Electrical Activity of the Heart
Rahul's Noteblog
Notes on Cardiology
Electrical Activity of the Heart
Cardiac Conduction Channels:
• Ungated-K: always open, unless membrane reaches -94mV.
• Voltage-gated Na: fast; depolarization opens them; won't respond to second stimulus until cell repolarizes.
• Voltage-gated Ca: slow; depolarization opens them.
• Voltage-gated K: open under resting conditions; close at depolarization.
Ventricular Muscle Action Potential:
Cardiac Conduction Ion Flow:
| Sodium | Potassium | Chloride | Calcium | |
| Phase 0 | inflow | |||
| Phase 1 | inactive | outflow | inflow | |
| Phase 2 | inactive | closed/outflow | inflow | inflow |
| Phase 3 | inactive | reopening/outflow | closed | closed |
| Phase 4 | Na/K active | open/outflow |
Cardiac muscle:
• Length of refactory period = length of mechanical event; no tetany. The specialized cells of the heart possess an unstable phase 4.
Sinoatrial Nodal (Pacemaker) Action Potential:
Sinoatrial Nodal Cells:
• Pacemaker potential or prepotential.
• Unstable phase 4; Na influx.
• Phase 0: Ca spike.
• Phase 3: rapid K efflux.
• Sympathetics: intrinsic firing rate increases.
• Parasympathetics: intrinsic firing rate decreases.
Sinoatrial: pacemaker greater than atrial muscle greater than AV (slow) greater than Purkinje (fast) greater than ventricles.
Automaticity:
• Sinoatrial: 100-120/min.
• AV: 40-60/min.
• Purkinje: 30-40/min.
ECG Waves:
• P: atrial depolarization.
• QRS: ventricular depolarization and atrial repolariztion.
• T: ventricular repolarization.
• PR: AV conduction delay.
Heart Blocks:
Degree 1 Heart Block:
• Slowed AV conduction; no treatment necessary.
Degree 2 Heart Block:
• Some impulses not transmitted through AV node. Mobitz I (progressively prolonging PR interval until a QRS is dropped); Mobitz II (PR normal; QRS dropped). Treat with pacemaker.
Degree 3 Heart Block:
• No impulses conducted from atria or ventricles; atria and ventricles beat independently; no correlation between P and QRS. Treat with pacemaker.
Random Notes on Cardiac Conduction:
• Conduction velocity in atrial/ventricular fibers is 0.3-0.5 m/sec.
• Velocity in purkinje fibers is 4 m/sec.
• Absolute refractory: heart cannot conduct even if stimulus is very strong.
• Relative refractory period: heart can conduct if stimulus is strong; this results in premature contractions.
• Cardiac muscle has larger T-tubules; only in cardiac muscle are T-tubules coupled with sarcoplasmic reticulum through sarcoplasmic tubules; and this causes extra Ca to be released into the cardiac muscle sarcoplasm producing prolonged depolarization.
• Ruptured chordae tendineae cause weak connection between cardiac valves and papillary muscles, producing mitral regurgitation.
• Note that chordae tendineae are not present with aortic and pulmonary valves.
• Excess K ions lead to slowed heart rate, or even CHF.
• Excess Ca ions lead to a faster heart rate, or spastic contractions.
Additional Reading:
Basic Cardiology
1. Electrical Activity of the Heart
2. Heart Muscle Mechanics
3. Heart Sounds and Murmurs
4. Cardiac Conduction Diagram
5. Blood Pressures in Cardiac Chambers
6. What is Pulsus Paradoxus?
Cardiology Videos
1. Video of Cardiology Examination in a Clinical Setting
Medical Images
Useful Medical Images & Diagrams (link opens in a new window)
Related Topics
1. Thorax Anatomy
2. Vascular Disorders
3. Heart Disorders
4. Histology of the Cardiovascular System
5. Jugular Venous Distention Workup
6. ER Chest Pain Workup
7. Cardiac Examination for Internal Medicine
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