Episode 009: The 2 View, L.A.S.T.

For the entire episode 009, please feel free to listen on Apple iTunes (search 2 view + emergency on podcasts) or click the links here: YouTube or Fireside FM.

Episode 009: Segment 4: Lidocaine Toxicity / LAST

I feel like when we do teaching to our new APPs, we often say - “we are just gonna numb this person up and then do xyz procedure”. But it’s important to hone in on dosing and safe practice of lidocaine. I always ask myself 3 questions before I use this drug to repair a laceration:

1. What is the extent of the injury and do I really need lidocaine for injection?

2. If I do need it, how much of it am I going to use?

3. Has the patient had it before ?? because some patients don’t have great pain relief from it.

There are studies that show when bupivicaine and lidocaine go head to head - bupivicaine had some benefits over lidocaine. Some patients have better pain relief from bupivicaine over lidocaine and at times, even less medication volume needs to be used to achieve the same anesthesia effects. Plus, it’s longer acting so consider again point number one - what is the extent of the injury - where is it how much surface area is involved and weigh your pros and cons then. 

Let’s talk safety about lidocaine and discuss a little bit about what the mechanism of action is and how it works. Remember Lido also comes in different forms - and that brings me to point number 1 again - can I use a LET - to get the anesthesia I need? And that is the topical formulation that can be set it, let it and leave it until, I”m ready to do what I need to do. I often will use it in combination with the injection to achieve a solid working surface free of pain. But, that also requires some calculation and planning. If there is a small lac on the face, that needs just a few sutures, then it might be a good choice alone as the LET. 

Lidocaine prevents pain by blocking the signals at the nerve endings in the skin. I feel like an infomercial here, but Lido comes in many forms and you should keep it in your arsenal for a variety of ailments. It’s a nice refresher for those not so common procedures we do where lidocaine can make an impact. It can be found in forms such as in a

• Spray

• Patch, extended release versions

• Gel/Jelly

• Cream

• Ointment

• Lotion

• Pad

• Swab

• Powder

• And of course, a solution for injection

So let’s talk about just laceration repair and safe uses of lidocaine. Specifically, local anesthetic systemic toxicity or LAST. Mike brought to my attention a 2018 article in the Journal of Local and Regional Anesthesia.

Local anesthetic systemic toxicity (LAST) is a life-threatening adverse event that may occur after the administration of local anesthetic drugs through a variety of routes. Not just localized injection of course. 

This article identifies recent data that demonstrated the underlying mechanisms of LAST are multi-factorial, with diverse cellular effects in the central nervous system and cardiovascular system. They note that although neurological presentation is most common, LAST often presents atypically, and one-fifth of the reported cases present with isolated cardiovascular disturbance. There are several risk factors that are associated with the drug used and the administration technique. They suggested that LAST can be mitigated by targeting the modifiable risk factors, including the use of ultrasound for regional anesthetic techniques and restricting drug dosage. There have been significant developments in our understanding of LAST treatment. 

In fact, I walked into the department the other day at Zuckerberg San Fran General ER, and lone behold what did I see? A white board - all on this topic. I was like, wow - Mike and I were just talking about this! So I took some time to hit up what our local ED pharmacists were saying about this issue. 

We are going to do some mathematics. Local anesthetic guidelines of lidocaine 1-2% is max 4-5mg/kg. So, to put that into perspective, if we have a 20 kg kid, we would never use more than 100 mg. For an adult patient that is say, 70 kg, never more than 350 mg. To break that down even further, we know that 1 ML of lidocaine 1% contains 10 mg of lidocaine and 2% contains 20 mg of lidocaine. So therefore, one would deduce here that a 20 kg kid gets no more than 10 mLs of 1% lidocaine or 5mLs of 2% lidocaine. The 70 kg adult would get no more than 35 mLs of 1% and roughly 17 mL of 2%. Now of course, 35 mLs of anything seems like a lot right? But what about those GIANT gashes that we go in with a start of 8-10 mLs and they need more? We have to be conscious of how much we are giving and what concentration. So that is simply the reminder here. And of course, those numbers are on the higher side of dosing. We should really start to worry when we are giving someone anything more than 3-4 mg/kg in an adult or pediatric patient. 

Let’s talk tox now - so symptoms of progression are as followed: slurred speech and parathesiases lead to drowsiness and confusion then muscle twitching - finally seizures, prolonged QRS and then the big old cardiac arrest. Yikes. SO this is not soemthing to fool around with. 

Neuro break down description of what is happing is this: Increasing plasma concentrations of lidocaine initially compromises cortical inhibitory pathways by blockade of NaV channels, disrupting inhibitory neuron depolarization. Inhibiting these pathways leads to excitatory clinical features of sensory and visual changes, muscular activation, and subsequent seizure activity. As the plasma concentrations of lido rise, excitatory pathways are affected, producing a depressive phase of neurological toxicity, with loss of consciousness, coma, and respiratory arrest.

Cardiovascular wise - this complex toxic pathway causes conduction disturbances, myocardial dysfunction, and lability of peripheral vascular tone. The primary effects are likely to arise from rhythm disturbance, prolonged QRS - with other effects being secondary. Normal conduction is disrupted by direct sodium channel blockade, chiefly at the bundle of His. By driving the resting membrane potential to a more negative level, action potential propagation is impaired, leading to prolonged PR, QRS, and ST intervals. Re-entrant tachyarrhythmias and bradyarrhythmias ensue, which may be worsened by further potassium channel blockade, prolonging the QT interval.

Myocardial dysfunction has several contributory mechanisms. Calcium channel and Na+–Ca2+ exchange pump blockade reduces intracellular calcium stores and, thus, diminishes contractility. The paper goes on to talk about the neuronal control mechanisms of baroreceptors, as well as a negative effect on systemic vascular tone. You can get nerdy and really dive deep into this one if you want!

The paper also takes a look at which patients are more at risk, so put the following patient on your radar…. Extreme age, elderly patients, who may have reduced clearance of lidocaine or any other drug for that matter. In fact, you should consider using a 10-20% reduction of your typical safe dose in these patients. 

Other concerning patients may be those who are pregnant and their doses need to be reduced as well. The paper also suggests that those with known hepatic dysfunction also get less of a dose. Patients with severe renal disease have a hyperdynamic circulation and reduced clearance of lidoaine. As a result, free plasma concentrations are largely unchanged and dose reduction is often unnecessary, unless the patient is uremic with metabolic acidosis. 

And of course, caution in those with cardiac disease. Patients with cardiac disease are at an increased risk of LAST. Those with pre-existing conduction disorders may be predisposed to cardiovascular toxicity, and careful dosing as well as the use of less cardiotoxic drugs such as ropivacaine or levobupivacaine is recommended.

Now this is also rare to have LAST, the incidence currently estimated to be 0.03%, or 0.27 episodes per 1,000 peripheral nerve blocks. The cure you ask if you suspect LAST? It’s intralipid, 20% 1.5 mg/kg IVP! A note: Lipid emulsion therapy may shuttle any toxic anesthetic agent from high blood flow organs – such as the heart or brain – to storage or detoxification organs such as muscles or the liver. That is essentially how this treatment works. Lipid emulsion therapy may also improve the cardiac output and blood pressure (hence further facilitating the shuttling effect), while postconditioning myocardial protection may also occur.

Immediate management involves the general safety and resuscitation measures that are essential in any emergency. The immediate priority is to manage the airway, breathing, and circulation.

Prompt and effective airway management is crucial to prevent hypoxia, hypercapnia, and acidosis (metabolic or respiratory), which are known to potentiate LAST. The airway should be secured and 100% oxygen administered, bearing in mind that hyperventilation and respiratory alkalosis have also been demonstrated to be injurious.

For more information, be sure to check us out and our other episodes on The 2 View! - Martha and Mike