Cardiopulmonary bypass (CPB) circuits are typically primed with ~1500 cc of volume (crystalloid, colloid, and/or blood). If a 70 kilogram male (~ 5 liter blood volume) is placed on CPB, one would expect his hematocrit to drop by 30-40% (hemodilution) with a subsequent decrease in systemic vascular resistance (SVR) – the primary mechanism why patients also become hypotensive immediately after initiating CPB.
In fluid dynamics, we all remember that the pressure gradient between two points is equal to the product of flow and resistance.
Going one step further, Poiseuille’s Law relates resistance to laminar flow having fluid viscosity, η, through a vessel having radius, r, and length, x, as follows:
Hemodilution causes the viscosity (η) to drop substantially thereby decreasing the resistance (R) to flow. This resistance is essentially the patient’s SVR. By holding flow (Q) constant, a decrease in SVR (R) will decrease pressure (P) resulting in hypotension.
Because the mechanism is a drop in SVR, we treat this hypotension with vasoactive agents which increase SVR (norepinephrine, phenylephrine, vasopressin).