14.4: Electrocardiogram Fundamentals

Introduction

An electrocardiogram (ECG) is a diagnostic tool for identifying cardiac conditions such as arrhythmias, conduction abnormalities, and myocardial ischemia.

Definition

An electrocardiogram (ECG) visualizes the heart's electrical activity by tracing the electrical movement associated with each heartbeat on a graph or monitor. As the heart beats, an electrical wave passes through it, correlating with the cardiac cycle events.

Parts of an ECG

An ECG utilizes electrodes on the skin to detect electrical currents from the depolarization and repolarization within cardiac muscle cells.

Electrodes are physical patches placed on the skin of the chest and limbs to capture electrical activity from different angles.

Leads are the electrical views of the heart generated by the electrodes.

Standard 12-lead ECG

A standard 12-lead ECG provides twelve views of the heart's electrical activity, offering insights into various cardiac conditions. Each lead provides a unique perspective, identifying specific areas affected by disease. Additional leads, like 15-lead and 18-lead ECGs, include extra chest leads for better detecting right ventricular and posterior myocardial infarctions.

Lead Placement

Proper lead placement is crucial for an accurate ECG. A standard 12-lead ECG uses ten electrodes: four limbs and six precordial (chest) electrodes.

Limb Electrode Placement

The four limb electrodes minimize interference from muscle movement:

1. Right Arm (RA): Inner aspect of the right wrist or upper arm.
2. Left Arm (LA): Inner aspect of the left wrist or upper arm.
3. Right Leg (RL): Right ankle or lower leg.
4. Left Leg (LL): Left ankle or lower leg.

These electrodes form six leads:

1. Lead I: Connects configuration between right and left arm.
2. Lead II: Connects configuration between right arm and left leg.
3. Lead III: Connects configuration between left arm and left leg.
4. aVR: Augmented Voltage Right arm.
5. aVL: Augmented Voltage Left arm.
6. aVF: Augmented Voltage Foot (left leg).

Chest Electrode Placement

The six precordial electrodes provide detailed heart views from the horizontal plane:

V1 is positioned at the fourth intercostal space along the right sternal border.

V2 is situated at the fourth intercostal space along the left sternal border.

V3 is located precisely in the middle of V2 and V4.

V4 is found in the fifth intercostal space and aligns with the left midclavicular line.

V5 matches the level of V4 but is positioned along the left anterior axillary line.

Lastly, V6 is on the same level as V4 and V5, positioned in the left midaxillary line.

Normal Waveform

A wave is a positive or negative deflection from a baseline that indicates a specific electrical event. The waves on an ECG are P, Q, R, S, T, and U waves.

Interval refers to the duration between two specific events on an ECG reading. Commonly measured intervals in an ECG comprise the PR interval, the duration of the QRS complex (often referred to as the QRS interval), the QT interval, and the interval between R waves (RR interval).

Segment refers to the distance between two points on an ECG, ideally located at the baseline amplitude, meaning they are neither above (positive) nor below (negative) this level. The segments found on an ECG comprise the PR segment, ST segment, and TP segment.

Complexity is the combination of multiple waves grouped. The only primary complex on an ECG is the QRS complex.

There is only one point on an ECG, the J point, where the QRS complex ends and the ST segment begins.

Representation of cardiac cycle events

The ECG waveform components represent a part of the cardiac cycle:

1. P wave: Atrial depolarization.
2. PR interval: Time from atrial depolarization to ventricular depolarization.
3. QRS complex: Ventricular depolarization.
4. ST segment: Early phase of ventricular repolarization.
5. T wave: Ventricular repolarization.
6. QT interval: Total time for ventricular depolarization and repolarization.
7. U wave: Might represent repolarization of the Purkinje fibers, often invisible.

Clinical Significance

Each component provides essential information about the heart's function and can indicate abnormalities when durations or shapes deviate from the normal.

An electrocardiogram, or ECG, uses a standard 12-lead with four-limb electrodes placed on the inner wrists and ankles, creating six limb leads: I, II, III, aVR, aVL, and aVF.

Six precordial chest electrodes include:

V1 at the fourth intercostal space on the right sternal border, V2 at the left sternal border on the fourth intercostal space, V4 at the fifth intercostal space on the left midclavicular line, V3 between V2 and V4, V5 at the left anterior axillary line, and V6 at the left midaxillary line.

The ECG waveform includes the P, Q, R, S, T, and U waves.

The P wave indicates atrial depolarization.

The PR interval measures the time from atrial to ventricular depolarization.

The QRS complex shows ventricular depolarization.

The ST segment reflects early ventricular repolarization

The T wave shows ventricular repolarization.

The QT interval shows total ventricular activity.

Finally, the U wave represents Purkinje fiber repolarization.