Cardiac Electrophysiology and ECG Interpretation: Key Concepts for Students

What are the two main cardiac cell types and their functions?

Conducting cells: SA node, AV node, His-Purkinje fibers (conduct impulses); Contractile cells: Atrial and ventricular muscle (generate force); Both types have gap junctions and conduct action potentials.

What is the correct sequence of cardiac conduction?

1. SA node → atrial muscle; 2. AV node (delay); 3. Bundle of His → Purkinje fibers; 4. Ventricular muscle

What are the phases of the SA node action potential?

Phase 4: HCN channels (If/Ih) → slow Na⁺ influx; T-type Ca²⁺ channels → slow Ca²⁺ influx; Phase 0: L-type Ca²⁺ channels → fast Ca²⁺ influx; Phase 3: K⁺ channels open → K⁺ efflux; L-type Ca²⁺ channels inactivate

What are key features of the SA node?

Automaticity; No true resting membrane potential; No prominent Na⁺ channels; Gradual phase 0 via L-type Ca²⁺ channels

What regulates HCN channels and what is their effect?

Open with hyperpolarization; Regulated by cAMP; Permeable to Na⁺ and K⁺; Cause slow depolarization

What are the intrinsic firing rates of pacemakers?

SA node: 60-100 bpm; AV node: 40-60 bpm; His-Purkinje: 20-40 bpm

What happens with SA or AV node ablation?

SA node ablation: Loss of P-wave, HR = 40-60 bpm; AV node ablation: Loss of P-wave, wide QRS, HR = 20-40 bpm

What are the four autonomic effects on the heart?

Chronotropy: Rate (HCN, L-type Ca²⁺); Dromotropy: Conduction speed (AV node); Inotropy: Contractility (DHPRs, RyRs); Lusitropy: Relaxation (SERCA, NCX)

How does parasympathetic stimulation affect the heart?

↓ cAMP → ↓ HCN opening; ↑ K⁺ permeability → more negative Vm; Slower pacemaker rise → bradycardia

What do the ECG wave components represent?

P wave: Atrial depolarization; P-R segment: AV node delay; QRS complex: Ventricular depolarization; S-T segment: Ventricular contraction; T wave: Ventricular repolarization

What are the limb and precordial leads?

Limb leads: I, II, III, aVR, aVL, aVF; Precordial leads: V1-V6 (horizontal plane)

What does axis deviation indicate?

Right axis: RVH, COPD; Left axis: LBBB, LVH; Normal axis: -30° to +90°

What are the types of heart block?

1st degree: Prolonged PR interval; 2nd degree: Some P waves not conducted; 3rd degree: P waves and QRS disconnected

What are ECG findings in MI?

STEMI: ST elevation, transmural injury (non-reversible); NSTEMI: ST depression, T inversion, subendocardial injury (reversible)

What are ECG effects of hyperkalemia?

↓ K⁺ gradient → less negative resting potential; Complex ECG changes

What drugs cause bradycardia and how is it treated?

Diltiazem/Verapamil: Block L-type Ca²⁺ channels; Beta blockers: Block β1 receptors; Treatment: Atropine (anticholinergic)