InfusionFox
Cardiology · Clinical background

Diltiazem CRI

Non-dihydropyridine calcium channel blocker (Class IV antiarrhythmic). Blocks L-type calcium channels in cardiac tissue, particularly the AV node, slowing AV nodal conduction, prolonging refractoriness, reducing sinus rate, and producing mild-to-moderate negative inotropy. Limited vascular smooth muscle effect compared to dihydropyridines (amlodipine, nifedipine), so produces less peripheral vasodilation. Onset 1–3 min IV; duration 30–60 min after single bolus, longer accumulation with prolonged CRI. Hepatic metabolism via CYP3A4. Clearance is reduced in hepatic dysfunction.

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Clinical background

Diltiazem is a non-dihydropyridine calcium channel blocker used in vet ICU for acute rate control in supraventricular tachyarrhythmia, principally atrial fibrillation. The clinical pivot is selectivity: diltiazem acts predominantly on the cardiac AV node with minimal peripheral vasodilation, distinguishing it from the dihydropyridines (amlodipine, nifedipine) that produce hypertensive-grade peripheral dilation with little direct AV-nodal effect. This makes diltiazem the right calcium channel blocker for rhythm control, where the target is the AV node, and the dihydropyridines the right choice for blood pressure control, where the target is the vasculature. Diltiazem carries a denser safety surface than most ICU CRI drugs, and the contraindications below are not soft cautions but hard “do not do this” rules.

Pharmacology

Class IV antiarrhythmic. Blocks the L-type calcium channel preferentially in cardiac tissue, particularly the AV node:

Onset is within 1–3 minutes IV. Single bolus duration is 30–60 minutes; the CRI provides sustained effect that accumulates somewhat over the first few hours of infusion. Hepatic metabolism via CYP3A4 is the principal clearance pathway. Plasma half-life is 3–5 hours but the active metabolite (desacetyldiltiazem) extends clinical effect.

Hepatic dysfunction slows clearance; renal dysfunction has minimal impact on diltiazem itself but may affect metabolite clearance with prolonged infusion.

Indications

Primary use cases:

Diltiazem is not appropriate for ventricular tachyarrhythmia (the AV node is downstream from the focus, so blocking it does not suppress the rhythm) and is not appropriate as an antihypertensive (peripheral vasodilation is too limited for that role).

Dosing

Loading dose: 0.05–0.15 mg/kg IV slowly over 2–3 minutes. May repeat 0.05–0.1 mg/kg every 5–15 minutes to a cumulative loading dose of 0.25 mg/kg if rate control is inadequate.

Skip the loading dose in patients with marginal blood pressure (MAP < 70 mmHg) or with compromised contractility at baseline; start at the lower end of the CRI range instead. The loading bolus carries most of the acute hypotension and AV-block risk; if the patient is already on the edge, the CRI alone delivers diltiazem more gradually and safely.

Cat dosing uses the same range as dogs. The clinical caveats around HCM and LVOT obstruction (below) drive the indication selection more than dose adjustment.

Critical contraindications and “do not combine” rules

This is the section that needs the most attention. The drug-interaction adverse events around diltiazem are not theoretical; they have produced cardiogenic shock and arrest in clinical use.

Never combine with IV beta-blockers. Esmolol, propranolol, atenolol IV, or any other parenteral beta-blocker plus diltiazem produces additive AV-nodal blockade and additive negative inotropy. The combination can precipitate complete heart block or cardiogenic shock within minutes. This is the most important single rule for diltiazem use.

For patients on oral beta-blockers (atenolol, sotalol, carvedilol): hold the next oral dose, consult cardiology if available, and start diltiazem at the low end of the CRI range with continuous BP and ECG monitoring. The pharmacokinetics of the oral beta-blocker can be managed; the IV combination cannot.

Contraindicated in pre-existing high-grade AV block (Mobitz II second-degree or third-degree AV block). Diltiazem worsens AV conduction; in a patient who is already not conducting reliably, this can produce asystole.

Contraindicated in sick sinus syndrome for the same reason: the additional slowing can produce hemodynamically significant bradycardia or asystole.

Wolff-Parkinson-White syndrome with rapidly conducting accessory pathway: diltiazem can paradoxically increase ventricular rate. Blocking AV-nodal conduction while the accessory pathway remains active accelerates conduction through the accessory pathway, producing very rapid (and potentially preexcitation-mediated VF) ventricular response. Confirm rhythm interpretation (look for delta waves, short PR, atypical bundle branch block pattern) before using diltiazem in any unusual tachyarrhythmia. When in doubt, choose esmolol (blocks both pathways) or amiodarone.

HCM cats with LVOT obstruction: avoid. The negative inotropy worsens the dynamic obstruction. In HCM cats without significant outflow obstruction, diltiazem is useful for rate control; in HCM cats with significant gradient, it can precipitate decompensation. Confirm the absence of significant LVOT obstruction by echocardiogram (or, if echo is not available, by absence of a dynamic systolic murmur with characteristic acceleration in late systole) before starting diltiazem CRI in HCM cats.

Severely reduced systolic function (EF < 30% by echo, severe DCM, severe MMVD with reduced EF): use cautiously and at low doses. The negative inotropy can produce decompensation. Sometimes the rate control is worth the inotropic cost; the decision is a clinical judgment, not a rule.

Administration

Stock is 5 mg/mL injectable in a 5 mL vial (25 mg per vial). The InfusionFox calculator preselects three weight-banded preparations (1, 0.5, or 0.2 mg/mL).

Diluent: 0.9% sodium chloride or 5% dextrose, both compatible.

The 0.5 mg/mL preparation uses one full vial in 50 mL of diluent, which is convenient for the typical A-fib rate-control workflow. The 1 mg/mL preparation requires two vials; the 0.2 mg/mL dilute preparation uses 2 mL of stock and is for cats and small dogs.

Compatibility constraints:

Storage: refrigerate the prepared syringe if not used immediately. Stability is 24 hours at room temperature, longer refrigerated. Discard if any precipitate appears.

Drug interactions

Adverse effects

Monitoring

Weaning and discontinuation

For acute rate control where the underlying driver has been corrected (transient A-fib in the setting of severe gastric dilatation that has been treated, hyperthyroid storm that has been controlled), wean by 25–50% every 4–6 hours and discontinue once rate is sustained on a lower dose or on oral therapy.

For chronic A-fib management, transition to oral diltiazem (1–4 mg/kg PO q8h) before stopping the IV infusion. The oral overlap should be at least 6–12 hours to ensure oral plasma levels are established before withdrawing the IV.

Abrupt cessation can produce rebound tachyarrhythmia, particularly in patients with longstanding A-fib who have been rate-controlled for the duration of the CRI. Watch for rate rebound in the first hour after discontinuation.

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