Our club member and AIDA instructor, Brad from The Freedive Project, recently had the chance to help in a scientific experiment with a number of legendary divers and researchers, namely Dave Mullins, Dr. Tom Scott, Prof Simon Mitchell, and Pete Mesley among others. The experiment set out to take arterial blood gas measurements at both maximum depth and at resurfacing prior to breathing to scientifically observe the behaviours of O2 and CO2 during a dive. This has never actually been done before to a depth of 60m!

Dave Mullins did two 60m dives and had blood samples taken at depth and at the surface before breathing, an amazing feat in itself. Read a detailed account and images by Dr. Tom Scott below, check out the 1NEWS article and video here and read the full study here.


Summary by Dr. Tom Scott, published with permission:

"The paper has now been published in the Journal of Applied Physiology which is pretty exciting from our point of view and means we can now talk about the results. I will post the link for those interested at the bottom.

A bit of background as to why we did this and what we were really hoping to prove (or disprove):
Shallow water blackout is a phenomenon that has been widely accepted in the freediving and dive medicine communities for a long time now, however until now there has not been any objective data on a single diver in a real-world environment to back it up.

The theory is essentially that we are (relatively) protected from a hypoxic loss of consciousness at depth due to the increasing ambient pressure on our bodies as we descend through the water column. Despite being apneoic and consuming 02 the increasing ambient pressure would compress your lungs and preserve (or possibly increase) your Pa02.

The problem with this is that as we ascend and consume more oxygen (especially on the deep dives where the freediver has become negatively buoyant on his/her descent) the ambient pressure also decreases, so we see a precipitous drop in Pa02 both from consumption and decreasing ambient pressure. On occasion this drop is severe enough that it puts the diver below a critical 02 threshold and they black out.

Our aim was therefore to take arterial blood samples of a freediver in a real-life situation and map the changes in Pa02 throughout the course of the dive and compare them with blood samples of exactly the same timing from a simple static apnoea on the surface.


The brown line is the surface static apnea, blue lines are the two deep dives.

PaC02. Initial point is a resting PaC02 prior to any pre-dive routing. The point at time zero is PaC02 following lung packing. Brown line is then the steady rise in PaC02 during static apnoea, blue lines are the two deep dives.

The results:

Dave's Pa02 to start with was 11.8kPa (88.5mmHg) which increased to 15.8kPa (118.5mmHg) with lung packing.

He then descended to 60m during which his Pa02 increased massively to 42.8 and 33.3kPa (321 and 249.8mmHg) on two consecutive dives.
The samples back on the surface prior to Dave resuming respiration were 8.2 and 8.6kPa (61.5 and 64.5 mmHg).

These have been shown on the graphs attached against samples from his static apnoea which caused a slow, steady decrease in Pa02 from 15.8 to 14.9kPa.

Interestingly we also saw a small rise in PaC02 at the bottom of the dive that was not explained by metabolism alone as this decreased slightly on ascent again. Previously it had been assumed that C02 was so soluble we would not see an increase in PaC02 with the change in ambient pressure. So this is the first time this has ever been shown.

These figures give us very clear, objective data and proof of the longstanding theory. They also highlight the danger of freediving and spearfishing. The dive Dave performed was 60m so under half his previous record dive 126m. there was certainly a bit of extra time at the turn for the collection of the blood sample but I suspect we would be quite alarmed if we looked at his Pa02 immediately after a very deep dive.

For us recreational spearos who can only dream of getting to those sort of depths, I think it still highlights a pretty important safety message. Just because you are feeling fine during your bottom time that does not necessarily translate to a safe ascent, especially if you are pushing yourself. Perhaps the extra 10 or 20 seconds trying to grab that cray or chase that fish isn't worth it.

Finally a massive thanks to Dave. There are not many people in the world who could do what he did for us, and I suspect there are even less who would actually do it. A professional athlete in the truest sense, and made the whole process so much easier with his relaxed attitude."


Dave Mullins posted on NZ Spearo forum:

"Most divers know the theory that we're protected from blackouts at depth by increased oxygen partial pressure, but then pay back the oxygen 'debt' very rapidly as we approach the surface and ambient pressure reduces. This (theoretically) is why virtually all blackouts happen near or right at the surface. Significantly though that effect had never actually been tested.

So Tom set up an experiment for a tech diver (Simon) to take a blood sample from a freediver (me) at depth and then a second sample to be taken back at the surface before breathing. Blood gas testing equipment was on the boat so they'd have oxygen and carbon dioxide pressures (and presumably a few other values if they wanted them) within minutes.

We settled on 60m as being deep enough for meaningful results, but shallow enough that I still had plenty in reserve to deal with the pauses and various other stresses. I'll leave it to him to discuss results if he wants, but at a first glance they seem really interesting. It was a very cool thing to be part of and will be awesome to have experimental proof either way regarding the 'vacuum effect'."