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Cardiac Electrophysiology

Cardiac Electrophysiology. Qiang XIA ( 夏强 ), PhD Department of Physiology School of Medicine Tel: 88206417, 88208252 Email: xiaqiang@zju.edu.cn. The major types of cardiac muscle: Atrial muscle Ventricular muscle Specialized excitatory and conductive muscle. Contractile cells （收缩细胞）.

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Cardiac Electrophysiology

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  1. CardiacElectrophysiology Qiang XIA (夏强), PhD Department of Physiology School of Medicine Tel: 88206417, 88208252 Email: xiaqiang@zju.edu.cn

  2. The major types of cardiac muscle: • Atrial muscle • Ventricular muscle • Specialized excitatory and conductive muscle Contractile cells （收缩细胞） Autorhythmic cells （自律细胞）

  3. Conducting system of the heart

  4. Cardiac muscle

  5. Sequence of cardiac excitation The Bundle of His and other parts of the conducting system deliver the excitation to the apex of the heart so that ventricular contraction occurs in an upward sweep. The sinoatrial node is the heart’s pacemaker because it initiates each wave of excitation with atrial contraction.

  6. General process of excitation and contraction of cardiac muscle • Initiation of action potentials in sinoatrial node • Conduction of action potentials along specialized conductive system • Excitation-contraction coupling • Muscle contraction

  7. Transmembrane potentials recorded in different heart regions

  8. Transmembrane potential of ventricular cells and its ionic mechanisms Resting Potential: -90 mV Action Potential • Phase 0: Depolarization • Phase 1: Early phase of rapid repolarization • Phase 2: Plateau（平台期） • Phase 3: Late phase of rapid repolarization • Phase 4: Resting phase

  9. Ionic mechanisms • Resting potential • K+ equilibrium potential • Na+-inward background current • Electrogenic Na+-K+ pump

  10. The action potential of a myocardial pumping cell. • Phase 0 • Threshold potential (-70mV) • Opening of fast Na+ channel • Regenerative cycle（再生性循环）

  11. Phase 1 • Transient outward current, Ito K+ current • activated at –20 mV • opening for 5~10 ms

  12. Phase 2 Inward current Outward current (Ca2+ & Na+) (K+ current)

  13. Types of Ca2+ channels in cardiac cells: • (1) L-type (long-lasting) (Nowycky, 1985) • (2) T-type (transient) (Nowycky, 1985)

  14. Ca2+ channels • L-type T-type Duration of current long-lasting transient Activation kinetics slower faster Inactivation kinetics slower faster Threshold high (-35mV) Low (-60mV) cAMP/cGMP-regulated Yes No Phosphorylation-regulated Yes No Openers Bay-K-8644 - Blockers varapamil Tetramethrin nifedipine, diltiazem Ni2+ Inactivation by [Ca2+]i Yes slight Patch-clamp recording run-down relatively stable

  15. Outward current (K+ current): • (1) inward rectifier K+ current (IK1) • (2) delayed rectifier K+ current (IK)

  16. Phase 3 Inactivation of Ca2+ channel Outward K+ current dominates IK: Progressively increased IK1: Regenerative K+ Outward Current

  17. Phase 4 Na+-Ca2+ exchange Sarcolemmal Ca2+ pump SR Ca2+ pump Na+-K+ pump

  18. a, The key ion channels (and an electrogenic transporter) in cardiac cells. K+ channels (green) mediate K+ efflux from the cell; Na+ channels (purple) and Ca2+ channels (yellow) mediate Na+ and Ca2+ influx, respectively. The Na+/Ca2+ exchanger (red) is electrogenic, as it transports three Na+ ions for each Ca2+ ion across the surface membrane. b, Ionic currents and genes underlying the cardiac action potential. Top, depolarizing currents as functions of time, and their corresponding genes; centre, a ventricular action potential; bottom, repolarizing currents and their corresponding genes. From the following article: Cardiac channelopathies Eduardo Marbán Nature 415, 213-218(10 January 2002) doi:10.1038/415213a

  19. Transmembrane potentials recorded in different heart regions

  20. Transmembrane potential of autorhythmic cells and its ionic mechanisms

  21. Purkinje cells: Fast response autorhythmic cells 4

  22. Contractile cellsAutorhythmic cells Phase 4 stable potentialPhase 4 spontaneous depolarization （4期自动去极化） Resting potential Maximal repolarization potential （最大复极电位）

  23. Ionic mechanism • Phase 0~3：similar to ventricular cells • Phase 4： • (1) If – Funny current, Pacemaker current（起搏电流） • (2) Ik Decay（钾电流衰减）

  24. Characteristics of If channel • Na+, K+ • Voltage- & time-dependent Activation── Repolarized to -60mV Full activation── Hyperpolarized to -100mV Inactivation── Depolarized to -50mV • Blocked by Cs, not by TTX

  25. Sinoatrial cells

  26. 0 3 4 Sinoatrial cells: Slow response autorhythmic cells • Maximal repolarization potential -70mV • Threshold potential -40mV • Phase 0, 3, 4

  27. 0 3 4 Ionic mechanism • Phase 0: ICa (ICa,L)

  28. 0 3 4 • Phase 3: • Inactivation of L-type Ca2+ channel • Outward K+ current (Ik)

  29. Phase 4： • Ik decay Inactivated when repolarized to -60mV • ICa,T Activated when depolarized to -50mV • If

  30. The action potential of anautorhythmic cardiac cell.

  31. Electrocardiogram (ECG)（心电图） The electrocardiogram (ECG) measures changes in skin electrical voltage/potential caused by electrical currents generated by the heart

  32. The relationship between the electrocardiogram (ECG), recorded as the difference between currents at the left and right wrists, and an action potential typical of ventricular myocardial cells. Electrocardiogram (ECG)

  33. I aVR aVL V1 V2 V3 V4 V5 V6 III II aVF The standard 12 lead ECG Einthoven’s Triangle Limb leads (Bipolar) (I, II, III) Augmented limb leads (Unipolar) (aVR, aVL, aVF) Chest leads (Unipolar) (V1, V2, V3, V4, V5, V6) Willem Einthoven: Dutch physiologist. He won a 1924 Nobel Prize for his contributions to electrocardiography.

  34. Placement of electrodes in electrocardiography

  35. 0.04 sec • ECG interpretation • Measurements • Rhythm analysis • Conduction analysis • Waveform description • Comparison with previous ECG (if any) Normal ECG

  36. P wave: the sequential depolarization of the right and left atria • QRS complex: right and left ventricular depolarization • ST-T wave: ventricular repolarization • U wave: origin for this wave is not clear - but probably represents "afterdepolarizations" in the ventricles

  37. PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS complex) • QT interval: duration of ventricular depolarization and repolarization • ST segment: the time period between the end of the QRS complex and the beginning of the T wave,  during which each myocyte is in the plateau phase (phase 2) of the action potential 

  38. Normal Partial block Complete block

  39. Physiological properties of cardiac cells • Excitability • Autorhythmicity • Conductivity • Contractility Electrophysiological properties （电生理特性） Mechanical property （机械特性）

  40. Excitability（兴奋性） • Factors affecting excitability • Resting potential • Threshold potential • Status of Na+ or Ca2+ channels

  41. Hyperkalemia（高钾血症） • The QRS complexes may widen so that they merge with the T waves, resulting in a “sine wave” appearance. The ST segments disappear when the serum potassium level reaches 6 mEq/L and the T waves typically become tall and peaked at this same range. The P waves begin to flatten out and widen when a patient‘s serum potassium level reaches about 6.5 mEq/L; this effect tends to disappear when levels reach 7-9 mEq/L. Sinus arrest may occur when the serum potassium level reaches about 7.5 mEq/L, and cardiac standstill or ventricular fibrillation may occur when serum levels reach 10 to 12 mEq/L.

  42. Periodic changes in excitability

  43. Premature systole & compensatory pause (extrasystole)

  44. A 39-year-old lady presenting with frequent palpitations lasting a few months • A 39-year-old lady presents to you with frequent palpitations lasting a few months, which are not associated with dizziness, syncope or angina. She has enjoyed good health and is not on any medication or herbal medicine. She is a non-smoker and has no known diabetes, hypertension or hypercholesterolaemia. Her menses is regular and physical examination is unremarkable other than a few premature beats. This is her ECG. Answers: Ventricular premature beats are noted.

  45. Premature ventricular beats unmask the P waves

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