Does SAC rate have any impact on NDL?

[Mr. Manfrenjensenden]

Apologies if this has been answered before; perhaps my searching wasn't using the correct terms. Also apologies if I'm not posting this in the correct area.

There's a thread over in the Basic forum with a poll about whether people exceed their NDL on a single Al80 dive, and it prompted a question for me. Does the rate at which you breathe what's in your tank affect your NDL? My computer says, "no," because it has no idea how much gas is in my tank or at what rate I'm breathing it, so it's calculating my NDL without considering it. But if you breathe less volume of air per unit of body mass (or whatever the correct way to normalize this might be) in a given period of time, are you exposing your lungs to less nitrogen, which would mean that less nitrogen is being absorbed into your body, and therefore your NDL calculated by your computer is too short?

Imagine that you could dive for a very long period at a given depth on a single breath. There's a finite amount of nitrogen that you breathed in, which will be absorbed over time. Would the amount of nitrogen absorbed per second decrease as time went on because the amount of nitrogen per square area your lungs were exposed to would decrease as more of it (in total) is absorbed? Some kind of asymptotic absorption curve, perhaps? Take this idea back to something more realistic, and it would suggest that people who breathe more slowly are having NDL times understated by their computers because the computer assumes nitrogen absorption from a gas with a constant amount of nitrogen, when in fact what's in the lungs of a slow breathing diver is less nitrogen on average than what the computer assumes?

Or, even if I might be right about the physiology of this, are the changes so minute that it doesn't matter when it comes to calculating something like NDLs?

 

[tbone1004]

short answer no.

long answer
Bubbles diffuse out of your tissues regardless of your breathing rate. Scubapro seems to think heart rate might have something to do with it, and they may or may not be right. Your blood saturates almost immediately in the lungs and since that saturation only cares about partial pressure in the lungs, holding your breath at 100ft has the same effect for inert gas diffusion as breathing at 100ft. Difference is you can't hold your breath down there long enough to get bent.
Other factors that are tied to SAC rate have contributing factors, but it isn't sac rate in and of itself

Super long answer, read up on decompression sickness in sperm whales who are obviously holding their breath

 

[Akimbo]

Indirectly yes, if the higher RMV (Respiratory Minute Volume) relates to higher workloads and/or respiratory and circulatory inefficiency. On the other hand, high RMV can be a good thing if it results in reduced CO2 retention.

I believe the diving community would be be better served if we were to underplay low SAC/RMV numbers as an indication of experience because it can lead to increased CO2 retention -- which probably contributes to lower resistance to Nitrogen Narcosis and increased panic. Another failure to emphasizing SAC/RMV is it is absurd to compare a petite female to a 250 Lb male. it is far safer to burn up too much gas than not enough.

Time and depth are the major factors (by far) but workload, temperature, and physical condition are are high on the list of secondary factors that influence optimum decompression.

 

[wheeledgoat]

DCS in whales - fun reading!

Animals may ascend rapidly to the surface to escape a predator, for example, though it is difficult to comprehend a predator that would cause a large set of whales to ascend to the surface.

indeed...

 

[Akimbo]

This is also interesting: The Hazards of Freediving

DCS incidence in the competitive apnea diving community is not unusual and well recognized. I wouldn't want to be a sea lion, but I sure admire their diving prowess.

 

[boulderjohn]

A quick summary of the first two responses you got.

Breathing rate in itself does not impact your NDL. If, on the other hand, you are breathing more because you are working harder and your blood is flowing more due to that workload, that will impact your NDL. In that case, it is the increased blood flow that is the reason, not the rate at which you are breathing. That is just a symptom of the workload.

 

[Mr. Manfrenjensenden]

Thanks all. The sperm whale stuff is interesting.

A quick summary of the first two responses you got.

Breathing rate in itself does not impact your NDL. If, on the other hand, you are breathing more because you are working harder and your blood is flowing more due to that workload, that will impact your NDL. In that case, it is the increased blood flow that is the reason, not the rate at which you are breathing. That is just a symptom of the workload.

I read and re-read the responses and eventually figured this out. Then you summarized it far better than I'd done for myself. Thanks.

 

[Dr Simon Mitchell]

Indirectly yes, if the higher RMV (Respiratory Minute Volume) relates to higher workloads

Akimbo makes the key point. Merely breathing more (or less) without an associated change in exercise levels will not materially affect tissue gas loading and will therefore not affect the decompression risk for a dive. But if you breathe less because you are at rest, or you breathe more because you are exercising, then this would be expected to lower or increase (respectively) the tissue gas loading during the dive, and it would alter the decompression stress of a subsequent ascent if the same NDL were adopted.

There was a very similar thread here just recently which was remarkable for a number of reasons, but which discusses this physiology in more detail.

Simon M