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Hypotension With Initiation Of Cardiopulmonary Bypass

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.

Δ(P) = Q*R

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:

R = [8*η*Δ(x)] / (π*r4)

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).

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2 Comments

  1. How do you feel about different components of prime? Plasmalyte vs Lactated Ringers? And also Mannitol being added to the prime as an osmotic diuretic?

    Also whats your opinion on adding crystalloid vs colloids when volume is required? Whether on bypass or in the ICU.

    1. The crystalloid versus colloid debate is easily one of the hottest points of debate in perioperative and critical care medicine, so I’m not going to go in depth here except saying I give the patient what I think they need in the context of anticipated changes. For example, a small patient with tenuous distal perfusion (ie, carotid disease) warrants blood prime consideration. I like Plasmalyte, but I’ll take any balanced crystalloid solution over saline. I’ve trained at institutions that prime with mannitol and others who don’t. In my mind, if a patient’s urine output is adequate, I don’t really see a need (if anything, they keep diuresing in the ICU and becoming hypovolemic). If urine output is sluggish, then I’ll definitely consider it on CPB.

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