Electrocardiogram (ECG)

Despite the continuous renewal of technologies used to perform medical diagnostics, electrocardiogram (ECG), available since early last century, still it has a central role in the investigation of various cardiac diseases.

The ECG is an important interpretation of cardiac rhythm and to detect heart ischemia further examination. The ECG is also valuable in evaluating other types of cardiac abnormalities, including heart valve disease, cardiomyopathy, pericarditis and cardiac sequelae of hypertension.

In this text we will try to explain the ECG to the general public. Our goal, obviously, is not to teach anyone to interpret an electrocardiogram, or exhaust the subject, even because it is too complex to be addressed in a single text.


What is an electrocardiogram?

The electrocardiogram is a test that detects the electrical activity of the heart. We can say that our heart is an organ driven by electricity. Each heartbeat, each contraction of the heart muscle, every movement of the heart valves are commanded by tiny electrical impulses generated by the heart itself (I’ll explain more details below).

Thanks to the ECG can identify normal patterns of transmission and generation of these electrical impulses. Abnormalities in cardiac electrical activity are clear signs that there are problems with your heart.

The electrocardiogram is best placed to assess cardiac arrhythmia and for the initial investigation of ischemic cardiac examination.

Does the electrocardiogram is used to treat disease?

No. The ECG is just a test. There is no illness or symptoms. In the same way a patient with pneumonia does not improve when chest x-ray, the patient with heart problems suffers no alteration when the electrocardiogram.

How electrocardiogram performed?

The resting electrocardiogram is done with the patient lying down and bare trunk. Ideally, the patient has not made any effort in the last 10 minutes, or have smoked in the last 30 minutes preceding the test.

Four electrodes are set at six members and fixed to the chest through adhesives, as in the illustration next. Usually it used some gel between each electrode and the skin to increase electrical conduction.

In some cases, the adhesive 6 with electrodes attached to the chest are replaced by rubber bulb with a metal base. This metal base is fixed on the skin once we press the rubber bulb, ie, a suction of air leaving the skin attached to the metal base is made.

After proper placement of the electrodes on the patient, they are connected to the machine that will make the reading of electrical activity of the heart.

Do not panic, there is no risk that you receive a shock during the test. The electrocardiogram presents no health risk; the worst that can happen is that you have a slight allergy in place of adhesives.

If the patient has a lot of chest hair, it may be necessary to shave it before, so that the electrodes can be fixed.

The test is very fast. A ready time, the result comes out in seconds. The machine picks up the heart’s electrical signals and prints a layout on graph paper.

Electrical activity of the heart

To understand a little electrocardiogram results, we need to first know how the generation occurs and propagacion of electrical impulses in the heart. The explanation below may seem confusing, but it is important to understand concepts such as ‘sinusual pace”, “alterations in ventricular repolarization”, “bundle branch block” often reported in the awards of the ECG.

Electrical stimulation is born in their hearts, in a region called sinus node, located at the apex of the left atrium. The sinus node continuously and regularly produces electrical impulses that propagate throughout the heart, leading to the contraction of the heart muscles.

The electrical impulses to be distributed throughout the heart muscle to induce influx of calcium ions into cells of the heart, a process called electrical depolarization. Depolarization stimulates muscle contraction. After the contraction, large amounts of potassium ion out of cells, in a process called repolarization, muscle cells preparing for a new depolarization. While there is no repolarization, the muscle cell does not shrink back, even receiving electrical stimulation.

The normal electrical activity arises in the sinus node, depolarized the right atrium and the left atrium after first. After going through the courts, the electrical impulse reaches the atrioventricular node, the division between the atria and ventricle. At this time, the momentum suffers a small delay, used to the atria contract before the ventricles. In the atrioventricular node, after waiting a few milliseconds, the electrical pulse is transmitted to both ventricles, so that the cells to depolarize, causing cardiac contraction and pumping of blood through the heart. The electrical impulse takes 0.19 seconds to go all the heart.

Basic ECG tracing

Let’s talk just the basics, trying to address what most appears in awards of electrocardiograms.
ECG tracing

Accompany the reading with the figure alongside. Electrocardiogram tracing basically it consists of five elements: P wave, PR interval, QRS complex, ST-T wave Namely:

  • The P wave is the line that corresponds to the depolarization of the atria (contraction of the atria).
  • The PR interval is the time between the start of the depolarization of the atria to the ventricles.
  • The QRS complex is the depolarization of the ventricles (contraction of the ventricles).
  • The ST segment is the time between the end of the depolarization and the start of repolarization of the ventricles.
  • The T wave is the repolarization of the ventricles, which become eligible for further contraction.

Each heartbeat consists of a P wave, a QRS complex and T wave

Note 1: The repolarization of the atria occurs at the same time the depolarization of the ventricles, why not in the ECG, being concealed by the QRS complex.

Note 2: the QRS complex can have various appearances depending shunt that is displayed.

ECG leads

All electrical impulse that journey is captured and interpreted by the electrocardiogram tracings through. The various positions of the electrodes are used to capture different angles heart, as though they were multiple cameras turned toward each of the parts of the body.

The normal 12-lead ECG has, which are as 12 different angles simultaneously accompanying the spread of electrical activity. These 12-lead cover much of cardiac tissue. They are called D1, D2, D3, aVR, aVL, aVF, V1, V2, V3, V4, V5 and V6.

Examples: the bottom wall of the ventricle can be evaluated by the D2, D3 and aVF; the anterior wall by V1 to V4 and high lateral wall D1 and aVL. Therefore, an alteration of the electrical conduction that is repeated in D2, D3 and aVF, for example, indicates a problem in the lower region of the ventricle.


It is impossible to explain all the possible impairment of an ECG. However, we can tell what the normal values ​​and the most common alterations are.

A normal electrocardiogram presents the following information:

  • Heart rate between 60-100 beats per minute.
  • P wave present, indicating sinus rhythm. Normal P wave usually less than 0.12 seconds duration.
  • PR interval lasts between 0.12 and 0.20 seconds.
  • QRS duration is between 0.06 and 0.10 seconds.
  • QRS duration is between 0.06 and 0.10 seconds.
  • Normal electrical shaft between -30° and 90°.

Common ECG changes

1. Bundle Branch Block

Left bundle branch block (LBBB) means that electrical conduction is committed to the nerve branch which leads the electrical impulse into the left ventricle.

The left bundle branch splits into left and left anterior posterior branch. Therefore, if only part of the branch were committed, it is also possible diagnoses left anterior branch block (LAHB) or left rear heimbloqueo (HBPE).

Right bundle branch block (BRD) means that electrical conduction is committed to driving industry electrical pulse to the right ventricle. The right branch does not branch, so there is only one type of BRD.

Left anterior branch block (LAHB) + Right bundle branch block (BRD) is a situation that means that the transmission of electrical impulses to the ventricle is being made only by the middle of the left branch (just to the left posterior branch). This is a patient who is about to lose electrical conduction to the ventricles.

Bundle branch block are common in patients with ischemic heart disease. Usually they occur in people who have had heart attacks and / or have heart failure.

2. Forward the electrical shaft

The normal electrical axis varies from -30° to 90°.

If shaft 90 were from -30 say that there is a turning shaft to the left. The main causes are the BRD, left ventricular hypertrophy, pulmonary emphysema, Wolff-Parkinson-White syndrome and previous infarction. The detour to the left may also occur in healthy people.

If the electrical axis were between 90 and 180, there is a turn to the right axis. The main causes are the BRI, prior infarction and right ventricular hypertrophy. And the deviation to the left, the deviation to the right can also occur in people without heart disease.

3. Sinus Arrhythmia

Although the term “arrhythmia” can frighten, sinus arrhythmia is a benign condition that occurs often in young people. It is usually a heart rhythm disorder caused by breathing. As sinus, it indicates that despite having an irregular rhythm, the electrical impulse is being properly generated by the sinus node. It is a condition that usually disappears with time.

4. Extrasystoles

Heartbeat beats are isolated out of rhythm. In these cases the heart beats regularly, but suddenly a heartbeat isolated unexpected arises. The extrasístole is called supraventricular if the focus of this abnormal heartbeat arises somewhere in the atrium (outside the sinus node) and if the abnormal ventricular extrasystole focus arises somewhere in the ventricle.

Isolated extrasystoles often have no clinical significance. If they were frequent, they can cause feelings of palpitation. In these cases, the cause should be investigated. 5. Alterations in ventricular repolarization

Alteration of ventricular repolarization is a relatively common finding. They are alterations in the T wave of the electrocardiogram and can be present in the case of hypertension, aortic valve stenosis or cardiac ischemia.

However, when the award is described as nonspecific changes in ventricular repolarization, usually the box has no clinical significance. The T wave abnormalities suggesting heart disease have a characteristic appearance to distinguish them from non-specific alterations, without clinical value. 6. Atrial fibrillation

The atrial fibrillation (AF) is a common arrhythmia, especially in the elderly. AF is a non-sinus rhythm, where a chaotic generation of electrical stimuli occur throughout the atrium which makes them get not contract. The atrium is shaking, as if in convulsion. As the atrioventricular node exists, those chaotic impulses are aborted before reaching the ventricle. Thus, the patient has no P wave, the heartbeat is irregular, but is normal QRS. 7. Left ventricular hypertrophy (LVH)

Also known as left ventricular overload, LVH is an increase in muscle mass of the left ventricle caused by stress on the heart to pump blood in patients with arterial hypertension (read: high blood pressure).

The left ventricular hypertrophy usually as signs increased the amplitude of the QRS complex, an alteration of the T wave and the QRS axis deviation to the left.

Electrocardiogram Paper in myocardial infarction

Electrocardiogram, being cheap and easily accessible, is the first review in patients who present with complaints of chest pain. There are several findings that may indicate an ischemic disease, including ST-segment elevation (ST-segment elevation), reduction of ST segment (ST segment depression), or inverted T waves peaked T waves.

The classic sign of heart attack by electrocardiogram is the ST- segment elevation, called myocardial above. However, it is important to note that not every heart attack occurs with ECG findings. A normal electrocardiogram is not sufficient to rule out a heart attack. If the patient has chest pain and especially if you have risk factors such as age above 50 years, obesity, diabetes, hypertension, smoking, etc., should perform blood tests (usually measured in troponin) to better investigate a possible myocardial box.


The electrocardiogram and any additional diagnostic means, must be addressed as just one piece in the puzzle of a diagnosis. A puzzle with just one piece is not complete. The ECG should be performed by a doctor who has experience with the exam, always taking into account other information such as medical history, symptoms, physical examination, laboratory tests and other complementary tests.

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