Cardiac Electrophysiology

Cardiac Electrophysiology Saturday, February 17, 2024 2:41 AM • Electric Activity of the Heart • All cardiomyocytes are myogenic (spontaneously generate action potential) ○ Heart beats on its own • Conducting System ○ Sinoatrial (SA) node – heart pacemaker ▪ Sends electrical signals throughout the heart • Timing of heartbeat (HR) • Cause heart to beat in coordinated, rhythmic pattern ▪ If damaged – AV node takes over (less frequency than SA node) ○ Atrioventricular (AV) node ○ Starts w/ SA node → atrium → impulses reach AV node → ventricles → contraction ▪ AV contracts but does not reach ventricle contracts ▪ Once atrium relaxes → ventricle contracts • Electrophysiological Categories of Cardiac Cells ○ Fast fibers (fast response AP) ▪ ↑ gap junction, ↓ resistance = high diameter ▪ Na+ dependent • Atrial + ventricular myocytes ▪ 5 phases • Phase 4 – resting potential ○ More K+ inside, Na+ outside • Phase 0 – rapid depolarization (either fast or slow fibers) ○ Fast Na+ channels ○ Voltage + time dependent • Phase 1 – partial repolarization (initial state) ○ Rapid but incomplete repolarization b/c transient outward current ito (open + close immediately) by voltage-gated K+ channels • Phase 2 – plateau ○ Slow inward Ca2+ current by L-type Ca2+ channels ▪ Abundant in T-tubules ▪ Open at phase 0 ○ Balanced by outward currents ▪ K+ delayed (open late) • Phase 3 – repolarization ○ Inactivation of LTCCs 9 (closed) ○ Repolarization by continued K+ efflux ▪ Kv channels (iKv) ▪ Kir channels relax • Refractory Period – time required for recovery of normal excitability t ○ Absolute: (0-200 msec) myocytes completely refractory to excitation 250 ○ Relative (200-250 msec): myocytes can be excited by strong stimulus a 200 ○ Slow fibers (slow response AP) ▪ ↓ gap junction, ↑ resistance = small diameter ▪ Ca2+ dependent **SA node + AV node DO NOT CONTRACT = no need for plateau • SA node • AV node ▪ Phase 4: gradual depolarization • Funny Na+ channels (If, HCN) spontaneously open • Transient T-type Ca2+ channels open at –50mV ○ Slow Ca2+ influx brings AP to threshold –40mV (more charges to +) ▪ Phase 0: rapid depolarization • L-type Ca2+ channels open at –40mV ▪ Phase 3: repolarization • Voltage-gated delayed K+ channels (K+ outside) 0.29a Oo • Transmembrane Potential • Depolarization (triggers AP) - inside from –ve to +ve = Na+ influx • Repolarization – inside from +ve to –ve = K+ efflux • Hyperpolarization (makes everything back to normal) = more K+ efflux • Cardiomyocytes Contraction • CICR (calcium-induced calcium release) ○ AP activates L-type CC • EKG • Measures heart depolarization + repolarization • 12 ECG leads divided in 2 types ○ 3 bipolar limb leads ▪ (-) → (+) ○ 9 unipolar ▪ 3 augmented limb leads ▪ 6 chest or precordial leads • AP of all cardiac cells • P wave – atrial depolarization ○ 0.08s (normal < 0.12s) • QRS – ventricular depolarization ○ 0.08-0.1s • T wave – ventricular repolarization • Standard ECG recording paper speed = 25 mm/sec ○ 1 small square is 1 mm (0.04 sec/square) ventriculardepolarization ventriolerropom orteraedoporbrizoon • D ○ PR interval – depolarization time of AV node and His-Purkinje system ▪ AV node delay (0.12-0.2s), > 0.2s = conduction pathway defect = heart block ○ ST segment ▪ Coincides w/ plateau ▪ Isoelectric – whole ventricle is depolarized (no current is flowing) ▪ Abnormal = ischemic heart disease ○ QT interval ▪ Q – depolarization ▪ T – repolarization ▪ Time b/w ventricular depolarization and repolarization (0.36 - 0.44s) • Ischemic Heart Disease ○ ST segment displacement (injury currents from depolarized ischemic to normal cardiomyocytes ○ Angina = ST segment depression ○ STEMI = elevates ST segment • RR interval ***EXAM: Calculate HR ○ HR = 1500/# of mm between beats or 1500/# of boxes between R's ▪ Ex: there are 23 mm between the first 2 beats = 1500/23 = 65 bpm É