What is the most reliable way to perform a quantitative analysis of the ECF volume in patients with metabolic alkalosis?
Accurate quantitative data about the ECF volume are not reliably obtained by the physical examination; instead, the hematocrit or total protein level in plasma can often be used to obtain this information.
The hematocrit or the concentration of hemoglobin in blood will rise when there are more red blood cells (RBCs) or fewer liters of plasma. If one ignores the Starling forces across capillaries, the changes in the plasma volume will reflect alterations in the ECF volume.
The hematocrit on admission provides helpful information to guide initial decisions about intravenous fluid therapy and to decide when to slow down this infusion rate.
For example, assume that the hematocrit is 0.40 and the blood volume is 5 L. Therefore the plasma volume will be 3 L and volume of RBCs is 2 L:
If the hematocrit on admission is 0.60 in a patient who does not have polycythemia, the new plasma volume can be calculated as follows:
On rearranging this equation, 0.6 X = 2.0 L and X is 3.3 L. Because 2 of these liters are red blood cells, the remaining volume is the plasma volume (3.3 L – 2.0 L = 1.33 L).
Thus the plasma volume has fallen from 3 to 1.33 L, and, by inference, the ECF volume is reduced to 44% of its normal volume (i.e., 100 × 1.33 L/3.0 L = 44%). Because hematocrit values of 0.6 can be seen in patients with diabetic ketoacidosis or in states with severe diarrhea (e.g., cholera), the measured [HCO 3 − ] will be more than twofold higher than in a similar patient who did not have such a contracted ECF volume.
