Lacosamide, an anti-seizure medication, primarily affects the heart by slowing electrical conduction, which shows up most clearly as a lengthening of the PR interval on an electrocardiogram (ECG). At standard doses, this change is usually small and harmless, but in certain people and certain circumstances it can escalate into clinically significant rhythm disturbances. The cardiac story around lacosamide is more layered than the prescribing label alone suggests, and understanding the range of possible heart effects helps patients and clinicians make better-informed decisions.
Why Lacosamide Affects the Heart at All
Lacosamide controls seizures by targeting sodium channels in the brain, but the heart also relies on sodium channels to generate and conduct its electrical signals. The specific cardiac sodium channel it interacts with is called Nav1.5, which is the main channel responsible for the rapid upstroke of the heartbeat’s electrical impulse. Laboratory studies show that lacosamide binds to this channel in a distinctive way: rather than simply blocking channels that are resting or fully active, it preferentially targets channels that are in a transitional, “preopen” state along the activation pathway.1PubMed Central. Block of human cardiac sodium channels by lacosamide: evidence for slow drug binding along the activation pathway This slow-binding characteristic is what makes lacosamide’s cardiac effects dose-dependent and generally mild at therapeutic levels, but potentially troublesome when drug concentrations rise.
PR Interval Prolongation at Therapeutic Doses
The most consistently documented cardiac effect of lacosamide is a small, dose-related increase in the PR interval, which reflects how long it takes for the electrical signal to travel from the upper chambers of the heart to the lower chambers. In pooled clinical trial data from patients taking lacosamide for partial-onset seizures, the average PR increase was about 1.4 milliseconds at 200 mg per day, roughly 4.4 milliseconds at 400 mg per day, and about 6.6 milliseconds at 600 mg per day, compared to essentially no change with placebo.2PubMed. Lacosamide cardiac safety: clinical trials in patients with partial-onset seizures To put those numbers in context, a normal PR interval ranges from about 120 to 200 milliseconds, so a shift of a few milliseconds in either direction is usually clinically insignificant.
A study in hospitalized patients found a similar picture: after starting lacosamide, the average PR interval went from about 179 milliseconds to roughly 186 milliseconds, a bump of around 6 milliseconds.3PubMed Central. The safety of lacosamide for treatment of seizures and seizure prophylaxis in adult hospitalized patients Most patients do not feel anything from this kind of change and never need their treatment adjusted because of it. Where the concern grows is when the PR interval was already borderline long before starting the drug, or when other medications that also slow conduction are on board.
When Conduction Problems Become More Serious
Beyond mild PR prolongation, lacosamide has been linked to various degrees of atrioventricular (AV) block, where the electrical signal between the upper and lower heart chambers is delayed or intermittently dropped. In the large clinical trial dataset, first-degree AV block (a PR interval above 200 milliseconds but with every beat still conducting) was reported in under one percent of patients on lacosamide, and second-degree or higher AV block was not observed at all during those trials.2PubMed. Lacosamide cardiac safety: clinical trials in patients with partial-onset seizures
Real-world use, however, paints a somewhat more complicated picture. Case reports document second-degree AV block (Mobitz type 1, also called Wenckebach) developing in otherwise healthy young adults on lacosamide. In one such case, a healthy young person developed progressively lengthening PR intervals followed by dropped beats on telemetry monitoring; the block resolved within a day after lacosamide was stopped, and a follow-up ECG showed a nearly normal PR interval.4PubMed Central. Lacosamide-associated second-degree atrioventricular block in a healthy, young adult Complete (third-degree) heart block has also been reported, particularly in elderly patients.5PubMed Central. Lacosamide Associated Complete Heart Block in Elderly
A disproportionality analysis of the FDA’s Adverse Event Reporting System identified 17 types of cardiac adverse events reported more frequently with lacosamide than expected. Bradycardia was the most commonly reported cardiac event, followed by cardiac arrest, AV block, and atrial fibrillation. Among the signals identified, complete AV block stood out with the strongest statistical association.6Scientific Reports. Cardiac adverse events associated with lacosamide: a disproportionality analysis of the FAERS database Reporting databases like this cannot prove that lacosamide caused each event, but the disproportionate clustering of conduction-related problems is consistent with what the drug’s mechanism predicts.
Arrhythmias Beyond Conduction Slowing
Lacosamide’s cardiac effects are not limited to slow conduction. A systematic review of published case reports found that ventricular tachycardia was the most frequently reported arrhythmia linked to lacosamide, accounting for roughly 29 percent of reported rhythm disturbances. New-onset atrial fibrillation and complete heart block each accounted for about 18 percent, followed by Mobitz type 1 AV block and sinus pauses at about 12 percent each.7PubMed Central. Lacosamide-Related Arrhythmias: A Systematic Analysis and Review of the Literature
One case that illustrates the more dangerous end of the spectrum involved a 70-year-old woman with valvular heart disease and worsening heart failure. Two hours after her second dose of lacosamide, she developed a wide QRS complex that transitioned into sustained ventricular tachycardia requiring electrical cardioversion. Ventricular tachycardia recurred after her third dose. Once lacosamide was stopped, her QRS normalized and the arrhythmia did not return.8PubMed. Lacosamide-Induced Recurrent Ventricular Tachycardia in the Acute Care Setting Cases like this tend to cluster in patients who already have structural heart disease, heart failure, or electrolyte abnormalities, suggesting that lacosamide may push a vulnerable heart past its tipping point rather than creating entirely new electrical problems from scratch.
Intravenous Loading Carries Higher Short-Term Risk
Lacosamide is sometimes given intravenously, especially in hospital settings where seizures need to be controlled quickly. This route delivers the drug faster than oral dosing, which can amplify its effect on the heart. In a study of 85 patients who received rapid IV loading, about a third experienced at least one cardiac adverse event. The most common were new first-degree AV block (in 19 patients) and a drop in blood pressure (in seven). A few patients developed atrial fibrillation, bradycardia, or atrial flutter. On average, PR intervals increased from about 169 milliseconds to about 185 milliseconds after loading, and heart rates dropped modestly. Older patients experienced bigger PR shifts.9PubMed. Cardiac effects of rapid intravenous loading of lacosamide in patients with epilepsy
A separate evaluation of IV lacosamide found that cardiac side effects occurred in about 6 percent of patients, with one developing new first-degree AV block and another developing premature atrial beats. Importantly, none of the observed changes in that study required clinical intervention, and there were no significant changes in QRS duration, corrected QT interval, or heart rate.10PubMed Central. Evaluation of Cardiovascular Concerns of Intravenous Lacosamide Therapy in Epilepsy Patients The inconsistency between these studies likely reflects differences in loading speed, total dose, and the cardiac health of the patients involved. The practical takeaway is that IV loading warrants closer cardiac monitoring than routine oral dosing.
How Lacosamide Compares to Other Anti-Seizure Drugs
It might seem like lacosamide is uniquely dangerous to the heart, but the broader picture is more reassuring than individual case reports suggest. A study of patients undergoing simultaneous EEG and ECG monitoring found that lacosamide was actually associated with fewer cardiac arrhythmias overall compared to patients not on the drug. The same was true for carbamazepine and lamotrigine, two other sodium channel-targeting anti-seizure medications. None of the commonly prescribed anti-seizure drugs in the study were associated with an increased risk of arrhythmias, and none were linked to significant changes in the QTc interval.11Seizure – European Journal of Epilepsy. Association between anti-seizure medications and cardiac arrhythmias in patients undergoing ambulatory electroencephalographic and electrocardiographic monitoring
This finding seems counterintuitive given the case reports, but it makes sense when you consider the context. Most people taking lacosamide have epilepsy, and epilepsy itself is associated with cardiac rhythm abnormalities, particularly during and after seizures. Better seizure control may actually reduce the overall burden of arrhythmias, even if the medication has a modest direct effect on cardiac conduction. The net result, for most patients, appears to be neutral or even slightly favorable.
Overdose and Toxic Exposures
When lacosamide concentrations climb well above the therapeutic range, the heart effects become much more dangerous. In overdose situations, the drug can cause severe conduction slowing, with widening of the QRS complex and prolongation of the PR interval to a degree that threatens hemodynamic stability. The most concerning scenarios involve ventricular tachycardia progressing to cardiac arrest.
A case report of an adolescent who intentionally ingested a large amount of lacosamide illustrates how extreme this can get. The patient developed pulseless ventricular tachycardia followed by asystole, requiring CPR. Spontaneous circulation returned but the rhythm collapsed again, needing a second round of resuscitation. Her blood lacosamide level on admission was over 106 micrograms per milliliter, far above therapeutic levels.12Seizure – European Journal of Epilepsy. Cardiac arrest following lacosamide intoxication in an adolescent; a case report Severe bradycardia and shock as primary overdose presentations, while rare, have also been documented.13PubMed Central. Severe Bradycardia and Shock With Delayed Onset Due to Lacosamide Overdose: A Case Report
Reversibility and How Cardiac Events Are Managed
One consistently encouraging feature across the literature is that lacosamide-related cardiac effects tend to be reversible once the drug is stopped or concentrations fall. In the therapeutic-dose case reports involving AV block, conduction typically normalized within hours to a day after discontinuation. Even in overdose scenarios, recovery is possible with supportive care.
The adolescent patient who experienced cardiac arrest during a lacosamide overdose was extubated 48 hours after ICU admission, fully regained consciousness, and had no lasting neurological damage. Her lacosamide levels declined naturally without the need for plasma exchange.14Seizure – European Journal of Epilepsy. Cardiac arrest following lacosamide intoxication in an adolescent; a case report In another severe case involving combined lacosamide and levetiracetam toxicity, the patient developed worsening QRS prolongation and hemodynamic instability. Treatment with intravenous sodium bicarbonate and continuous renal replacement therapy was successful: QRS duration normalized and seizures came under control.15BMJ Case Reports. Management of combined lacosamide and levetiracetam toxicity with intravenous 8.4% sodium bicarbonate and continuous renal replacement therapy
Sodium bicarbonate works in this setting because it helps overcome sodium channel blockade by increasing the sodium gradient across heart cell membranes. It is the same strategy used for tricyclic antidepressant overdoses and other drug-induced wide-complex tachycardias. Renal replacement therapy can help clear lacosamide from the blood when the drug levels are extremely high and the heart is struggling.
Genetic Vulnerability and Unmasked Conditions
Perhaps the most alarming cardiac risk with lacosamide involves people who carry genetic variants in the SCN5A gene, which codes for the Nav1.5 cardiac sodium channel. Some of these variants cause conditions like Brugada syndrome, a disorder that predisposes the heart to dangerous rhythms and sudden cardiac death. Many people with Brugada-type genetic variants are completely asymptomatic until something provokes the characteristic ECG pattern, and sodium channel-blocking drugs are among the most potent triggers.
A published case report describes lacosamide unmasking SCN5A-associated Brugada syndrome in a young woman with epilepsy. The authors characterize this as a rare but potentially lethal adverse event in patients with unrecognized genetic susceptibility.16PubMed Central. Case Report: Lacosamide unmasking SCN5A-associated Brugada syndrome in a young female with epilepsy This is a genuinely difficult clinical problem because most patients with latent Brugada syndrome do not know they have it, and routine screening before starting lacosamide is not standard practice. A family history of unexplained sudden cardiac death, especially in young relatives, should prompt extra caution and possibly a baseline ECG evaluation before initiation.
Cardiac Signals in Pediatric Patients and Neonates
Lacosamide is approved for use in patients as young as one month of age with partial seizures, so its cardiac safety in children matters. A disproportionality analysis of the FDA adverse event database looking specifically at pediatric reports found some striking signals. The strongest were cardiac arrest in neonates, second-degree AV block, and a somewhat harder-to-interpret signal labeled “cell death.” While the absolute numbers were small (only three reports for neonatal cardiac arrest), the reporting odds ratios were dramatically elevated compared to other drugs, and Bayesian analysis confirmed these as strong signals.17PubMed Central. The real-world safety of lacosamide and perampanel in children: a disproportionality analysis of the FDA Adverse Event Reporting System
Neonatal cardiac arrest was flagged as an “unexpected” signal, meaning it was not previously well-recognized in the drug’s safety profile. This does not prove lacosamide caused those events; critically ill neonates are at baseline risk of cardiac arrest from many causes, and confounding is always a concern with adverse event databases. Still, the finding adds a reason for careful cardiac monitoring when lacosamide is used in very young infants, particularly those under one month of age who fall outside the drug’s labeled age range.
Who Needs Extra Monitoring
Given everything above, a reasonable question is: who should be watched more closely when starting lacosamide? The evidence points to several groups where the risk of cardiac complications is elevated:
- Older adults: Age was a significant predictor of larger PR interval changes after IV lacosamide loading, and case reports of complete heart block cluster in elderly patients.
- People with pre-existing cardiac disease: Structural heart disease, heart failure, and baseline conduction abnormalities all lower the threshold for lacosamide to provoke clinically meaningful arrhythmias.
- Patients on other conduction-slowing drugs: Beta-blockers, calcium channel blockers, and other sodium channel blockers (including anti-seizure drugs like carbamazepine) can compound lacosamide’s effect on the PR interval and AV node conduction.
- People receiving rapid IV loading: The faster the drug reaches peak levels, the greater the acute stress on cardiac conduction. Slower infusion rates and continuous monitoring reduce risk.
- Anyone with a family history of sudden cardiac death: This raises the possibility of a latent channelopathy like Brugada syndrome that lacosamide could unmask.
For patients outside these groups, the cardiac risk at standard oral doses appears quite low. The clinical trial data, where first-degree AV block occurred in well under one percent of patients and no higher-degree block was seen, remains the most representative picture for the average person taking the drug by mouth at approved doses.2PubMed. Lacosamide cardiac safety: clinical trials in patients with partial-onset seizures A baseline ECG before starting treatment and periodic follow-up ECGs, especially after dose increases, are sensible precautions that catch problems before they become dangerous.
Symptoms That Should Prompt Attention
Most lacosamide-related cardiac effects are asymptomatic and discovered only on ECG. When symptoms do occur, they reflect impaired blood flow from a heart that is beating too slowly or conducting too irregularly. One hospitalized patient developed an altered mental state alongside an exceptionally prolonged PR interval (212 milliseconds, up from a baseline of 178), which resolved after the drug was stopped.3PubMed Central. The safety of lacosamide for treatment of seizures and seizure prophylaxis in adult hospitalized patients Dizziness, lightheadedness, feeling faint, unusual fatigue, and an awareness that the heart is beating slowly or irregularly are all signals worth reporting promptly. These symptoms can overlap with common neurological side effects of lacosamide like dizziness and drowsiness, which makes them easy to dismiss. If you’re on lacosamide and notice these, particularly after a dose increase, an ECG can quickly distinguish a benign neurological side effect from a cardiac conduction change that needs attention.
Fainting (syncope) is the red flag that warrants urgent evaluation. A sudden loss of consciousness in someone taking lacosamide should trigger an immediate ECG and cardiac monitoring, because syncope can be the first clinical sign of high-degree AV block or a dangerous arrhythmia. The good news, as the case literature consistently shows, is that stopping or reducing lacosamide almost always reverses these cardiac effects, often within hours.