Cardiac Conduction SA Node Primary Pacemaker Spontaneous Depolarization

Cardiac Conduction

• SA Node: Primary Pacemaker • • • Spontaneous Depolarization Depolarizes faster = Higher Automaticity 60 -100 depolarizations per minute 75 action potentials per minute (resting) Internodal pathways Bachmann Bundle

• AV Node: Secondary Pacemaker • Depolarizes 40 -60 times per minute • Slight delay at AV node to allow for ventricular filling • Vagus nerve

• AV Node: Secondary Pacemaker • Depolarizes 40 -60 times per minute • Slight delay at AV node to allow for ventricular filling • Vagus nerve

• His-Purkinje System: • • Synchronized ventricular stimulation and contraction Heart rate originating in ventricles 30 -40 beats per minute RBB vs LBB Purkinje Fibers

• Autonomic Nervous System Control • • Sympathetic activity increases rate of AV conduction Recognized as a decreased PR interval Norepinephrine binding to beta-adrenergic receptors Beta-blockers cause…? ? ? Regulation of Conduction

• Parasympathetic (Vagal) • Slows conduction • Acetylcholine binding to muscarinic receptors • AV Block

• • Depolarization Repolarization Membrane Potential: SA Node vs Myocardial Cells Refractory Period Action Potentials

• Fast Potentials • Originate: • His Perkinje Fibers • Cardiac Myocytes • Only in abnormal conditions do these determine the heart rate • Unpredictable and lead to dysrhythmias Action Potentials

• 4 PHASES • • • Phase 0 = rapid depolarization (Na influx) Phase 1 = partial repolarization Phase 2 = plateau (Ca dependent) Phase 3 = rapid repolarization (K extrusion) Phase 4 = rest or rapid depolarization

• Slow Potentials • Originate: • SA Node • AV Node

• 3 PHASES • Phase 0 = slow depolarization (Ca influx) • Phase 3 = repolarization • Phase 4 = spontaneous depolarization (pacemaker potential)

• • Sinus Tachycardia Sinus Bradycardia AV Block Atrial Fibrillation Atrial Flutter Ventricular Fibrillation Ventricular Flutter Dysrhythmias

• Sinus Tachycardia >100 beats per minute

• Sinus Bradycardia < 60 beats per minute

• Atrial Fibrillation

• Atrial Flutter

• Supraventricular Tachycardia • 150 -250 beats per minute

• Ventricular Tachycardia • 150 to 250 beats per minute

• Ventricular Fibrillation

• • P WAVE: Atrial Depolarization PR INTERVAL: AV node delay (P wave to QRS) QRS COMPLEX: Ventricular Depolarization QT INTERVAL: (QRS beginning to T wave completion) • T WAVE: Ventricular Repolarization • ST SEGMENT: (QRS end to T wave start) Electrocardiogram

• Sustained? • Symptomatic? • Ventricular? When to treat?