11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing

ABIM Critical Care Medicine Study Guide Exam

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1 Chapter 1: Exam Orientation: Format, Blueprint & Study Strategy

1.1 ABIM Critical Care Medicine Exam: Format, Blueprint & Study Strategy

2 Chapter 2: Acute Respiratory Failure & ARDS

2.1 ARDS Definition, Pathophysiology & Phenotypes 2.2 Lung-Protective Mechanical Ventilation 2.3 Refractory ARDS: Prone Positioning, Neuromuscular Blockade & VV-ECMO

3 Chapter 3: Obstructive Airways, Pulmonary Vascular & Pleural Emergencies

3.1 Severe Asthma & COPD Exacerbations in the ICU 3.2 Massive & Submassive Pulmonary Embolism 3.3 Pulmonary Hypertension & Right Ventricular Failure in the ICU 3.4 Massive Hemoptysis, Complex Pleural Effusions & Thoracic Drainage

4 Chapter 4: Airway, Parenchymal & Nonthrombotic Pulmonary Emergencies

4.1 Airway Management, Upper-Airway Obstruction & Artificial-Airway Complications 4.2 Diffuse Alveolar Hemorrhage, Interstitial Lung Disease, Pulmonary Vasculitis & Noncardiogenic Edema 4.3 Severe Community-Acquired & Viral Pneumonia 4.4 Nonthrombotic Embolism (Air, Fat, Amniotic Fluid, Tumor, Septic) & Acute Chest Syndrome

5 Chapter 5: Mechanical Ventilation Modes, Dyssynchrony & Liberation

5.1 Mechanical Ventilator Modes & Advanced Breath Delivery 5.2 Patient-Ventilator Dyssynchrony & Waveform Analysis 5.3 Ventilator Liberation, Spontaneous Breathing Trials & Post-Extubation Support

6 Chapter 6: Shock Syndromes & Hemodynamic Principles

6.1 Shock Classification, Cellular Pathophysiology & Biomarkers 6.2 Hemodynamic Monitoring: Arterial Lines, CVP & Pulmonary Artery Catheter 6.3 Dynamic Assessment of Fluid Responsiveness & Resuscitation Targets

7 Chapter 7: Vasoactive Therapies & Mechanical Circulatory Support

7.1 Vasopressors & Inotropes: Pharmacology & Clinical Titration 7.2 Temporary Mechanical Circulatory Support: IABP, Impella & TandemHeart 7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia 7.4 Post-Cardiac Arrest Care, Targeted Temperature Management & Neuroprognostication

8 Chapter 8: Acute Kidney Injury & Renal Replacement Therapy

8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation 8.2 Continuous Renal Replacement Therapy (CRRT) vs Intermittent Hemodialysis 8.3 Indications & Timing for RRT in Critical Illness: AKIKI & STARRT-AKI

9 Chapter 9: Electrolyte, Acid-Base & Endocrine Emergencies

9.1 Severe Dysnatremias: Hyponatremia, Hypernatremia & Osmotic Demyelination 9.2 Potassium, Calcium, Magnesium & Phosphate Disorders 9.3 Complex & Mixed Acid-Base Disorders: Boston vs Copenhagen vs Stewart 9.4 Endocrine Crises: DKA, HHS, Thyroid Storm, Myxedema Coma & Adrenal Crisis

10 Chapter 10: Glycemic Control, Hyperammonemia, Pituitary & Adrenal Disorders

10.1 ICU Glycemic Control & Severe Hypoglycemia 10.2 Non-Hepatic Hyperammonemia & Metabolic Encephalopathy 10.3 Pituitary Emergencies, Adrenal Excess & Critical Illness-Related Corticosteroid Insufficiency

11 Chapter 11: Acute Coronary Syndromes & Cardiac Arrhythmias

11.1 Acute Coronary Syndromes & Mechanical Complications of Myocardial Infarction 11.2 Critical Tachyarrhythmias: Atrial Fibrillation with RVR, VT & VF 11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing

12 Chapter 12: Acute Heart Failure, Valvular & Pericardial Emergencies

12.1 Acute Decompensated Heart Failure & Cardiogenic Pulmonary Edema 12.2 Acute Valvular Regurgitation & Prosthetic Valve Emergencies 12.3 Pericardial Tamponade & Acute Aortic Syndromes

13 Chapter 13: Myocardial Disease, Hypertensive & Cardiac Device Emergencies

13.1 Fulminant Myocarditis, Stress & Peripartum Cardiomyopathy, and Hypertrophic Cardiomyopathy 13.2 Hypertensive Emergencies & Ruptured Abdominal Aortic Aneurysm 13.3 Implanted Cardiac Devices, LVAD Emergencies & Electrical Storm

14 Chapter 14: Sepsis & Systemic Infectious Diseases

14.1 Sepsis-3 Definitions & Surviving Sepsis Campaign 2026 Guidelines 14.2 Refractory Septic Shock & Adjunctive Therapies 14.3 Multidrug-Resistant Pathogens & Antimicrobial Stewardship in the ICU

15 Chapter 15: ICU-Acquired & Severe Focal Infections

15.1 Hospital-Acquired & Ventilator-Associated Pneumonia 15.2 Central Line-Associated Bloodstream Infections & Catheter-Associated UTIs 15.3 Intra-Abdominal Sepsis, Necrotizing Soft Tissue Infections & CNS Infections

16 Chapter 16: Focal, Special-Host & Systemic Infections

16.1 CNS Infections Beyond Bacterial Meningitis: Encephalitis, Brain Abscess & Epidural Abscess 16.2 Infections in the Immunocompromised Host, Tropical & Bioterror Infections 16.3 Infective Endocarditis, Device Infections & Other Focal Cardiovascular Infections 16.4 Antimicrobial Pharmacokinetics, Allergy, Toxicity & Infection Prevention 16.5 Gastrointestinal & Hepatobiliary Infections: Fulminant C. difficile, Typhlitis, CMV Colitis & Abscesses

17 Chapter 17: Neurocritical Care & Coma

17.1 Coma, Altered Mental Status & Acute Delirium in the ICU 17.2 Status Epilepticus & Non-Convulsive Status Epilepticus 17.3 Acute Ischemic Stroke & Intracranial Hemorrhage

18 Chapter 18: Cerebrovascular Syndromes, Encephalopathy & Withdrawal States

18.1 Cerebral Venous Sinus Thrombosis, PRES & Reversible Cerebral Vasoconstriction Syndrome 18.2 Alcohol & Sedative Withdrawal, Wernicke Encephalopathy & Drug-Induced Encephalopathy

19 Chapter 19: Intracranial Hypertension & Neuromuscular Weakness

19.1 Intracranial Pressure Monitoring, Cerebral Perfusion & Tiered ICP Management 19.2 Acute Neuromuscular Respiratory Failure, ICU-Acquired Weakness & Brain Death Determination

20 Chapter 20: Traumatic Brain Injury, Spinal Cord Injury & Toxin-Mediated Paralysis

20.1 Traumatic Brain Injury Management & Paroxysmal Sympathetic Hyperactivity 20.2 Acute Spinal Cord Injury, Neurogenic Shock & Autonomic Dysreflexia 20.3 Toxin-Mediated Paralysis & Neuromuscular Crises: Botulism, Tetanus, Tick Paralysis & Myasthenic Crisis

21 Chapter 21: Gastrointestinal, Hepatic & Abdominal Emergencies

21.1 Acute Upper & Lower Gastrointestinal Bleeding in the ICU 21.2 Acute Liver Failure & Severe Acute Pancreatitis 21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

22 Chapter 22: Luminal GI Emergencies & Complications of Cirrhosis

22.1 Esophageal, Gastric & Bowel Emergencies: Perforation, Obstruction, Ogilvie Syndrome & Colonic Ischemia 22.2 Complications of Cirrhosis: Alcohol-Associated Hepatitis, Hepatorenal Syndrome & Pulmonary Vascular Syndromes

23 Chapter 23: Trauma, Burns & Perioperative Critical Care

23.1 Polytrauma Resuscitation, Damage Control & Massive Transfusion 23.2 Severe Burn Injury, Inhalation Injury & Environmental Emergencies 23.3 Perioperative Critical Care for High-Risk Surgical Patients

24 Chapter 24: Thoracic Trauma, Environmental Injury & Obstetric Critical Care

24.1 Thoracic Trauma: Flail Chest, Pulmonary Contusion, Great Vessel & Tracheobronchial Injury 24.2 Environmental Injury: Drowning, Diving, Altitude, Heat, Cold, Electrical & Radiation 24.3 Obstetric & Genitourinary Emergencies and the Physiology of Pregnancy

25 Chapter 25: Transplantation, Donor Management & Post-Cardiac Surgery Care

25.1 Solid Organ Transplant Recipients & Deceased Donor Management 25.2 Post-Cardiac Surgery Critical Care: Bleeding, Tamponade, Low Output & Vasoplegia

26 Chapter 26: Hematologic Emergencies, Toxicology, Critical Ultrasound & ICU Liberation

26.1 Hematologic Emergencies in the ICU: HIT, DIC, TMA & Anticoagulant Reversal 26.2 Critical Care Toxicology, Toxidromes & Specific Antidotes 26.3 Critical Care Point-of-Care Ultrasound (POCUS) 26.4 ICU Liberation, Analgosedation, Goals of Care & Critical Care Ethics

27 Chapter 27: Transfusion Medicine, Hematologic & Oncologic Disorders

27.1 Transfusion Medicine: Products, Thresholds, Reactions & Apheresis 27.2 Red and White Cell Disorders, Hemoglobinopathies, Factor Deficiencies & Hypercoagulable States 27.3 Oncologic Emergencies, Hematopoietic Cell Transplant Complications & Cellular Therapy Toxicity

28 Chapter 28: Clinical Pharmacology, Adverse Drug Reactions & Specific Poisonings

28.1 Pharmacokinetic Principles, Dose Adjustment & Drug Interactions in Critical Illness 28.2 Adverse Drug Reactions: Anaphylaxis, Severe Cutaneous Reactions & Hyperthermic Syndromes 28.3 Specific Poisonings: Sodium-Channel Blockers, Lithium, Valproate, Theophylline, Iron, Sulfonylureas & Methemoglobinemia

29 Chapter 29: Ethics, Research Methods, Administration & Abdominal Ultrasound

29.1 Advance Care Planning, Nonbeneficial Treatment, Informed Consent & Confidentiality 29.2 Critical Care Research Methods, Statistics, IRB Oversight & Quality Improvement 29.3 Abdominal, Vascular & Procedural Point-of-Care Ultrasound

11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing

Key Takeaways Explain This Topic AI

Symptomatic bradycardia in the ICU (heart rate < 50 bpm accompanied by hypotension, acute altered mental status, shock, ischemic chest pain, or acute pulmonary edema) is managed with intravenous Atropine 1 mg every 3 to 5 minutes (maximum 3 mg) as first-line therapy; atropine is effective for sinus and AV nodal bradyarrhythmias, but completely ineffective for infranodal blocks (Mobitz II second-degree and third-degree AV block with wide QRS) and denervated transplanted hearts.

Second-line pharmacotherapy for atropine-refractory or infranodal symptomatic bradycardia consists of continuous chronotropic infusions: Dopamine (5-20 mcg/kg/min), Epinephrine (2-10 mcg/min), or Isoproterenol (2-10 mcg/min), while preparations for immediate transcutaneous or transvenous electrical pacing are completed.

Transcutaneous Pacing (TCP) provides an immediate non-invasive bridge to transvenous pacing: pads must be placed in an anterior-posterior configuration to lower transthoracic impedance, and true mechanical capture must be verified by palpating a right femoral pulse or observing an arterial line waveform, never by checking the carotid pulse (where skeletal muscle contractions can mimic a pulse).

Temporary Transvenous Pacing (TVP) is indicated for symptomatic high-grade AV block, Mobitz II AV block, symptomatic sinus arrest, and new bifascicular block with acute MI; the right internal jugular vein provides the straightest, most stable route to the RV apex, where stimulation threshold should be < 1.0 mA, output set to 2 to 3 times threshold (typically 3-5 mA), and sensitivity calibrated to half the measured intrinsic R-wave amplitude.

Pacemaker troubleshooting requires differentiating failure to capture (pacing spike without subsequent QRS/T wave), undersensing (spikes fire asynchronously on native complexes, risking fatal R-on-T ventricular fibrillation; corrected by increasing sensitivity / decreasing the numerical millivolt threshold), and oversensing (pacing inappropriately inhibited by myopotentials or artifact; corrected by decreasing sensitivity / increasing the numerical millivolt threshold).

Last updated: October 2026

ACLS Bradycardia Algorithm & Clinical Assessment

Bradycardia is defined as a heart rate < 50 bpm < 50\text{ bpm} < 50 bpm . In the intensive care unit, bradycardia is common and may represent a benign physiological adaptation (e.g., in trained athletes or during deep sleep) or a life-threatening conduction disturbance leading to profound hypoperfusion, cardiogenic shock, and cardiac arrest.

Clinical Assessment of Hemodynamic Compromise

The fundamental clinical imperative is determining whether bradycardia is producing acute end-organ hypoperfusion. An unstable, symptomatic bradycardia is defined by the presence of any of the following five cardinal features:

Systemic Hypotension : Systolic blood pressure < 90 mmHg < 90\text{ mmHg} < 90 mmHg or mean arterial pressure < 65 mmHg < 65\text{ mmHg} < 65 mmHg .

Acutely Altered Mental Status : Confusion, lethargy, encephalopathy, or syncope from cerebral hypoperfusion.

Signs of Shock : Diaphoresis, cool mottled extremities, delayed capillary refill, oliguria, or rising serum lactate.

Ischemic Chest Discomfort : Angina or ischemic ST-segment deviations secondary to diminished coronary perfusion pressure.

Acute Heart Failure / Flash Pulmonary Edema : Bilateral rales, tachypnea, severe hypoxemia, and elevated jugular venous pressure. ACLS BRADYCARDIA ALGORITHM (HR < 50 BPM)

│
┌──────────────────┴──────────────────┐
▼ ▼
STABLE PATIENT UNSTABLE PATIENT
Monitor and observe • Signs of shock, hypotension,
Identify underlying triggers ischemia, altered mental status
Obtain 12-lead ECG │
▼
FIRST-LINE: ATROPINE
1 mg IV push q3-5 min (max 3 mg)
Ineffective in Mobitz II, wide 3rd
degree block, & heart transplants
│
┌────────────────┴────────────────┐
▼ ▼
ATROPINE INEFFECTIVE INFRANODAL BLOCK
Transcutaneous Pacing (TCP) • Prepare immediate
IV Chronotropic Infusion: Transvenous Pacing
Dopamine: 5-20 mcg/kg/min (TVP)
Epinephrine: 2-10 mcg/min
Isoproterenol: 2-10 mcg/min
Anatomical Level of Conduction Impairment

Understanding the anatomical level of the conduction block dictates therapeutic responsiveness and prognosis:

Supra-Nodal and AV Nodal Blocks : Sinus bradycardia, sinoatrial exit block, sinus pause/arrest, and Second-Degree AV Block Type I (Mobitz I / Wenckebach). Conduction delay occurs within the sinus or AV node, which is heavily innervated by sympathetic and parasympathetic (vagal) fibers. The escape rhythm arises from the junction (narrow QRS, rate 40 – 60 bpm 40\text{--}60\text{ bpm} 40 – 60 bpm ), is hemodynamically stable, and responds promptly to vagolytic therapy (atropine).

Infranodal Blocks (His-Purkinje System) : Second-Degree AV Block Type II (Mobitz II) and Third-Degree (Complete) AV Block with wide QRS complexes ( > 120 ms > 120\text{ ms} > 120 ms ). Conduction failure occurs below the AV node within the bundle of His or bundle branches, which lack parasympathetic innervation . The escape pacemaker is ventricular (broad, bizarre QRS, rate 20 – 40 bpm 20\text{--}40\text{ bpm} 20 – 40 bpm ), intrinsically unstable, prone to sudden asystolic arrest, and completely refractory to atropine .

ICU Reversible Etiologies

Acute Myocardial Infarction : Inferior MI (RCA occlusion) induces AV nodal ischemia or heightened vagal tone (Bezold-Jarisch reflex); typically transient and responds to atropine. Anterior MI (LAD septal perforator occlusion) causes extensive septal necrosis destroying both bundle branches, causing sudden, catastrophic infranodal complete AV block with a wide QRS, severe cardiogenic shock, and high mortality.

Medications : Beta-adrenergic blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone, clonidine, and dexmedetomidine.

Metabolic Derangements : Severe hyperkalemia ( K + > 6.5 mEq/L K^+ > 6.5\text{ mEq/L} K + > 6.5 mEq/L ; sine waves, complete block), severe hypothermia ( < 32 ∘ C < 32^\circ\text{C} < 3 2 ∘ C ; Osborn J-waves), severe acidosis, profound hypoxemia, and severe hypothyroidism (myxedema coma).

Intracranial Hypertension (Cushing Triad) : Life-threatening elevation of intracranial pressure manifesting as severe bradycardia, systolic hypertension with widened pulse pressure, and irregular respirations.

Pharmacological Management of Symptomatic Bradycardia
First-Line Therapy: Intravenous Atropine

Mechanism : Competitively blocks muscarinic M 2 M_2 M 2 acetylcholine receptors on the sinoatrial and atrioventricular nodes, abolishing parasympathetic vagal tone, accelerating SA node firing, and enhancing AV nodal conduction velocity.

Dosing : Administer 1.0 mg 1.0\text{ mg} 1.0 mg rapid IV push every 3 to 5 minutes to a maximum cumulative dose of 3.0 mg 3.0\text{ mg} 3.0 mg (equivalent to 0.04 mg/kg 0.04\text{ mg/kg} 0.04 mg/kg , which achieves full muscarinic receptor blockade).

Critical Pitfalls and Contraindications :

Subtherapeutic Dosing Trap : Doses < 0.5 mg < 0.5\text{ mg} < 0.5 mg are strictly avoided because low doses centrally stimulate the vagal nucleus or block presynaptic inhibitory M 1 M_1 M 1 receptors, paradoxically worsening bradycardia.

Infranodal Conduction Block : Ineffective in Mobitz II second-degree AV block and complete heart block with wide QRS, as the His-Purkinje system lacks vagal innervation. In these patients, administration of atropine can accelerate the atrial rate without improving ventricular conduction, paradoxically increasing the degree of block.

Heart Transplantation : The denervated donor heart has no vagal nerve supply; atropine is completely ineffective and may precipitate paradoxical AV nodal block.

Acute Myocardial Ischemia : Excessive atropine-induced tachycardia drastically accelerates myocardial oxygen consumption, exacerbating infarct size.

Second-Line Chronotropic Vasoactive Infusions

When atropine fails, is contraindicated, or while preparing for electrical pacing, initiate continuous intravenous chronotropic infusions:

Intravenous Dopamine Infusion : Continuous infusion at 5 – 20 μ g/kg/min 5\text{--}20\mu\text{g/kg/min} 5 – 20 μ g/kg/min . At 5 – 10 μ g/kg/min 5\text{--}10\mu\text{g/kg/min} 5 – 10 μ g/kg/min , β 1 \beta_1 β 1 -adrenergic stimulation increases heart rate and cardiac contractility. At > 10 μ g/kg/min > 10\mu\text{g/kg/min} > 10 μ g/kg/min , α 1 \alpha_1 α 1 -adrenergic vasoconstriction increases systemic vascular resistance, elevating blood pressure.

Intravenous Epinephrine Infusion : Continuous infusion at 2 – 10 μ g/min 2\text{--}10\mu\text{g/min} 2 – 10 μ g/min (or 0.1 – 0.5 μ g/kg/min 0.1\text{--}0.5\mu\text{g/kg/min} 0.1 – 0.5 μ g/kg/min ). Provides potent direct β 1 \beta_1 β 1 chronotropy, β 1 \beta_1 β 1 inotropy, and α 1 \alpha_1 α 1 systemic vasoconstriction. Highly effective in severe bradycardia accompanied by profound hypotension.

Intravenous Isoproterenol Infusion : Continuous infusion at 2 – 10 μ g/min 2\text{--}10\mu\text{g/min} 2 – 10 μ g/min . Pure non-selective β 1 \beta_1 β 1 and β 2 \beta_2 β 2 adrenergic agonist that markedly accelerates SA nodal discharge and AV conduction without α \alpha α -mediated vasoconstriction. Highly effective in orthotopic heart transplants, beta-blocker toxicity, and pause-dependent Torsades de Pointes. However, β 2 \beta_2 β 2 -mediated peripheral vasodilation can worsen hypotension if systemic vascular resistance is already diminished.

Targeted Antidotes for Specific Toxicities

Beta-Blocker Overdose : High-Dose Insulin Euglycemia Therapy (HIET: regular insulin 1 unit/kg 1\text{ unit/kg} 1 unit/kg IV bolus, then 0.5 – 1.0 unit/kg/hr 0.5\text{--}1.0\text{ unit/kg/hr} 0.5 – 1.0 unit/kg/hr infusion with dextrose support), intravenous Glucagon ( 3 – 10 mg 3\text{--}10\text{ mg} 3 – 10 mg slow IV bolus; bypasses adrenergic receptors by directly activating adenylate cyclase), and IV calcium salts.

Calcium Channel Blocker Overdose : Intravenous calcium chloride ( 1 – 2 g 1\text{--}2\text{ g} 1 – 2 g via central line) or calcium gluconate ( 3 – 6 g 3\text{--}6\text{ g} 3 – 6 g ), HIET, and vasopressors.

Digoxin Toxicity : Digoxin-specific Fab antibody fragments (DigiFab; empiric dose 10 to 20 vials for acute cardiac arrest or life-threatening instability).

Severe Hyperkalemia : Intravenous calcium gluconate ( 1 – 2 g 1\text{--}2\text{ g} 1 – 2 g ) to stabilize myocardial resting membrane potential, followed by regular insulin, dextrose, nebulized albuterol, and hemodialysis.

Transcutaneous Cardiac Pacing (TCP)

Transcutaneous pacing delivers repetitive electrical current across the chest wall to depolarize the myocardium non-invasively, providing an immediate life-saving bridge to transvenous pacing in unstable, atropine-refractory bradycardia.

Electrode Pad Placement

Pad placement directly dictates transthoracic electrical impedance:

Anterior-Posterior (AP) Configuration (Preferred) : The anterior pad is placed over the cardiac apex / left lower parasternal border, and the posterior pad is placed on the back in the left infrascapular area. The AP configuration delivers significantly lower transthoracic impedance, requires lower pacing current (mA), and achieves higher capture rates than anterolateral placement.

Anterolateral Configuration : Right upper sternal pad and left mid-axillary pad; utilized only when spinal immobilization or patient positioning prevents posterior pad placement.

Procedure and Capture Verification

Attach pacing pads and connect ECG monitoring leads. Set pacer mode to Demand (synchronous).

Set pacing rate between 60 and 80 bpm 60\text{ and }80\text{ bpm} 60 and 80 bpm .

Turn pacing current (mA) on at minimum and gradually increase output (typically 50 – 100 mA 50\text{--}100\text{ mA} 50 – 100 mA , up to 150 – 200 mA 150\text{--}200\text{ mA} 150 – 200 mA in hyperinflated or obese patients) until electrical capture is achieved.

Electrical Capture on ECG : A distinct, tall pacing spike followed immediately by a widened, bizarre QRS complex ( > 120 ms > 120\text{ ms} > 120 ms ) and a broad, discordant T-wave of opposite polarity.

Mechanical Capture Confirmation (CRITICAL) :

Electrical capture does not equal mechanical cardiac output.

Never verify capture using the carotid pulse . Transcutaneous current causes vigorous skeletal muscle contraction of the pectoralis, intercostals, and sternocleidomastoid that mimics a carotid pulsation.

True mechanical capture must be verified by palpating the right femoral artery pulse , observing an arterial line pulsatile pressure waveform, or verifying a pulsatile pulse oximeter plethysmograph waveform correlated precisely with the paced rate.

Analgesia and Sedation : TCP is excruciatingly painful due to intense chest wall muscle tetany and cutaneous nerve stimulation. Administer intravenous analgesia (fentanyl 25 – 50 μ g 25\text{--}50\mu\text{g} 25 – 50 μ g ) and sedation (midazolam 1 – 2 mg 1\text{--}2\text{ mg} 1 – 2 mg or propofol) as soon as hemodynamic stability is achieved.

Warning

Mistaking electrical capture or pacing-induced skeletal muscle twitching for mechanical capture is a fatal error. Always confirm a concordant arterial pulse via femoral palpation or an indwelling arterial line waveform. If no mechanical pulse is present despite electrical capture, the patient is in pulseless electrical activity (PEA) arrest—immediately initiate CPR.

Temporary Transvenous Pacing (TVP)

Temporary transvenous pacing involves percutaneously advancing a bipolar pacing electrode catheter into the right ventricular endocardium to achieve direct, reliable electrical capture.

Indications for Transvenous Pacing

Symptomatic high-grade AV block (Mobitz II, complete heart block) refractory to medical management.

Symptomatic sinus pauses > 3 seconds > 3\text{ seconds} > 3 seconds or sinus arrest refractory to atropine.

New alternating bundle branch block or new bifascicular block (e.g., LBBB or RBBB with left anterior/posterior hemiblock) in the setting of acute anterior myocardial infarction.

Refractory pause-dependent ventricular arrhythmias (overdrive pacing at 90 – 110 bpm 90\text{--}110\text{ bpm} 90 – 110 bpm for Torsades de Pointes).

Prophylactic standby pacing during pulmonary artery catheter placement in patients with pre-existing LBBB (risk of complete AV block from transient RBBB).

Vascular Access Selection

Right Internal Jugular Vein (RIJ; Gold Standard) : The anatomical path of choice. Provides the straightest, most direct vertical trajectory through the superior vena cava, right atrium, and tricuspid valve into the RV apex, maximizing catheter stability and minimizing accidental dislodgement.

Left Subclavian Vein : Favorable alternative; provides a smooth, gentle anatomic curve through the innominate vein and SVC.

Femoral Veins : Third-line route; associated with significantly higher deep venous thrombosis, immobilization, line infection, and frequent catheter dislodgement whenever the patient flexes the hip.

Insertion Technique & Threshold Testing

Under sterile conditions, a 5 – 6 French 5\text{--}6\text{ French} 5 – 6 French introducer sheath is placed. A balloon-tipped, flow-directed bipolar pacing catheter is advanced into the RV apex:

Advance catheter into the right atrium, inflate balloon with 1.5 mL 1.5\text{ mL} 1.5 mL air, and float across the tricuspid valve into the right ventricle. Fluoroscopy or bedside intracardiac ECG monitoring (showing marked ST-segment elevation "injury current" when contacting endocardium) confirms apical placement.

Deflate balloon, secure catheter, and connect pins to the external pulse generator (distal electrode to negative terminal [cathode], proximal electrode to positive terminal [anode]).

Stimulation (Capture) Threshold Testing :

Set pacing rate 10 bpm 10\text{ bpm} 10 bpm above intrinsic heart rate.

Gradually decrease output current (mA) until electrical capture is lost, then increase until 1:1 capture is restored. This minimum current is the capture threshold .

A reliable threshold in the RV apex is < 1.0 mA < 1.0\text{ mA} < 1.0 mA (ideally < 0.5 mA < 0.5\text{ mA} < 0.5 mA ). If threshold is > 1.0 mA > 1.0\text{ mA} > 1.0 mA , the catheter tip is poorly seated and must be gently repositioned.

Operational Output Setting : Set the working output to 2 to 3 times the measured capture threshold (or at least 3 – 5 mA 3\text{--}5\text{ mA} 3 – 5 mA ) to provide an adequate safety margin against physiological threshold shifts.

Sensitivity Threshold Testing :

Sensitivity determines the minimum voltage (mV) the generator must detect to identify an intrinsic ventricular depolarization and inhibit pacing in demand mode (VVI).

Measure the intrinsic R-wave amplitude (sensing threshold).

Set generator sensitivity to half of the measured sensing threshold (e.g., if intrinsic R-wave is 4 mV 4\text{ mV} 4 mV , set sensitivity to 2 mV 2\text{ mV} 2 mV ). Note that a smaller numerical mV setting makes the pacemaker more sensitive ; a larger numerical mV setting makes it less sensitive (more "blind").

Troubleshooting Pacemaker Malfunctions

When caring for a patient with a temporary transvenous pacemaker, clinicians must rapidly recognize and correct three classic malfunction patterns: TEMPORARY PACING TROUBLESHOOTING

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┌───────────────────────┼───────────────────────┐
▼ ▼ ▼
FAILURE TO CAPTURE UNDERSENSING OVERSENSING
Spike without QRS • Fires on native QRS • Missing expected spikes
Mechanism: Lead and T waves (R-on-T) • Mechanism: Generator too
dislodgement, high • Mechanism: Generator sensitive (detects myo-
threshold, dead batt "blind" (mV too high) potentials / artifact)
Action: Max output, • Action: INCREASE • Action: DECREASE
tighten pins, left SENSITIVITY (turn SENSITIVITY (turn mV
lateral decubitus, mV dial DOWN) dial UP), switch to
reposition catheter VOO for electrocautery
1. Failure to Capture

ECG Manifestation : Pacing spikes appear on the rhythm strip at the programmed rate, but are not followed by an evoked QRS complex or T-wave .

Etiology :

Catheter tip dislodgement / displacement from the RV endocardium (most common).

Loose wire pin connection in the generator terminals or broken bridging cable.

Depleted generator battery.

Acute rise in myocardial stimulation threshold: severe hyperkalemia, severe metabolic acidosis, myocardial infarction/ischemia at the electrode contact site, or class Ic / amiodarone toxicity.

Right ventricular free wall perforation (pacing catheter punctures through thin RV into pericardial space; manifests as failure to capture, loss of sensing, pericardial friction rub, or cardiac tamponade).

Immediate Bedside Actions :

Increase generator current output (mA) to maximum.

Check all physical connections: verify pins are firmly seated in correct terminals, ensure cable connections are tight, and replace battery.

Position patient in the left lateral decubitus position , which promotes gravity-assisted contact of the catheter tip against the RV apex.

Obtain immediate bedside echocardiogram and chest X-ray to evaluate catheter position and exclude pericardial effusion/tamponade.

Check and correct serum electrolytes ( K + K^+ K + ) and arterial blood gases.

If capture cannot be restored, immediately initiate backup transcutaneous pacing or chronotropic infusion while preparing for sterile catheter repositioning.

2. Undersensing (Failure to Sense Native Depolarizations)

ECG Manifestation : The pacemaker fails to detect the patient's intrinsic R-waves and fires pacing spikes asynchronously during or immediately following native QRS complexes and T-waves.

Lethal Clinical Hazard : An inappropriately timed pacing spike landing on the peak or downslope of a native T-wave triggers the R-on-T phenomenon , precipitating polymorphic ventricular tachycardia or refractory ventricular fibrillation!

Etiology : Sensitivity threshold is set too high (the numerical millivolt number is too large, e.g., set to 10 mV 10\text{ mV} 10 mV when native R-waves are only 3 mV 3\text{ mV} 3 mV , rendering the generator "blind" to native cardiac signals); lead displacement to an area of poor endocardial voltage; low intrinsic R-wave amplitude.

Immediate Bedside Actions : Increase sensitivity by decreasing the numerical millivolt (mV) setting on the generator dial (e.g., turn down from 5 mV 5\text{ mV} 5 mV to 1 – 2 mV 1\text{--}2\text{ mV} 1 – 2 mV ). If intrinsic R-wave is chronically < 1 mV < 1\text{ mV} < 1 mV , reposition the catheter or switch temporarily to asynchronous VOO mode only if the patient has zero intrinsic ventricular activity.

3. Oversensing (Inappropriate Pacing Inhibition)

ECG Manifestation : Pacing spikes are absent when expected despite a native heart rate below the programmed pacing rate. The pacemaker appears "dead" or intermittently pauses, resulting in prolonged asystolic pauses and syncope in pacemaker-dependent patients.

Etiology : The sensitivity threshold is set too low (the numerical millivolt number is too small, e.g., set to 0.5 mV 0.5\text{ mV} 0.5 mV , making the pacemaker "hypersensitive"). The generator misinterprets low-voltage non-cardiac electrical signals as intrinsic cardiac depolarizations and inappropriately inhibits pacing. Common culprits include:

Pectoral or diaphragmatic skeletal muscle myopotentials (shivering, coughing, deep breathing).

Electromagnetic interference (electrocautery during surgery, cell phones, radiofrequency devices).

Internal lead wire fracture or loose cable connection generating static artifact.

Tall, peaked T-waves or P-waves detected as R-waves.

Immediate Bedside Actions : Decrease sensitivity by increasing the numerical millivolt (mV) setting on the generator dial (e.g., turn up from 0.5 mV 0.5\text{ mV} 0.5 mV to 2 – 4 mV 2\text{--}4\text{ mV} 2 – 4 mV ) so the pacemaker ignores low-amplitude noise; eliminate external electrical interference; tighten loose connections. In an emergency or during surgical electrocautery in a pacemaker-dependent patient, switch the generator to asynchronous mode (VOO) to prevent fatal asystolic pauses.

Conduction Abnormality Anatomical Level of Block Characteristic ECG Features Clinical Course & Hemodynamic Risk Temporary Pacing Indication

Sinus Bradycardia / Arrest Sinoatrial (SA) node Normal P wave morphology, rate < 50 bpm < 50\text{ bpm} < 50 bpm ; sinus pauses Typically benign; symptomatic if cerebral/systemic hypoperfusion occurs Indicated only if hemodynamically unstable and refractory to Atropine / Dopamine

First-Degree AV Block AV node ( > 85 % > 85\% > 85% ) or His bundle Prolonged PR interval ( > 200 ms > 200\text{ ms} > 200 ms ); 1:1 AV conduction preserved Benign; excellent prognosis No indication for temporary pacing

Second-Degree Mobitz I (Wenckebach) AV node ( > 75 % > 75\% > 75% ) Progressive PR prolongation until a P wave fails to conduct; narrow QRS Benign; reversible; rarely progresses to complete heart block Rarely indicated; atropine if symptomatic; pacing only if severe hemodynamic instability

Second-Degree Mobitz II Infranodal (His-Purkinje system) Constant PR intervals with intermittent abrupt non-conducted P waves; wide QRS ( > 70 % > 70\% > 70% ) Highly malignant; unpredictable rapid progression to complete heart block / asystole Temporary transvenous pacing is indicated (atropine is often ineffective)

Third-Degree (Complete) AV Block AV node (congenital / inferior MI) or Infranodal (anterior MI / degenerative) Complete dissociation between P waves and QRS complexes; atrial rate > > > ventricular rate If infranodal: ventricular escape rate 20 – 40 bpm 20\text{--}40\text{ bpm} 20 – 40 bpm , wide QRS, severe shock, high mortality Immediate temporary transcutaneous, then transvenous, pacing

New Bifascicular Block with Acute MI Infranodal (His-Purkinje) RBBB with Left Anterior or Posterior Hemiblock, or alternating BBB High risk of sudden complete AV block and cardiogenic shock Prophylactic temporary transvenous pacing is reasonable

Malfunction Pattern Rhythm Strip Manifestation Underlying Causes Immediate Bedside Corrective Actions

Failure to Capture Pacing spikes fire at scheduled rate without subsequent QRS or T-wave Lead dislodgement from RV apex; high stimulation threshold (acidosis, hyperkalemia, ischemia); loose pins; dead battery; RV perforation 1. Increase current output (mA) to maximum; 2. Tighten pin connections and replace battery; 3. Position patient in left lateral decubitus; 4. Urgent echo/CXR to check position and rule out perforation; 5. Correct acidosis and hyperkalemia

Undersensing (Failure to Sense) Spikes fire asynchronously during native QRS and T-waves; risk of R-on-T VF Sensitivity dial set too high (mV threshold too high; generator "blind"); lead displacement; low native R-wave amplitude ( < 2 mV < 2\text{ mV} < 2 mV ) 1. Increase sensitivity by decreasing mV threshold dial (e.g., turn down from 5 mV 5\text{ mV} 5 mV to 1 – 2 mV 1\text{--}2\text{ mV} 1 – 2 mV ); 2. Reposition lead if intrinsic R-wave amplitude is inadequate; 3. Switch to VOO mode only if patient has zero native ventricular rhythm

Oversensing (Inappropriate Inhibition) Pacing spikes absent when expected; long pauses; heart rate slower than set rate Sensitivity dial set too low (mV threshold too low; generator "hypersensitive"); detects myopotentials (shivering), electrocautery, tall T-waves 1. Decrease sensitivity by increasing mV threshold dial (e.g., turn up from 0.5 mV 0.5\text{ mV} 0.5 mV to 2 – 4 mV 2\text{--}4\text{ mV} 2 – 4 mV ); 2. Eliminate electrical interference and muscle shivering; 3. Switch to asynchronous mode (VOO) during surgical electrocautery