Through the Looking Glass: Chirocaine vs Bupivicaine. St.Emlyn’s

I’ve seen a few paediatric femoral shaft fractures in the last couple of years, and while they always make my spidey-sense tingle for non-accidental injury, most (even the spiral fractures) have been explained away by plausible mechanisms of injury; they are usually late toddling age, have got their feet stuck, twisted and fallen over.

I’ve even seen a few which were pathological – undoubtedly the result of the vitamin d deficiency which stems from the miserable attempt at sunshine we have to put up with here in the North West.

The way I’ve managed these patients has changed with my experience and skill level; as an SHO I looked at them with sympathy while the oramorph, ibuprofen and paracetamol were absorbed slowly from their (no doubt static) GI tracts. As a registrar, they now get a squirt of intranasal diamorphine and an application of ametop over the ipsilateral groin, in anticipation of an ultrasound-guided femoral nerve block when they return from x-ray. I mix up a 50:50 solution of 1% lidocaine with 0.5% bupivacaine and, before ultrasound guidance, used 1mL per year of age. It works out as less than 2mg/kg of lidocaine (usually) and less than 1mg/kg bupivacaine – so relatively small doses. I’ve done it a few times, and it works pretty well, especially in combination with a Thomas splint. The great thing about ultrasound-guidance besides lower failure rates is that you can use even smaller drug volumes – it ends up more like 0.5mL per year of age.

So as I examined the two year old, who was refusing to weight bear, with little deformity to his leg but clear femoral tenderness, I reached for the trusty bupivacaine/lidocaine combination to make splint application less of an auditory assault. But shock! Horror! No bupivacaine!

Instead, there was chirocaine – which completely confused me, as I had no idea what it was (and there was apparently no bupivacaine to be had anywhere). I asked around – and no-one seemed to be entirely sure how the two drugs related to one another. So, after some research, I thought I’d share.

Chirocaine (levobupivacaine), as you may know (I didn’t), is “the pure S-enantiomer” of bupivacaine. The story is that some drugs exist in a 50:50 (racaemic) mixture of two stereo-isomers; molecules which are mirror images of one another. Where the mirror images cannot be superimposed, the molecules are said to be chiral, and the most common cause for this is an asymmetrical carbon atom. There is a right (dex-, or d-) and left (levo-, or l-) version of the molecules which cannot be made symmetrical no matter the orientation in the same way as your left foot just doesn’t fit in your right shoe [NB: an important differential in the non-weight bearing child]. Where drugs bind to receptors, one isomer may be able to bind, while the other cannot – or may generate alternative effects.

You probably knew this already, right? Every time you prescribe levothyroxine, esomeprazole, escitalopram or levofloxacin, you think proudly about how you are selecting out the correct isomer for your patient, yes? And how you are therefore giving half the dose, as they aren’t getting the 50% of isomers which don’t give a clinical effect? Well, it doesn’t quite work as predictably as that. In fact, in some cases the dose is significantly lower for the same clinical effect (notably escitalopram, 30 times more potent than citalopram) and in others confers no therapeutic advantage at equivalent doses (see esomeprazole) or may even be harmful (see thalidomide).

So what’s so special about chirocaine? It’s twice the price of standard bupivacaine per 10mL ampule, so it’s twice as good, isn’t it?

Well, no – it seems to be equivalent at providing analgesia in a variety of blocks (caudal, ilioinguinial, rectus sheath… The list goes on…), but every study is focussed on the reduction in adverse (cardiac) effects with levobupivacaine. It seems the rationale for selecting out this isomer is reducing cardiac toxicity. Where does this come from?

Most data comes from animal studies (in case you didn’t know, children are no more little adults than they are little rats, pigs, rabbits…).

You have to dig quite deep for original studies. All roads seem to lead back to this one paper. 14 healthy, adult, male subjects were injected with bupivacaine or levobupivacaine infusion and asked “do you have any symptoms?”. OK, let’s be fair, they’ve powered the study to detect a 10% difference in their primary outcome (stroke volume), but their bottom line conclusion is “that levobupivacaine may be a safer drug than rac-bupivacaine for procedures requiring high doses of local anaesthetic.” So not really our fractured femur toddlers, then.

Which brings us back to the beginning. Studies in children have suggested that levobupivacaine produces equivalent anaesthetic efficacy, but what they haven’t done is convince me that we should ditch bupivacaine when we are using such tiny doses. It’s cheaper, and with ultrasound guidance we can use a miniscule amount to achieve local anaesthesia long enough to reduce the fracture in a Thomas splint.

So the bottom line is this; use what you’ve got unless there’s specific cardiac risk for your paed patient, and using 50:50 with 0.5% chirocaine provides the same analgesia as bupivacaine (in our hospital the maximum dosing for ilioinguinal block is apparently 1.25mg/kg/side). Whatever you use, do the block; your patients will probably thank you (but we might have to wait to find out).

Natalie May

Ref

• A comparison of the cardiovascular effects of levobupivicane and rac-bupivicane following intravenous administration to healthy volunteers. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1873676/pdf/bcp0046-0245.pdf
• EPG data sheet on Chirocaine http://www.epgonline.org/drugs/chirocaine/
• Regional anaesthesia for kids with femoral fractures at BestBets http://www.bestbets.org/bets/bet.php?id=130

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