Flexeril (Cyclobenzaprine) and Atrial Fibrillation
Flexeril (cyclobenzaprine) is not contraindicated in patients with atrial fibrillation, as no major cardiology guidelines or drug safety data identify AF as a contraindication to its use. However, caution is warranted due to its anticholinergic and tricyclic antidepressant-like properties.
Key Safety Considerations
Cardiac Effects to Monitor
Cyclobenzaprine has structural similarity to tricyclic antidepressants, which can prolong the QT interval and potentially affect cardiac conduction, though this is not specifically mentioned in AF management guidelines 1.
The primary cardiac concerns with cyclobenzaprine include:
- Tachycardia (due to anticholinergic effects)
- Potential QT prolongation in susceptible patients
- Arrhythmia risk in patients with pre-existing cardiac conduction abnormalities
When to Exercise Caution
Avoid or use with extreme caution if the patient is on antiarrhythmic drugs that prolong the QT interval (class IA or III agents like quinidine, sotalol, dofetilide, amiodarone), as the corrected QT interval should be kept below 520 ms 1.
Monitor closely in patients with:
- Structural heart disease
- Heart failure
- Concurrent use of multiple QT-prolonging medications
- Electrolyte abnormalities (hypokalemia, hypomagnesemia) 1
Rate Control Implications
Cyclobenzaprine's anticholinergic effects may counteract rate control strategies in AF patients, particularly those on digoxin, beta-blockers, or calcium channel blockers for ventricular rate control 1.
First-line rate control agents for AF include beta-blockers or nondihydropyridine calcium channel antagonists (diltiazem, verapamil), and adding a medication with anticholinergic properties could theoretically reduce their effectiveness 1, 2.
Safer Alternatives for Muscle Spasm
Acetaminophen should be considered first-line for musculoskeletal pain in AF patients, as it has no cardiac effects or drug interactions with anticoagulants 3.
Topical analgesics (lidocaine patches, diclofenac gel) provide localized relief without systemic cardiac effects 3.
Physical therapy and heat/cold therapy are non-pharmacological options that avoid any potential cardiac complications 3.
Common Pitfalls
Do not assume all muscle relaxants are equivalent - cyclobenzaprine's tricyclic structure makes it unique among muscle relaxants in terms of cardiac considerations.
Avoid combining cyclobenzaprine with other anticholinergic medications in elderly AF patients, as this increases fall risk and may complicate rate control 3.
If cyclobenzaprine must be used, ensure adequate rate control is achieved first and monitor for increased heart rate or palpitations 1.