>As you are likely well familiar at this point, polycythemia is associated with an elevated blood cell count with an associated increase in blood viscosity that increases the chances of blood clot formation, reduced tissue oxygenation and may reduce blood flow to certain areas. For divers, this is a potential concern as diving has an intravascular (within blood vessels) dehydrating effect, largely due to elevations in blood volumes that get passed through the kidneys. With increases in external pressure as well as cold water immersion (even in warm tropical water), blood is shunted away from the peripheries and towards our vital organs and core. This altered distribution can increase the work load on the heart and also increases the amount of blood sent to the kidneys. If you've ever wondered why diving seems to make you have to urinate more, that's the reason.
>The net effect is that divers, especially with repetitive dives, will experience reduced intravascular blood volumes (fluid, not blood cells). Decompression stress, the physiological stress the body experiences with the release of on-gassed inert gas (nitrogen)in the form of bubbles, can also have an intravascular inflammatory effect that independently triggers the clotting cascade. The presence of intravascular bubbles (bubbles within blood vessels) is a common occurrence post dive, but can trigger an inflammatory response within the blood vessels which has a knock-on effect of causing platelet activation which can result in sludging of blood. This sludging, (thickening of blood) can be worsened by underlying dehydration and your already elevated risk for increased blood viscosity. The bottom line here is that decompression sickness is associated with intravascular inflammation and the risk of experiencing DCS appears to increase with reductions in intravascular volume associated with dehydration or blood disorder that increases viscosity.
>While I am unaware of any specific research on the topic of polycythemia and DCS, textbooks on diving medicine state that "diving is not recommended" (please see subsequent email with this reference) because of the anticipated increase in decompression sickness risk as described above.
>The next part of your question is how to reduce DCS risk – with or without polycythemia. The established risk reduction methods include: reduction of depth and bottom time, increasing surface interval times (>1 hour), stay well hydrated and use nitrox on air tables. Added techniques can include those you propose which would be to stay above 20fsw and use 100% oxygen or other elevated oxygen mixture for safety stops which would reduce your inert gas load and encourage off-gassing.
>It is clear to me by your question, that you appreciate the increased risk of DCS associated with polycythemia. If you choose to continue diving, aware of this increased risk, the DCS risk reduction methods stated above are potential options. If you choose to use 100% oxygen underwater, please remember that you are also at risk of sudden oxygen toxicity and should never use oxygen at depths below 20fsw and if used, you should try to stay at shallower depths like 10-15fsw.
>As with all medical issues, we encourage you to consult with your physician and that person is also welcome to contact us for further discussion at (919-684-2948).
>Nicholas Bird MD, MMM