When anemia develops chronically, over a prolonged period of time, and blood volume is maintained, there are four primary mechanisms of compensation.
- Increased cardiac output: The two principal determinants of SVR are vascular tone and viscosity of blood and in isovolemic hemodilution from chronic anemia, the hematocrit decreases and reduces SVR through decreased viscosity of blood. The decrease in SVR then increases stroke volume and therefore cardiac output and blood flow to tissues. Oxygen delivery usually remains constant at a hematocrit between 30 and 45%. Further reductions in hematocrit are accompanied by increases in cardiac output (up to 180%) baseline as hematocrit nears 20%.
- Redistribution of cardiac output: When isovolemic hemodilution occurs in chronic anemia, blood flow is redistributed to the tissues with higher extraction ratios (brain and heart, for example). This blood is redistributed to the coronary circulation in a healthy heart and coronary blood flow can increase up to 600% of baseline. When the heart reaches the point at which it can no longer increase either cardiac output or coronary blood flow, then it is subjected to possible myocardial injury from decreased oxygen delivery.
- Increased oxygen extraction: In times when the hematocrit reaches less than 25%, the oxygen extraction ratio increases in multiple tissue beds, leading to an increase in the total body oxygen extraction ratio and to a decrease in mixed venous oxygen saturation. The brain and heart already have a high extraction ratio and are unable to increase oxygen delivery by this mechanism, but tissues such as the kidney, skeletal muscle, and skin compensate in this manner.
- Changes in oxygen-hemoglobin affinity: The oxyhemoglobin dissociation curve relates the partial pressure of oxygen in the blood to the percent saturation of hemoglobin with oxygen. The P50 at 37 degrees celcius and a pH of 7.4 is 27mmHg. When anemia develops over a long period of time, the oxyhemoglobin dissociation curve is shifted to the right, whereby hemoglobin has a decreased affinity for the oxygen molecule and releases oxygen to the tissues at higher partial pressures. Since this process occurs only after increased 2,3 DPG, it occurs only with chronic anemia and NOT when patients undergo isovolemic hemodilution (see below).
Isovolemic hemodilution is a process in which a patient’s blood is drawn during the perioperative and replaced with an equal volume of colloid. The autologous blood is then saved and transfused back to the patient at the end of the procedure.