Bradycardia

"Bradycardia" literally means a heart rate slower than normal — clinically, a resting heart rate under 60 beats per minute. That single threshold alone does not distinguish a normal, healthy finding (a fit 25-year-old with a resting rate of 48 bpm) from a dangerous one (an 80-year-old with syncope and a resting rate of 45 bpm), which is why this chapter organizes bradycardia around three overlapping categories that actually dictate management: sinus node dysfunction (SND — the modern term for what is still often called "sick sinus syndrome"), atrioventricular (AV) block, and — the angle pharmacists are most often first to recognize — drug-induced or drug-exacerbated bradycardia, which can affect either mechanism, or both at once, most often when interacting drugs are layered together.

Two questions dominate every bradycardia work-up, whether it starts at the bedside or at the pharmacy counter: is the bradycardia genuinely producing symptoms, and is there a reversible or physiologic cause? Neither the CCS/CHRS nor the ACC/AHA/HRS approach uses a minimum heart rate or pause duration to trigger permanent pacing in an asymptomatic patient — establishing a real temporal correlation between symptoms and the bradycardia is what matters.^[1][2] The major exception is Mobitz type II second-degree AV block, advanced/high-grade AV block, and third-degree (complete) AV block: once a reversible or physiologic cause has genuinely been excluded, permanent pacing is recommended for these specifically regardless of symptoms.^[2][1]

For a community or hospital pharmacist, the practical stakes run in both directions. Missing that a new drug or dose increase — a non-dihydropyridine calcium channel blocker added to an existing beta-blocker, a cholinesterase inhibitor started in a frail older adult, a digoxin level drifting up as renal function declines — is driving a patient's fatigue or falls is a real, correctable harm. But the opposite error is just as real: assuming that any bradycardia in a patient taking one of these drugs is simply "the drug" and can be managed by stopping it alone. Genuine drug-induced advanced AV block is uncommon, and recurrence after stopping the presumed culprit is common — particularly in older, frailer patients whose conduction disease was most likely present, if quiet, before the drug was ever started.^[3]

Leads I and II showing third-degree (complete) AV block — the P waves (atrial activity) and QRS complexes (ventricular activity) are each regular but completely dissociated from each other, the ECG hallmark of complete heart block. (MoodyGroove, public domain, Wikimedia Commons)

Typical presentation

Symptoms range from none at all to a genuine cardiac emergency, which is exactly why establishing a real correlation between symptoms and the bradycardia — not the heart rate number alone — is the central diagnostic task.

• Entirely asymptomatic, found incidentally on a routine ECG, a pharmacy blood-pressure-machine pulse reading, or a wearable device.
• Fatigue, reduced exercise tolerance, and dyspnea on exertion — often attributed to "getting older" or deconditioning before bradycardia is recognized as the actual cause.
• Chronotropic incompetence: an inability to raise the heart rate appropriately with exertion, described by patients as exercise intolerance rather than a "slow pulse."
• Lightheadedness, presyncope, or frank syncope — syncope from bradycardia is a red flag (see Red Flags/Refer).
• Confusion or a subtle cognitive decline in an older adult, sometimes the only presenting feature.
• Symptoms of worsening heart failure, when a slow rate can no longer maintain adequate cardiac output in a patient with reduced ventricular function.

Physiologic vs. pathologic bradycardia — the first branch point

Classification — three overlapping categories

Non-drug causes

• Age-related degenerative fibrosis of the sinus node and conduction system — the single most common cause of sinus node dysfunction.^[2]
• Ischemic or nonischemic cardiomyopathy, acute myocardial infarction (especially inferior MI, which supplies the AV node in most patients), and congenital heart disease.^[2]
• Infection/inflammation of the conduction system — Lyme carditis, infective endocarditis with a perivalvular abscess, myocarditis, Chagas disease, acute rheumatic fever, diphtheria.^[2]
• Infiltrative disease — cardiac amyloidosis, sarcoidosis, hemochromatosis, lymphoma.^[2]
• Rheumatologic disease — systemic lupus erythematosus, rheumatoid arthritis, systemic sclerosis.^[2]
• Surgical or procedural trauma — valve surgery, transcatheter aortic valve replacement (TAVR), septal myomectomy, catheter ablation.^[2]
• Metabolic/endocrine — hyperkalemia, hypokalemia, acidosis, hypothyroidism, hyperthyroidism, adrenal disease; also hypothermia and hypoxemia/hypercarbia (including obstructive sleep apnea).^[2]
• Autonomic causes — carotid sinus hypersensitivity, neurally-mediated (vasovagal) syncope, situational syncope (cough, defecation, micturition, vomiting), sleep, and high-level athletic conditioning.^[2]
• Congenital complete AV block associated with maternal systemic lupus erythematosus (anti-Ro/SSA antibodies) — see Special Populations.^[2]

The pharmacist-critical differential — drug-induced and drug-exacerbated bradycardia

Medications are one of the most common, and most correctable, contributors to bradycardia — but as the treatment-decision sections below make clear, "the drug list includes this patient's medication" is a starting point for investigation, not a diagnosis on its own.

History, examination, and medication reconciliation

Characterize the symptoms and their temporal relationship to activity, rest, and sleep; ask specifically about syncope and exertional tolerance. A complete medication reconciliation is a core diagnostic step here, not an afterthought — include every prescription and OTC product, and specifically ask about ophthalmic beta-blocker drops (e.g., timolol) for glaucoma, which are easy to miss on a list that only scans oral medications but are absorbed systemically enough to cause clinically significant bradycardia.^[2] Ask about recent dose changes, newly started interacting drugs, and any recent decline in renal function.

Working definitions

^[2]

Extended monitoring and functional testing

• Ambulatory ECG monitoring (24-48 hour Holter, external event/patch monitor, or implantable loop recorder) when symptoms are intermittent and not captured on a resting ECG.
• Exercise stress testing to assess the chronotropic response, and — where the AV block site is ambiguous (e.g., 2:1 block) — to help distinguish AV-nodal from infranodal conduction.^[2]
• Electrophysiology study, reserved for cases where the site of block remains unclear after noninvasive testing and the answer would genuinely change management.^[2]

Laboratory testing

• TSH (hypothyroidism is a reversible cause) and electrolytes (potassium, magnesium, calcium).
• Renal function in every patient, and specifically a digoxin level (drawn at least 6-8 hours post-dose) and renal function in any patient on digoxin presenting with bradycardia or new AV block.
• Troponin and serial ECGs if myocardial ischemia is suspected as the precipitant.
• Screen for obstructive sleep apnea symptoms in nocturnal or otherwise unexplained bradycardia.^[2]

Echocardiography

Recommended newly identified left bundle branch block, Mobitz type II second-degree AV block, advanced/high-grade AV block, or third-degree AV block, with or without apparent structural or coronary disease. Reasonable other bradycardia or conduction disorders if structural heart disease is otherwise suspected. Not indicated routinely asymptomatic sinus bradycardia or first-degree AV block with no other clinical evidence of structural heart disease.^[2]

• Distinguish physiologic from pathologic bradycardia, and reversible/drug-induced from intrinsic conduction disease, before committing to permanent therapy.
• Establish a genuine temporal correlation between symptoms and bradycardia — neither guideline requires a minimum heart rate or pause duration to justify pacing in an asymptomatic patient.^[1][2]
• Stabilize acute symptomatic bradycardia promptly with pharmacologic therapy and pacing as needed, without delaying pacing preparation while waiting to see whether a drug effect wears off.^[2]
• Identify and correct genuinely reversible causes — including medications, hypothyroidism, electrolyte disturbance, ischemia, and sleep apnea.^[1][2]
• Recognize when a drug is unlikely to be the sole explanation — particularly for Mobitz II, advanced, or third-degree AV block — and avoid delaying a genuinely indicated pacemaker while waiting for a drug washout trial that is unlikely to resolve the underlying problem.^[3]
• Provide permanent pacing without unnecessary delay for Mobitz II, advanced/high-grade, or third-degree AV block not explained by a reversible or physiologic cause, regardless of symptoms.^[2][1]
• Avoid unnecessary permanent pacing for physiologic or genuinely asymptomatic bradycardia.^[2]
• Screen for and treat obstructive sleep apnea in patients with sleep-related or otherwise unexplained bradycardia.^[2]
• Practice shared decision-making with the patient (and, where relevant, their substitute decision-maker) once a guideline-based indication for pacing exists.^[2]

• Permanent pacemaker implantation is the definitive treatment for bradycardia meeting the indications above — it is genuinely first-line therapy here, not a last resort.
• Choose a physiologic pacing mode (dual-chamber pacing, or His-bundle/conduction-system pacing) over single-chamber right ventricular pacing where an appreciable ventricular pacing burden is expected in a patient with reduced-to-mid-range ejection fraction, to avoid pacing-induced cardiomyopathy.^[2]
• Temporary transcutaneous pacing, escalating to transvenous pacing, bridges acute symptomatic bradycardia until the cause resolves or a permanent device is placed.
• Screen for and treat obstructive sleep apnea (CPAP, weight loss) in patients with sleep-related or otherwise unexplained bradycardia — this is a Class I recommendation, not an afterthought, and treating the sleep apnea itself may resolve the bradycardia.^[2]
• Reassurance without treatment is appropriate for asymptomatic physiologic bradycardia — most commonly athletic conditioning — once an appropriate heart-rate response to exertion is confirmed and no red-flag symptoms are present.
• Address the underlying structural, ischemic, infiltrative, or rheumatologic disease directly when one is identified (e.g., treat active Lyme carditis, manage sarcoidosis) rather than treating the bradycardia in isolation.
• Do not order echocardiography or other advanced cardiac imaging routinely in asymptomatic sinus bradycardia or first-degree AV block without other evidence of structural heart disease.^[2]

Pregnancy

Congenital complete heart block associated with maternal anti-Ro/SSA antibodies (most often in the context of Sjögren syndrome or systemic lupus erythematosus) is a recognized, though uncommon, cause of fetal and neonatal bradycardia.^[2] Avoid unnecessary changes to a stable maternal cardiac medication regimen in pregnancy; where a bradycardia-associated drug genuinely must be continued or started, monitor maternal heart rate and symptoms as in any other patient and coordinate care with obstetrics and cardiology.

Breastfeeding

Atropine, dopamine, epinephrine, and isoproterenol are acute-setting drugs and not a routine breastfeeding consideration. Digoxin at standard maintenance doses is generally considered compatible with breastfeeding; confirm specifics against its own Drug Database entry.

Older adults, frailty, and cholinesterase inhibitors

Advancing age is itself the leading risk factor for both sinus node dysfunction and AV block, through progressive fibrosis of the conduction system.^[2] This is also where the cholinesterase-inhibitor question comes up most often, and the honest answer has two genuinely true parts that need to be held together rather than picked apart. At the individual-patient level, a US Veterans Affairs cohort of 11,328 patients with dementia found cholinesterase inhibitor use associated with a real increase in bradycardia risk (adjusted HR 1.4, 95% CI 1.1-1.6), with a clear dose-response relationship for donepezil specifically — the highest-dose group carried the greatest risk (HR 2.1, 95% CI 1.5-2.9) — and patients who developed bradycardia were more likely to fall, have syncope, or need a pacemaker.^[5] At the population level, a much larger Ontario, Canada study of 2,353,909 community-dwelling seniors found the opposite direction of effect on the outcome that matters most practically: cholinesterase inhibitor use was associated with a significantly lower, not higher, rate of pacemaker insertion (unadjusted HR 0.58, 95% CI 0.55-0.61), a finding that held after adjustment.^[6] Both are correct at once — most patients tolerate these drugs without ever needing a pacemaker, but individual bradycardia, falls, and syncope are real, dose-related risks worth actively monitoring for, especially at higher donepezil doses, rather than dismissing the concern because the population-level signal looks reassuring.

Renal impairment

Reduce the digoxin maintenance dose and monitor levels more closely as renal function declines — see the Drug Database for specific adjustment. Consult a renal dosing reference or pharmacist for other renally cleared agents used in this chapter.

Hepatic impairment

Ivabradine is contraindicated in severe hepatic impairment.^[4]

Post-cardiac surgery and TAVR

Transient AV block is common immediately after valve surgery, TAVR, or catheter ablation, and temporary epicardial or transvenous pacing wires are typically placed proactively; a meaningful minority of patients go on to need a permanent pacemaker. Monitor per institutional protocol before assuming the block will resolve on its own.^[2]

• Confirm resolution with a follow-up ECG and symptom review after any dose reduction or deprescribing undertaken because a drug was suspected of causing bradycardia — do not assume success without checking.
• Counsel patients started or continued on a bradycardia-associated drug on symptom awareness (dizziness, near-syncope, unusual fatigue) and when to seek care.
• Provide fall-risk counselling and, where relevant, driving-restriction guidance after any syncopal or near-syncopal episode attributable to bradycardia — follow local/provincial driving-fitness requirements.
• For patients with a new permanent pacemaker: confirm device follow-up (in-clinic and/or remote monitoring) is arranged, and counsel on recognizing signs of a device or lead complication (unusual pocket pain, swelling, or recurrent symptoms).
• Reinforce adherence and confirm understanding of the dose-titration instructions for ivabradine specifically, given its heart-rate-triggered up/down titration schedule.^[4]
• Screen for, and revisit adherence with, obstructive sleep apnea treatment at follow-up when relevant.^[2]
• For patients on digoxin: confirm a follow-up level and renal function check after any dose change or the addition of an interacting drug.

1. Atropine given to the maximum dose without adequate response: move directly to a second-line infusion (dopamine or epinephrine) or isoproterenol, and prepare for transcutaneous pacing in parallel rather than sequentially.^[2]
2. Bradycardia or AV block persisting or recurring after stopping the presumed culprit drug: do not repeat or extend the "wait and see" period — refer for pacing evaluation, particularly for Mobitz II, advanced, or third-degree block.^[3]
3. Digoxin toxicity not responding to digoxin immune Fab within the expected timeframe (most patients improve within about an hour): reassess the dose given against the estimated total body load, and reconsider the diagnosis.
4. Recurrent symptoms in a patient with an existing permanent pacemaker: arrange device interrogation before assuming a new, unrelated cause — lead or device malfunction, or a programming issue, is a genuine possibility.
5. Ivabradine started for heart failure with a resting heart rate that will not rise above 50 bpm, or persistent bradycardia symptoms, despite the lowest dose: discontinue per the product monograph rather than persisting at the lowest dose indefinitely.^[4]
6. Suspected sleep-related bradycardia with an inadequate response to confirmed, adherent obstructive sleep apnea treatment: reassess for an independent cause of sinus node dysfunction rather than attributing the ongoing bradycardia to unresolved sleep apnea alone.^[2]

• Assuming that any bradycardia in a patient on a beta-blocker, non-dihydropyridine calcium channel blocker, digoxin, or antiarrhythmic is simply "the drug," and stopping it alone without further work-up — especially dangerous for Mobitz II, advanced, or third-degree AV block, where true drug causation is uncommon and recurrence after discontinuation is common.^[3]
• Giving atropine to a heart transplant recipient — the denervated heart will not respond.^[2]
• Missing an ophthalmic beta-blocker (e.g., timolol eye drops for glaucoma) on medication reconciliation because the review only scanned oral medications.^[2]
• Treating asymptomatic athletic bradycardia as a disease requiring investigation or pacing.^[2]
• Combining a non-dihydropyridine calcium channel blocker with a beta-blocker, or either with ivabradine, without a clear plan for monitoring heart rate — these combinations are additive on the AV node and, for ivabradine plus verapamil or diltiazem specifically, contraindicated outright.^[4]
• Starting or continuing digoxin without checking renal function and a level when bradycardia or new AV block appears, or restarting at the same dose without addressing the precipitant.
• Delaying pacing evaluation in a frail or older patient with advanced AV block "to see if it's the medication," when the evidence shows recurrence is common and true drug causation is the exception rather than the rule for this specific block pattern.^[3]
• Ordering echocardiography or other advanced imaging routinely for asymptomatic sinus bradycardia or first-degree AV block without other evidence of structural heart disease.^[2]
• Dismissing a genuine, dose-related bradycardia signal from a cholinesterase inhibitor because a population-level study shows an overall lower pacemaker rate — the population finding does not rule out a real reaction in the specific patient in front of you.^[5][6]
• Overlooking obstructive sleep apnea screening in a patient with nocturnal or otherwise unexplained bradycardia.^[2]

Acute Management of Symptomatic Bradycardia

First-line and second-line agents for moderate-to-severe symptomatic bradycardia, per the 2018 ACC/AHA/HRS acute management table.

Antidotes for Drug-Induced Bradycardia and AV Block

Targeted therapy for a specific, identified toxicologic cause — used alongside, not instead of, atropine and pacing readiness.