▪ Sarcoidosis, amyloidosis, hemochromatosis

○Inflammatory

▪ Pericarditis, myocarditis

○Familial diseases

▪ Myotonic dystrophy, Friedrich ataxia, Na⁺ channel mutations

○Collagen vascular disease

▪ Systemic lupus erythematosus, rheumatoid arthritis, scleroderma, ankylosing spondylitis

○Trauma or surgery

▪ Valve replacement, ablation, atrial septal defect (ASD) repair, heart transplantation

Extrinsic

○Drugs

▪ AADs (class I, class III), β-blockers, diltiazem/verapamil, digoxin, ivabradine

▪ Sympatholytic (reserpine, methyldopa, clonidine), alcohol, lithium

○Electrolyte imbalances

▪ Potassium, calcium, or magnesium

○Metabolic

▪ Hypothyroidism, hypothermia

○Myocardial ischemia

▪ Inferior myocardial infarction (MI): Neural reflex

○Autonomic-mediated syndromes

▪ Neurocardiogenic syncope, carotid-sinus hypersensitivity

▪ Situational: Coughing, micturition, defecation, vomiting

○Infection

▪ Chagas, endocarditis, Salmonella, diphtheria, rheumatic fever, viral myocarditis

Classification

○ Inappropriate sinus bradycardia

▪ Sinus bradycardia in the absence of an appropriate cause or that results in symptoms

○ Tachycardia-bradycardia syndrome

▪ Atrial tachycardia (AT; usually AF) alternating with sinus bradycardia, sinus pauses, and/or an AV block

○ Intermittent sinus pauses include:

▪ Sinus arrest: Transient cessation of SA node firing (>2 seconds)

▪ Sinoatrial (SA) exit block: Depolarization fails to exit the SA node (the RR interval is unchanged).

• Type 1 (Mobitz): The time between sinus firing and when the atrial capture progressively prolongs leading to gradual shortening in the PP intervals before the pause (pause PP < two preceding PP).

• Type 2 (Mobitz): The PP interval is constant before and after the pause and is a multiple (2×, 3×…) of the basic PP interval.

▪ Absolute: Inability to increase heart rate to 60% age-predicted target (220 – age) or to >100–120 bpm.

▪ Relative: Able to reach target heart rate but at a significant delay with reduced exercise tolerance.

12-Lead ECG

○Heart rate <60 bpm with characteristics of sinus rhythm

▪ 1:1 P:QRS relationship

• Every P wave is followed by a QRS complex and every QRS complex is preceded by a P wave.

▪ Sinus P-wave morphology and axis

• Upright in I, II, aVF, and V2–V6 (best seen in II)

• Inverted in aVR

• Upright or biphasic in V1 and V2; upright in V3–V6

Other Investigations

○Carotid sinus massage or tilt table testing to identify neurocardiogenic causes

○Treadmill stress test

▪ Assesses the chronotropic response to exercise

○Ambulatory electrocardiogram (ECG) monitoring (Holter, event monitor, ILR)

▪ Correlates symptoms with the electrical disorder

○Assessment of intrinsic heart rate (IHR: Rarely performed)

▪ IV atropine 0.04 mg/kg + IV propranolol 0.2 mg/kg

▪ Predicted IHR = 118.1 – [0.57 × age]:

• Low IHR = intrinsic sinus node dysfunction (SND)

• Normal IHR = autonomic imbalance

○Electrophysiology study (see Assessment of SA Node Function section)

Acute Management

○Atropine or isoproterenol

Chronic Management

○Stop medications that suppress the SA node (e.g., β-blockers, verapamil/diltiazem, digoxin).

○Pacemaker indications:

▪