Grayanotoxin poisoning is, above all, a cardiac conduction problem. The compound holds cardiac voltage-gated sodium channels open, amplifies vagal tone, and disrupts the heart’s pacemaker and conduction system. The electrocardiogram, or ECG, is the single most informative bedside tool for assessing how severely the system has been affected, because it shows the conduction disturbance directly.
For a clinician, the ECG does three things in a suspected mad honey case. It confirms the nature of the problem as a bradyarrhythmia and conduction disturbance rather than something else. It grades the severity, from simple slowing of the heart rate to complete failure of conduction between the atria and ventricles. And it guides treatment escalation, because the specific finding determines whether observation, atropine, or pacing is the appropriate next step. This article describes the findings in order of increasing severity and what each one means.
The Underlying Mechanism, Briefly
Grayanotoxin produces its cardiac effects through two connected routes. It increases vagal tone, and vagal acetylcholine acting at M2 muscarinic receptors slows the sinoatrial node and impairs conduction through the atrioventricular node. It also disrupts sodium channels directly in the nodal and conduction tissue. The result is a heart that fires too slowly at its natural pacemaker and conducts impulses poorly from the atria to the ventricles. Every ECG finding below is an expression of one or both of these effects. For the full mechanism, the cardiovascular effects article is the companion reference.
The ECG Findings, From Mild to Severe
The findings below form a spectrum. A given patient may show one or progress through several, and the severity of the ECG finding is the primary driver of how aggressively the case is managed.
| ECG finding | What it shows | Typical rate | Escalation trigger |
| Sinus bradycardia | Sinus node firing slowly; normal conduction path preserved | 40-60 bpm | Observe if above 50 and stable; atropine if below 50 or symptomatic |
| Sinus bradycardia with hypotension | Slow rate, plus reduced cardiac output and vasodilation | 40-50 bpm | IV fluids plus atropine |
| First-degree AV block | Every atrial impulse conducts, but with a delay (PR over 200 ms) | Variable | Atropine; monitor for progression |
| Second-degree AV block, Mobitz I (Wenckebach) | Progressive PR lengthening until a beat drops | Variable | Atropine; prepare for possible progression |
| Second-degree AV block, Mobitz II | Intermittent dropped beats without progressive PR change | Variable | Higher risk of progression; prepare pacing |
| Complete (third-degree) AV block | No atrial impulses reach the ventricles; an independent escape rhythm | 20-40 bpm | Atropine plus catecholamine; temporary pacing if unresponsive |
| Junctional/ventricular escape rhythm | Backup pacemaker takes over at a slow, often inadequate rate | 20-40 bpm | Temporary pacing usually required |
| Atrial fibrillation | Less common; chaotic atrial activity, sometimes with slow ventricular response | Variable | Rate and hemodynamic management; caution with atropine |
Sinus Bradycardia: The Baseline Finding
Sinus bradycardia is the most common ECG finding in grayanotoxin poisoning and is present in the large majority of cases. The rhythm remains sinus, meaning the sinoatrial node is still the pacemaker and the conduction pathway is intact, but the rate is slow. The P wave morphology and the relationship between P waves and QRS complexes are preserved. This is the mild end of the spectrum, and in a stable patient with a rate above 50 and no hypotension, it may require only monitoring and supportive care.
AV Block: The Finding That Grades Severity
The degree of atrioventricular block is the most important severity marker in grayanotoxin poisoning, because it reflects how badly conduction between the atria and ventricles has been impaired.
First-degree AV block
Every atrial impulse still reaches the ventricles, but conduction is delayed, shown as a prolonged PR interval beyond 200 milliseconds. This indicates AV nodal involvement but preserved conduction. It usually responds to atropine and warrants monitoring for progression.
Second-degree AV block
Some atrial impulses fail to conduct. In Mobitz I, or Wenckebach, the PR interval lengthens progressively until a beat is dropped, then the cycle resets. This pattern is typically nodal and often atropine-responsive. In Mobitz II, beats drop intermittently without the progressive PR lengthening, which suggests conduction failure below the AV node and carries a higher risk of sudden progression to complete block. Mobitz II is a signal to prepare for pacing.
Complete (third-degree) AV block
This is the severe end. No atrial impulses reach the ventricles at all. The atria and ventricles beat independently, and a slow escape rhythm from the junction or ventricles maintains a rate that is usually only 20 to 40 beats per minute, often too slow to sustain adequate blood pressure. Complete AV block that does not respond promptly to atropine and catecholamine support is the classic indication for a temporary pacemaker in grayanotoxin poisoning.
A Point of Caution: ST and T Wave Changes
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How the ECG Guides Treatment
The ECG finding maps directly onto the treatment ladder. Sinus bradycardia that is stable and above 50 beats per minute may need only observation and fluids. Bradycardia below 50, or any degree of AV block with symptoms, is the trigger for atropine, which counteracts the vagal component of the conduction problem. When atropine is insufficient, which is more likely with higher-grade block and in patients with pre-existing conduction disease, catecholamine support is added. Complete AV block unresponsive to pharmacological treatment is the indication for a temporary transvenous pacemaker, which bridges the patient until the grayanotoxin clears and normal conduction returns.
A defining feature of grayanotoxin conduction block is that it is reversible. The block reflects a pharmacological effect on the conduction system, not structural damage to it. As the compound is cleared, conduction recovers, which is why pacing is temporary and permanent pacemaker implantation is not indicated. The full stepwise approach is set out in the treatment protocol reference.
Monitoring and Course
Continuous ECG monitoring is standard for any patient with bradycardia or conduction block from suspected grayanotoxin poisoning, because the rhythm can change and lower-grade block can progress. The reassuring counterpart to that vigilance is that the course is typically self-limiting with appropriate support. In the documented case record, conduction abnormalities resolve as the compound clears, generally within the first day, and patients who are monitored and treated appropriately recover conduction without lasting cardiac effects.
What We Don’t Know Yet
The precise frequency of each ECG finding across a large, systematically collected series is not well established, because most data comes from case reports and smaller series rather than prospective registries. The factors that predict which patients progress to high-grade block, as opposed to remaining at sinus bradycardia, are not formally modeled. And because no controlled pharmacokinetic data exists, the exact relationship between blood grayanotoxin concentration and ECG severity has not been quantified.
Summary
On the ECG, grayanotoxin poisoning presents as a spectrum of bradyarrhythmia and conduction disturbance. Sinus bradycardia is the most common and mildest finding. The degree of AV block grades the severity, from first-degree delay through second-degree dropped beats to complete third-degree block with a slow escape rhythm. The finding drives treatment, from observation to atropine to catecholamine support to temporary pacing. Grayanotoxin can also produce ST and T wave changes that should not be mistaken for acute coronary syndrome in a patient with a honey history. The conduction block is reversible, and with appropriate monitoring and treatment, conduction recovers as the compound clears.
