How to read an ECG report with basic interpretation

Posted On August 13, 2020September 6, 2021

Overview 

In this article, we will understand how to read an ECG report with basic interpretation.

You must have seen the zigzag line on the ECG paper.

Did you ever try to understand it?

Or, do you want to understand it?

If you want to know how to read an ECG strip, keep reading to find every parameter of an ECG graph.  In this post, you will know – 

• concept of depolarisation and repolarisation
• normal electrical activity of the heart
• significance of cardiac vector
• how to read an ECG graph
• how to read an ECG value and readings

Let’s understand how to read an ECG  with basic concepts – 

What is an ECG?

ECG (or EKG) stands for electrocardiogram. It is an important medical test that records the electrical activity of your heart and represents it in a graphic pattern. 

The whole process is called Electrocardiography. 

This procedure provides a complete picture of the electrical activity of your heart. 

ECG is one of the most essential tools for clinical purposes.  It is a helpful technique to detect heart abnormalities like myocardial infarction, angina pectoris, ischemic heart disease, arrhythmia, chest pain, etc.

What is the position of ECG leads, and how do they work?

ECG works on the principle of a Galvanometer, which detects the heart’s electrical activity by positive and negative electrodes.

The heart’s electrical activity is recorded by attaching electrodes (negative and positive electrodes) to the surface of the chest and limbs. 

The positive lead (or electrode) is applied on the left foot. Whereas negative lead is used on the right hand. These leads help to records the heart’s electrical activity graphically. 

When you visit for an ECG test, there are many leads applied to your body.  

The standard ECG is in 12 leads that include – 

• three limb leads (I, II, and III), 
• three augmented limb leads (aVR, aVL, and aVF)
• six chest leads (V1, V2, V3, V4, V5, and V6). 

The correct placements of ECG lead help to record your electrical activity in 12 different views of the heart. 

Limb leads

Limb leads are bipolar (negative and positive electrodes) that record the current direction in the frontal plane – superior (up), inferior (down), right side, and left side of your heart. 

Augmented vector leads 

Augmented vector leads are unipolar (single positive electrode). They help to record the electrical potential difference in the frontal plane of your heart. 

Chest leads

Chest leads are also unipolar leads (single positive electrodes) that record the current direction in the transverse plane (anteroposterior view) of your heart. The difference in every chest lead placement appro. 30 degrees.

How does electric current flow in the heart?

Before understanding the concept of ECG, you need to learn the normal electrical activity of your heart.

Electrical activity is normally dependent on depolarisation and repolarisation, that called an action potential.

Is it difficult to understand?

Ok. Let me make it easy. Firstly, we understand depolarisation and repolarisation.

Suppose your heart muscle is not active. It is in a resting or negative polarised state. It must have some voltage.

It will be – 90 mV.

Means, Resting Membrane Potential (RMP) = – 90 mV.

In the resting phase, your heart contains more potassium ions and less sodium or calcium ions. Whereas sodium and calcium ions are generally higher outside the heart muscle. 

It means if your heart has no electrical activity. Then your heart has – 

• More intracellular potassium (inside your heart muscle)
• More extracellular sodium and calcium (outside your heart muscle)

When your heart is stimulated by a sympathetic nerve, it reaches threshold potential (initiation of depolarization). 

Threshold potential = – 70 mV.

As soon as your heart touches the threshold, the voltage-gated sodium channel gets open. Then sodium ions are started to travel from outside to the inside of your heart. These troops of sodium ions (Na⁺) reduced the electronegativity of your heart cells because sodium has positive charge ions. 

If more sodium influx, then myocardium cells (heart muscle) become depolarised (-90 mV to 0 mV).

When heart muscle loses negative polarity due to the moving of sodium ions from outside to inside your heart muscles. The whole process is called depolarization. 

During depolarization, your heart gets an electrically positive charge. As a result, the heart starts to contract. 

When your heart gets completely depolarised, then potassium and calcium channel are opened.

After that, potassium ions (K⁺) move from inside to outside (efflux), and calcium ions (Ca⁺²) move from outside to inside (influx). 

During this process, your heart muscles lose potassium ions and gain calcium ions.

Due to the loss and gain of cations, it makes balances the potential that gives the plateau phase in the action potential graph for a brief time. 

But as time passes, the calcium channel gets closed, and potassium ions keep moving out. It is progressively lost until the membrane becomes electronegative or RMP (resting membrane potential). 

This process is called repolarisation. 

It means cardiac membrane back to negative polarised state due to potassium efflux. 

These depolarisation and repolarisation processes represent the electrical activity of your heart muscles.

During depolarization, your heart muscle gains positive charges (Na⁺ ions), and it reflects contraction. In contrast to repolarisation, your heart muscle loses positive charges (K⁺ ions), representing relaxation of the heart muscles. 

What are the relations of a cardiac vector in ECG?

Our heart is made up of special cardiac muscle cells are called cardiomyocytes. 

It has four main chambers – right atrium, left atrium, right ventricle, and left ventricle.

• Both atrium and ventricles = separated by atrioventricular (AV) valves
• Right atrium and left atrium = separated by the interatrial septum
• Right ventricle and left ventricle = separated by an interventricular septum (IVS)

The shape of your heart is slightly cone, and it is located in the chest, behind the sternum (mediastinal cavity). 

The heart is fixed in your ribcage (3rd to 5th ribs). The top portion in the heart’s 3rd and base (apex) is present in 4 to 5th ribs. 

The apex of your heart is tilted forward, downward, and leftward. So, the electric current is produced leftward and downward of your heart.

Normally, every electric force is represented by a vector. 

Thus, a virtual vector is produced in your heart which shows the magnitude and direction of electric current. This virtual vector is called a cardiac vector. 

Generally, electric force is directly proportional to the strength of the vector.

Due to this cardiac vector, the ECG machine can easily determine the electrical activity of your heart by the real vector (standard 12 leads).

Here, the limb lead has an excellent role in detecting the movement of cardiac vectors. 

Limb leads are bipolar electrodes. Lead-I is a negative electrode applied on the right arm. In contrast, Lead-II is a positive electrode used on the left foot. 

The pointer of the vector shows the direction of electrical activity.

As I stated earlier, ECG works on the principle of a Galvanometer. ECG machine needles will fluctuate based on your cardiac activity.

According to the galvanometer principle, “if similar charges move towards the similar electrode, then deflection will be positive.” 

• If your heart is resting (RMP), then the needle won’t fluctuate.
• When the wave of positive charge (or heart muscle gain of Na⁺ ions) moves towards the positive electrode during depolarization, it represents positive deflection (long vector).
• If the positive charges (Na⁺ ions) move towards the negative electrode, it will show a negative deflection (short vector)
• If your heart muscle gets completely depolarised, it will show no deflection (vector will disappear)
• When the wave of negative charge (or heart muscle loss of K⁺ ions) moves towards the negative electrode, it represents positive deflection (A very strong vector produce due to thick ventricles muscles).  

What is the electrical pathway of the heart in order? How do ECG parameters relate to it?

Here, we will see how many electric phases occur in your heart during one cardiac cycle. These electric events produce many cardiac vectors, which translated into an ECG pattern.

1st Phase – Electric event in the atria

When your heart is stimulated by the sympathetic nerve, the SA (sinoatrial) node fires depolarization waves (gain of positive charge or influx of Na+ ions) in the atrium chambers. 

SA node is present on the right side and upper side of the top atrium. So, it sweeps the electric current downward and leftward. It generates current with moderate velocity.

Due to this firing of depolarization waves, the current flow and the atrium chamber start to contract. As a result, you generally see P waves on ECG paper. 

2nd Phase – Stimulation of AV (atrioventricular) node

When both atria are completely depolarised, the depolarization waves hit the fibrous area of atrioventricular valves (the valve between the atria and ventricles) and the AV node.

AV node is generally present in the center of the atrioventricular valve. This area is a bad conductor of electricity.

AV node is specialized in slow conduction of current. It takes 0.1 seconds to reach the atria to ventricles.

It holds the electric current for a few seconds because the AV node contains small cells with a narrow diameter. And, depolarisation depends on Ca+2 ions (not dependent on Na+). 

Due to this slow conductivity of electric current in the AV node, you may see a small flat line (no wave) after the P wave on ECG paper.

3rd Phase – Electrical activity in the ventricles 

Our cardiac muscles are comprised of three layers – subendocardium (inner), myocardium (middle), and epicardium (outer).

There is very specialized tissue present in a sub-endocardium region of the heart, which is known as Purkinje fibers. 

Purkinje fibers are present in the center of both ventricles. Purkinje fibers are specialized in the fast conduction of current because they have large cells, a wide diameter, and depolarization is mainly dependent on Na+ ions.

Naturally, the depolarized current is strongly produced on the left side because the left ventricles are 3 times thicker than the right.

The wave of depolarization normally produces from the endocardium to epicardium (inside to outside). In ventricles, current flow is in 3 stages –

a. Septum depolarization

b. Major ventricular depolarization

c. Basal ventricular depolarization

As soon as the electric current reaches the ventricles region. Firstly, it depolarized the interventricular septum area (valve separates right and left ventricles). 

In the septum area, the depolarization wave (positive charge of Na+) moves towards the negative electrode. Due to this, you may see a slight negative deflection that represents the Q wave on the ECG paper. 

After the depolarization of the septum area, the strong current flows in the major area of the ventricles. Because ventricles are thicker cardiac muscles. It will show a large vector. 

Due to this, you may see a long and wide spike of the QRS complex on the ECG paper. 

Eventually, it depolarizes the lateral (or side portion) of the ventricles. The depolarization waves will move towards the negative electrode because the current flows from inside to outside in normal ventricles. Due to this, you may see a slight negative deflection of the S wave on the ECG paper. 

Repolarization – After the depolarization (or contraction of the heart) process, heart muscle starts to relax by repolarization. 

During this process, your heart muscles lose the positive charge (K⁺ ions) and back to electronegative. The waves of repolarization move gradually slow in right and upward.

According to the Galvanometer principle, the deflection will be positive if similar charges move towards a similar electrode. 

Here, your heart has an electronegative charge due to the loss of potassium ions. It produces a negative vector that moves towards a negative electrode. You may see the positive deflection of the T wave on the ECG strips. 

What is normal ECG report look like? How to read an ECG wave, segments, and intervals?

The ECG machine records the electric events of your heart, and it represents graphically in ECG paper in the forms of waves, segments, and intervals. 

Let’s understand how to read an ECG wave, segments, and intervals.

If you see a straight line in an axis of ECG paper represents no electrical activity is called the Isoelectric line. 

You must have observed a small straight line comes before the beginning of the P wave. This straight line indicates RMP (Resting membrane potential). 

Waves 

The ECG paper represents the 5 significant waves – P, Q, R, S, and T. 

P wave – Atrial depolarization. It indicates an atrium contraction, and the heart is ready to push blood into the ventricles. 

QRS wave or complex – Ventricular depolarization. It indicates the contraction of ventricles, and the heart ejects blood from ventricles. 

If we see individual waves as it produces electrical activity in 3 stages –

a. Q wave represents ventricular septum depolarization. It shows negative deflection as it produces a minor vector in the negative electrode direction and right side.  

b. R wave represents major ventricular depolarization. It produces large deflection rapidly as fast conduction in the apex of the heart due to Purkinje cells. 

c. S wave represents basal ventricular depolarization. It is the last stage of depolarisation and produces slight negative deflection as these positive vectors move towards the negative electrode and on the right side.

T wave – It represents ventricular repolarization (relaxation of ventricles). It produces large positive deflection with slow velocity because a negative vector moves towards a negative electrode. The repolarization starts from outside to inside (epicardium to endocardium). 

Segments

You must have observed two crucial isoelectric lines (flat line) in ECG paper that indicates segments –

PR segment – It usually reflects because the AV node holds the current for 0.1 seconds. It shows in between the end of the P wave and before the beginning of the QRS wave.

ST-segment – It represents the end of ventricular depolarization and the beginning of ventricular repolarization. It shows in between the end of the S wave and before starting the T wave.

Intervals 

There are three intervals reflect in the ECG paper – PR interval, QRS interval, and QT interval. In ECG intervals, it also includes waves and lines. 

PR interval – It shows that atrial depolarisation (wave) and AV node conduction (no wave). It starts from the beginning of the P wave to the beginning of the QRS wave. 

PR interval = P wave + PR segment 

QRS interval – It signifies the duration of current in – 

QRS interval = Q wave + R wave + S wave

• Ventricular septum depolarization (Q wave)
• Major ventricular depolarization (R wave)
• Basal ventricular depolarization (S wave)

QT interval – It reflects at the beginning of Q wave and end of T wave.

QT interval = QRS complex + ST-segment + T wave

It shows that 

• Ventricular septum depolarization (Q wave)
• Major ventricular depolarization (R wave)
• Basal ventricular depolarization (S wave)
• Isoelectric line of depolarisation (ST-segment)
• Ventricular repolarization (T wave)

How to read an ECG time interval and durations?

The electrical activity of your heart is recorded in ECG graph paper, which is also called an ECG strip. Let’s understand how to read electrocardiogram results with normal time and duration.

ECG paper consists of many horizontal and vertical lines in grid form. 

It contains many large squares and small squares. Each large square consists of 5 small squares.

Your ECG is usually recorded at a speed of 25 mm/sec (5 large squares/sec). Here, we need to understand some calculations.

If ECG strips or paper runs, then – 

300 large squares pass in during 1 minute.

300 large square covers = 60 seconds

1 large square = 60/300 = 0.2 seconds

1 small square = 0.2/5 = 0.04 seconds (most important time)

P wave covers two and half small squares. So, if we calculate time – 

0.04 + 0.04 + 0.02 = 0.1 second.

PR segment also covers two and half small squares and takes time 0.1 seconds.

PR interval (5 small squares) = P wave + PR segment 

                                    = two and half small square + two and half small square

                                    = 0.1 + 01.

                                    = 0.2 seconds

QRS wave also covers two and half small squares and takes time 0.1 seconds.

QT interval covers 10 small squares 

0.04 x 10 = 0.4 seconds.

Conclusion 

Now you have got clarity on how to read an ECG report. We have learned every basic concept of ECG. 

It was the complete information of a normal ECG pattern from basic to advance. In my next post, I will be explaining the abnormalities of ECG. In that post, we will understand how to read an ECG for heart attack, arrhythmia, ischemic heart disease, and other heart medical conditions.

I always try to provide complete information to my readers. 

If you found this post (how to read an ECG) informative, please share it on social media. 

Reference – Lippincott Williams and Wilkins. ECG interpretation. Made incredibly easy. Wolters Kluwer Health, 5^th edition, 2011.

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