2: Electrocardiogram

Electrocardiogram Lecture 02

• The electrocardiogram (ECG) measures potential differences between recording electrodes generated by electrical currents spreading from the heart throughout the body during depolarization and repolarization.
• Electrodes are placed on the body in standardized locations.

Learning Objectives

• Name different ECG waves, segments, and intervals, describing their representation of cardiac electrical activity.
• Describe normal electrode placement for leads I, II, III, aVL, aVR, and aVF, and their representation within the axial reference system.
• Explain how precordial leads differ from limb leads.
• Determine the mean electrical axis from standard limb leads.

Learning Resources

• Klabunde, Cardiovascular Physiology Concepts, 3e, Chapter 3 (pages 46-56)
• Klabunde RE. Cardiac electrophysiology: normal and ischemic ionic currents and the ECG. Adv Physiol Educ 41:29-37, 2017. (PDF copy available on Canvas, Week 1)
• ECG instruction on cvphysiology.com; further resources available on slides

Basic Electrocardiogram (ECG)

• The ECG, a recording of the heart's electrical activity, provides information about heart rhythm, conduction, heart injury, and overall heart muscle mass.

What Does the ECG Measure?

• The ECG measures electrical potential differences between recording electrodes as electrical currents spread from the heart throughout the body, during both depolarization and repolarization.

Clinical Value of ECG

• Different ECG leads (views) provide information about electrical activity in various heart regions.
• ECG primarily shows rhythm (rate and synchrony), conduction, injury currents, and muscle mass (like ventricular hypertrophy).

Volume Conductor Principles (Vectors)

• Electrical depolarization and repolarization spread through the heart via specialized conduction pathways, reflected in electrical signals sensed by ECG electrodes.
• ECG signal amplitude and direction are determined by the direction and magnitude of the mean electrical vectors relative to the electrode.
• Waves traveling toward a positive electrode yield positive voltage; away, negative. Perpendicular waves register no net voltage, while magnitude relates to tissue mass.

Sequence of Ventricular Depolarization

• The instantaneous voltage results from the direction and magnitude of the mean electrical vector relative to a specific lead axis.
• The sequence of ventricular depolarization takes approximately 100 milliseconds (QRS duration)

The QRS Complex and Mean Electrical Axis

• The QRS complex's shape depends on the sequence and timing of ventricular depolarization, abnormal conduction, QRS duration, and ventricular mass (e.g., hypertrophy).
• Mean electrical axis is the summation vector of the entire sequence of ventricular depolarization.

Ventricular Repolarization - T Wave

• Ventricular repolarization reflects as a T wave after the QRS complex.
• Sub-epicardial cells exhibit faster repolarization than sub-endocardial cells. This directional movement usually produces a positive T wave.

ECG Tracing - Definitions

• Key ECG definitions include P-wave (atrial depolarization, hidden in QRS), PR interval (atrial depolarization + AV delay), QRS (ventricular depolarization), QT interval (from initial depolarization until complete repolarization), ST segment (isoelectric), and T wave (ventricular repolarization).

QRS Morphology: Naming Convention

• QRS complexes may not always have Q, R, and S components. The first positive deflection is the R wave; any subsequent negative deflection is an S wave; the first negative deflection is a Q wave; a negative deflection without an R wave is a Q wave or QS complex, a second positive deflection is R'.

ECG Tracing - Normal Wave Durations and Intervals

• Normal standard durations and intervals, including P wave, PR interval, QRS duration, and QTc interval are defined, and the units of measure (mm, s). This information also includes important guidelines on recording speed.

Abnormally Long PR Intervals

• Prolonged PR intervals (e.g., >0.20 seconds) indicate impaired conduction within the AV nodal region, often termed first-degree AV block. Causes include excessive vagal stimulation, β-adrenergic receptor blockade, or AV nodal ischemia/disease.

Altered Shape and Increased Duration of the QRS Complex

• Altered QRS shape and increased duration suggest altered conduction within the ventricles. Examples include bundle branch blocks, lengthening conduction pathways, or ectopic beats.

Heart Rate Determination

• Heart rate is calculated from the ECG by measuring the number of small boxes per minute between consecutive R waves (25 mm/sec). Normal adult rates range between 60-100 beats per minute.

Sequence of ECG Interpretation

• Essential interpretation steps include rhythm, rate, intervals (PR, QRS, QT), mean electrical axis (QRS), P-wave abnormalities, QRS abnormalities, and ST-segment or T-wave abnormalities.

12-lead ECG

• Bipolar standard (leads I, II, III) and unipolar augmented leads (aVL, aVR, aVF) are used to view the heart from different angles on an ECG. Unipolar chest leads (V1-V6) provide complementary information.

Axial Reference System

• Understanding the axial reference system involves knowing the electrode placement on the body corresponding to specific angles and degrees to visualize the heart's electrical activity from different perspectives.

Estimating the Mean Electrical Axis

• Methods for determining the mean electrical axis involve identifying the lead that is equally biphasic and calculating the 90-degree angle from that lead in the direction of the lead with the greatest net positive QRS amplitude.

Normal Sinus Rhythm (NSR)

• Normal sinus rhythm (NSR) is characterized by a heart rate within 60-100 bpm, controlled by the SA node. Key characteristics include normal P-waves that precede QRS complexes, and a regular rhythm with appropriate PR, QRS, and QT intervals.

Summary of ECG concepts

• Electrical depolarization and repolarization spread through the heart via specialized conduction pathways.
• ECG measures time-dependent electrical potentials (dipoles).
• Instantaneous mean electrical vectors reflect the direction and amplitude of these electrical dipoles.
• The mean electrical axis is the average vector during depolarization.

Questions and Answers

• These are detailed questions and answers on ECG topics. These contain specific information regarding the analysis and interpretation of ECGs.

This quiz covers the fundamentals of electrocardiograms (ECGs), including the various waves, segments, and intervals representing cardiac electrical activity. You will also learn about standard electrode placements and their significance in interpreting the ECG. Master the difference between precordial and limb leads while determining the mean electrical axis from standard limb leads.