For purpose of consideration of the use of hyperbaric oxygen (HBO₂) therapy, exceptional blood-loss anemia is by definition loss of enough red blood cell mass to compromise sufficient oxygen delivery to tissue in patients who cannot be transfused for medical or religious reasons.  Medical reasons may include the threat of blood product incompatibility or concern for transmissible disease.  Religious beliefs may prohibit the receipt of transfused blood products.

Red blood cells (RBCs) contain the respiratory pigment hemoglobin (Hb).  Hemoglobin has the powerful ability to pick up oxygen as RBCs pass through the blood vessels of the lungs. Hemoglobin then has the equally powerful ability to off-load oxygen in the tissues of the body’s organ systems.  If plasma were the only vehicle to deliver dissolved oxygen, each 100 ml of blood flowing to an organ system would carry only 0.3 ml of gaseous oxygen.  The consumption of oxygen by human tissues far exceeds this.  For instance, the kidney extracts approximately 2 ml of oxygen for every 100 ml of blood which circulates through it.  From the same 100 ml of blood, the brain extracts approximately 6.5 ml and the heart 10.5 ml of oxygen.

In most instances, humans average 15 grams of hemoglobin per 100 cc of blood.  Each gram of hemoglobin transports 1.34 ml of oxygen.  This is in addition to the oxygen carried by plasma.  So, 100 ml of blood, by containing 15 grams of hemoglobin, can carry approximately 20 ml of gaseous oxygen (1.34 ml X 15 g Hb = 20 ml of oxygen).

In the 1960s, the Dutch thoracic surgeon Boerema demonstrated that one could exchange transfuse piglets with a simulated plasma mixture of buffered normal saline (Ringer’s Lactate solution), dextrose and dextran.  In this process, blood was removed from the blood vessels and the substitute liquid (without hemoglobin) replaced.  He then pressurized the piglets in a hyperbaric chamber while the animals breathed 100% oxygen.  By the trick of pressurization, enough oxygen could be dissolved in the simulated plasma mixture to supply tissue oxygen requirements.  This was enough to adequately sustain the animal, as evidenced by the fact that the animals survived and could be brought out of the chamber to be successfully re-exchange transfused with their previously extracted blood.

As hyperbaric oxygen (or for that matter normobaric oxygen) administered for long periods can become toxic, intermittent administration of HBO₂ is essential.  This point has been demonstrated clinically by the American thoracic surgeon, George Hart.  In 1974, he reported a series of 26 severe blood loss patients who were treated with HBO2 as an alternative to otherwise disallowed red blood cell transfusion.  The survival rate was 70%.