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Fundamental of EP
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Fundamental of Electrophysiology

    Normal Conduction System


medmovie.com ▲
Conduction System of the Heart
Specialized conducting components of the heart include the sinoatrial node, the internodal pathways, Bachmann's bundle, the atrioventricular node, the atrioventricular bundle, the right and left bundle branches, and the Purkinje fibers.
ref: OpenStax, Anatomy & Physiology. ▲

    The Action Potential

This is the action potential of...?
This is the action potential of non-pacemaker cels such as myocardium and His-Purkinje fiber. ▲
Four Phases of Action Potential.
Phase 0: Na+ ↑
Phase 1: K+ ↓
Phase 2: Ca2+ ↑
Phase 3: K+ ↓
Phase 4: K+ ↓
▲
Temporal Correlation between Action Potential and ECG.
QRS = phase 0-1, ST-T = phase 2, T = phase 3.
from Nat Rev Drug Discov 2, 439–447 (2003). ▲
Action Potential of Non-pacemaker cells (left) vs. Pacemaker cells (right)
Pacemaker cells = nodal cells including sinoatrial (SA) and atrioventricular (AV) nodal cells.
Noted that non-pacemaker cells have true resting membrane potential (phase 4) and rapid phase 0.
In pacemaker cells, there are only 4 phases. Phase 4 is the spontaneous depolarization phase and is mainly Calcium dependent. Phase 0 is depolarization and phase 3 is repolarization. ▲
    Pacemaker Cells Action Potential
  • Calcium dependent
  • Slow up and down stroke
  • Pacemaker activity is generated automatically.
  • Can be modified by many factors, such as autonomic nervous system, and etc.
  • Specific current = If or Funny Current
Specific Properties of Action Potential of Pacemaker Cells
...including Calcium depedent, slow up & down strokes, modifiable by autonomic nervous system. ▲
PropertiesPacemaker CellsNon-pacemaker Cells
LocationSA and AV NodesMyocardium, His-Purkinje, Bypass Tracts
Normal Resting Potentials-40 to -65mV-80 to -95mV
Phase 0 CurrentsPrimarily CalciumSodium
Conduction VelocitySlow: 0.01-0.1 m/sFast: 0.5-5 m/s
Conduction PropertyDecrementalAll or None
EP Properties of Pacemaker vs Non-pacemaker cells.
▲
    Refractory Period (RP)
  • Absolute RP (ARP): nothing can pass.
  • ARP = phase 0, 1, 2, and early part of phase 3.
  • Effective RP: allow local depolarization; not propagated one.
  • Relative RP: stronger stimulus may depolarize.
Refractory Period
Absolute Refractory Period (ARP) = nothing can pass = phase 0-2 and early 3. ▲

    Mechanism of Arrhythmias

Enhance Automaticity
= accelerated generation of an action potential by either normal pacemaker tissue (enhanced normal automaticity) or by abnormal tissue within the myocardium (abnormal automaticity).
Examples: sinus tachycardia, atrial tachycardia, junctional tachycardia, or idioventricular rhythm. ▲
    Enhanced Automaticity
  • from normal pacemaker cells, such as sinus tachycardia.
  • from abnormal cells, such as atrial tachycardia.
  • Warm up & cool down phenomenon.
  • Response to autonomic nervous activity and many drugs.
Enhanced Automaticity
Warm up & cool down phenomenon. Modifiable by drugs and autonomic nervous system. ▲
Reentry
= continous activity around the circuit within myocardium to re-excite the heart after the local refractory period has ended.
Examples: atrial flutter, AVNRT, or scar related VT. ▲
    Reentry
  • Rotate around an anatomical barrier.
  • 2 pathways with 2 different EP properties.
  • Initiate by premature beat.
  • Rapid pacing can entrain and terminate.
Reentry Mechanism
Can be entrained and overdrive suppressed. ▲
Triggered Activity
= abnormal action potentials that are triggered by the preceding action potential.
Examples: idiopathic PVCs/VT, torsades de pointes, or digitalis toxicity. ▲
Triggered Activity
EAD: Long QT, Brady-induced TdP, HypoK+
DAD: Digitalis toxicity, idiopathic VT. ▲

    Antiarrhythmic Agents

Class I: Sodium Blocker
Ia
Ib
Ic
Class I: Sodium Blocker
categorized into 3 classes, by Na+ channel asso-/dissociation rate.
From rapid to slow: IB, IA, IC. ▲
    Class Ia
  • Intermediate asso-/dissociation rate of sodium channel.
  • Quinidine, Procainamide, Disopyramide, Ajmaline.
  • Prolong QT and QRS duration
    Class Ib
  • Rapid asso-/dissociation rate of sodium channel.
  • Lidocaine, Mexilitine, Phynytoin
  • Shorten QT. Minimal effect on QRS duration.
    Class Ic
  • Slow asso-/dissociation rate of sodium channel.
  • Flecainide, Encainide, Propafenone, Moricizine
  • Prolong QRS. Minimal effect on QT
    Class II: Beta-Blocker
  • Only class that saves life.
  • Bisoprolol, Carvedilol, Metoprolol, and etc.
  • Inhibit sympathetic activity.
  • Increase action potential duration.
    Class III: Potassium-Blocker
  • Amiodarone, Dofetilide, Ibutilide, Sotalol
  • Prolong QT
  • Amiodarone inhibits multiple channels including Na+, beta, and Ca2+.
    Class IV: Calcium-Blocker
  • Diltiazem, Verapamil
  • Block SA and AV nodes
  • Negative inotropic effect
    Ivabradine
  • Inhibit cardiac pacemaker current or If.
  • If  controls diastolic depolarization of SA node.
  • Ivabradine selectively block If  and slow sinus rate.
  • Has no effects of BP or cardiac contractility.
    Adenosine
  • Multiple effects via multiple Adenosine receptor subtypes.
  • Inhibits impulse generation in SA and AV node.
  • Vasodilation and bronchoconstriction.
  • Commonly used to terminate PSVT.
JACC Cardio Interv 2014
    Prolong QTc and torsades de pointes.
  • Prolong QTc can "trigger" torsades de pointes.
  • Causes: drugs, electrolytes abnormalities, and congenital.
  • Drugs that prolong QTc: Class III AADs (think of Sotalol), haloperidol, methadone, and etc.
Yan et al. JACC 2003.
Drugs and Devices
FunctionIncrease Threshold Decrease Threshold
Pacing Class Ic (think of flecainide), beta blockers, quinidine, hypoxemia, acidosis, alkalosis, hyperglycemia Adrenaline (Epinephrine), Isoproterenol, Atropine, steroids
Defibrillation amiodarone, lidocaine, flecainide, sildenafil (Viagra), alcohol sotalol, dofetilide

    Qs and As

Question 1: Tracing
Question 1 ▲
A 54-year-old woman with symptomatic paroxysmal AF is admitted to the hospital for sotalol loading. On the third hospital day, a two-channel rhythm strip is recorded (as shown previously).
Which of the following should you recommend?
  1. Reduce the dose of sotalol by 50%, discharge the patient.
  2. ICD implantation.
  3. Perform immediate defibrillation
  4. Start IV amiodarone and continue for the duration of five half-lives of sotalol.
  5. Discontinue sotalol and give IV magnesium.
Q1: The Answer is...
Answer: e
Tracing = sinus rhythm with prolong QT with PVCs which later triggers torsades. Sotalol is a class III AAD and known to increase QT interval. Treatment includes discontinue the causal agent, give IV magnesium regardless of the magnesium level. Rapid pacing may be required. ▲
Which of the following drugs is most likely to elevate the myocardial stimulation threshold after implantation of a dual chamber pacemaker?
  1. Digoxin
  2. Diltiazem
  3. Flecainide
  4. Mexilitine
  5. Sotalol
Q2: The Answer is...
Answer: c
Class IC is notoriously known to increase pacing threshold. ▲
In a patient suffering from VT after myocardial infarction, which of following medication should be avoided?
  1. Amiodarone
  2. Dofetilide
  3. Flecainide
  4. Metoprolol
Q3: The Answer is...
Answer: c
Class IC is associated with increase risk of death in patients with history of MI. ▲
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Heart Rhythm Box is a collection of web-paged style presentations in clinical cardiac electrophysiology topics. The page is created mainly for educational purpose. I believe that presentations should be more interactive and easily accessible. No need to register or download. You can access everything at your fingertips.

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