Electrocardiography Lecture: ECG Basics

Electrocardiography Lecture 3 Tamar Aladashvili Chapter 11 (Guyton)  Electrocardiography - An ECG is a graphic representation of the sum of all the electrical activity of the heart - It is usually recorded from the surface of the body. - It can h...

Electrocardiography Lecture 3 Tamar Aladashvili Chapter 11 (Guyton)  Electrocardiography - An ECG is a graphic representation of the sum of all the electrical activity of the heart - It is usually recorded from the surface of the body. - It can help your doctor find out if you have heart muscle damage or a problem with the conduction of electrical impulses  Electrical Potentials in ECG Recording A wave of The electrical Part of the current depolarization current is reaches the surface spreads through the transmitted to of the body. heart surrounding tissues The electrocardiogram (ECG) records the electrical activity of the heart using electrodes placed on the skin. The recorded signals represent the depolarization and repolarization of cardiac muscle cells. Completely Depolarized Cardiac Muscle → Flat Line (No Wave Deflection) Fully Repolarized Cardiac Muscle → Flat Line (No Wave Deflection) Condition Electrical Activity ECG Appearance Fully Depolarized No voltage difference Flat line Fully Repolarized No voltage difference Flat line Depolarization in Voltage changes as wave Wave deflection (P wave, Progress spreads QRS complex) Repolarization in Voltage changes as heart Wave deflection (T wave, U 1. Heart Rate (HR) Determines whether the heart rate is normal (60-100 bpm), bradycardic (100 bpm). Measured by counting the R-R intervals on ECG paper. 2. Heart Rhythm Identifies normal sinus rhythm or arrhythmias such as: Atrial fibrillation (irregularly irregular rhythm) Atrial flutter (sawtooth pattern) Ventricular tachycardia or fibrillation (life-threatening) Heart blocks (AV blocks) (e.g., 1st-degree, 2nd-degree, 3rd-degree) 3. Electrical Axis of the Heart Determines the heart’s electrical axis in the frontal plane using limb leads. Left axis deviation (LAD) – Possible left ventricular hypertrophy (LVH) or conduction blocks. Right axis deviation (RAD) – Possible right ventricular hypertrophy (RVH) or pulmonary disease. 4. Cardiac Chamber Enlargement: Atrial enlargement: Right atrial enlargement (RAE) → Tall P waves in lead II (P pulmonale). Left atrial enlargement (LAE) → Bifid P waves in lead II (P mitrale). Ventricular hypertrophy: Left ventricular hypertrophy (LVH) → Increased R wave amplitude in leads like V5/V6. Right ventricular hypertrophy (RVH) → Dominant R wave in V1. 5. Ischemia and Infarction Myocardial Ischemia (Reduced blood supply): ST depression (subendocardial ischemia). T-wave inversions in leads corresponding to affected areas. Myocardial Infarction (Heart Attack): ST elevation in specific leads (STEMI). Pathologic Q waves (old infarction). 6. Conduction Abnormalities Bundle branch blocks (BBB): Right bundle branch block (RBBB) → RSR’ pattern in V1. Left bundle branch block (LBBB) → Broad QRS with deep S wave in V1. AV blocks (e.g., prolonged PR interval in 1st-degree AV block). 7. Electrolyte Imbalances Hyperkalemia → Peaked T waves and wide QRS. Hypokalemia → U waves. Hypercalcemia → Short QT interval. Hypocalcemia → Prolonged QT interval. 8. Drug Effects and Toxicity Digoxin effect → Downsloping ST depression (scooped appearance). QT prolongation → Due to drugs like antiarrhythmics, antidepressants, or electrolyte imbalances.  Time Measurement A millisecond (ms) is one-thousandth of a second (1/1000 s or 10⁻³ s). From larger to smaller units: 1 second (s) = 1,000 milliseconds (ms) 1 millisecond (ms) = 1,000 microseconds (µs) 1 microsecond (µs) = 1,000 nanoseconds (ns) Distance/Length Measurement A millimeter (mm) is one-thousandth of a meter (1/1000 m or 10⁻³ m). From larger to smaller units: 1 meter (m) = 1,000 millimeters (mm) 1 millimeter (mm) = 1,000 micrometers (µm) 1 micrometer (µm) = 1,000 nanometers (nm) Electrocardiogram paper  Horizontal Axis (Time)  Small Square: 1 mm × 1 mm, representing 0.04 seconds (40 milliseconds) at a standard paper speed of 25 mm/s.  Large Square: 5 mm × 5 mm (5 small squares), representing 0.20 seconds (200 milliseconds).  Standard Paper Speeds:25 mm/s (default): 1 second = 5 large squares.  50 mm/s (faster setting): 1 second = 10 large squares, used for detailed rhythm analysis. Vertical Axis (Voltage) 1 mm (small square) = 0.1 mV10 mm (two large squares) = 1 mV, the standard calibration for ECG machines.  Einthoven's Law and the Triangle It is an equilateral triangle formed by the three standard limb leads (I, II, III) when electrodes are placed on the right arm (RA), left arm (LA), and left leg (LL). The triangle represents the way electrical impulses travel through the heart and how they are detected in different orientations. Limb Lead Orientation: Lead I: Measures the electrical difference between RA (-) and LA (+). Lead II: Measures the electrical difference between RA (-) and LL (+). The right leg (RL) electrode in a standard 12-lead Lead ECG III:does Measures not thecontribute electrical difference to thebetween LA electrical measurements of the leads (I, II, III, aVR, aVL, aVF, or (-) and LL (+). precordial leads V1–V6). However, it plays a crucial role as the electrical reference (ground).  A standard ECG contains 12 leads. 3 Standard Limb leads (Bipolar) 3 Enhanced Limb leads (Unipolar) 6 Chest leads (unipolar)  Standard Limb Leads (Bipolar) Negative Positive Electrode Direction of Lead Electrode (-) (+) Measurement Lead I Right Arm (RA) Left Arm (LA) Right to Left Lead II Right Arm (RA) Left Leg (LL) Right to Lower Left Upper Left to Lower Lead III Left Arm (LA) Left Leg (LL) Left Record electrical differences between two electrodes. Provide information about the frontal plane of the heart. Follow Einthoven’s Triangle.  Enhanced unilateral limb leads Two limbs are connected to the negative terminal of the ECG, and the third limb is connected to the positive. aVR : Attached to the right arm. It is inverted because the current flow is opposite direction of cardiac potential. aVL : Attached to the left arm. aVF : Attached to the left leg.  Precordial or Chest leads V1: 4th intercostal space right chest. V2: 4th intercostal space in the left chest area. V3: Midway between V2 and V4. V4: 5th intercostal space at the midclavicular line. V5: Between V4 and V6 in the anterior axillary line. V6: lateral to V4 and V5 in the midaxillary line.  Chest Leads V1 and V2: The QRS complex is mostly negative because the thoracic abductions are closer to the base of the heart. V3, V4, V5 and V6: - The QRS complex is mostly positive because the thoracic abductions are closer to the apex of the heart.  ECG structures P wave : Depolarization of the atria. PR interval : The delay between the impulse from the atria to the ventricles and the AV node, decreases with increasing heart rate. Duration: 0.12-0.2 (average: 0.18) QRS complex : Ventricular depolarization.(Purkinje fibers) Duration: 0.08-0.1 ST segment: isoelectric and shows the period between ventricular depolarization and repolarization. (plateau portion) Duration: 0.32 T wave: Ventricular repolarization (usually in the same direction as the QRS). QT interval : It includes the beginning of depolarization until the end of repolarization of the ventricles. Duration: 0.4-0.43. (May be shorter depending on the heart rate).  Effects of electrodes one the wave: If the depolarizing signal from the heart is going towards the active electrode (+ ve ) it will make an upward wave. If the repolarizing signal from the heart is going towards the active electrode (+ ve ) it will make a downward wave. If the depolarizing signal from the heart is going away from the active electrode(- ve ) it will make a downward wave. If the repolarizing signal from the heart is going away from the active electrode(- ve ) it will make an upward wave. https://ecgwaves.com/ecg-qrs-complex-qrs-wave-duration-interval/  ECG structures P wave Atrial depolarization produces a " P wave" on the ECG. The impulse then travels to the AV node. The AV node, which is known for its short delay in impulses, called the AV node delay, does not cause depolarization or repolarization, so it forms an isometric line.  ECG structures QRS complex: After the impulse passes through the bundle of Hiss, then it goes to the right bundle branch. The first part that depolarizes in the ventricle is the septum, and the depolarization comes from the left bundle branch, which means that the direction of depolarization is away from the active electrode. So it creates a negative wave called the Q wave.  ECG structures T wave: This is the repolarization of the ventricles that occurs from outside to inside. Which means that it is removed from the active electrode and therefore emits a positive wave.  PR interval This is the time from the beginning of the P wave and the beginning of the QRS complex. This is the interval between the beginning of the electrical excitation of the atria and the beginning of the excitation of the ventricles. The PR interval is approximately 0.16 seconds QT interval The contraction of the ventricles lasts from the beginning of the Q wave to the end of the T wave The QT interval is the time from the beginning of the Q wave to the end of the T wave. The QT interval is approximately 0.35 seconds In the case of ventricular repolarization, repolarization begins at the epicardium (outer surface) and spreads forwards to the endocardium (inner surface), therefore, it moves away from the electrode and accordingly receives its positive deflection. On an ECG recording that is not the same as atrial repolarization.