Ch 24/25 Cardiac Rhythm & Contractility Pharmacology

SA Node Action Potential

3 Phases: 403

• Phase 4: Recovery/resting

• Phase 0: Depolarization

  • Action potential caused by Ca2+ influx

• Phase 3: Hyperpolarization

Myocyte Action Potential

5 Phases: 40123

• Phase 4: Recovery/resting

• Phase 0: Depolarization

  • Action potential caused by Na+ influx

• Phase 1: Early repolarization

• Phase 2: Plateau period

  • Ca2+ influx

• Phase 3: Late rapid depolarization

P Wave

Atrial depolarization

QRS Complex

Ventricular depolarization (shows phase 0 of myocyte depolarization, Na+ influx)

T Wave

Ventricular repolarization

ST

Plateau of action potential (shows phase 2 of myocyte repolarization, Ca2+ influx)

Formation Defects

Early/delayed afterdepolarization period

Conduction Defects

Damaged tissue causes abnormal arrhythmias

Class 1 Antiarrhythmic Agents

Na+ channel blockers

3 subclasses:

• Class 1a: Moderate Na+ blocking

  • Prefers OPEN Na+ channels

    • Procainamide

• Class 1b: Minimal Na+ blocking

  • Prefers OPEN OR INACTIVE Na+ channels

    • Lidocaine

• Class 1c: Major Na+ blocking

  • Flecainide

Class 2 Antiarrhythmic Agents

Beta-Adrenergic Antagonists

• Propranolol

• Pindolol (partial agonist, too)

Class 3 Antiarrhythmic Agents

K+ channel blockers

Prolong repolarization & refractory period

• Ibutilide

Class 4 Antiarrhythmic Agents

Ca2+ channel blockers

• SA & AV nodes, specifically (phase 0)

• Slows AV node action potentials

***NOT GOOD FOR USE WITH CLASS 2 ANTIARRYTHMICS

• Verapamil

Contractile Dysfunction at the Cellular Level

1. Dysregulation of Ca2+ homeostasis

2. Contractile protein expression changes

3. B-Adrenergic signaling pathway changes