Which of the following are chemical examples that can cause abnormal conduction?

• A. Beta adrenoreceptor agonist or antagonist
• B. Muscarinic agonist or antagonist
• C. Antiarrhythmic drugs
• D. All of the above

Answer: (D)

Abnormal conduction can be changed by chemicals that alter the heart’s electrical properties either through autonomic regulation or direct ion-channel effects.

Beta adrenoreceptor–targeting agents, whether they stimulate or block sympathetic signaling, modify the dromotropic effect—that is, how quickly impulses travel through the conduction system. A stimulant raises conduction and heart rate, but excessive sympathetic influence or abrupt changes can lead to irregular conduction patterns; a blocker slows AV nodal conduction and can produce bradycardia or AV block.

Muscarinic agents do the opposite through parasympathetic pathways: agonists slow AV nodal conduction and lower heart rate, potentially causing blocks, while antagonists remove that slowing and can increase conduction.

Antiarrhythmic drugs are designed to modify conduction and refractoriness directly by affecting ion channels and other cardiac electrical properties. They can slow intra-cardiac conduction, prolong PR or QRS intervals, or create certain blocks, depending on the mechanism and drug class.

Since all these chemical classes can alter conduction, they can all contribute to abnormal conduction.