Keith Van Meter, M.D., FACEP
Patients who have marked loss of red blood cell mass by hemorrhage, hemolysis, or aplasia run the risk of lacking adequate oxygen carrying capacity by blood. The more quickly the severe anemia develops, the less tolerant the patient may be of the insult.
Hemoglobin (Hgb), a powerful carrier for oxygen, transports 1.34 ml of oxygen per gram. The amount of oxygen that will dissolve in one milliliter of plasma is 0.003 ml per mmHg of the partial pressure of oxygen (O2) in inhaled gas. CaO2 and CvO2 respectively represent the arterial or venous content of oxygen in blood.
On the average, the body extracts 5 to 6 ml of O2 for every 100 ml of blood that sweeps through the microvasculature of most organ systems. Physiologic normal levels of Hgb readily supply tissue oxygen extraction rates of 5 to 6 volume percent. As Hgb drops to 6 g/dL, oxygen delivery, to offset these baseline oxygen extraction rates, becomes problematic and is clearly inadequate at Hgb levels below 3.6 g/dL.
Accumulative oxygen debt is defined as the time integral of the VO2 measured during and after shock insult minus the baseline VO2 required during the same time interval. Clinical research in evaluation of patients with severe hemorrhage, demonstrates no chance of survival if the accumulative oxygen debt exceeds 33 L/m2. Multiorgan failure (MOF) occurs if the accumulative oxygen debt exceeds 22 L/m2. All patients who have an accumulative oxygen debt of 9 L/m2 survive without residual disability.1
1 Hyperbaric Oxygen 2003: Indications and Results, The Hyperbaric Oxygen Therapy Committee Report by John J. Feldmeier, D.O., Chairman and Editor. Copyright 2003, Undersea and Hyperbaric Medical Society, Inc., Kensington, MD.