Adult Bradycardia

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

Bradycardia in adults is strictly defined as a heart rate of less than 50-60 beats per minute (bpm). However, the pathophysiological threat occurs when the heart rate drops so significantly that it produces a critically low cardiac output, leading to systemic hypoperfusion and vital organ ischemia.

At the cellular and mechanical level, the etiology is typically driven by an intrinsic failure of the sinoatrial (SA) node, a conduction block at the atrioventricular (AV) node, or a secondary external depressant. Acute myocardial ischemia (especially inferior STEMIs) can cause direct ischemic injury to the AV node or trigger profound reflex parasympathetic (vagal) tone. Toxicological ingestions (e.g., calcium channel blockers, beta-blockers, digoxin) directly inhibit nodal depolarization. Metabolic derangements, specifically hyperkalemia, alter the resting membrane potential of the myocardium, slowing electrical conduction and widening the QRS complex. Narrow QRS bradycardias generally imply a supraventricular origin and carry a better prognosis, whereas a wide QRS indicates infranodal blocks or severe metabolic/toxic disruptions.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: Assess airway, breathing, and circulation (ABCs). Provide supplemental oxygen if the patient is hypoxemic. Establish intravenous (IV) access and place the patient on a continuous cardiorespiratory monitor.
• Preparation: Place transcutaneous pacing pads on the patient's chest immediately upon arrival, even if they currently appear stable. Have the resuscitation cart at the bedside.
• Identify Stability: You must immediately determine if the bradycardia is causing cardiopulmonary compromise. Instability is defined by the presence of hypotension, acutely altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure.
• The Unstable Pathway (First-Line): Administer Atropine 1 mg IV bolus. This may be repeated every 3 to 5 minutes, up to a maximum total dose of 3 mg.
• The Unstable Pathway (Second-Line): If Atropine is ineffective (or contraindicated), immediately transition to:
• Transcutaneous Pacing (TCP).
• AND/OR Dopamine IV infusion: 5 to 20 mcg/kg/minute, titrated to patient response.
• OR Epinephrine IV infusion: 2 to 10 mcg/minute, titrated to patient response.
• Definitive Therapy: Consult cardiology for potential transvenous pacing or pacemaker placement.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• "Can't-Miss" Differential Diagnoses (The Reversible Causes):
• Myocardial Ischemia/Infarction: Particularly inferior STEMIs affecting the right coronary artery.
• Electrolyte Abnormalities: Severe hyperkalemia is a lethal, rapidly reversible mimic of primary conduction blocks.
• Toxicologic Exposures: Beta-blockers, calcium channel blockers, and digoxin.
• Hypoxia & Hypothermia: Ensure the bradycardia is not a secondary reflex to severe anoxia or environmental exposure.
• Prioritized Diagnostic Workup:
• 12-Lead ECG: The gold standard to identify the specific block, evaluate QRS width, and look for ischemic ST-segment changes.
• Point-of-Care Potassium: Check for hyperkalemia early and at the bedside, as waiting for central lab results can delay life-saving membrane stabilization.
• Laboratory Panel: Core labs include a basic metabolic panel (electrolytes), cardiac biomarkers (troponin) to assess for ACS, and specific drug levels (e.g., digoxin) if suspected.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• 12-Lead ECG Checklist:
• Assess the P-waves: Evaluate the atrial rate and check if every P-wave is followed by a QRS complex.
• Assess the PR Interval: A progressively lengthening PR interval followed by a dropped beat indicates a Mobitz I (Wenckebach) second-degree block. A constant PR interval with sudden dropped QRS complexes indicates a Mobitz II block.
• Assess AV Dissociation: If the atria (P-waves) and ventricles (QRS complexes) are beating entirely independently of one another, this confirms a Third-Degree (Complete) Heart Block.
• Assess QRS Width: A wide QRS (>0.12 seconds) in the setting of bradycardia is highly concerning for hyperkalemia or an infranodal block.
• Point-of-Care Ultrasound (POCUS): Utilize rapid bedside echocardiography to evaluate for underlying structural causes, assess left ventricular contractility, or identify a pericardial effusion/tamponade.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• AHA ACLS Adult Bradycardia Algorithm: The critical decision node relies entirely on the presence of cardiopulmonary compromise.
• If the patient exhibits hypotension, altered mental status, shock, ischemic chest pain, or heart failure, they are categorized as Unstable and require immediate pharmacological or electrical intervention (Atropine, Pacing, Epinephrine/Dopamine).
• If the patient lacks these features, they are Stable. They require continuous observation and a targeted workup to identify and treat the underlying cause. "Treat the patient, not the number".

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• Cognitive Trap (The Atropine Reflex): Reflexively pushing Atropine for all unstable bradycardias. Correction: Atropine is a vagolytic agent and will be ineffective for non-parasympathetic causes, specifically high-degree infranodal blocks (Mobitz II or Third-Degree heart blocks). For these rhythms, you must prepare immediately for pacing or chronotropic infusions.
• Cognitive Trap (Treating the Blood Pressure): Administering antihypertensives or vasodilators to a patient who presents with severe bradycardia and concurrent hypertension. Correction: This can induce catastrophic hemodynamic collapse. The hypertension is often a compensatory mechanism to maintain perfusion.
• Cognitive Trap (Isoproterenol in ACS): Utilizing Isoproterenol to increase the heart rate in a patient experiencing acute myocardial ischemia. Correction: Isoproterenol drastically increases myocardial oxygen demand and will exacerbate the ischemic injury.
• Critical Action: Always place transcutaneous pacing pads on the patient immediately upon arrival to the resuscitation bay, even if they appear temporarily stable.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzwords: "Cardiopulmonary compromise" (defines instability), "Third-degree heart block" (Atropine will fail), "Point-of-care potassium" (first test in wide-complex bradycardia).
• Classic Distractor (The Complete Heart Block): A clinical vignette describes a 70-year-old male with a heart rate of 35 bpm, a blood pressure of 70/40 mm Hg, and an ECG showing independent P waves and QRS complexes. The distractor option will suggest "Administer Atropine 1 mg IV." Explanation: The ECG describes a third-degree (complete) heart block. Atropine is largely ineffective for third-degree blocks. The correct answer is to initiate transcutaneous pacing or an epinephrine/dopamine infusion.
• Classic Distractor (The Asymptomatic Patient): A 55-year-old marathon runner presents with a twisted ankle. His monitor shows a sinus bradycardia of 45 bpm, but his blood pressure is 120/80 mm Hg and he is conversing normally. The distractor offers "Transcutaneous pacing." Explanation: Treat the patient, not the number. Asymptomatic, stable patients do not require acute intervention for bradycardia.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Initial Approach: "This patient is presenting with severe bradycardia. I will immediately assess their ABCs, place transcutaneous pacing pads on their chest, establish IV access, provide supplemental oxygen, and obtain a stat 12-lead ECG and bedside potassium level.".
• Assessing Stability: "I am explicitly evaluating the patient for signs of cardiopulmonary compromise—specifically looking for hypotension, acutely altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure.".
• Executing the Resuscitation (Unstable): "Because the patient is hemodynamically unstable, I am immediately administering Atropine 1 mg IV push. If the patient does not respond to Atropine, or if the ECG reveals a Mobitz II or third-degree AV block, I will immediately initiate transcutaneous pacing and start an Epinephrine infusion at 2 to 10 mcg/min to maintain cardiac output, while consulting cardiology for transvenous pacing.".