Electrocardiography Basics

Basic ECG Principles

• An ECG records the heart's electrical activity from different angles to identify pathology.
• Electrodes (conductive pads) are placed on the patient to record electrical activity.
• The ECG leads provide a graphical representation of the heart's electrical activity.
• A 12-lead ECG produces 12 separate graphs, using only 10 physical electrodes.

ECG Cycle Components

P Waves

• Represent atrial depolarization (contraction).
• In healthy individuals, a P wave should precede each QRS complex.

PR Interval

• Starts at the beginning of the P wave and ends at the beginning of the Q wave.
• Represents the time for electrical activity to move between the atria and ventricles.

QRS Complex

• Represents the depolarization (contraction) of the ventricles.
• Consists of three waves: Q, R, and S.

ST Segment

• Isoelectric line between depolarization and repolarization of the ventricles

T Wave

• Represents ventricular repolarization.
• Appears as a small, upward deflection after the QRS complex.

QT Interval

• Begins at the start of the QRS complex and finishes at the end of the T wave.
• Represents the time taken for the ventricles to depolarize and then repolarize.

Initial Checks Before Interpretation

• Confirm the patient's name and date of birth match the ECG details.
• Check the date and time the ECG was performed.
• Check the calibration of the ECG.
• Understand the clinical context (e.g., patient symptoms like chest pain, breathlessness).

Heart Rate Calculation

• Normal heart rate: 60-100 beats per minute.
• Tachycardia: Heart rate > 100 beats per minute.
• Bradycardia: Heart rate < 60 beats per minute.

Heart Rate Calculation with RR Interval (Regular rhythm)

• Count the number of large squares in one RR interval.
• Divide 300 by the number of large squares to calculate the heart rate.
• Example: 7 large squares, so 300 / 7 = 48 beats per minute.

Heart Rate Calculation - Irregular Rhythm

• Count the number of QRS complexes on a 10-second ECG strip.
• Multiply the number of QRS complexes by 6 to get the heart rate per minute.
• Example: 11 QRS complexes, so 11 * 6 = 66 beats per minute.

Heart Rate Rhythms

• Regular rhythms have consistent intervals between heartbeats.
• Irregular rhythms have variable intervals between heartbeats.
• Regularly irregular rhythms have a recurrent pattern of irregularity.
• Irregularly irregular rhythms are completely disorganized (e.g., atrial fibrillation).

Regularly Irregular Rhythm

• Recurrent pattern of irregularity, seen in AV blocks

Irregularly Irregular Rythm

• Very disorganized with variable RR intervals

Cardiac Axis

• Represents the overall direction of electrical activity through the heart.
• Normal cardiac axis: -30° to +90°.

Cardiac Axis - Positive Deflection

• Net direction of electrical activity towards a lead is a positive deflection.

Cardiac Axis - Negative Deflection

• Whenever electrical activity goes away from an ECG lead

Normal Cardiac Axis

• Lies between -30° and +90°.
• Positive deflection in leads one and two, with the most positive in lead two.
• Lead 3 may be slightly positive, isoelectric, or slightly negative.
• Most negative deflection in AVR.

Right Axis Deviation

• Depolarization distorted to the right (+90° to +180°).
• Common cause: right ventricular hypertrophy.
• Lead 1 becomes negative, leads AVF and Lead 3 become more positive.
• Associated with pulmonary hypertension.
• Can be normal in very tall individuals.

Left Axis Deviation

• Depolarization distorted to the left (-30° to -90°).
• Lead 1 becomes positive, lead three negative.
• Lead two becomes negative
• Usually caused by left ventricular hypertrophy or conduction abnormalities.

P Wave Assessment

• Are P waves present?
• Is each P wave followed by a QRS complex?
• Do the P waves look normal in duration, direction, and shape?
• If P waves are absent, is there any atrial activity: flutter waves (sawtooth baseline), fibrillatory waves (chaotic baseline), or flatline?

PR Interval Assessment

• Normal duration: 120-200 milliseconds (3-5 small squares).
• Prolonged PR interval suggests AV block.

First-Degree AV Block

• Consistent prolongation of the PR interval (>200 milliseconds).
• Every P wave is followed by a QRS complex.
• Often an incidental finding; patients usually asymptomatic.

Second-Degree AV Block Type 1 (Mobitz Type 1 or Wenckebach)

• Progressive prolongation of the PR interval until a QRS complex is dropped.
• AV nodal conduction resumes, and the sequence repeats.
• Usually benign, rarely causes hemodynamic compromise, and typically requires no intervention if asymptomatic.

Second-Degree AV Block Type 2 (Mobitz Type 2)

• Consistent PR interval duration with intermittently dropped QRS complexes.
• Follows a repeating cycle (e.g., 3:1 or 4:1 block).
• Always pathological; can occur at the bundle of His (20%) or bundle branches (80%).
• Patients at risk of progressing to complete AV block.

Third-Degree (Complete) AV Block

• No electrical communication between the atria and ventricles; they function independently.

Shortened PR Interval

• P wave originates closer to the AV node.
• Atrial impulse uses a faster shortcut (accessory pathway).

Wolf-Parkinson-White (WPW) Pattern

• Accessory pathway leads to premature stimulation of the ventricles (preexcitation).
• Shortened PR interval, delta wave, and widened QRS complex.
• Delta wave is a slurred upstroke of the QRS.
• Patients at risk of developing tachyarrhythmias.

QRS Complex Assessment

• Width: Normal QRS complex should be <0.12 seconds (3 small squares).
• Height: Small complexes are <5mm in limb leads or <10mm in chest leads. Tall complexes imply ventricular hypertrophy.
• Morphology: Look for delta waves, broad QRS complexes (abnormal depolarization sequence).

Broad QRS Complexes

• Occur with abnormal depolarization sequences.
• Examples: Ventricular ectopic beats, bundle branch blocks.

Bundle Branch Block ECG Features

• Hallmark feature: Broad QRS complexes.
• William Morrow Pneumonic

Right Bundle Branch Block (RBBB)

• rsr’ pattern (M shape) in V1.
• Broad S wave (W shape) in V6.

Left Bundle Branch Block (LBBB)

• Deep S wave (W) in V1 (may be notched).
• Broad, M-shaped R wave in V6.

ST Segment

• Part of the ECG between the end of the S wave and the start of the T wave.
• Normally an isoelectric line.
• Abnormalities (elevation or depression) indicate ischemia or infarction.

ST Elevation

• Significant when >1 mm in two or more contiguous limb leads.
• Significant when >2 mm in two or more chest leads.
• Most commonly caused by acute full-thickness myocardial infarction (STEMI).
• High takeoff / Benign early repolarization is a normal variant

ST Depression

• 0.5 mm or greater in two or more contiguous leads indicates myocardial ischemia.
• May be seen in NSTEMI.

Anatomical Territories

• Knowing which leads represent each heart region allows localization of pathology.
• Example: ST elevation in leads V3 and V4 suggests anterior myocardial infarction.

T Waves

• Represent repolarization of the ventricles.
• Tall T waves can be associated with hyperkalemia or a hyperacute STEMI.
• T-wave inversion can represent normal variation or underlying pathology.
• Isolated T-wave inversion in AVR and V1 is normal.
• Pathological T-wave inversion is often a nonspecific sign but can indicate myocardial ischemia or myocarditis.

U Waves

• 0.5 mm deflection after the T wave, best visualized in V2 or V3.

• Seen in electrolyte imbalances, hypothermia, and secondary to antiarrhythmic therapy (e.g., digoxin, amiodarone).

Documenting ECG Interpretation

• Patient details, date and time the ECG was performed, your interpretation, and overall impression/plan.

Case Study Key Points

Rate

• 102 beats per minute.

Rhythm

• Irregularly irregular.

Cardiac Axis

• Normal.

P Waves

• Absent

QRS

• Normal shape and width

ST Segment

• Normal

T waves

• Normal

Indications

• Atrial Fibrillation