position may be more stable, load hanging below support, and not by the side. But this is a personal preference. I have never seen any proof of more efficient deco in horizontal position, neither have a looked for any.

---------- Post added January 28th, 2015 at 10:28 AM ----------

However, unless you are extremely used to this type of rapid descent in a head down position, I think it is more likely to induce narcosis. The head up - feet first or even spread eagle descent is a more natural orientation for most people and I suspect that they feel less narcosis in that position.

Ha! Maybe the TDI manuals were originally written in 1990's in an area where people did not own drysuits, and the choice was between head down and head up descents? Then the rule to descend feet first would make sense. But how did they get that horizontal option in then?
 
Last edited:
There ARE data that rapid descents lead to more narcosis (or at least more awareness of it) which makes some kind of sense.

Work of breathing WILL be slightly higher if the chest is below the regulator. Most people will be capable of generating that increased work, possibly with some feeling of mild discomfort (ever breathed a double-hose reg upside down?). It might lead to a little increased CO2, in people who are CO2 tolerant, because we really DON'T like increased work of breathing, so if the CO2 doesn't have a strong drive effect in a particular person, they might permit it to rise rather than breathe harder. In that case, a vertical descent should lead to some INCREASE in narcosis, as CO2 is strongly narcotic.

All the effects are small, and I doubt they have any significance compared with the individual variation in narcosis susceptibility, between people and within a person on different days and under different diving conditions.

Don't get me started on the mechanism of arterial gas embolism . . .
 
My suspicion has thus been confirmed. Many scuba instructors do not understand what they teach, but recite a book of myths. For this we pay, and for that worthless card of plastic, a sign but of customership.
 
https://www.shearwater.com/products/teric/

Back
Top Bottom