Section 3 · Cardiology

Chapter 1 –

Bradycardia

[!case]+ Clinical Vignette An 87-year-old man with coronary artery disease, hypertension, and hyperlipidemia is found unresponsive at home and admitted for evaluation. He is light-headed, pulse 42 bpm, BP 85/47 mm Hg. ECG (▶ Fig 1-1, not shown) demonstrates a narrow-complex regular bradycardia.

1. Anatomy & Physiology

[!question]- What is the normal pathway of electrical conduction? [!answer]- SA node → right & left atrial myocardium AV node (low right atrium) Bundle of His (membranous septum) Right & Left bundle branches → Purkinje network → ventricular myocardium

[!question]- Which coronary artery usually supplies the nodes?

[!answer]-

Node	Arterial supply	Prevalence
SA node	Proximal RCA	65 %
	Circumflex	25 %
	Dual (RCA + Cx)	10 %
AV node	RCA (posterior desc.)	80 %
	Circumflex	10 %
	Dual	10 %

2. Heart-Rate Basics

[!info] Definition

Bradycardia = HR < 60 bpm (adults). Adjust for age, fitness, medication, & comorbidity.

Concept	Key fact
Normal resting HR	≈ 70 bpm
Sleep-related drop	−24 bpm in young adults · −14 bpm in elderly
Cardiac output	CO = SV × HR
Symptoms	Fatigue, weakness, light-headedness, syncope
Exam clues	Slow pulse, hypotension, cool extremities, ± cannon A-waves (with AV dissociation)

3. ECG Classification

[!summary]+

Bradycardia categories

Narrow QRS (< 120 ms)
Wide QRS (≥ 120 ms)

Each may be regular or irregular.

At 25 mm · s⁻¹ paper speed:

1 small box = 40 ms · 5 small = 1 large = 200 ms

3 A. Narrow-Complex Bradycardia — Regular Rhythm

Common Cause	Hallmark ECG Finding
Sinus bradycardia	Upright P in I & aVF, 1:1 AV conduction
Atrial flutter + AV block	Saw-tooth flutter waves (II, III, aVF) ± 2–5:1 block
Atrial tachycardia + AV block	Ectopic P-wave morphology, atrial > 100 bpm
Junctional escape	Absent/inverted P or P after QRS, rate 40-60 bpm

[!tip] Flutter rate math

Atrial ≈ 300 bpm → Ventricular rate = 300 / (block ratio)

e.g. 300 / 5 = 60 bpm (bradycardic).

3 B. Narrow-Complex Bradycardia — Irregular Rhythm

Cause	Key Points
Sinus arrhythmia	Cyclic ↑HR on inspiration / ↓HR on expiration
Sinus arrest / SA exit block	Pause ≥ 3 s. Type II exit block pause = integer × basic P-P
A-fib with slow VR	No P waves, irregularly irregular
Mobitz I/II AV block	Progressive PR prolongation (Type I) or fixed PR with dropped QRS (Type II)

3 C. Wide-Complex Bradycardia — Regular Rhythm

Cause	Distinguishing ECG
Sinus brady + BBB	P waves 1:1 with broad QRS
Complete (3°) AV block + ventricular escape	AV dissociation, rate 20-40 bpm

3 D. Wide-Complex Bradycardia — Irregular Rhythm

Cause	Scenario
A-fib / flutter with BBB	Irregular broad QRS with flutter/fibrillatory waves
Mobitz II with BBB	Baseline wide QRS, intermittent non-conducted P

Pacemaker patients may exhibit relative bradycardia if rate fails to rise with metabolic demand (e.g. sepsis, anemia).

4. Case-Based Q&A

[!question]- What rhythm does the index patient have?

[!answer]- Junctional escape rhythm (narrow, regular, no clear P).

[!question]- What additional major ECG finding is present?

[!answer]- Inferior ST-segment elevation ⇒ acute inferior MI.

[!question]- Likely cause of the escape rhythm?

[!answer]- Ischemia of SA/AV nodes from RCA-territory inferior MI.

[!question]- Immediate management priorities?

[!answer]-

IV fluids to support preload
Atropine if symptomatic
Transcutaneous ➜ transvenous pacing if needed
Reperfusion (PCI or fibrinolysis)

[!question]- Long-term strategy if bradycardia persists? Permanent pacemaker implantation.

5. Key Points (High-Yield)

[!note] Memorise Bradycardia = HR < 60 bpm (context-dependent). Symptoms range from none → syncope. Always classify by QRS width & rhythm regularity. Narrow + regular = think sinus vs flutter/tachy with block vs junctional. Wide + regular = sinus with BBB vs ventricular escape. Node ischemia (RCA) is a common, reversible cause.

6. References

Mangrum JM, DiMarco JP. N Engl J Med. 2000;342:703-709.
Berne RM, Levy MN. Physiology, 4e. Mosby; 1998.
Brodsky M et al. Am J Cardiol. 1977;39:390-395.
Kantelip JP et al. Am J Cardiol. 1986;57:398-401.
Cunha BA. Clin Microbiol Infect. 2000;6:633-634.
Page RL et al. Circulation. 2016;133:e506-e574.
Yasuma F, Hayano J. Chest. 2004;125:683-690.
Benditt DG et al. Circulation. 1987;75:II-93-III-102.
Falk RH. N Engl J Med. 2001;344:1067-1078.
Levine HD, Smith C. Cardiology. 1970;55:2-21.
Riera AR et al. Indian Pace Electrophysiol J. 2010;10:40-48.
Vogler J et al. Rev Esp Cardiol. 2012;65:656-667.

[!quote]

“Slow and steady” is not always safe—understand the rhythm, search for the cause, and pace when the pause becomes perilous.