Electrocardiography (ECG) Basics

Electrocardiography

• Electrocardiography is a graphic representation of the sum of all the heart's electrical activity
• ECG's are typically recorded from the body's surface
• ECG's can help find heart muscle damage and problems with the conduction of electrical impulses
• Electrodes are placed on the skin to record the heart's electrical activity
• The ECG signals represent the depolarization and repolarization of the cardiac muscle cells

Electrical Potentials in ECG Recording

• A wave of depolarization spreads through the heart
• Electrical current is transmitted to surrounding tissues
• Part of the current reaches the body's surface

Time Measurement

• A millisecond (ms) is one-thousandth of a second, equivalent to 1/1000 s or 10^-3 s
• 1 second (s) equals 1,000 milliseconds (ms)
• 1 millisecond (ms) equals 1,000 microseconds (µs)
• 1 microsecond (µs) equals 1,000 nanoseconds (ns)

Distance/Length Measurement

• A millimeter (mm) is one-thousandth of a meter, equivalent to 1/1000 m or 10^-3 m
• 1 meter (m) equals 1,000 millimeters (mm)
• 1 millimeter (mm) equals 1,000 micrometers (μm)
• 1 micrometer (μm) equals 1,000 nanometers (nm)

ECG Paper Measurements

• Paper speed is typically 25mm/sec
• One small square vertically equals 1mm ie 0.1mV
• One large square vertically equal 5mm ie 0.5mV
• Two large squares vertically equal 1mV
• One small square horizontally is 0.04 sec or 40 msec
• One large square horizontally is 0.2 sec or 200 msec
• Five large squares horizontally equal 1 sec or 1000 msec

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) where 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 mV
• 10 mm (two large squares) = 1 mV, which is the standard calibration for ECG machines

Einthoven's Law and the Triangle

• The three standard limb leads (I, II, III) form an equilateral triangle 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 ECG measures neither the electrical nor vectorial measurements of the leads (I, II, III, aVR, aVL, aVF, or precordial leads V1-V6)
• The RL electrode plays a crucial role as the electrical reference (ground)

Standard ECG Leads

• A standard ECG contains 12 leads
• These can be broken down into
  • 3 Standard Limb leads (Bipolar)
  • 3 Enhanced Limb leads (Unipolar)
  • 6 Chest leads (unipolar)

Standard Limb Leads (Bipolar)

• Standard Limb Leads record electrical differences between two electrodes
• These provide information about the frontal plane of the heart
• These follow Einthoven's Triangle
• Lead I measures from the Right Arm(RA) Negative electrode to the Left Arm (LA) positive electrode, travels Right to Left
• Lead II measures from the Right Arm(RA) Negative electrode to the Left Leg (LL) positive electrode, travels Right to Lower Left
• Lead III measures from the Left Arm (LA) Negative electrode to the Left Leg (LL) positive electrode, travels Upper Left to Lower Left

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 and 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: Delay between the impulse from the atria to the ventricles and the AV node, decreasing 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: 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 on the Wave

• If the depolarizing signal from the heart goes towards the active electrode (+ ve), it will make an upward wave
• If the repolarizing signal from the heart goes towards the active electrode (+ ve), it will make a downward wave
• If the depolarizing signal from the heart goes away from the active electrode (- ve), it will make a downward wave
• If the repolarizing signal from the heart goes away from the active electrode (- ve), it will make an upward wave

ECG wave components

• Atrial depolarization creates the "P wave" on the ECG, the impulse then travels to the AV node
• The AV node, known for its short delay in impulses, is called the AV node delay, it does not cause depolarization or repolarization, so it forms an isometric line
• After the impulse passes through the bundle of His, 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
• This creates a negative wave called the Q wave
• Ventricular 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
• This is not the same as atrial repolarization

ECG Analysis

Heart Rate (HR) Determination

• The heart rate is either normal (60-100 bpm), bradycardic (<60 bpm), or tachycardic (>100 bpm)
• It is measured by counting the R-R intervals on ECG paper

Heart Rhythm Identification

• 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)

Electrical Axis of the Heart Assessment

• 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

Cardiac Chamber Enlargement Evaluation

• 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

Ischemia and Infarction Detection

• Myocardial Ischemia (Reduced blood supply): ST depression (subendocardial ischemia) or T-wave inversions in leads corresponding to affected areas
• Myocardial Infarction (Heart Attack): ST elevation in specific leads (STEMI) or Pathologic Q waves (old infarction)

Conduction Abnormalities Identification

• 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)

Electrolyte Imbalances Analysis

• Hyperkalemia → Peaked T waves and wide QRS
• Hypokalemia → U waves
• Hypercalcemia → Short QT interval
• Hypocalcemia → Prolonged QT interval

Drug Effects and Toxicity Detection

• Digoxin effect → Downsloping ST depression (scooped appearance)
• QT prolongation → Due to drugs like antiarrhythmics, antidepressants, or electrolyte imbalances