Definition: Venom/poison effects that arise due to the direct action of venom/poison on the cardiac conduction system and/or cardiac muscle.

Signs and symptoms:

• Bradycardia, tachycardia, arrhythmias,
• arterial hypotension, arterial hypertension,
• cardiac insufficiency, heart failure,
• ECG changes,
• cardiac muscle enzymes ↑.

Little is known about the direct action of animal venoms/poisons on the human heart (cardiac muscle, cardiac conduction system).

In contrast, numerous indirect effects of envenoming/poisoning involve the myocardium and the myocardial conduction system:

• Hypoxia as a consequence of neurotoxic envenoming/poisoning that leads to paralysis of the respiratory musculature and thus to respiratory insufficiency or failure.
• Bleeding, e.g. subendocardial haemorrhage or cardiac tamponade, as a consequence of haemostatic venoms/poisons. 
• Myocardial effects following autopharmacological venom/poison activity via the release of endogenous histamine, bradykinins etc. (see "Autopharmacological venom/poison effects") and subsequent neurological effects due to the release of acetylcholine and catecholamines.
• Hyperkalaemia as a consequence of rhabdomyolysis or acute renal failure.

A primary cardiac effect has been explicitly shown for a few species of venomous/poisonous animals. However, the clinical observations that were made are extremely non-specific, for example arterial hypotension, bradycardia or cardiac arrhythmias. The same is true for the recorded ECG findings, for example T inversion.

Primary cardiac effects are speculated to occur in envenoming/poisoning caused by the following animals:

• poisonous animals: ciguatera, tetrodotoxin poisoning;
• venomous fish: Stonefishes, among others;
• cnidarians: Chironex fleckeri, Physalia sp., among others (disturbance of Ca²⁺ ion transport?);
• scorpions: e.g. Leiurus quinquestriatus (controversy as to whether, besides cardiac and vascular effects of acetylcholine and catecholamine, direct cardiac effects of the venom also play a role);
• terrestrial snakes: European vipers, Echis coloratus, Bitis arietans, Bitis gabonica, Naja kaouthia.