Diving after Blood Donation

By Dr. Sawatzky

Blood has many functions in the body. It transports oxygen from the lungs to the cells and carbon-dioxide from the cells to the lungs. It transports nutrients (food) from the gut to the liver and the rest of the body, and waste products from the cells to the liver and kidneys. Blood transports disease fighting cells and proteins around the body, and is used in heat regulation. Blood is the body's transport system.

Blood is comprised of cells and fluid. The cells are primarily red blood cells (RBC) with some white blood cells (WBC - the cellular part of the immune system) and platelets (involved in plugging leaks). The RBCs contain hemoglobin and the hemoglobin transports most of the oxygen and some of the carbon-dioxide. The fluid part of the blood is called "plasma". It transports the dissolved gases and molecules.

The amount of blood in your body is determined by your size. The standard adult (70 kg male) has approximately 5.0 litres of blood. The cells make up 40 to 54% of the volume with the rest being plasma (typically, 44% RBCs, 1% WBCs and platelets, 55% plasma). Each litre of blood usually contains 135 to 180 grams of hemoglobin with males typically having slightly more hemoglobin than females.

When a person donates blood, they loose 450 mls of whole blood. The body responds to this blood loss by moving fluid into the circulation and replacing the "volume" lost by the next day. This dilutes the cells and proteins in the blood. Over the next few weeks, the body will make more cells and proteins to replace those that were lost and bring the "concentration" of the cells and proteins in the blood back to normal.

So what is the significance of donating blood on diving? We will have to look at this question from several perspectives. First, diving is exercise, so we must look at the effect of donating blood on exercise. Red blood cells last for approximately 120 days before they become fragile and break when they are squeezed through capillaries. Therefore, 1/120 or 0.8% of the RBCs in the body have to be replaced every day to maintain the total number of RBCs in the body constant. These new RBCs are manufactured in the marrow in the long bones. After a blood donation, the body will respond to the reduced concentration of RBCs in the blood by sending a message to the bone marrow to increase the rate of RBC production. Over a few weeks, the concentration of RBCs in the blood will return to normal.

Before the RBCs are replaced, the total number of RBCs in the body will be reduced by approximately 9% (450 ml / 5,000 ml). Therefore, the amount of oxygen that can be transported by a unit of blood will be reduced by 9% the day after the blood donation. Over the next few weeks, the oxygen carrying capacity of the blood will return to normal. Therefore, you would expect that maximum aerobic performance would be reduced by 9% the day after donating blood and slowly return to normal over the next few weeks. Unfortunately, life is not so simple!

The amount of oxygen that is delivered to the tissues is determined by the concentration of hemoglobin (RBCs) in the blood, the amount of blood flowing to the tissues and the amount of oxygen that is removed from the blood by the tissues (we will assume
for this discussion that the lungs are working normally and most of the hemoglobin is being loaded with oxygen in the lungs, a valid assumption in a healthy diver breathing air under pressure where the partial pressure of oxygen is elevated). Reducing the concentration of RBCs in the blood causes a reduction in the viscosity (thickness, resistance to flow) of the blood. As the viscosity of the blood is reduced, the blood flows more easily through the circulation and the cardiac output increases (at the same cardiac work load). Therefore, more blood is delivered to the tissues, at least partially offsetting the reduced concentration of hemoglobin. It is impossible to measure these changes with sufficient accuracy to determine if the delivery of oxygen to the tissues is reduced or actually increased by blood donation but there is some relevant research.

When cardiac output is measured as a function of hemoglobin (RBC) concentration (changing blood viscosity) and the oxygen delivery calculated as a function of the cardiac output times the hemoglobin concentration a surprising discovery is made. The maximum oxygen delivery occurs at a hemoglobin concentration around 100 gm/l, much lower than that found in the average healthy adult! Therefore, oxygen delivery to the tissues might actually be increased after a blood donation, even though the amount of oxygen that can be carried by each unit of blood is reduced because more units of blood are sent to the tissues.

Another factor that must be examined is circulating blood volume. Immediately after donating a unit of blood, the circulating blood volume is reduced by 9%. This will reduce the blood pressure and make the person more susceptible to fainting, one of the reasons you are escorted and watched closely immediately after donating blood, and why you are fed and forced to drink fluids. The fluid lost by donating blood is replaced in a few hours, however, divers are also prone to dehydration/fainting. They might become very hot preparing for the dive, they might intentionally limit their fluid intake before diving (so as to limit urine production during the dive), they will lose significant amounts of fluid during the dive due to breathing dry air, being immersed in water, and being exposed to cold. By definition, all divers are dehydrated after a dive and thereby more susceptible to fainting. It makes sense to not go diving until the blood volume lost by donating blood is replaced.

How long should you wait? The fluid is usually replaced by 24 hours (especially if you drink lots of fluids) but it seems reasonable to be slightly more conservative to ensure that the body has re-established its equilibrium before diving. Pilots in the Canadian Military are not allowed to fly for 72 hours after donating blood and that would certainly be a safe limit for diving. For "easy" diving, you would probably be okay in 24 to 48 hours.

How about susceptibility to nitrogen narcosis after donating blood? Nitrogen is dissolved in the fluid portion of the blood. After donating blood, the fluid portion of the blood will be slightly larger (the 200 mls of cells removed by the donation will initially be replaced by water) and the delivery of blood to the tissues slightly increased (due to the reduced viscosity). Therefore, the amount of nitrogen delivered to the tissues will be slightly increased.

Nitrogen narcosis is a result of the partial pressure of nitrogen in the brain. The brain has an excellent blood supply and therefore, a slight increase in the rate nitrogen is delivered should not change the partial pressure of nitrogen in the brain. Donating blood should not have an effect on nitrogen narcosis.

How about oxygen toxicity? Like nitrogen narcosis, oxygen toxicity is a function of the partial pressure of oxygen in the tissues (primarily the brain). Slightly more oxygen may be delivered to the tissues after a blood donation (as explained above) and therefore we might hypothesize a slight increase in susceptibility to oxygen toxicity after a blood donation. However, the difference is so small and so many other variables come into play that I can not convince myself that the difference has any practical
significance. Therefore, I do not expect donating blood "significantly" changes susceptibility to oxygen toxicity.

How about decompression sickness? More nitrogen will be delivered to the tissues after a blood donation (as explained above) and therefore, the diver should be more susceptible to DCS. However, many other factors have large effects on the risk of DCS and although blood donation probably does effect DCS risk, that effect is most likely very small.

In conclusion, how does donating blood affect diving? For the first few hours/days after donating blood, the person will be more susceptible to fainting and will have a reduced exercise capacity. After the blood volume that was lost in the donation is replaced, the person will no longer be prone to fainting and might actually have a slightly improved exercise capacity. There are theoretical slight increases in the risk of oxygen toxicity and DCS but this change in risk most likely has no practical significance. You should never dive (nor do other strenuous activity) the day of a blood donation and should be sure to drink lots of nonalcoholic fluids after donating blood. It should be okay to do conservative dives the next day but if you want to be absolutely certain you have recovered from the acute effects of donating blood, wait 72 hours.