ECG criteria
- Sinus bradycardia: HR <60 bpm, P wave preceding each QRS, PR interval may be normal or prolonged (typical for verapamil, diltiazem)
- Prolonged PR interval >200 ms — a sign of depressed AV conduction (verapamil, diltiazem)
- Second-degree AV block is possible; with typical nodal toxicity, Type I features are more commonly observed, but classification can be difficult and should not replace the assessment of hemodynamics and the level of block
- Complete (third-degree) AV block with AV dissociation and a junctional or idioventricular escape rhythm
- QRS is usually not widened, typically <110 ms (threshold for wide QRS is ≥120 ms); widening may occur in severe intoxication, hyperkalemia, or co-ingestion of other cardiotoxic drugs
- With dihydropyridines (amlodipine, etc.), HR may be normal or elevated against the background of hypotension; marked bradycardia is uncharacteristic, except in massive overdose
- In severe intoxication with any CCB — junctional or idioventricular escape rhythm, up to asystole
- In severe intoxication, hyperglycemia and metabolic/lactic acidosis may occur; hyperkalemia should be checked for in renal failure, severe acidosis, insulin deficiency, or combined pathology
What it looks like on ECG
The pattern depends on the drug class. With nondihydropyridine CCBs (verapamil, diltiazem), sinus bradycardia is typical: heart rate less than 60 bpm, P wave preserved and preceding each QRS. PR prolongation, first-degree AV block, second-degree block (more often with Type I features), and complete third-degree AV block with AV dissociation are possible — these conduction disturbances may occur in various sequences, not necessarily step-by-step, and require dynamic monitoring rather than waiting for a predictable progression.
With dihydropyridines (amlodipine, nifedipine), peripheral vasodilation and arterial hypotension typically dominate; HR may be normal or elevated (reflex tachycardia), and marked bradycardia and AV block are not mandatory signs. In massive overdose, selectivity of action is lost, and bradycardia, depressed AV conduction, and reduced contractility join in — the picture approaches that of verapamil/diltiazem intoxication.
The QRS generally remains narrow, but in severe intoxication with any CCB, it may widen; wide complexes require ruling out hyperkalemia, co-ingestion of sodium channel blockers/TCAs, and severe myocardial depression. Nondihydropyridine CCBs block L-type calcium channels of the sinoatrial and atrioventricular nodes rather than myocardial sodium channels of the ventricles; therefore, in typical cases, QRS width changes less than with sodium channel blockers and tricyclic antidepressants, though this is not an absolute distinction.
In severe overdose (especially with large doses of verapamil or diltiazem, or sustained-release formulations), progression to a junctional escape rhythm, idioventricular rhythm, and asystole is possible.
| Sign | ECG Manifestation |
|---|---|
| Rhythm (verapamil, diltiazem) | Sinus bradycardia, junctional or idioventricular escape rhythm |
| Rhythm (dihydropyridines) | Usually normal HR or tachycardia; bradycardia occurs in massive overdose |
| PQ | May be prolonged >200 ms, especially with verapamil/diltiazem |
| AV conduction | From first-degree AV block to complete AV block |
| QRS | Usually narrow (<110 ms), widening is possible in severe intoxication |
| Hemodynamics | Arterial hypotension (vasodilatory and/or cardiogenic) |
Mechanism
Nondihydropyridine CCBs (verapamil, diltiazem) block slow L-type calcium channels in the cells of the sinoatrial and atrioventricular nodes, where depolarization in phase 0 of the action potential depends specifically on the inward calcium current rather than sodium. This slows sinus node automaticity (bradysardia) and conduction through the AV node (PR prolongation, AV blocks). They also cause a negative inotropic effect and vasodilation. Dihydropyridines (amlodipine, nifedipine) in therapeutic doses selectively affect vascular smooth muscle calcium channels, causing vasodilation with minimal effect on pacemaker nodes and conduction; in overdose, this selectivity is lost, and bradycardia, depressed AV conduction, and reduced contractility may supervene.
Causes
- Verapamil or diltiazem overdose (accidental or intentional/suicidal)
- High-dose dihydropyridine CCB overdose (amlodipine, nifedipine)
- Sustained-release formulations — delayed and prolonged action
- Co-ingestion with beta-blockers or digoxin (mutually reinforcing bradycardia and AV conduction depression)
- Drug interactions via cytochrome P450 (increased toxicity)
Differential diagnosis
- Beta-blocker poisoning: clinically similar bradycardia and AV block with verapamil/diltiazem; with CCBs, hyperglycemia due to suppressed insulin secretion is more common, whereas hypoglycemia may occur with beta-blockers — these signs do not replace history-taking and toxicological assessment. Bronchospasm is characteristic primarily of non-selective beta-blockers, but is not mandatory
- Digoxin toxicity: ventricular extrasystoles, accelerated junctional rhythm, scooping ST depression are characteristic — such ST-T changes are typically absent in CCB toxicity
- Hyperkalemia on ECG: QRS widening, tall peaked T waves — in pure CCB toxicity, the QRS is usually not widened and the T wave is unchanged unless secondary hyperkalemia has developed
- Sick sinus syndrome: organic bradycardia unrelated to drug ingestion and without hypotension of this severity
- Complete organic AV block: no history of CCB ingestion, no accompanying refractory hypotension
Clinical significance and management
CCB toxicity is dangerous due to the development of refractory cardiogenic and vasodilatory shock, and bradyarrhythmias up to asystole. The physician should record serial ECGs (rhythm, HR, PR, degree of AV block), monitor hemodynamics, and assess blood glucose and lactate levels. Guidelines-level management: airway maintenance and hemodynamic support, consideration of calcium administration, high-dose insulin therapy, vasopressors, and mechanical circulatory support for refractory shock. In refractory hemodynamically significant bradycardia or high-grade AV block, temporary cardiac pacing is considered alongside intensive specific therapy; its efficacy may be limited in the presence of marked myocardial depression. Specific doses and regimens are the domain of toxicology and critical care protocols, not ECG diagnosis.
Common interpretation errors
- Assuming bradycardia and AV block are mandatory signs of poisoning with any CCB — dihydropyridines are more often dominated by vasodilation and hypotension without marked bradycardia
- Expecting strict step-by-step progression from PR prolongation to Mobitz I and further to complete block — in reality, various patterns and sudden deterioration can occur without passing through all stages
- Believing QRS widening is impossible in CCB toxicity and thereby ruling out the diagnosis — in severe intoxication, hyperkalemia, or mixed poisoning, widening is possible
- Failing to link bradycardia and AV block to the history of drug ingestion, interpreting them as primary organic conduction system disease
- Failing to account for concurrent hyperkalemia in severe intoxication with acidosis or renal failure, which may alter the QRS and ST-T morphology
1. Which ECG finding is most typical for poisoning with nondihydropyridine calcium channel blockers (verapamil, diltiazem)?
Verapamil and diltiazem block calcium channels of the sinus and AV nodes, causing bradycardia and impaired AV conduction, whereas QRS width changes very little in typical cases.
2. How does the ECG in CCB toxicity most commonly differ from that in sodium channel blocker poisoning?
Sodium channel blockers slow ventricular depolarization and widen the QRS complex, whereas CCBs act primarily on calcium channels in the nodes; QRS widening in CCB toxicity is possible but atypical for the standard presentation.
3. What management approach is considered for severe symptomatic AV block due to CCB poisoning refractory to medical therapy?
For refractory hemodynamically significant bradyarrhythmia, temporary cardiac pacing is considered in combination with specific therapy (calcium, insulin, vasopressors); its efficacy may be limited in the presence of severe myocardial depression.
Frequently asked questions
How to distinguish CCB poisoning from beta-blocker poisoning on an ECG?
The ECG presentation of verapamil/diltiazem and beta-blocker toxicity is very similar: bradycardia and AV block in both cases. Differentiation relies on the medication history and laboratory findings—hyperglycemia is more common in CCB poisoning, whereas hypoglycemia and bronchospasm (primarily with non-selective agents) may occur with beta-blockers. Distinguishing them solely by ECG is challenging.
Does the QRS widen in calcium channel blocker poisoning?
Usually not: non-dihydropyridine CCBs act on calcium channels of the sinus and AV nodes rather than sodium channels of the ventricular myocardium, so the QRS complex remains narrow (<110 ms) in typical cases. However, QRS widening does not rule out CCB toxicity—it can occur in severe intoxication, hyperkalemia, or concurrent ingestion of sodium channel blockers/TCAs, meaning a widened QRS requires severity assessment and electrolyte evaluation.
Is amlodipine overdose dangerous for heart rhythm?
Amlodipine overdose is typically dominated by peripheral vasodilation and hypotension, while bradyarrhythmias are less pronounced than with verapamil; the heart rate may be normal or elevated due to reflex tachycardia. In massive overdose, selectivity is lost, and bradycardia, AV block, and myocardial depression can occur alongside profound hypotension.
What degree of AV block is most characteristic of verapamil poisoning?
The entire spectrum is possible—ranging from PQ prolongation and first-degree AV block to complete third-degree AV block with an escape rhythm; these variants may not occur in a strict sequential order. The severity depends on the dose and degree of intoxication, thus requiring serial ECG monitoring.
Is temporary cardiac pacing necessary in CCB poisoning?
It is considered for refractory hemodynamically significant bradycardia or high-degree AV block unresponsive to medical therapy, alongside intensive specific therapy (calcium, high-dose insulin, vasopressors). Pacing efficacy may be limited in the presence of severe myocardial contractility depression.
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Related terms
More in "Pharmacology and Toxicology"
Sources
- Hampton J. The ECG in Clinical Practice
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.