ECG general

• Electrophysiology of the heart
• ECG calculation: Heart frequency can be calculated using the formula: 300 HF/min = 1RR x 0.2s
• Electro-mechanical properties of the heart:

  • Automaticity: Heart cells spontaneously generate action potential (impulses), with the highest frequency in the SA node
  • Excitability: The ability of heart cells to respond to impulses
  • Conductivity: The impulse propagates rapidly from the SA node to the entire conduction system and then more slowly through the working myocardium
  • Contractility: The ability to contract, applies to the working myocardium containing myofibrils (muscle fibers)
• Conduction system of the heart:

  • Supraventricular conduction system includes the SA node, Bachmann's bundle, and Internodal pathways
  • Atrio-ventricular junction includes the AV node and Bundle of His
  • Ventricular conduction system includes the Right bundle branch, Left bundle branch, Left anterior fascicles, Left posterior fascicles, and Purkinje fibers
• Cardiac action potential:

  • Phases of ventricular myocyte action potential include Phase 4 (resting potential), Phase 0 (rapid depolarization phase), Phase 1 (beginning of repolarization), Phase 2 ("plateau" phase), Phase 3 (rapid repolarisation)
• SA node action potential:

  • Phases of cardiac action potential include Phase 4 (resting potential), Phase 0 (rapid depolarization phase), Phases 1 and 2, Phase 3 (rapid repolarisation)
• Refractory period:

  • Absolute refractory period: from the beginning of phase 0 until phase 3
  • Relative refractory period: from the end of phase 3, during this phase a stronger-than-usual stimulus is required to produce another action potential
• Leads in ECG:

  • The standard 12-lead electrocardiogram represents the heart's electrical activity recorded from electrodes on the body surface
  • Different leads "view" the heart from different angles, including Bipolar limb leads, Augmented unipolar limb leads, and Unipolar chest leads
• Interpretation of ECG:

  • P-wave: atrial depolarization
  • PR interval: conduction of the impulse from atria to the ventricles
  • QRS complex: depolarization of ventricles
  • ST segment: plateau, ventricles are fully depolarized
  • T-wave: repolarization of ventricles
  • QT interval: duration of ventricular depolarization and repolarization
• Ventricular depolarisation and repolarisation:

  • Opposite depolarisation and repolarisation direction but the same electrical vector
• Electrical axis of heart:

  • Calculation from leads I, II, and III can determine Normal, Left axis deviation, or Right axis deviation
• Practice:

  • Calculate heart frequency using ECG paper speed of 25 mm/s.
  • there are 7 large blocks between RR tops.

  25 mm = 5 large blocks = 1 s

  5 mm = 1 large block = 0,2 s

  7 x 0.2 = 1.4 s 

  60 s / 1.4 s = 42.85 = 43