Wednesday, October 21, 2009

Renal Failure in the Neuro ICU

Here are a few important guidelines for prescribing RRT in the Neuro ICU gleaned from this review. First off, a recap of the physiology...

Intracranial pressure increases are buffered by CSF, such that increased ICP leads to increased CSF entry to the spinal cord, increased reabsorption and reduced production. This buffering works best for slowly rising ICP; once this buffering capacity is overcome, the ICP increases steeply, exceeding cerebral perfusion pressure and preventing blood flow.

Secondly, even stable outpatients develop subclinical cerebral edema during hemodialysis. This can be enough to precipitate a catastrophe in a patient with raised ICP. Intermittent HD can worsen intracranial hypertension via systemic hypotension (ICP rises when MAP falls), faster removal of urea from plasma than brain and intracellular acidosis (CO2 crosses BBB faster than HCO3).

Finally, the integrity of the BBB is a key piece of information for the nephrologist in the NICU. In vasogenic cerebral edema (such as is seen in traumatic brain injury (TBI), acute intracerebral hemorrhage, small vessel vascular disease, hypertensive encephalopathy and infection) the BBB is broken down, and substances commonly used in the dialysis prescription such as mannitol and albumin can cross into the brain and worsen cerebral edema. This may explain the inferior outcomes seen in TBI patients treated with albumin in the SAFE study. Use hypertonic saline instead.

Based on the above, here are my top tips for a surprise-free existence during your NICU rotation:

1. Choose CVVH if possible

2. Hemodialysis only if stable CV, and no ICP or midline shift:

- Low blood flow rate and cooled dialysate to minimise hypotension

- Small dialyzer to slow the rate of change of plasma osmolality

- High dialysate [Na] (150-160 meq/l)

- Low [bicarbonate] ~ 30 meq/l

- Maintain a pre-dialysis urea of 30 mg/dL

- Heparin-free

- This is essentially a SLED or hybrid therapy

3. Peritoneal dialysis

- Avoid icodextrin for the same reasons as albumin and mannitol

- Low fill volumes to prevent raised intra-abdominal pressure


PatelUTMEM said...

Two things are not clear to me in above excellent note

Pre dialysis BUN: ?

Icodextrin in PD should remain in peritoneal cavity, how can it exert same effect as albumin/mannitol??

Conall O' Seaghdha said...

Thanks for the feedback.
1. Regarding the Predialysis BUN, the idea is that maintaining a lower pre-dialysis BUN causes less osmotic shifts per session. This may require daily treatments to acheive. Here's the reference (Davenport A: Renal replacement therapy in the patient with acute
brain injury. Am J Kidney Dis 37:457–466, 2001)

2. As for icodextrin, there is systemic absorption observed, which accounts for cross-reactivity with glucometer readings etc. This has been shown to affect plasma osmolality, although the effect is likely small.