Protecting the Brain

Viscosity:

(Keeping it Brief…)


How far do we HemoDilute?

I got into a discussion with a colleague of mine who was going to relieve me during an aortic reconstruction case that would require Deep Hypothermic Circulatory Arrest (DHCA).

Hypothermia even when mild (34°-36°C) is established to reduce cerebral metabolism and allows the brain to tolerate longer periods of hypoperfusion or ischemia.  When considering hypothermia In tandem with the implied viscosity of cooler circulating blood, I was always of the mindset that micro-capillary perfusion was bound to be affected.  Subsequently, to offset potential thickening or “sludging” of cooler blood, I hemodilute my patients down to a hematocrit of 20%  when targeting 20 degrees Celsius or lower.

He was not of that mindset, and was more of a leave the blood where it is kind’uv guy.

So I ask you.  What’s your position on hemodilution, viscosity and tissue perfusion?


Blood viscosity modulates tissue perfusion: sometimes and somewhere

“Each organ possesses specific properties for controlling microvascular perfusion. Such specificity provides an opportunity to design transfusion fluids that target thrombo-embolic or vasospasm-induced ischemia in a particular organ or that optimize overall perfusion from systemic shock. The role of viscosity in the design of these fluids might be underestimated, because viscosity is rarely monitored or considered in critical care decisions. Studies linking viscosity-dependent changes of microvascular perfusion to outcome-relevant data suggest that whole blood viscosity is negligible as a determinant of microvascular perfusion under physiological conditions when autoregulation is effective. Because autoregulation is driven to maintain oxygen supply constant, the organism will compensate for changes in blood viscosity to sustain oxygen delivery.

In contrast, under pathological conditions in the brain and elsewhere, increases of overall viscosity should be avoided – including all the situations where vascular autoregulatory mechanisms are inoperative due to ischemia, structural damage or physiologic dysfunction…” 

To Read the Full Article Click Here.

Hemodilution & Priming Solutions…

“These physiologic effects are potentially quite harmful if CPB is performed without considering viscosity. In most centers, modern CPB involves the use of flow rates that are somewhat lower than those of “normal” blood: 50 mL/kg per minute or 2.0 L/m2 per minute.

Frequently, hypothermia is also used if flow rates are further reduced to provide a bloodless operative field or, more commonly, to augment myocardial protection. Reductions in both flow rate and perfusion pressure would tend to increase viscosity and thus peripheral resistance; this in turn would decrease tissue perfusion. Hemodilution works to limit the adverse effects of CPB by significantly reducing blood viscosity during bypass. There is a direct relationship between viscosity and hematocrit.

Therefore, reducing hematocrit produces a marked decrease in total resistance and results in an increase in tissue perfusion.

For example, in canine experiments, a decrease in hematocrit from 42% to 25% produces a 50% increase in flow at the same pressure (14). Extremes of hemodilution (hematocrit <10%) permit blood to act as a newtonian fluid .”

To read the Full Article Click Here.

(Elementary Video Below – But Fun …)

[youtube=https://www.youtube.com/watch?v=PgI80Ue-AMo]

Effects of hypothermia, hemodilution, and pump oxygenation on organ water content, blood flow and oxygen delivery, and renal function

JR Utley, C Wachtel, RB Cain, EA Spaw, JC Collins and DB Stephens

“Hypothermia, hemodilution, and the pump-oxygenator each contribute important effects during cardiopulmonary bypass. We studied their separate effects with a 2(3) factorial, completely fixed experimental design in 16 adult male mongrel dogs. Animals undergoing hypothermia were cooled to 25 degrees +/- 1 degree C. In dogs having hemodilution, hematocrit was adjusted to 25 +/- 2%. An analysis of variance was used to determine the effects of hypothermia, hemodilution, and pump oxygenation.

The experiments show that hemodilution produces increased water content in tissue and that edema is greatest in heart and gastrointestinal organs. The pump-oxygenator decreased flow to the subendocardium, whereas hemodilution increased subendocardial flow. Both hypothermia and pump oxygenation diminished flow to the outer kidney cortex, and hemodilution augmented flow to this region.

Hypothermia and pump oxygenation decreased and hemodilution raised renal free-water clearance. Although none affected glomerular filtration rate, hypothermia increased filtration fraction while hemodilution diminished it. Hypothermia lessened cerebral cortical flow, an effect opposite that of hemodilution.

Thus, hemodilution opposes the adverse effect of hypothermia or pump oxygenation on blood flow, oxygen delivery, or renal function. Increased water content in gastrointestinal organs and myocardium accompanies the beneficial vascular and renal effects of hemodilution.”

(The Annals of Thoracic Surgery, Vol 31, 121-133, Copyright © 1981 by The Society of Thoracic Surgeons )