• Temple EM

Phenobarbital Overdose

Temple GEM of the Week By: Jessica Fujimoto Edited by: Alexei Adan, Jessica Jackson & Maura Sammon

Case: 47 year old male presenting after intentional overdose on phenobarbital. EMS brought him in with an empty pill bottle that contained 60x 60mg tabs of phenobarbital. Time of ingestion is unknown. VSS. GCS 12.

Q: In addition to airway management and supportive care, what are the toxin-specific treatments for this patient? (see below)

A: For phenobarbital (though no other barbiturates), you can consider alkalinizing the urine to promote excretion! Technique: – Bolus 1-2mEq/kg of 8.4% sodium bicarb (1 amp is 50mEq of 8.4% sodium bicarb) – Then start a drip of 150mEq sodium bicarb in D5W @200-250cc/hr – Titrate drip to goal urine pH 7.5+ and serum pH no higher than 7.55-7.60 (Ideally, check baseline urine pH and serum pH prior to initiating bicarb) This is the same technique that you would use to alkalinize the urine to promote salicylate excretion.

Note: While urine alkalinization has been proven to be an effective method of salicylate excretion, it has not been proven as a solo therapy to shorten period of unconsciousness in phenobarbital overdose.

Extra Credit:

To appreciate how alkalinization works, it is important to note that salicylic acid (HSal) is a weak acid. In the equilibrium reaction

 H+   +   Sal-    <—>     HSal

if the systemic pH is increased, the equation will move to the left. The ensuing fall in the plasma HSal concentration will allow HSal in the CNS and other tissues to diffuse into the extracellular fluid down a favorable concentration gradient where it will be trapped as Sal-. The decrease in the CNS salicylic acid concentration then causes the first equation to move to the right in the brain cell. This increase in the cellular salicylic acid concentration promotes further drug movement out of the CNS.


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