Corrected bicarbonate concentration in significant ECF volume contraction
The hematocrit or total protein levels in plasma may be used to obtain a quantitative estimate of the ECF volume.
Sample calculation: If the patient does not have preexisting anemia or polycythemia, use the hematocrit (though the concentration of albumin or total proteins in plasma may also be used) to obtain a quantitative estimate of the ECF volume and calculate its content of HCO 3 − . In a normal adult, the usual hematocrit is 0.40 (40%); this represents a blood volume of 5 L (2 L of red blood cells [RBC] and 3 L of plasma):
Therefore when the hematocrit is 0.50, the plasma volume equals the of RBC volume (i.e., 2 L):
The present blood volume is 4 L, and thus the plasma volume is 2 L. Hence the plasma volume is reduced by 1 L from its normal value of 3 L. Ignoring changes in Starling forces for simplicity, the ECF volume is diminished by ∼33%; this implies that rapid restoration of ECF volume would lead to a [HCO 3 − ] that is 33% of its measured value.
