Since I've been queried now several times about the relative risks of of succumbing to DCI with respect to diving on full compared to empty, I thought to lay my thoughts on the topic for those who may be interested, namely deep divers.
Several factors help limit the rate of (inert) gas uptake on `empty´ versus full lungs, thereby reducing the risk of DCI:
- A rapidly diminishing alveolar surface area, which decreases more quickly on empty versus full lungs
- An increasing alveolar membrane thickness, since it is un-stretched compared to the full lung state
- Cardiac output: (a component of the DR) is decreased more quickly in the empty state, which results in a shallower alveolar-capillary gas concentration gradient. This would suggest that a prompt and strong DR may be an important requisite for limiting gas uptake. It would also be an important factor in reducing the risk of SWB, but that another conversation altogether.
With respect to gas uptake through the upper (respiratory) cavities, of the sinuses (including middle-ears and mastoid air-cells) and nose, it is entirely feasible that copious amounts of gas are absorbed (cf. Einer-Jensen on the ability of gases and low molecular weight substances to pass through into the cerebral circulation and, subsequently, enter the brain, for example). This is rendered all the more plausible if we consider that the volume of these cavities are on the order of maybe 150-200cc and equalization (with gas) would increase this substantially, more or less in direct proportion to the partial pressure experienced; at a depth of 20 atm the gas load could be in the order of 20 x 150-200cc! A way to circumvent this problem is of course is to switch to water equalization early in the dive, which also potentiates the DR. Furthermore, since a prompt and accentuated DR results in peripheral vasoconstriction (which also limits uptake by the lungs), gas uptake via this uppermost route would be further limited. This is readily noticeable as a stuffy/congested nose becomes patent during a breath-hold maneuver.
In short, therefore, there is I would suggest a pretty strong positive correlation between the amount of gas that is taken on board and the risk of DCI that can be expected. It’s not just that a large lung volumes increase the risk because of the sheer amount of gas (about 5: 1 between full and empty), but also because of the effect that such a large ventilatory load this has on the dynamics of gas uptake in relation to blood perfusion and flow through ventilated areas, which is how the gas gets in, in the first instance. Even in seals, considered well adapted for venturing to depths in excess of 300m, the risk is very real as was eloquently, albeit accidentally shown by Scholander in the 1940s, when a seal was submerged on full lungs to 300m and promptly died from CAGE upon surfacing. I appreciate that exhaling into a bottle at 15m may mitigate this risk, as might jumping on O2 post-dive, but the issue of gas uptake via the uppermost routes of the respiratory tree is not lost. Perhaps freedivers should take heed and do their homework first before blindly venturing, especially with repeated venturing with inadequate surface intervals or worse deep warm-ups!
Now, I haven't spoken to Herbert about his dive or his future plans, and so cannot speak on his state of mind, but as our approaches seem to be at least superficially widely different, the notion of stepping up to 300m with a full lung approach is simply a case of playing Russian roulette with a fully loaded chamber. Eitherway, this approach will require serious rethinking. One suggestion I might offer is to at least to slow the (speedy) descent, at least until the DR has kicked-in. Staying on the surface, under breath-hold, for 20-30 seconds wouldn't be bad dive planning.
Best
S
Several factors help limit the rate of (inert) gas uptake on `empty´ versus full lungs, thereby reducing the risk of DCI:
- A rapidly diminishing alveolar surface area, which decreases more quickly on empty versus full lungs
- An increasing alveolar membrane thickness, since it is un-stretched compared to the full lung state
- Cardiac output: (a component of the DR) is decreased more quickly in the empty state, which results in a shallower alveolar-capillary gas concentration gradient. This would suggest that a prompt and strong DR may be an important requisite for limiting gas uptake. It would also be an important factor in reducing the risk of SWB, but that another conversation altogether.
With respect to gas uptake through the upper (respiratory) cavities, of the sinuses (including middle-ears and mastoid air-cells) and nose, it is entirely feasible that copious amounts of gas are absorbed (cf. Einer-Jensen on the ability of gases and low molecular weight substances to pass through into the cerebral circulation and, subsequently, enter the brain, for example). This is rendered all the more plausible if we consider that the volume of these cavities are on the order of maybe 150-200cc and equalization (with gas) would increase this substantially, more or less in direct proportion to the partial pressure experienced; at a depth of 20 atm the gas load could be in the order of 20 x 150-200cc! A way to circumvent this problem is of course is to switch to water equalization early in the dive, which also potentiates the DR. Furthermore, since a prompt and accentuated DR results in peripheral vasoconstriction (which also limits uptake by the lungs), gas uptake via this uppermost route would be further limited. This is readily noticeable as a stuffy/congested nose becomes patent during a breath-hold maneuver.
In short, therefore, there is I would suggest a pretty strong positive correlation between the amount of gas that is taken on board and the risk of DCI that can be expected. It’s not just that a large lung volumes increase the risk because of the sheer amount of gas (about 5: 1 between full and empty), but also because of the effect that such a large ventilatory load this has on the dynamics of gas uptake in relation to blood perfusion and flow through ventilated areas, which is how the gas gets in, in the first instance. Even in seals, considered well adapted for venturing to depths in excess of 300m, the risk is very real as was eloquently, albeit accidentally shown by Scholander in the 1940s, when a seal was submerged on full lungs to 300m and promptly died from CAGE upon surfacing. I appreciate that exhaling into a bottle at 15m may mitigate this risk, as might jumping on O2 post-dive, but the issue of gas uptake via the uppermost routes of the respiratory tree is not lost. Perhaps freedivers should take heed and do their homework first before blindly venturing, especially with repeated venturing with inadequate surface intervals or worse deep warm-ups!
Now, I haven't spoken to Herbert about his dive or his future plans, and so cannot speak on his state of mind, but as our approaches seem to be at least superficially widely different, the notion of stepping up to 300m with a full lung approach is simply a case of playing Russian roulette with a fully loaded chamber. Eitherway, this approach will require serious rethinking. One suggestion I might offer is to at least to slow the (speedy) descent, at least until the DR has kicked-in. Staying on the surface, under breath-hold, for 20-30 seconds wouldn't be bad dive planning.
Best
S