A question arose from the US NAVY diving manual

[impulse]

Hello,

I have been reading the chapter two of the US NAVY Diving Manual. The below paragraph is a question to me.

Quotation from the Chapter 2, Page 11 : Diver Body Temperature. A diver will start to become chilled when the water temperature falls below a seemingly comfortable 70°F (21°C). Below 70°F, a diver wearing only a swimming suit loses heat to the water faster than his body can replace it. Unless he is provided some protection or insulation, he may quickly experience difficulties. A chilled diver cannot work efficiently or think clearly,
and is more susceptible to decompression sickness.

Can anyone tell me, why a chilled diver is more prone to decompression sickness?

Thank you.

 

[giozon]

Hi, I'm not experienced diver, my OWD is still pending but an explanation to your question could be that the blood vessels shrink due to the cold temperature and do not allow to the exess level of nitrogen to clear the same way if you dive in higher temperatures waters so if you dive in cold waters the safety stop perhaps should be increased a little bit.

 

[carrielsal]

I am by no means an expert, but your post made me go back and review a study I had found in Rubicon last year. The article is THE INFLUENCE OF THERMAL EXPOSURE ON
DIVER SUSCEPTIBILITY TO DECOMPRESSION SICKNESS.

It concludes
the notion that gas exchange kinetics in tissues involved in DCS are slowed by
vasoconstriction during cold exposure and hastened by vasodilation during warm
exposure.2 Thus, warm conditions at bottom may hasten gas uptake and increase DCS
risk, whereas the same conditions during decompression hasten gas elimination and
decrease DCS risk.

Diver thermal status during different phases of a dive can greatly influence diver
susceptibility to DCS. Cold conditions during BT and warm conditions during
decompression (e/W) are optimal for minimizing DCS risk and maximizing BT.
Divers should be kept cool during dive BT and warm during subsequent
decompression

You can find the study at Rubicon Research Repository: Item 123456789/5063.

 

[impulse]

Thanks for the replies. I am also not an experienced diver but I like to learn the scientific details of diving. The article about the influence of thermal exposure on diver susceptibility to decompression sickness is really interesting. Thanks for that. Knowing all these facts is good. At the end of the article it is concluded that, "Beneficial effects of warm conditions during decompression were more pronounced than deleterious effects of warm conditions during BT, while effects of a 10°C increase in Twd (water temperature during decompression) were comparable to effects of halving BT.". So If I am not mistaken, the paragraph on the US NAVY diving manual is generalising the effect of the water teperature on a diver in terms of decompression sickness. Warm water conditions during bottom time is also a factor of the decompression sickness. I now wonder if the diving computers calculate water tempreture factors for NDL limits? This is lots of science.

 

[carrielsal]

Before I had read that study, I had only heard the cold water increased the risk of DCS with no explanations. The paper is pretty dry, but I read it through several times. It's the only study I have been able to find online. If there are more, maybe someone will post a link.

 

[Akimbo]

Simply put, the body automatically restricts blood flow to extremities to protect core temperature. This protects organs at the expense of limbs, which also happen to have a high ratio of surface area to mass; thus conduct heat more readily. As hypothermia progresses, circulation to the head is also compromised as it also has a high surface to mass ratio. Brains are important, but heart and lungs are at the top of the survival heap.

Poor circulation = bad for decom and clear thinking.

 

[giozon]

Does it mean that if one use electrical heating elements during decompression the DSC risck is reduced?

 

[ucfdiver]

Does it mean that if one use electrical heating elements during decompression the DSC risck is reduced?

It means that if the core temperature is kept high enough that it's not causing vascular constriction that it balances your risk with someone in warmer conditions. The heated vest methods have some of their own risks however, so i would think it out before using one if you don't need it.

 

[rjack321]

Does it mean that if one use electrical heating elements during decompression the DSC risck is reduced?

More like "divers wearing adequate thermal protection for the water temperature and dive duration have a reduced risk of hypothermia and DCS".

Heated undergarments have their place in technical diving, but only after proper passive insulation is provided.

 

[Bubbletrubble]

I have been reading the chapter two of the US NAVY Diving Manual. The below paragraph is a question to me.

Quotation from the Chapter 2, Page 11 : Diver Body Temperature. A diver will start to become chilled when the water temperature falls below a seemingly comfortable 70°F (21°C). Below 70°F, a diver wearing only a swimming suit loses heat to the water faster than his body can replace it. Unless he is provided some protection or insulation, he may quickly experience difficulties. A chilled diver cannot work efficiently or think clearly, and is more susceptible to decompression sickness.

This excerpt might be a little misleading. 70°F is not a magical number by any stretch. Given time, a person without exposure protection will get chilled if the water is lower than body temperature. Being comfortable temperature-wise in the water entails wearing the appropriate exposure protection for the amount of time one intends to spend in water at a certain temperature.

Can anyone tell me, why a chilled diver is more prone to decompression sickness?

Being chilled, or feeling subjectively cold, implies that physiological blood shifts are occurring in the body -- blood shifts that others have already explained. On a theoretical level, it is believed that blood circulation is involved in the inert gas on-gassing and off-gassing processes which occur during a dive. If this is the case, it's not a huge leap to infer that blood shifts in the wrong direction during a particular phase of on-/off-gassing may increase DCS risk. There may also be certain molecular "middle men" involved. One hypothesis is that cold stress can increase concentrations of stress hormones in the bloodstream, which may heighten DCS risk. Another hypothesis is that cold stress can increase platelet activation giving rise to a prethrombotic state leading to DCS.

That being said, the U.S. Navy has conducted extensive trials to show that, given the conditions they describe in a population of fit, young males, a "chilled" diver has an increased risk of DCS. Many of these studies on cold stress in divers have been published. Feel free to do a few searches on the Rubicon Research Repository.