Standard hemodialysis can increase cerebral water content and lead to edema. The pressure in the brain or ICP shouldn’t have too many influences. There is strong buffer system in the brain but if that fails- then the ICP can increase rapidly. It has been noted that even small subclinical cerebral edema that occurs in IHD can slowly raise the ICP. Why is that and how does the BP and acidosis make this possible?
A review by Davenport on this topic suggests few cautions:
1. Drops in blood pressure or intradialytic hypotension can lead to ICP rises( Figure 1 from the Davenport review)
2. Big fluctuations in urea shifting
3. Intracellular acidosis in the brain as CO2 is removed in dialysis leads to paradoxical Co2 absorption in the brain and causing imbalance.
4. Exogenous substances can enter brain ( albumin) during brain injury as BBB is broken and lead to worsening edema.
Above Figure from Davenport article.
Some key points regarding what should be done then?
1. All standard intermittent therapies, hemofiltration, hemodialysis, and hemodiafiltration, will lead to increased cerebral swelling, and if the patient has suffered a severe injury and is unconscious, then most centers would deem continuous renal replacement or hybrid therapies as the preferred treatment. ICPs have shown to remain much more preserved and constant in CRRT forms rather than IHD forms of therapy.
2. Treatment should be designed to both slow down the rate of change of serum urea and osmolality, and to maintain cardiovascular stability( so perhaps daily IHD might be needed to minimize BUN shifts and prevent edema from getting worse)
3. Given breakdown of BBB, avoid heparin based dialysis in this setting
4. If doing HD, would do daily and use a lower BFR and cooler dialysate( 35C), smaller dialyzer and high Na bath(≤10 mEq/L above serum sodium), bicarb of 30meq/l , higher K and calcium baths).
5. Some might suggest keeping a pre dialysis BUN low( less osmotic shifts – close to 30 and keep on supplemental oxygen.
Interestingly, regarding cooled dialysis- a recent JASN 2014 article sheds some light on that as well. In total, 73 patients on incident hemodialysis starting within 6 months were randomized to dialyze with a dialysate temperature of either 37°C or 0.5°C below the core body temperature and followed up for 1 year. Cooled dialysate improved hemodynamic tolerability, and changes in brain white matter were associated with hemodynamic instability and patients who dialyzed at 0.5°C below core body temperature exhibited complete protection against white matter changes at 1 year.
Preventing hypotension and rapid osmotic shifts is essentially what is required.
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