Electrophysiology of the Heart J.M. Cairo, Ph.D. Telephone:

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 TissueBeta-AdrenergicCholinergic 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 CurrentNameIon that is carried Reversal Potential (mV) Inhibitor I Na Na+ CurrentNa++60TTX Lidocaine I Ca Ca++ current Ca+++120Nifedipine Verapamil IKIK I KR, I KS, I to, G-protein K+-100Ba++ Cs+ TEA

Membrane Currents for the Various Phases of the Ventricular Myocyte Action Potential Phase 0 (Rapid Upstroke) –I Na and I Ca Phase 1 (Early Repolarization) –Inactivation of I Na and I Ca, and I K (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) –I K Phase 4 (Diastolic period) –I K and Na-K pump

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 CurrentNameIon that is carried Reversal Potential (mV) Inhibitor I Na Na+ CurrentNa++60TTX Lidocaine I Ca Ca++ current Ca+++120Nifedipine Verapamil IKIK I KR, I KS, I to, G-protein K+-100Ba++ Cs+ TEA IfIf Pacemaker current Na+, K+-35Cs+

Membrane Currents for the Various Phases of the SA node and AV node Action Potentials Phase 0 (Slow Upstroke) –I Na and I Ca Phase 3 (Repolarization) –I K Phase 4 (Diastolic depolarization) –I K, I Ca, I f

Principle Time Dependent and Voltage Dependent Currents TissueCurrents SA nodeI Ca, I K, I f AV nodeI Ca, I K, I f Purkinje fiberI Na, I Ca, I K, I f Atrial muscleI Na, I Ca, I K Ventricular muscleI Na, I Ca, I K

Effect of Adrenergic and Cholinergic Stimulation on Pacemaker Activity TissueBeta-AdrenergicCholinergic 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 Tissue Conduction Velocity (m/s) SA node0.05 Atrial Pathways1 AV node0.05 Bundle of His1 Purkinje System4 Ventricular muscle 1