Lecture 6.1 - Basics of ECG

ECG Waves

• T wave: ventricular repolarization, with action potential durations shorter in the anterobasal region
• Negative charge moving towards the negative electrode gives a positive deflection after a short period of time
• U wave may be present due to delayed repolarization of papillary muscles and chordinae tendinae

ECG Waves and Cardiac Cycle

• PR interval: conduction of AV node
• Conduction delay/block elongated: links to PR interval
• Widening of QRS interval: ventricles are delaying depolarization
• ST segment: should always be isoelectric (conditions like MI could stop the straight line showing)
• J point: used to determine if the ST segment is isoelectric or elevated
• QT interval: indicates the entire depolarization and repolarization of ventricles

ECG and Electrodes

• ECG can be recorded by limb electrodes or including chest electrodes
• 12 lead ECG: 12 views of the heart
• Chest leads: 6 electrodes giving 6 tracings/views (horizontal plane)
• Unipolar leads: only have positive electrodes
• Limb leads: 4 electrodes giving 4 tracings/views (frontal plane)

Impulse Conducting System of the Heart

• Cardiac cycle relies on wave of excitation spreading across the myocardium in a sequential process
• Depolarizing (+ve) current is generated in SA node
• Spreads to atrium and then to the AV node
• From the AV node, the +ve current moves to bundle branches depolarizing the interventricular septum
• The +ve current then moves to the purkinje fibres depolarizing the ventricles

ECG Graphical Representation

• Captures the electrical activity of the heart (movement of currents)
• Graphical representation of electrical activity of the heart
• The electrodes placed in the body capture movement of ions and record it
• +ve current moving towards +ve electrode will give a +ve deflection
• +ve current moving away from +ve electrode will give a -ve deflection
• +ve current moving perpendicular to the electrodes will give isoelectric line

Normal ECG

• Lead II: most commonly used to produce the sinus rhythm tracing (rhythm strip)
• P wave: formed by atrial depolarization
• P-R segment: 0.1s delay in conduction in AV node leading to P-R segment
• Q wave: septal depolarization
• R wave: ventricular (apex) depolarization
• S wave: ventricular wall depolarization
• QRS complex: ventricular depolarization
• S-T segment: entire ventricular myocardium is depolarized and is not repolarized yet

Limb Leads

• Three bipolar limb leads: Lead I, Lead II, and Lead III
• Lead I: LA (+) to RA (-)
• Lead II: LL (+) to RA (-)
• Lead III: LL (+) to LA (-)
• Leads I, II, and III can be represented schematically in terms of a triangle, called Einthoven's triangle
• The area of heart covered by the leads:
  • Lead I: high lateral wall of left ventricle
  • Lead II and III: inferior portion of right and left ventricle

Unipolar Limb Leads

• The abbreviation 'a' refers to augmented; V to voltage/vector; and R, L, and F to right arm, left arm, and left foot (leg) respectively
• The area of heart covered by:
  • aVR: right ventricle and basal septum
  • aVL: high lateral wall of left ventricle
  • aVF: inferior wall of the heart

Precordial Chest Leads

• In horizontal/transverse plane
• Placement: V1 to V6
• The area of heart covered by:
  • V1-V2: right ventricle
  • V2-V3: basal septum
  • V2-V4: anterior wall of the heart
  • V5-V6: lateral wall of the heart
• The ECG waves:
  • R wave increases in size from V1-V6
  • S wave decreases in size from V1-V6

This quiz covers the different waves and intervals in an electrocardiogram (ECG), including the T wave, PR interval, and QRS interval, and their relation to the cardiac cycle.