Why this poison is interesting

Every other poison in this library arrives by ingestion, inhalation or bite. Local anaesthetic systemic toxicity arrives by injection, from a clinician's own hand, and it is the toxicity of doing a routine thing slightly wrong. Local anaesthetics are sodium-channel blockers — that is how they stop a nerve conducting — and the entire therapeutic manoeuvre is to place a high concentration next to a nerve and nowhere else. LAST is what happens when the drug reaches the bloodstream instead: inadvertent intravascular injection, or systemic absorption of an excessive dose.² The same molecular action that numbs a finger then blocks sodium channels in the brain and the heart, where it is not wanted and not survivable in excess.

This makes LAST the deliberately-injected member of the sodium-channel blockade family — the archetype, in fact, because local anaesthetics are the drugs that family is named after. It sits beside the tricyclics and lamotrigine in producing the same cardiac lesion by the same mechanism, and beside cocaine, which is a local anaesthetic and poisons the heart the same way. What sets LAST apart is the tempo: an intravascular bolus can produce seizures within seconds and an arrest within a minute, faster than any swallowed poison.

And it is the one poisoning in this library with an antidote built specifically for it. Intravenous lipid emulsion was developed as a rescue for local anaesthetic cardiotoxicity and remains its defining treatment: a bag of 20% lipid that can restore a circulation a bupivacaine overdose has stopped. No other page here has a therapy invented for that single toxin. That antidote, and the mechanism that makes it work, are most of what this page is for.

A poison is a drug whose kinetics have escaped its pharmacology.

The toxic principle

The toxic principle is the intact local anaesthetic blocking voltage-gated sodium channels in tissues that were never meant to receive it. In a peripheral nerve this stops the propagation of an action potential — the intended effect. In the central nervous system and the myocardium the same block is a poisoning, and the two systems fail in a characteristic order because they have different thresholds. El-Boghdadly and colleagues summarise the modern understanding: the mechanisms are multifactorial, with diverse cellular effects in the central nervous system and cardiovascular system² — sodium-channel block is the core, but potassium- and calcium-channel effects and impairment of myocardial mitochondrial energy metabolism all contribute to the cardiac picture.

The CNS usually fails first and in two stages. Inhibitory interneurones are more sensitive to sodium-channel block than excitatory ones, so a rising concentration first removes inhibition — producing the classic prodrome of perioral numbness, metallic taste, tinnitus, visual disturbance, agitation and muscle twitching, and then a generalised seizure — before, at higher concentrations, blocking excitatory transmission too and causing CNS depression, coma and respiratory arrest.² The excitation is disinhibition, not stimulation, which is why it is a warning sign of an approaching toxic concentration rather than a benign side effect. Established

Why bupivacaine is the one that arrests the heart

Not all local anaesthetics are equally cardiotoxic, and the difference is kinetic. Bupivacaine binds the cardiac sodium channel avidly and dissociates from it slowly — the fast-in, slow-out behaviour — so it accumulates in the channel during diastole and is not cleared between beats, producing profound conduction block and ventricular arrhythmias that are notoriously resistant to defibrillation. Lidocaine, by contrast, is fast-in, fast-out and far less cardiotoxic; ropivacaine and levobupivacaine are single-enantiomer agents developed specifically to be less cardiotoxic than racemic bupivacaine. The whole reason bupivacaine cardiac arrest became the disaster that lipid emulsion was invented for is that its channel binding outlasts the cardiac cycle. Established

Toxicokinetics

The kinetics that matter for LAST are not those of clearance but those of delivery: how fast the drug reaches the systemic circulation, which depends entirely on the route of the error. An intravascular injection delivers a bolus in seconds; absorption from a tissue depot delivers it over many minutes. Clearance, by contrast, is barely relevant in the acute event, because the emergency is decided long before hepatic metabolism can change the concentration.

Local anaesthetic — the kinetics of delivery, not of clearance, decide LAST
Parameter	Therapeutic	In overdose	Why it changes
Route of exposure	Inadvertent intravascular injection (seconds) or systemic absorption from a tissue depot (minutes to over an hour)²	The route sets the tempo — instant collapse vs delayed toxicity	This is the single most important kinetic fact. An intravascular bolus produces near-immediate seizures or arrest; a large-volume block absorbed from tissue can produce toxicity well after the injection is finished, which is why LAST is not excluded by an uneventful first few minutes.
Absorption from tissue	Depends on vascularity of the site, the total dose, the drug and whether a vasoconstrictor was added	Highly vascular sites and large volumes raise the peak concentration	Intercostal and paracervical blocks reach higher peaks than subcutaneous infiltration for the same dose; adrenaline in the mixture slows absorption. The dose limit is a function of the site, not a single number.
Distribution	Rapidly to well-perfused organs — brain and heart first	The target organs are reached before slower compartments	The brain and myocardium receive the highest early concentrations, which is why they, not the liver or kidney, are where the toxicity appears.
Protein binding	High for bupivacaine (~95%); toxicity relates to the free fraction	Acidosis and a high free fraction worsen cardiotoxicity	A seizure produces acidosis, which raises the free (active) fraction and worsens the cardiac toxicity — one reason prompt seizure control matters beyond the seizure itself.
Elimination	Hepatic metabolism (amides) or plasma esterase hydrolysis (esters); irrelevant to the acute event	Too slow to alter the emergency	Clearance does not rescue a LAST arrest — the timescale is minutes and the answer is lipid emulsion and resuscitation, not waiting for the drug to be metabolised.
Dialysability	—	No role. The emergency is over in minutes to an hour and is managed at the bedside	There is neither time nor rationale for extracorporeal removal; lipid emulsion works far faster than any circuit could be established.

Metabolism and the metabolites

Metabolism is almost beside the point in LAST, because the toxicity is the parent drug's and the event is over before metabolism matters — but the two chemical classes are worth distinguishing because they fail differently and, in one case, produce a second toxidrome. The amide local anaesthetics (lidocaine, bupivacaine, ropivacaine, prilocaine) are metabolised in the liver; the ester local anaesthetics (procaine, tetracaine, and cocaine) are hydrolysed by plasma esterases.

Local anaesthetic — the block is the parent drug's; one metabolite makes a different poisoning

Local anaesthetic in the circulationThe toxic species — blocks Na channels in brain and heart²
Hepatic metabolismAmides — lidocaine, bupivacaine, ropivacaine, prilocaineToo slow to affect the acute event; the parent drug is what poisons
Plasma esterase hydrolysisEsters — procaine, tetracaine, cocaineCocaine is an ester local anaesthetic — see cocaine, the recreational face of the same Na-channel cardiotoxicity
MethaemoglobinPrilocaine's metabolite oxidises haemoglobin — a separate toxidrome; see methaemoglobin inducers

Elimination and accumulation

In the acute intravascular event there is no meaningful accumulation to discuss — the bolus is delivered and the toxicity is immediate. The accumulation that matters is the slower, more insidious one: repeated or continuous dosing, and absorption from a large tissue depot, both of which can raise the plasma concentration into the toxic range over minutes to hours rather than seconds.

The other accumulation risk is patient-dependent. Hepatic impairment slows amide clearance; low plasma protein or acidosis raises the free fraction; extremes of age and cardiac disease lower the threshold. None of these changes the acute intravascular event, but all of them lower the dose at which absorption from a depot becomes toxic — which is why the safe maximum dose is individualised rather than fixed. Inferred

Target organs — and why those

Brain

TargetVoltage-gated sodium channels, inhibitory interneurones first

Why hereThe organ that usually declares the toxicity, and the reason for the biphasic CNS picture. Inhibitory neurones are blocked at a lower concentration than excitatory ones, so the first effect is disinhibition — the prodrome and then a seizure — before higher concentrations depress all neuronal activity into coma and apnoea.² The brain is targeted first because it is highly perfused and because its inhibitory circuits are the most sensitive to sodium-channel block. Established

At the bedsidePerioral numbness, metallic taste, tinnitus, agitation, muscle twitching, then seizures, then CNS depression and respiratory arrest.² Under general anaesthesia this whole sequence is silent.

Heart

TargetCardiac sodium channels (bupivacaine fast-in, slow-out), plus K/Ca channels and mitochondrial metabolism

Why hereThe organ that kills, and the reason lipid emulsion exists. Sodium-channel block slows conduction and depresses contractility; bupivacaine's slow dissociation from the channel produces refractory ventricular arrhythmia and asystole. About one in five cases present here first, with no CNS warning.² The heart is targeted because the same Na channel the drug blocks in a nerve carries the cardiac impulse, and bupivacaine will not let go of it between beats. Established

At the bedsideQRS widening, bradycardia, heart block, ventricular tachycardia or fibrillation, myocardial depression and cardiac arrest resistant to defibrillation.²

Red cell — prilocaine only

TargetHaemoglobin, oxidised by the metabolite o-toluidine

Why hereIncluded because prilocaine belongs to two toxidromes. Its metabolite o-toluidine oxidises haemoglobin to methaemoglobin, so a large prilocaine dose can produce methaemoglobinaemia in addition to, or instead of, sodium-channel toxicity. This organ is targeted not by the block but by a metabolite, which is why it is a prilocaine-specific footnote rather than a general LAST feature. Established

At the bedsideCyanosis unresponsive to oxygen and a saturation gap after a prilocaine dose; managed as methaemoglobinaemia — see methaemoglobin inducers.

Timeline of effects

LAST — two clocks: the intravascular bolus that arrests in a minute, and the tissue depot that surfaces an hour later

Time

What you seeWhat is happening

Seconds (intravascular)Immediate toxicity
What you seeSudden seizure or cardiovascular collapse at, or within seconds of, injection.²
What is happeningAn intravascular bolus delivers a toxic concentration to brain and heart at once. There is no prodrome and no latent interval on this route — the needle was in a vessel.
1–30 minThe classic sequence
What you seePerioral numbness, metallic taste, tinnitus, agitation, twitching → seizure → CNS depression; then cardiac conduction change and arrhythmia.²
What is happeningRising concentration blocks inhibitory neurones first (disinhibition), then excitatory ones, then the heart. This is the textbook progression — and the one general anaesthesia hides.
30 min – over 1 h (tissue depot)The delayed presentation
What you seeA patient who was fine at the end of the block becomes toxic later — seizure or arrhythmia as the depot is absorbed.²
What is happeningA large-volume block, infusion or repeated top-up is absorbed from tissue over time, so the plasma concentration can cross the toxic threshold well after the injection is finished. The gap is pharmaceutical — slow absorption — not metabolic, and it is why LAST is not excluded by an uneventful first few minutes.
During treatmentRescue and recovery
What you seeRecovery with early lipid emulsion, seizure control and prolonged resuscitation; survival is now the usual outcome.¹
What is happeningLipid emulsion partitions the drug away from the channels and supports myocardial energetics; the modified resuscitation (reduced adrenaline, avoiding several usual drugs) carries the patient until the drug redistributes and is cleared.¹

A gap made by tissue, not by metabolism

Local anaesthetic systemic toxicity (LAST) — the tissue depot — a large-volume block absorbed slowly, so systemic toxicity can appear well after the injection is finished, not only at the moment of an intravascular one

The other 39 kinds of latent phase in this library

Amphetamines and MDMA — a hormone acting normally on a kidney behaving normally, while the patient supplies the water
Anticoagulants — clotting factors that were already made, still working — the poison stops production and nothing happens until the existing stock decays
Antipsychotics — a physical object in the stomach — extended-release quetiapine forming a pharmacobezoar
Arsenic — a tissue declaring on its own timetable rather than the poison's — the arsenic is excreted within days, but the nail that was growing while it circulated does not show its white transverse line for several weeks
Arsine and stibine — a red cell mass haemolysing faster than a kidney can cope with — the exposure is over, the haemolysis is silent until the urine changes colour, and the renal failure that follows is the cause of death
Baclofen — not the poisoning but its withdrawal — an implanted pump that has silently stopped delivering, with hours to days before anything appears
Beta-blockers — a repolarisation lesion waiting for an ectopic beat to fall inside it — sotalol prolongs the QT and then, for hours, nothing happens
Calcium-channel blockers — a tablet that has not yet dissolved
Carbamazepine — reduced gut motility from its own antimuscarinic effect, forming a reservoir that delays absorption and causes relapse on the second or third day
Carbon monoxide — an inflammatory process continuing after the poison itself has gone
Chlorine and ammonia — paraquat's cause at its shortest — the time an injured alveolar epithelium takes to leak
Cocaine — a package that has not yet failed — the calcium-channel blocker's cause moved outside the tablet, where wrapping integrity cannot be measured
Colchicine — antimitotic injury to fast-dividing tissues that declares itself over 24–72 hours, after a gastrointestinal phase that can appear to settle
Death cap and the amatoxin mushrooms — the interval before amatoxin-blocked transcription starves the hepatocyte of protein
Digoxin — distribution of the drug to its target — which that page argues is not a latent phase in the paracetamol sense at all
Ethylene glycol — paracetamol's cause with a different enzyme — glycolate accumulating behind glycolate oxidase while the patient looks merely drunk
GHB and GBL — not the poisoning but its withdrawal — a half-life under an hour means admission for any reason interrupts round-the-clock redosing
Hydrofluoric acid — an ion diffusing far enough to reach a nerve ending — and the thinner the solution, the further it travels before anybody feels it
Insulin — the long-acting analogue depot — a subcutaneous reservoir of glargine or degludec that releases insulin for a day or more, so hypoglycaemia can be delayed in onset and persist long after a soluble insulin would have cleared
Iron — a true remission — the corrosive injury settling while absorbed iron moves into the cells it will poison, so the patient improves on their way to the dangerous phase
Ketamine — nitrous oxide's cause on a longer clock — urothelial damage accumulating over months to years of repeated exposure
Lead — bone giving lead back — a store with a half-life of 10 to 30 years releasing its contents when pregnancy, lactation, menopause or osteoporosis resorbs it, so the blood concentration rises after the exposure has ended
Lithium — transport across cell membranes
Mercury — distribution on two clocks — tissue concentrations peaking within 24 hours everywhere except the brain, which is not reached until 2 to 3 days, and which then cannot let the poison out again
Metformin — drug accumulation and a lactic acidosis that build over hours, so a well-looking early patient can deteriorate
Methaemoglobin inducers — paracetamol's cause at its shortest — one to four hours manufacturing the toxic species
Methanol — paracetamol's cause again — formate accumulating behind a folate-dependent disposal step that primates perform poorly
Methotrexate — antifolate injury to fast-dividing tissues that declares itself over days, after an early phase that can be silent
Nitrous oxide — damage accumulating to a threshold
Opioids — an antidote wearing off before the poison does — renarcotisation, the only gap in this set that treatment creates rather than reveals
Organophosphate insecticides — a fat store emptying — and, separately, a second and unexplained lesion declaring itself at a neuromuscular junction the first phase had already left
Paracetamol — time spent manufacturing a toxic metabolite
Paraquat — the body responding to an injury that is already complete
Sodium-channel blockade — a gap that cannot be shortened
Sulfonylureas — delayed and recurrent hypoglycaemia, because the drug keeps releasing insulin long after the first glucose correction
Thallium — hair on its own clock rather than the poison's — sensory symptoms come first and the alopecia that makes the diagnosis obvious follows them, well after the interval in which treatment is recommended
Theophylline — a prolonged-release formulation whose peak may not arrive for twelve hours, so the concentration rises while the patient is still being assessed
Valproate — hyperammonaemia and carnitine depletion that build after ingestion, so encephalopathy can lag a reassuring early examination
Venlafaxine and the SNRIs — the slow-release capsule — a modified-release preparation whose absorption is spread over hours, so the seizure risk of a large ingestion is still climbing long after presentation

Most latent phases in this library are metabolic — a poison must be converted or must accumulate before anything happens. LAST's delayed presentation is pharmaceutical: a depot of drug in tissue is absorbed slowly, so a patient who tolerated the injection can become toxic as the reservoir empties into the circulation. It is drawn as a gap because during it there is genuinely nothing wrong — until, without any further dose, there is.

What the mechanism predicts at the bedside

Stop injecting and get lipid emulsion at the first sign. LAST is a real-time emergency; the specific antidote is intravenous 20% lipid emulsion given early — for the seizure as well as for cardiovascular collapse.¹² See lipid emulsion.
Do not wait for the tingling lip. One in five present with the heart and no CNS prodrome², and general anaesthesia removes the prodrome entirely — a collapse during or after a block is LAST until proven otherwise.
Control the seizure immediately with a benzodiazepine. Beyond stopping the seizure, this limits the acidosis that raises the free drug fraction and worsens cardiotoxicity.²
Modify the resuscitation. The ASRA checklist advises reducing adrenaline doses (to ≤1 µg/kg) and avoiding vasopressin, calcium-channel blockers, β-blockers and any further local anaesthetic.¹ The usual reflexes are wrong here.
Expect bupivacaine arrest to resist defibrillation and to need prolonged effort. Its slow dissociation from the cardiac sodium channel² means recovery can take a long resuscitation supported by lipid; cardiopulmonary bypass is a last resort.¹
Prevention is most of the safety. Ultrasound guidance, careful aspiration, incremental injection and adherence to weight-based dose limits reduce the risk before the drug is ever given.²
Remember prilocaine's second toxidrome. Cyanosis unresponsive to oxygen after a prilocaine block is methaemoglobinaemia, not hypoxia — see methaemoglobin inducers.
A late deterioration after a block is still LAST. Absorption from a tissue depot can surface toxicity more than half an hour after the injection²; an uneventful few minutes does not exclude it.

Why LAST has an antidote when almost nothing else here does

Intravenous lipid emulsion works by two mechanisms that fit this poison exactly. The lipid sink: local anaesthetics are lipophilic, so an expanded plasma lipid phase partitions the drug away from the sodium channels in the heart and brain, lowering the free concentration at the target. And a metabolic effect: the fatty-acid load supports the poisoned myocardium's energy metabolism and improves contractility. Both depend on the drug being lipophilic and the injury being acute and reversible — which is precisely the situation in LAST, and precisely why lipid was invented for it and works less predictably for the other lipophilic poisons that borrow it (tricyclics, lamotrigine). The antidote and the poison were designed for each other. Inferred

The antidote, from the poison's side

LAST is the rare poisoning in this library with a specific antidote, and an unusually good one, because it was developed for this exact toxin. The management is a package: lipid emulsion, seizure control, and a deliberately modified advanced life support, because several of the usual resuscitation drugs are the wrong choice against a sodium-channel-blocked heart.

Intravenous lipid emulsion (20%): The specific antidote, given early for seizures or cardiovascular toxicity: a bolus followed by an infusion, repeated for persistent instability. It partitions the lipophilic drug away from the channels (the lipid sink) and supports myocardial energetics. Invented for local anaesthetic cardiotoxicity and still its defining treatment.¹² See lipid emulsion.
Benzodiazepines: First-line for the seizure — and, by preventing the acidosis a seizure causes, a way of limiting the rise in free drug and the worsening of cardiotoxicity.²
Modified advanced life support: Reduce adrenaline (small doses, ≤1 µg/kg), and avoid vasopressin, calcium-channel blockers, β-blockers and further local anaesthetic.¹ A sodium-channel-blocked myocardium responds badly to the usual arrest algorithm, which is why LAST has its own checklist.
Airway and prolonged resuscitation: Ventilation to correct hypoxia and acidosis, and a willingness to continue resuscitation far longer than usual — bupivacaine arrests can recover after prolonged effort supported by lipid.¹
Cardiopulmonary bypass / ECMO: The last resort for an arrest refractory to lipid and resuscitation¹ — buying time for the drug to redistribute and be cleared.

Critical appraisal

The sodium-channel and bupivacaine 'fast-in, slow-out' mechanisms are Established. They are standard electrophysiology and underpin the differential cardiotoxicity of the agents; the Established badges reflect that, while the multifactorial cardiac contribution (K/Ca channels, mitochondrial effects) is drawn directly from the cited review.²
The '1 in 5 present cardiovascularly' figure is from reported cases and carries their bias. El-Boghdadly and colleagues state that one-fifth present with isolated cardiovascular disturbance²; reported-case series over-represent severe and unusual presentations, so the figure describes the published literature, and the clinical point — that a CNS prodrome cannot be relied upon — stands regardless of the exact proportion.
The lipid-emulsion mechanism is badged Inferred. The lipid-sink and metabolic explanations are well supported by animal and laboratory work and are the accepted account, but the precise contribution of each in a human LAST arrest is not settled, and the page badges the mechanism as inference rather than as demonstrated in poisoned humans.
The management specifics are guideline consensus, not trial results. The reduced-adrenaline and drug-avoidance advice comes from the ASRA checklist¹; there is no randomised evidence for these choices, and the page attributes them to the checklist rather than presenting them as proven superior.
Dose limits are deliberately not tabulated. Maximum safe doses depend on the agent, the site, the patient and whether a vasoconstrictor is used; the page states that the limit is individualised² and refers dosing to the relevant anaesthetic guidance rather than printing numbers that would be unsafe out of context.
No lethal dose or fatal concentration appears here. Severity is described by the clinical syndrome; risk assessment and any concentration data belong to TOXBASE, NPIS and the anaesthetic literature.

References

1
Neal JM, Woodward CM, Harrison TK. The American Society of Regional Anesthesia and Pain Medicine Checklist for Managing Local Anesthetic Systemic Toxicity: 2017 Version. Regional Anesthesia and Pain Medicine 2018 Feb;43(2):150–153. PMID 29356775. The ASRA management checklist — early lipid emulsion, seizure control, reduced adrenaline dosing and avoidance of vasopressin, calcium-channel blockers, β-blockers and further local anaesthetic.
2
El-Boghdadly K, Pawa A, Chin KJ. Local anesthetic systemic toxicity: current perspectives. Local and Regional Anesthesia 2018 Aug 8;11:35–44. PMID 30122981. Review of LAST mechanisms (multifactorial CNS and cardiovascular effects), the atypical presentations (one-fifth with isolated cardiovascular disturbance), risk factors, prevention (ultrasound, dose limits) and treatment (early lipid emulsion, seizure management, careful cardiovascular support).

Local anaesthetic systemic toxicity (LAST)

At a glance