Rapid Review·Cardiovascular
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PHARMACOLOGY
T1Must knowClass IV & Other Antiarrhythmics
FA P328
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Focus on
Calcium channel blockers as class IV, plus adenosine, magnesium, and ivabradine.
Key takeaways
Class IV, the calcium channel blockers
Drugs
diltiazem and verapamil.Mechanism
block L-type calcium channels, so spontaneous depolarization slows; conduction velocity falls, the effective refractory period lengthens, and the PR interval lengthens.Use
rate control in atrial fibrillation and flutter, and prevention of nodal arrhythmias.Adverse
constipation, gingival hyperplasia, flushing, edema, plus heart failure, AV block, and sinus node depression.| Context | Called | Use |
|---|---|---|
| Antihypertensive and antianginal | Nondihydropyridine CCB | Hypertension, angina |
| Antiarrhythmic | Class IV | Rate control in AF and flutter, nodal arrhythmias |
Adenosine
What it does
slows the pacemaker action potential, lowering heart rate and AV nodal conduction. Very short acting, about 15 seconds.How, through the A1 receptor (Gi-coupled)
- Less cAMP: the funny current (HCN) slows, phase 4 flattens, and the rate falls.
- Less cAMP also shuts L-type calcium channels: less calcium influx, a slower phase 0, and slower AV conduction.
- Direct opening of K+ channels: K+ leaves the cell, hyperpolarizing it and lengthening the pacemaker's refractory period.
Use
drug of choice for diagnosing and terminating certain forms of SVT.Adverse
flushing, hypotension, chest pain, a sense of impending doom, and bronchospasm.Its effect is blunted by theophylline and caffeine
both are adenosine receptor antagonists.| Antagonist | Effect on adenosine |
|---|---|
| Theophylline | Blunts it, being an adenosine receptor antagonist |
| Caffeine | Blunts it, being an adenosine receptor antagonist |
Magnesium
Use
effective in torsades de pointes and in digoxin toxicity.Ivabradine
Mechanism
selectively inhibits the funny sodium channels (I_f), prolonging slow depolarization in phase 4. "IVabradine works on phase IV."Use
chronic HFrEF. Adverse: luminous phenomena, i.e. visual brightness (the same channels sit in the retina), plus hypertension and bradycardia.The conceptual thread
ivabradine slows the heart without touching contractility or blood pressure, because it acts purely on the SA node's pacemaker current. That is what makes it useful once beta-blockers are maxed out or not tolerated.| Drug | Mechanism | Use | Signature |
|---|---|---|---|
| Adenosine | Raises K+ efflux and lowers the calcium current, slowing AV nodal conduction | Diagnose or terminate SVT | About 15 seconds; sense of impending doom, flushing, bronchospasm |
| Magnesium | Membrane stabilization | Torsades de pointes, digoxin toxicity | First-line in stable torsades |
| Ivabradine | Inhibits funny channels (I_f), prolonging phase 4 | Chronic HFrEF | Luminous phenomena (visual brightness) |
| Feature | Ivabradine | Beta-blockers |
|---|---|---|
| Target | I_f, the funny current, in the SA node only | Beta receptors throughout the body |
| Effect on heart rate | Falls | Falls |
| Effect on contractility | None | Falls |
| Effect on blood pressure | Rises, a listed adverse effect | Falls |
| Use | Chronic HFrEF | HFrEF, SVT, rate control, post-MI |
Adenosine fails to break an SVT in a patient who drinks eight coffees a day. Why?
Caffeine is an adenosine receptor antagonist (so is theophylline), so the receptor is already occupied and the 15-second drug never gets a grip.
How it's tested
Adenosine lasts about fifteen seconds and produces a memorable sense of impending doom, and its effect is blunted in patients on theophylline or heavy caffeine, because both antagonize the adenosine receptor. Ivabradine is the drug that slows the heart by inhibiting the funny current alone, hence the mnemonic that IVabradine works on phase IV, and its visual brightness side effect follows from the same channels being present in the retina.
Go deeper
First Aid 2026 — CV/Pharmacology (p.328) · B&B — Class IV antiarrhythmics, adenosine & ivabradine · Mehlman — HY Cardio (adenosine, funny current)
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