BLOCKERS
L-type voltage-dependent calcium channels (Cav1.2) are the primary mediators of the phase 0 upstroke in slow-response tissue (SA and AV nodes) and sustain the phase 2 plateau in ventricular myocardium. Non-dihydropyridine (non-DHP) CCBs, verapamil and diltiazem, block Cav1.2 with preferential selectivity for nodal tissue, making them highly effective for rate control and AV nodal-dependent tachycardias.1
Dihydropyridine CCBs (amlodipine, nifedipine) act predominantly on vascular smooth muscle Cav1.2 and have negligible cardiac electrophysiologic effects at therapeutic doses. They are not antiarrhythmic agents and must never be substituted for verapamil or diltiazem in tachyarrhythmia management.
Verapamil produces potent, use-dependent blockade of L-type calcium current (ICaL) in nodal tissue, slowing phase 0 upstroke velocity in the AV node, prolonging AV nodal conduction time (PR interval prolongation), and increasing AV nodal effective refractory period. Its effect on the SA node reduces spontaneous automaticity and sinus rate. At therapeutic concentrations, it has minimal effect on fast-response ventricular myocardium. Verapamil also has weak sodium channel blocking activity and direct negative inotropic effects that are clinically relevant at higher doses.1,2
Oral bioavailability: 20–35% due to extensive first-pass hepatic metabolism (CYP3A4). Significantly higher with repeat dosing (saturable first-pass). Half-life: 6–8 hours (immediate-release); 24-hour sustained-release formulations available. Metabolism: CYP3A4 to active metabolite norverapamil (~20% activity of parent). Inhibits P-glycoprotein and CYP3A4, raises digoxin levels ~70%; raises cyclosporine, simvastatin, and carbamazepine levels. IV formulation: 2.5–5 mg IV over 2–3 minutes; repeat 5–10 mg at 15–30 minutes if needed. Onset of AV nodal effect within 1–3 minutes.
Acute termination of AVNRT and AVRT: Effective in ~90% of cases by blocking the slow pathway (AVNRT) or the retrograde AV nodal limb (AVRT). Second-line to adenosine for acute termination; preferred when adenosine is contraindicated (severe asthma, high-degree AV block with caffeine interaction concerns).
Ventricular rate control in AF/AFL: IV verapamil produces rapid, effective rate slowing. Particularly useful when beta-blockers are contraindicated (reactive airway disease, severe bradycardia at rest). Verapamil-sensitive VT (idiopathic left VT / fascicular VT): A narrow-complex or relatively narrow wide-complex tachycardia arising from the left posterior fascicle (Belhassen VT) is uniquely sensitive to verapamil due to its Ca2+-dependent triggered mechanism. This is one of the few VT subtypes where verapamil is therapeutic rather than dangerous. Critical Contraindication: Verapamil/Diltiazem in Wide-Complex Tachycardia
Never administer IV verapamil or diltiazem empirically for wide-complex tachycardia of unknown origin. If the rhythm is VT (not SVT with aberrancy), AV nodal blockade eliminates the rate-slowing benefit while negative inotropy and vasodilation precipitate hemodynamic collapse. The one exception: confirmed fascicular (verapamil-sensitive) VT with a known history and characteristic narrow-ish RBBB+left axis morphology. If in doubt, treat wide-complex tachycardia as VT.
Diltiazem shares verapamil's AV nodal blocking mechanism but has a more favorable hemodynamic profile: its negative inotropic effect is significantly less pronounced, and peripheral vasodilation is less marked than with verapamil. This makes diltiazem the preferred non-DHP CCB when hemodynamic tolerance is a concern, including in patients with mildly impaired LV function where verapamil would be hazardous.2 IV dosing: 0.25 mg/kg IV over 2 minutes (typical 15–20 mg); repeat 0.35 mg/kg at 15 minutes if needed; maintenance infusion 5–15 mg/hr for rate control in AF. Oral: Immediate-release 30–90 mg QID; extended-release (Cardizem CD, Dilacor) 120–480 mg once daily. Hepatic CYP3A4 metabolism; half-life 3–4 hours (IR), ~8 hours (ER). Indications: Identical to verapamil for rate control and SVT management. Preferred over verapamil in patients with borderline LV function, hypertension requiring vasodilation, or post-MI (where verapamil's greater inotropy risk is a concern).
Shared contraindications with verapamil: HFrEF (EF <40%), pre-excited AF/AFL (WPW), wide-complex tachycardia of unknown origin, sick sinus syndrome without pacemaker, high-degree AV block without pacemaker.
Comparative Profile
Verapamil and diltiazem differ in several clinically relevant properties. Verapamil (phenylalkylamine class) produces more potent AV nodal blockade, more pronounced negative inotropy, and moderate peripheral vasodilation; oral bioavailability is 20 to 35%; it is preferred for AVNRT/AVRT termination and fascicular VT. Diltiazem (benzothiazepine class) produces slightly less AV nodal blockade but substantially less negative inotropy and comparable peripheral vasodilation; oral bioavailability is 40 to 67%; it is preferred for rate control when hemodynamic tolerance is a concern, in borderline LV function, and in the post-MI setting.
Adenosine is a naturally occurring purine nucleoside with an extraordinarily brief half-life (≤5–10 seconds in vivo) that makes it uniquely suited for acute diagnostic and therapeutic applications in supraventricular tachycardia.3
Adenosine acts on A1 purinergic receptors on SA and AV nodal cells. A1 receptor activation couples via Gi protein to: (1) activation of IKAdo (an acetylcholine-sensitive inwardly rectifying K+ current, also called acetylcholine-activated potassium current (IKACh)), hyperpolarizing the cell and opposing spontaneous phase 4 depolarization; and (2) inhibition of adenylyl cyclase, reducing cAMP and attenuating ICaL. The net effect is profound, transient slowing or complete block of AV nodal conduction, typically lasting 10–20 seconds.3,4
Half-life: 5–10 seconds; taken up by erythrocytes and vascular endothelium and rapidly deaminated to inosine. Not affected by hepatic or renal function. Route: Rapid IV bolus only, must be given as fast push followed immediately by a rapid saline flush (20 mL). Use the largest, most proximal peripheral vein available (antecubital preferred); distal hand veins produce unreliable delivery. Dosing: Initial dose 6 mg IV rapid bolus; if no response within 1–2 minutes, 12 mg; repeat 12 mg once more if needed. In patients receiving dipyridamole or carbamazepine (block adenosine uptake/breakdown), use 3 mg initial dose. In cardiac transplant recipients (denervated hearts with upregulated A1 receptors), use 3 mg with extreme caution; prolonged asystole risk is significant. Interaction with methylxanthines: Caffeine and theophylline are competitive A1 receptor antagonists. Patients who have consumed caffeine recently may require higher doses or may not respond.
First-line for acute termination of AVNRT and AVRT: Success rate ~90–95% for AV nodal-dependent re-entrant tachycardias. Terminates the circuit by producing transient AV nodal block, interrupting the re-entrant loop. Diagnostic tool in wide-complex tachycardia: If the rhythm is SVT with aberrancy, adenosine terminates it or reveals the underlying atrial rhythm. If VT, adenosine is ineffective (except in the rare adenosine-sensitive fascicular VT or RVOT VT driven by cAMP-mediated triggered activity). Of note, adenosine does not worsen VT hemodynamically due to its ultra-short half-life. Adenosine-sensitive VT: Right ventricular outflow tract (RVOT) VT and some idiopathic left VTs are driven by cAMP-mediated triggered activity and are uniquely terminated by adenosine.
Universal transient effects: Flushing, chest tightness, dyspnea, and a sense of impending doom occur in the majority of patients. Warn the patient beforehand; effects resolve within 30–60 seconds. Bronchospasm: A1 receptor activation in bronchial smooth muscle can precipitate severe bronchospasm in asthmatic patients. Adenosine is relatively contraindicated in symptomatic asthma; use verapamil instead.
Transient AV block and asystole: Expected and usually self-limiting (≤3–5 seconds). Prolonged asystole (>10 seconds) is uncommon but requires atropine readiness. Continuous cardiac monitoring and resuscitation equipment are mandatory. AF/AFL induction: Adenosine can induce AF in ~1–2% of patients by shortening atrial refractoriness. In WPW patients, adenosine-induced AF with rapid accessory pathway conduction can precipitate VF; therefore adenosine is contraindicated in pre-excited AF/AFL.
Digoxin is a cardiac glycoside derived from Digitalis lanata with over two centuries of clinical use. Despite a narrow therapeutic index and a complex interaction profile, it retains a defined role in rate control for AF and as a positive inotrope in HFrEF, though its mortality benefit in the latter indication remains absent from trial evidence.5
Digoxin inhibits the Na+/K+-ATPase (sodium pump) on cardiac myocyte membranes. Pump inhibition increases intracellular Na+, which secondarily reduces the Na+ gradient driving the Na+/Ca2+ exchanger (NCX). The NCX extrudes less Ca2+, raising intracellular Ca2+ and increasing SR Ca2+ stores. The result is enhanced contractility.5 Its antiarrhythmic (rate-slowing) action is primarily vagotonic: digoxin sensitizes cardiac baroreceptors and directly stimulates the vagal nucleus, increasing parasympathetic tone to the AV node. This slows AV nodal conduction and prolongs AV nodal refractoriness. This vagotonic mechanism has a critical limitation: it is largely ineffective during sympathetic activation (exercise, fever, stress), making digoxin a poor sole agent for rate control in active patients.5
Bioavailability: Oral tablets ~60–80%; liquid-filled capsules (Lanoxicaps) ~90–95%. Significant inter-individual variability.
Distribution: volume of distribution (Vd) ~7 L/kg; extensive tissue binding (skeletal muscle, heart). Initial distribution phase takes 6–8 hours, serum levels drawn before this are unreliable. Elimination: Primarily renal excretion of unchanged drug (70–80%). Half-life 36–48 hours in normal renal function; prolonged to days–weeks in renal impairment. Therapeutic range: 0.5–0.9 ng/mL for rate control in AF (lower range reduces toxicity without sacrificing efficacy). Levels >2.0 ng/mL are associated with toxicity. Draw trough levels ≥6 hours post-dose. Renal dosing: Reduce dose proportionally to GFR. In severe CKD or dialysis patients, use digoxin with extreme caution or avoid; accumulation is unpredictable.
Amiodarone inhibits both P-glycoprotein and renal clearance of digoxin, raising digoxin levels by 70 to 100%; halve the digoxin dose and recheck levels when amiodarone is added. Verapamil inhibits P-glycoprotein, raising digoxin levels by approximately 70%; reduce the digoxin dose and monitor closely. Quinidine inhibits P-glycoprotein and reduces renal clearance, approximately doubling digoxin levels; halve the digoxin dose. Spironolactone reduces renal tubular secretion of digoxin, raising levels by approximately 25%; monitor levels and reduce dose if needed. Cholestyramine reduces gastrointestinal absorption of digoxin, lowering levels; separate doses by at least 2 hours. Rifampicin induces P-glycoprotein and increases metabolism, reducing digoxin levels; a dose increase may be needed with monitoring.
Digoxin toxicity is a clinical syndrome of cardiac and non-cardiac manifestations that can occur even within the traditional therapeutic range, particularly when precipitating factors are present. Predisposing conditions include: hypokalemia, hypomagnesemia, hypercalcemia, hypothyroidism, renal impairment, advanced age, and cardiac ischemia.6 Digoxin Toxicity: Recognition and Management
NON-CARDIAC: Nausea, vomiting, anorexia (earliest symptoms); visual disturbances (yellow-green xanthopsia, halos); confusion, delirium (especially elderly). CARDIAC (any arrhythmia possible): Bradyarrhythmias most common (sinus bradycardia, AV block); paroxysmal atrial tachycardia with AV block is pathognomonic; accelerated junctional rhythm; bidirectional VT (sign of severe toxicity). MANAGEMENT: Stop digoxin. Correct hypokalemia (maintain K+ 4.0–5.0 mEq/L) and hypomagnesemia. Avoid calcium (worsens toxicity). Atropine or temporary pacing for symptomatic bradycardia. DIGOXIN-SPECIFIC ANTIBODY FRAGMENTS (Digibind/DigiFab): Indicated for life-threatening arrhythmias, serum K+ >5.5 mEq/L (sign of severe toxicity), or ingestion of >10 mg in adults. Dose based on estimated total body load or serum level. Effects within 30–60 minutes of IV administration.
Rate control in AF: Useful as adjunct therapy, particularly in sedentary or bedridden patients where exercise rate control is not a priority. Less effective than beta-blockers during physical activity. Current guidelines recommend it as add-on therapy rather than monotherapy for rate control. HFrEF: The DIG trial (1997) demonstrated that digoxin reduces HF hospitalizations but does not improve mortality in HFrEF with sinus rhythm. Post-hoc analyses suggest harm at higher serum levels (>1.2 ng/mL), particularly in women. Target the lower therapeutic range (0.5–0.9 ng/mL).
Magnesium sulfate is an essential electrolyte with direct membrane-stabilizing properties that make it a first-line agent in two specific arrhythmia contexts: torsades de pointes and digoxin-induced arrhythmias.7
ICaL blockade: Mg2+ reduces inward L-type Ca2+ current, suppressing the trigger current for early afterdepolarizations (EADs). This is the primary mechanism in torsades de pointes (TdP) — magnesium terminates TdP even when serum magnesium is normal. late sodium current (INaL) blockade: Reduces persistent late sodium current, further attenuating early afterdepolarization (EAD) formation. Na+/K+-ATPase modulation: In digoxin toxicity, magnesium may partially restore pump activity and reduce intracellular Ca2+ overload.
Torsades de pointes: IV magnesium sulfate 2 g (8 mmol) over 1–2 minutes as first-line, regardless of serum magnesium level. A second 2 g bolus may be given if TdP recurs. Maintenance infusion 1–2 g/hour if recurrent. Terminates TdP in ~80% of episodes. Digoxin toxicity (adjunct): 2 g IV over 20–30 minutes (slower infusion than TdP, as rapid administration may worsen AV block in this context). Avoid if significant AV block is present. Refractory VF: 2 g IV may be administered empirically in refractory VF during resuscitation if TdP or hypomagnesemia is suspected. Toxicity of rapid IV magnesium: Flushing, nausea, loss of deep tendon reflexes (first sign of toxicity at ~4 mmol/L). Respiratory paralysis at ~6–8 mmol/L. Monitor DTRs during infusion. Calcium gluconate (1 g IV) reverses magnesium toxicity.
Adenosine acts on A1 receptors to activate IKAdo, produces effect within seconds, is safe in HFrEF, must be avoided in WPW-AF (risk of AF induction with rapid accessory pathway conduction), and is first-line for AVNRT/AVRT termination and SVT diagnosis. Verapamil blocks ICaL in nodal tissue, produces effect in 1 to 3 minutes IV, must be avoided in HFrEF and WPW-AF, and is preferred for AVNRT/AVRT and fascicular VT. Diltiazem shares verapamil's ICaL nodal mechanism, produces effect in 2 to 5 minutes IV, should be used with caution in mild LV dysfunction, must be avoided in WPW-AF, and offers a better hemodynamic profile than verapamil for rate control. Digoxin acts via vagotonic Na+/K+-ATPase inhibition, has a slow onset of 30 to 60 minutes IV, is acceptable as an adjunct in HFrEF, must be avoided in WPW-AF, and is best suited for resting heart rate control in sedentary patients. Magnesium sulfate blocks ICaL and INaL, produces effect in 1 to 2 minutes IV, is safe in HFrEF and can be used in TdP, and is the first-line agent for TdP and an adjunct in digoxin toxicity. Beta-blockers act via β-adrenergic blockade, produce IV effect in 5 to 15 minutes, are safe in compensated HFrEF, must be avoided in WPW-AF, and have the broadest antiarrhythmic indication including catecholaminergic polymorphic ventricular tachycardia (CPVT), long QT syndrome (LQTS), and post-MI prophylaxis.
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