Electrocardiogram (ECG)

General Description

The electrocardiogram (ECG) is a non-invasive procedure used to record the heart’s electrical activity, which consists of impulses responsible for coordinating the rhythmic cardiac contractions necessary for blood pumping. The results may indicate normal cardiac function or, conversely, detect conditions that alter heart behavior.

During an electrocardiogram, cardiac activity is recorded as wavy lines representing the different phases of atrial and ventricular depolarization (activation) and repolarization (recovery). The resulting tracings, whether on paper or digitally, display the following waves:

• P wave: electrical activity during atrial depolarization, corresponding to the upper heart chambers.
• Q, R, and S waves (QRS complex): electrical impulses generated during ventricular depolarization, affecting the lower chambers and the interventricular septum.
• T wave: represents ventricular repolarization, i.e., the phase when the ventricles recover and prepare for the next beat.
• U wave: appears as a small wave in the same direction as the T wave. It is associated with ventricular repolarization, although its exact origin is unknown. It does not always appear but is more common in older individuals with low heart rates.

One of the most commonly used techniques in this procedure is electrocardiographic lead placement, where each lead provides a specific perspective of the heart’s electrical activity (voltage). This requires placing electrodes—typically twelve—at precise locations on the patient’s body.

Specialists can interpret the resulting ECG waves to detect abnormalities visually. Therefore, this test serves as a basic diagnostic procedure that often requires complementary tests to confirm the diagnosis.

When is it indicated?

An electrocardiogram is performed in patients presenting with any of the following symptoms:

• Chest pain
• Irregular heartbeats (palpitations, fluttering, skipped beats) detected during auscultation
• Rapid pulse
• Fatigue
• Weakness
• Shortness of breath
• Dizziness
• Lightheadedness
• Confusion
• Family history of cardiac diseases

Analysis of the results helps diagnose conditions such as:

• Arrhythmias (heart rate disorders):
• Tachycardia (abnormally fast heartbeats)
• Bradycardia (abnormally slow heartbeats)
• Irregular pulse
• Angina pectoris: chest pain due to reduced blood flow to the heart
• Myocardial infarction: ischemic heart disease resulting from insufficient blood supply, either recent or past
• Cardiomyopathies: conditions affecting the cardiac muscle (myocardium) that impair pumping capacity
• Heart failure: inability of the heart to pump blood efficiently
• Metabolic disturbances: imbalances of potassium or calcium affecting the heart’s electrical rhythm

How is it performed?

To perform an electrocardiogram, electrodes are placed at specific points on the body to capture cardiac signals. Electrodes are arranged in two main planes to provide a multidimensional perspective of cardiac activity:

• Frontal leads: located on the coronal plane, an imaginary vertical plane dividing the body into anterior (ventral) and posterior (dorsal) sections. Each lead records different information:
• Lead I: potential difference between the left and right arms
• Lead II: potential difference between the left leg and right arm
• Lead III: potential difference between the left leg and left arm
• aVR: electrical activity relative to the right arm
• aVL: electrical activity relative to the left arm
• aVF: electrical activity relative to the left leg
• Precordial leads: used to record activity in the horizontal plane:
• V1: evaluates the interventricular septum using an electrode at the fourth intercostal space, right parasternal line
• V2: records septal activity at the fourth intercostal space, left parasternal line
• V3: provides information from the anterior wall of the left ventricle, between V2 and V4
• V4: anterior wall of the left ventricle, fifth intercostal space at the left midclavicular line
• V5: lateral wall of the left ventricle, anterior axillary line
• V6: lateral wall of the left ventricle, midaxillary line

In Spain, electrode placement follows the International Electrotechnical Commission (IEC) guidelines:

• Limb electrodes (frontal leads):
• Right arm: R – red
• Left arm: L – yellow
• Right leg: N – black
• Left leg: F – green
• Chest electrodes (precordial leads):
• Fourth intercostal space, right parasternal line: V1/C1 – red
• Fourth intercostal space, left parasternal line: V2/C2 – yellow
• Midpoint between V2 and V4: V3/C3 – green
• Fifth intercostal space, left midclavicular line: V4/C4 – brown
• Anterior axillary line: V5/C5 – black
• Midaxillary line: V6/C6 – violet

Once placed, electrodes are secured with adhesive pads or suction cups and connected via cables to the electrocardiograph, which records the impulses as waveforms.

Risks

There are no risks associated with an electrocardiogram, as the electrodes do not emit electricity. Some patients may experience redness or a mild rash at the electrode sites, which typically resolves within a few hours.

What to expect from an electrocardiogram

To ensure proper electrode placement, the patient should expose the arms, chest, and legs or wear a hospital gown. It is recommended not to use creams or perfumes, as they may interfere with electrode adhesion and signal detection. In some cases, hair may be shaved to ensure proper contact.

During the test, which lasts two to three minutes, the patient lies supine on a stretcher, breathing normally, remaining still, and silent.

Once the necessary recordings are obtained, electrodes are removed. Depending on placement or hair presence, removal may cause slight discomfort similar to removing an adhesive bandage.

Specialists can detect abnormalities immediately upon reviewing the tracings; however, a follow-up appointment is often scheduled to analyze the results in detail and plan next steps.

Specialties in which an electrocardiogram is requested

The electrocardiogram is primarily a Cardiology test but may also be requested in other specialties to assess patient health, such as preoperative evaluations in any surgical specialty, Pediatrics, Family and community medicine, Sports medicine, or Occupational medicine.