How to read an electrocardiogram (ECG)

The electrocardiogram (ECG) is one of the simplest and earliest cardiac tests accessible, yet it can still offer a wealth of important data and is still a crucial component of determining the severity of heart defects.Modern equipment makes it rapid and simple to obtain surface ECGs at the patient's bedside, and they are based on electrophysiological principles that are quite simple. However, young doctors frequently find them hard to comprehend.This article is the first of several that will:

Enable readers to comprehend and analyze ECG records.

Reduce some of the fear that juniors frequently feel before an ECG

standard principles

What is an ECG?

The electrical activity of the cardiac muscle is only represented by an electrocardiogram (ECG), which is normally recorded on paper for easier study. The electrical depolarization of the muscle cells, as with other muscles, causes the heart muscle to contract. The entirety of this electrical activity that has been amplified and quickly recorded is referred to as an ECG.

Basic Electrophysiology of the Heart 

The sinus node, a region of unique tissue, depolarizes spontaneously to start the typical cardiac cycle.A wave of electric depolarization originates in the upper right atrium (RA), travels through the RA, and then crosses the interatrial septum into the left atrium (LA).Because the atria and ventricles are divided by an electrically inert fibrous ring, the only pathway for electrical depolarization to get from the atria to the ventricles in a healthy heart is through the atrioventricular (AV) node.The electrical signal is briefly detected by the AV node, and then a wave of depolarization spreads into the right (RV) and left (LV) ventricles via the His bundle, right and left bundle branches, and the interventricular septum (IVS). Therefore, when conduction is proper, the two ventricles contract synchronously, which is crucial for maximum cardiac output.Myocardium must first repolarize following complete depolarization of the heart before it is prepared to depolarize once more for the subsequent cardiac cycle.

Electrical axis and lead vectors for recording

The ECG is measured by placing a number of electrodes on the skin of the patient, hence the name "surface" ECG.The VS and ventricles are all affected by the wave of electric depolarization that originates in the atria. This depolarization therefore typically occurs from the superior to inferior side of the heart. Because the heart is positioned in the chest to the left and has more muscle mass than the right ventricle, the wave of depolarization often moves in a leftward direction. The electrical current is moving in this general direction.The electrical axis is the path of depolarization across the heart.This is how the electrical current will move generally.

The electrical axis is a region of depolarization in the heart.A basic tenet of ECG recording is that a positive or upward deflection occurs when the wave of depolarization approaches a recording lead. There is a negative or downward deflection when it moves away from a recording lead.

Although the electrical axis is typically left and downward, we can estimate it more precisely in individuals.

if we know from which 'direction' each recording lead measures the ECG for our patient

s.

Conventionally, we use 12 separate recording "directions" to capture the conventional surface ECG, though

Unfortunately, this can't be done with just 10 skin-surface recording electrodes. Six of these—the precordial leads—were recorded from the chest above the heart. The limb leads from the LMBS are recorded in four. It

Each of the 10 recording electrodes must be positioned correctly in order for the ECG to look correctly; otherwise, the appearance of the ECG will be dramatically altered, making accurate interpretation impossible.

Since the electric axis is typically only detected on the coronal pane, the LMB leads can be utilized to record the ECG in the coronal plane and to calculate the electric axis. Leads I, II, III, AVR, AVL, and AVF are referred to as limb leads, and they demonstrate the proper angles from which they "look" at the heart. drawing a heart-shaped horizontal line with a goal to the traditional label for the reference point of 0 degrees (or 0°C) is to the left (exactly in the direction of east).The angles in degrees from this baseline are used to characterize the directions from which other leads "look" at the heart.

Degrees are also used to describe the electrical axis of depolarization, which typically falls between -30° and +90°.Although it is outside the scope of this article to go into great depth about how to calculate the axes, the principles discussed here should make the concepts easier for readers to comprehend.

V1, V2, V3, V4, V5, and V6 are the chest leads, which capture the ECG in the transverse or horizontal plane.

Voltage intervals and timing

It is customary to record the ECG using benchmarks for the electrical signal's amplitude and the paper's movement speed during the recording. This enables:

Simple understanding of heart rates, cardiac intervals, and

ECGs taken at different times or using different ECG devices can be meaningfully compared.

On an ECG, the recorded electric signal's amplitude, or voltage, is displayed in millivolts (mV), with the vertical dimension. 1 mV is represented by a deflection of 10 mm on standard ECG paper. An increase in muscle mass, such as that seen in left ventricular hypertrophy (LVH), typically causes an increase in the electrical depolarization signal and, consequently, an increase in the amplitude of the vertical deflection on the electrocardiogram.

The ECG's ability to display how the heart's electrical activity changes over time is a crucial component. In addition

In other terms, the ECG can be compared to a graph where the horizontal axis represents time and the vertical axis represents electrical activity. During real-time recording, standard ECG paper moves at a speed of 25 mm per second. As a result, a distance of 25 mm along the horizontal axis corresponds to one second of time while seeing the printed ECG.

A grid of tiny and large squares is used to mark the ECG paper. Along the horizontal axis, each little square represents 40 milliseconds (ms), while each larger square contains five smaller squares, totaling 200 ms.

Cardiovascular timing intervals can be easily measured thanks to square markings and standard paper speeds. This makes it possible to calculate heart rates and spot heart aberrant electrical conduction.

A typical ECG

The right atrium is the first component to be noticed during a typical sinus beat.

The left atrium will come next, cautiously. Consequently, the atria are the source of the first electrical signal on a typical ECG, which 

is known as the P wave.

Although most ECGs only show one P wave, the P wave actually represents the sum of electrical signals from the two atria, which are typically overlaid.

The atrioventricular (AV) node then causes a little physiological delay before sending the electrical depolarization to the ventricles. The PR interruption, which appears on the ECG as a straight horizontal or "isoelectronic" line and lasts for a brief period, is caused by this delay.

Due to the higher muscle mass in the ventricles, depolarization of the ventricles typically causes the largest portion of the ECG signal, known as the QRS complex.

The initial 'negative' or downhill, or initial wave, deflection is the Q wave.

The next upward deflection is the R wave, assuming it crosses the isoelectric line and turns "positive."

If it temporarily passes the isoelectric inlet, the S wave is the next downward deflection.

before reverting to the isoelectric baseline, negative.

Additionally, an electrical signal indicating repolarization of the myocardium is present in the ventricles. The T wave and ST segment are used to illustrate this. Most leads have an upright deflection with varied amplitude and length for the T wave, but the ST segment is typically isoelectric.

Normal intervals

The length of time it takes for the various electrical depolarization phases to occur can be recorded, typically in milliseconds, by recording an ECG on standard paper. Such 'intervals' fall within the following norma range:

The PR interval is the distance between the commencement of the P wave and the onset of the QRS complex. Normal range: 3 little squares on the ECG paper, 120-200 ms.

Measured from the beginning of the QRS complex to its conclusion at the isoelectric line, the QRS duration

On the ECG paper, the normal range is up to 120 ms (3 little squares).

Measured at isoelectric line from the beginning of the QRS complex to the end of the T wave, the QT interval Although it varies with heart rate and could be slightly longer in females, the normal range is up to 440 ms.

Heart rate estimation with a standard ECG paper.

The heart rate (HR) can be roughly estimated using an ECG recording. On the horizontal axis, each second is represented by a square measuring 250 mm in size. The HR is 60 beats per minute if there are 5 squares between each QRS complex.

100 beats per minute is the HR.

150 beats per minute is the HR.