SCUBA: Breathing performance underwater

On one of my first dives to the Cumberland wreck (100m) I remember how incredibly hard I found the swim across the bottom to find the ship’s bow. These complaints backfired on me somewhat when we later viewed the video of the dive, and I had to admit it looked like I was out on a gentle ‘Sunday swim’. ‍

That was a real eye-opener to me. The dive felt hard and I was panting away on my rebreather but to an observer we looked like we were just idling along!

This raises the important issue of the factors that limit breathing performance under-water. Failure to appreciate these limitations can lead to problems ranging from unpleasant to deadly! The most relevant contributing factors are considered below.

1. Depth

Diving affects our ability to move air or other gas in and out of the lungs. As depth increases, so too does the density of the gas we are breathing. Increased gas density promotes turbulent flow in the airways and this in turn increases the work of breathing. As you might expect we are simply not able to shift as much gas at depth. Indeed, the maximum voluntary ventilation (MVV – the amount of gas you can breath in and out per minute if you pant as hard as you can) during air breathing at 4 ATA (30m) is half that at the surface!

Increasing depth is not the only factor that may reduce breathing capacity (see below), but this change in MVV alone is sufficient to ensure our work capacity at depth is much less than at the surface.

A reduction in our ability to ventilate is one potential reason why divers may develop high levels of carbon dioxide (CO₂) during periods of work at depth no matter whether they are using open circuit or rebreather equipment.

Getting rid of CO₂ from the blood is entirely dependent on moving sufficient gas in and out of the lungs. If you breathe more, you eliminate more CO₂ and vice versa. It is easy to appreciate the critical danger of a diver putting himself in a situation where he works hard while being unable to breathe sufficiently to eliminate the CO₂ produced by the work of his task. He may enter a deadly spiral of increasing CO₂ production (from desperate efforts to breathe more) and decreasing lung ventilation (from inefficient rapid, shallow breathing). If the body CO₂ level becomes high enough the diver will eventually become unconscious, stop breathing and drown. This sounds like an extreme and even fanciful scenario, but there is very good reason to believe that this has occurred in at least one technical diving accident at extreme depth.

Deep divers substitute non-narcotic and vastly less dense helium for nitrogen in breathing mixes in order to reduce both the narcosis and the work of breathing on very deep dives. This strategy will increase the work capacity of the deep diver. One of my recent articles for Dive addressed the issue of optimal gas density for deep diving.

2. Breathing resistance

Underwater breathing equipment almost inevitably imposes an extra resistance to breathing in addition to that imposed by increasing depth. This varies with the configuration and tuning of the equipment used.

For example, suboptimal tuning of a regulator may result in greater diver-initiated pressure changes being required to open valves. Similarly, poor design may see dense gas at depth having to flow through tubes that are narrower than necessary (= greater resistance) or that change direction in such a way as to create turbulent flow. In either case this will translate into greater breathing work. Extra breathing resistance imposed by equipment will reduce breathing capacity and increase the chance of increases in CO₂ as described above.

3. Dynamic efficiency of CO2 scrubbers

For those divers utilising rebreather equipment, the dynamic efficiency of the CO₂ scrubber is another potentially important limitation on underwater work. If a scrubber fails to remove all expired CO₂ then the diver will re-inhale it, and body CO₂ levels are likely to rise.

It is important to distinguish the predicted duration of a scrubber (which could be called its ‘static capacity’) from its ‘dynamic efficiency’. Dynamic efficiency is the scrubber’s moment to moment ability to remove a CO₂ load presented to it, and this might be exceeded even though there is considerable static capacity remaining. For example, we might start a dive with brand new soda lime in a rebreather scrubber and therefore expect to have a duration of several hours, yet early in its expected life there could be some CO₂ ‘breakthrough’ into the inhaled gas if the scrubber’s dynamic capacity is overwhelmed by a short period of very rapid CO₂ delivery and high gas flow due to exercise and heavy breathing.

Factors that can reduce dynamic capacity include decreased temperature, increased depth, partial flooding, partial expiry of scrubber material and improper packing of the scrubber.

The effect of temperature is easily explained on the basis that the reaction between CO₂ and the scrubber material is slower at low temperatures.

The effect of extreme depth is interesting. All other factors being equal, we still produce the same number of CO₂ molecules irrespective of the depth we are at. However, the deeper we go, the greater the pressure (and number of molecules) of other gas in the rebreather loop. One theory holds that in this setting there may be a lower chance of contact or interaction between the relatively sparse CO₂ molecules and the scrubber material because of ‘crowding’ from other gas molecules.

The effect of partial flooding of the scrubber with wetting of the material is almost certainly to impair contact between the scrubber material and CO₂ molecules.

The effect of partial expiry of scrubber material is reasonably obvious. If some of the scrubber material has already absorbed as much CO₂ as it can, then there are less actively absorbing particles of material available. The longer the scrubber is used, the more relevant this becomes. This effectively means that the scrubber has progressively less active absorbing surface area available, and the probability of dynamic capacity being exceeded rises with time.

The effect of improper packing is an uneven distribution of scrubber material resulting in so-called ‘channelling’ since gas flow will tend to favour the path of least resistance. This is most likely if the scrubber is packed too loosely, resulting in a gravity-dependent shift in material during the dive. CO₂ is less likely to be completely removed if larger volumes of gas are flowing rapidly through a small less resistant area of scrubber.

Obviously, dynamic capacity is more likely to be overwhelmed if CO₂ is presented to the scrubber at a greater rate, especially since an increased rate of CO₂ delivery is usually accompanied by a much greater flow rate of gas through the scrubber (because the diver is breathing harder) thereby reducing the ‘dwell time’ during which the CO₂ from each breath is in contact with the scrubber material.

4. Physiological factors

The effects of increased work of breathing on breathing control are potentially the most important. Normally, if body CO₂ levels increase, the brain will cause us to breathe more in order to eliminate more CO₂ and bring body levels back to normal.

However, if the work of breathing is increased, this normal control mechanism can be blunted and CO₂ levels will rise. There is considerable individual variation in this tendency but (unfortunately) no easy way of testing for it. This problem, known as ‘CO₂ retention’ was also reviewed in more detail in a recent Dive article.

Summary

Divers, and particularly deep technical divers, must be very aware that attempts to perform hard work at depth may be frustrated by limitations imposed by both the increased density of the gas they are breathing and their equipment.

Put another way, tasks and swim distances that may be easily completed in shallow depths may become very challenging when undertaken deeper. Failure to recognise this, and failure to adjust expectations or plans accordingly, may result in dangerous events such as CO₂ toxicity.