Electrophysiology of the Heart J M Cairo Ph

Electrophysiology of the Heart J. M. Cairo, Ph. D. Telephone: 568 -4246 Email: jcairo@lsuhsc. edu

Electrophysiological Properties of the Heart • Excitability – Ability of the heart to respond to a stimulus • Rhythmicity (Automaticity) – Ability of the heart to initiate a depolarization in the absence of external stimuli • Conductivity – Ability of the heart to propagate an impulse once it has been initiated

Other terms… • Chronotropy – Relates to heart rate: a positive chronotropic agent will increase heart rate; a negative chronotropic agent will decrease heart rate • Dromotropy – Relates to conduction velocity: a positive dromotropic agent will increase the conduction velocity of an impulse through the heart; a negative dromotropic agent will slow the conduction velocity of the impulse

Effect of Adrenergic and Cholinergic Stimulation Tissue Beta-Adrenergic Cholinergic SA node ↑ Pacemaker rate ↑ Conduction Velocity ↓ Pacemaker rate ↓ Conduction velocity AV node ↑ Pacemaker rate↑ Conduction Velocity ↓ Pacemaker rate ↓ Conduction velocity Purkinje fibers ↑ Pacemaker rate ↓ Pacemaker rate

Excitability

Factors Affecting Resting Membrane Potential in Cardiac Myocytes • Macromolecules • Diffusible ions (Na+, K+, Ca++, Cl-) – Electrochemical gradients • Sodium-Potassium ATPase pump

Major Cardiac Membrane Currents Current Name Ion that is carried Reversal Potential (m. V) Inhibitor INa Na+ Current Na+ +60 TTX Lidocaine ICa Ca++ current Ca++ +120 Nifedipine Verapamil IK IKR, IKS, Ito, G -protein K+ -100 Ba++ Cs+ TEA

Membrane Currents for the Various Phases of the Ventricular Myocyte Action Potential • Phase 0 (Rapid Upstroke) – INa and ICa • Phase 1 (Early Repolarization) – Inactivation of INa and ICa, and IK (minor contribution) • Phase 2 (Plateau) – Continued entry of Na and Ca through major channels and minor membrane current due to Na-Ca exchanger • Phase 3 (Repolarization) – IK • Phase 4 (Diastolic period) – IK and Na-K pump

Refractory Periods

Refractory Periods • Absolute Refractory Period – Beginning of phase 0 to the beginning of phase 3 – Refractory to all stimuli • Relative Refractory Period – Beginning of phase 3 to the beginning of phase 4 – A stronger than normal stimulus will result in an AP that is less than a normal AP

Rhythmicity/Automaticity

Major Cardiac Membrane Currents Current Name Ion that is carried Reversal Potential (m. V) Inhibitor INa Na+ Current Na+ +60 TTX Lidocaine ICa Ca++ current Ca++ +120 Nifedipine Verapamil IK IKR, IKS, Ito, G -protein K+ -100 Ba++ Cs+ TEA If Pacemaker current Na+, K+ -35 Cs+

Membrane Currents for the Various Phases of the SA node and AV node Action Potentials • Phase 0 (Slow Upstroke) – INa and ICa • Phase 3 (Repolarization) – IK • Phase 4 (Diastolic depolarization) – IK, ICa, If

Principle Time Dependent and Voltage Dependent Currents Tissue Currents SA node ICa, IK, If AV node ICa, IK, If Purkinje fiber INa, ICa, IK, If Atrial muscle INa, ICa, IK Ventricular muscle INa, ICa, IK

Effect of Adrenergic and Cholinergic Stimulation on Pacemaker Activity Tissue Beta-Adrenergic Cholinergic SA node ↑ Pacemaker rate ↓ Pacemaker rate AV node ↑ Pacemaker rate ↓ Pacemaker rate Purkinje fibers ↑ Pacemaker rate ↓ Pacemaker rate

Conductivity

Conduction Velocities in Different Cardiac Tissue Conduction Velocity (m/s) SA node 0. 05 Atrial Pathways 1 AV node 0. 05 Bundle of His 1 Purkinje System 4 Ventricular muscle 1