ΔK

A study in Anesthesia & Analgesia reminds us of why intubating some intensive care patients with succinylcholine can be a bad idea. It demonstrates how the potentially harmful potassium efflux and transient hyperkalemia we get after administering Succinylcholine increases in magnitude with length of stay in the ICU.

Background
Succinylcholine (sux) is the muscle relaxant of choice for almost every rapid sequence induction intubation ever done in the history of mankind. Most of the time it is an incredibly efficient and safe drug that we all know how to use. However, there are some pit-falls associated with it. One of them is the potential for hyperkalemia caused by the potassium efflux we get when sux binds nicotinic acetylcholine receptors.

Typically serum potassium increases by 0,5-1 mmol/L with a peak 3-5 minutes after injecting sux. If this increase (ΔK)  shoots  serum potassium to more than around 6,5-7,0 mmol/L the patient risks having a potentially lethal arrhythmia. Which is why we are taught not to use sux in hyperkalemia, burns, rhabdomyolysis , crush syndrome (whatever that is) or other hyperkalemia states.  Unfortunately we often forget a much more common reason for  hyperkalemia. Up-regulation of cholinergic receptors.

Such up-regulation of receptors happen in diseases and states like anatomical denervation, prolonged administration of neuromuscular blocking drugs, burn injury and prolonged immobilisation. The body is fooled into thinking it needs more cholinergic receptors and decides to make more of them. More receptors means more potassium ions are purged in depolarisation. This in turn means the ΔK risk being a lot larger than the expected 0,5-1 mmol/L.

Or something along those lines. As a purveyor of the miracle of anesthesia I am perfectly happy, content even, knowing stuff happens without me knowing how or why.

Anyway, this is important as we intubate a whole lot of critical care patients. Many of them suffering from critical care neuropathy/myopathy which results in prolonged immobilisation and denervation. Prior studies have shown up-regulation and exaggerated potassium efflux in these patients.

ICU immobilisation, critical care neuropathy and myopathy are progressive diseases. This means the ΔK abnormality should increase with time, and that is exactly what this study looks at.

 The study
A prospective observational study where serum potassium was measured  before and after intubation was measured. The study enrolled a general ICU population of 131 patients who were intubated for a total of 153 times.  The increase in serum potassium, ΔK, was plotted against the length of ICU stay.

Results
The ΔK from all the intubations are plotted against length of ICU stay in the graph to the right.

ΔK increases with increased length of ICU stay.

At around day 15 the average potassium surge is more than the expected 0,5-1,0. Some of the patients had massive ΔKs in the order of 3-5 mmol-L after 10 or so days.

Take home message
Think twice before using succinylcholine when intubating patients who have long ICU stays  or who have developed critical care neuropathy.

Study lives here:
Anesth Analg. 2012 Oct;115(4):873-9. Epub 2012 Jul 4. The limits of succinylcholine for critically ill patients. Blanié A, Ract C, Leblanc PE, Cheisson G, Huet O, Laplace C, Lopes T, Pottecher J, Duranteau J, Vigué B.

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