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7 Steps on How to Read an ECG: ECG Interpretation

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Phlebotomy Training Institute
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Picture a busy clinic. A patient comes in with chest pain or dizziness. The ECG (Electrocardiogram) is the first test doctors use to check the heart. Knowing the overall process of reading an ECG helps you find serious heart problems early, prevent risks, or give patients peace of mind.

If you’re a medical or healthcare professional or just someone who is interested in heart health, it’s important to learn how to read an ECG. 

To read an ECG, look at the wave patterns on the graph to check the heart’s —

  • rhythm, 
  • rate, 
  • and any signs of problems.

Focus on key parts like the P wave, QRS complex, and T wave to spot any abnormalities.

Let’s get into the details starting with ECG meaning first.

Fast electrocardiogram reading display where the heartbeats are getting more intense.

What is an ECG?

An ECG/ Electrocardiogram test is a graphical representation of your heart’s electrical signals. It shows the heart’s rhythm and reveals if there are any abnormalities. 

The ECG graph typically consists of waves and intervals that each represent different phases of your heartbeat.

How Does an ECG Work?

When you get an ECG, small sensors (called electrodes or leads) are placed on your chest, arms, and legs. These sensors pick up the electrical signals coming from your heart and show them on a graph. Each beat of your heart produces a pattern of lines.

Graph showing the results of the electrocardiogram (ECG) test on recording paper.

12-lead ECG

A 12-lead ECG is a simple, non-invasive test that checks your heart’s electrical activity from 12 different angles. It helps find heart problems like arrhythmias, heart attacks, or muscle damage

During the test, electrodes are put on your chest and limbs to pick up electrical signals, which show up as waveforms on a screen or paper.

V1, V2, V3, V4, V5, V6 in ECG

In a 12-lead ECG, V1 and V2 focus on the right ventricle, V3 looks at the septum, V4 shows the anterior wall, and V5 and V6 capture the lateral wall of the left ventricle. 

These six chest leads among 12-lead ECG give specific views of the heart’s electrical activity.

What are the other six limb leads in ECG?

In addition to the six chest leads (V1-V6), a 12-lead ECG includes six limb leads.

The six limb leads in an ECG are Lead I, Lead II, Lead III, aVR, aVL, and aVF. They capture the heart’s electrical activity from different angles. 

Check out the blog, “What is a 12-lead ECG?” to know about 12-Lead ECG in detail.

ECG Graph Basics

The ECG graph consists of different waves, segments, and intervals. Let’s break down what each part means:

The ECG Paper

An ECG graph is printed on a special type of grid paper. Here’s what you should know:

  • Small Squares: Each small square measures 1 mm by 1 mm.
  • Time on the X-axis: Each small square represents 0.04 seconds.
  • Voltage on the Y-axis: Each small square represents 0.1 millivolt (mV).

This grid can help you measure the intervals, time, and amplitude of the ECG wave which is super important for understanding the heart’s activity.

Graphical presentation of the parts of an ECG wave

Parts of an ECG Wave

Before starting the ECG interpretation, let’s break down the key components of an ECG wave:

Graphical presentation of the parts of an ECG wave like P QRS T U.

P Wave:

This shows the electrical signal that makes the upper chambers of the heart (the atria) squeeze or contract.

QRS Complex: 

This shows the electrical activity that makes the ventricles (lower chambers) contract. It has three parts:

  • Q wave: A small dip just before the spike.
  • R wave: A big upward spike.
  • S wave: A small dip right after the R wave.

ST Segment: 

The flat section of the ECG after the QRS complex and before the T wave.

T Wave: 

This represents the heart relaxing after a beat.

PR Interval: 

It measures how long it takes for the electrical signal to move from the upper chambers (atria) to the lower chambers (ventricles) of the heart.

QT Interval:

Measures the time it takes for the lower chambers or ventricles to contract and recover.

Hands holding A regular ECG with a stethoscope in the background

7 Steps on How to Read an ECG

Do you know that early deaths from heart problems in England are at their highest in over 10 years in 2024? This makes using the right ECG technique super important to find heart issues early and save lives.

Here is the step-by-step guide to reading an ECG. 

ECG Chart for Quick Reference
ECG Component Normal Range Abnormality
Heart Rate 60-100 bpm <60 bpm (Bradycardia), >100 bpm (Tachycardia)
P Wave Uniform and precedes every QRS Absent or varying shapes
PR Interval 0.12 – 0.20 seconds Shortened or prolonged
QRS Complex <0.12 seconds Wide QRS indicates ventricular issues
T Wave Upright in most leads Inverted or flattened
QT Interval 0.35 – 0.45 seconds Prolonged indicates risk of arrhythmia

Let’s begin!

Before reading an ECG, make sure to:

  • Confirm the patient’s name and date of birth match the ECG report.
  • Note the ECG date, time, any series number, patient symptoms like chest pain, and relevant info like potassium levels for accurate diagnosis.
  • Ensure the ECG calibration is correct (typically 25mm/s and 10mm/1mV).

Check the Heart Rate

Count the large squares between two R waves (the peaks in the QRS complex). Use the 300 rule: divide 300 by the number of large squares to find the heart rate. 

For example, if you count 5 large squares, the heart rate is 60 bpm (300/5 = 60).

Normal Heart Rate 60-100 beats per minute.
Bradycardia Less than 60 beats per minute.
Tachycardia More than 100 beats per minute.

Check the Rhythm

Heart rate estimation counts through the rule of 300 shown as normal ECG

Is the rhythm regular or irregular? Examine whether the R-R intervals (the distance between two R waves) are consistent. If they are, it suggests a regular rhythm. If not, it may indicate an arrhythmia.

Heart Rate Calculation
Regular Rhythm
  • Count the number of large squares between two R-R intervals.
  • Divide 300 by this number to find the heart rate.
Irregular Rhythm
  • Count the number of complexes on a 10-second rhythm strip.
  • Multiply this number by 6 to get the average beats per minute.

Analyse the P Wave

Check if the P waves are present, and if they are followed by QRS complexes. P waves should be smooth and upright. If missing or unusual, it could point to atrial abnormalities.

Measure the PR Interval

The PR interval is the space between the beginning of the P wave and the start of the QRS Complex. It should be between 0.12 to 0.20 seconds (or 3-5 small boxes on the ECG paper). 

Longer or shorter intervals can indicate issues with electrical signals reaching the ventricles. For instance, a prolonged PR interval may indicate a heart block.

Graphical presentation of the parts of an ECG wave

Check the QRS Complex

Look at the QRS Complex to see if it is narrow or wide. A normal QRS Complex is less than 0.12 seconds (about 3 small boxes). If it’s wider, it could indicate a problem in the ventricles.

Evaluate the T Wave

The T wave should be upright in most leads. Abnormalities in the T wave, like inverted or flattened waves, can suggest issues like ischemia (reduced blood flow).

Measure the QT Interval

The QT interval is the space from the start of the QRS Complex to the end of the T wave. This interval should be between 0.35 to 0.45 seconds or 9–11 small squares. A prolonged QT interval could be a sign of an increased risk of dangerous arrhythmias.

Graphical presentation of the parts of an ECG waveClose-up of Electrocardiogram (pulse trace) displayed on a digital tablet screen

Common ECG Findings and What They Mean

Below are some common ECG findings and their underlying meanings.

  • Normal Sinus Rhythm: Everything is working normally. The P wave is visible, the rhythm is steady, and the heart rate is between 60-100 beats per minute.
  • Atrial Fibrillation: The rhythm is not regular, and there are no clear P waves. This means the atria are beating in a chaotic way.
  • Ventricular Tachycardia: The heart is beating very fast, and the QRS Complexes are wide. This means the ventricles are pumping too quickly.
  • Myocardial Infarction (Heart Attack): If the ST segment on the ECG is elevated above the baseline, it could indicate a heart attack. This needs to be addressed immediately.

Wrapping Up

So, coming to this point, we can assume that you now have a pretty good idea of how to read an ECG. Reading an ECG might seem tough at first, but it gets easier once you understand its parts. The main things to focus on are finding the P wave, checking the spaces between waves, and looking closely at the QRS Complex and T wave. With time and practice, this will start to feel natural.

If you’re a phlebotomist, knowing about ECGs is important because you work with patients who might need help in heart-related situations. Taking training from a recognised institute like Phlebotomy Training Institute, which provides CPD-accredited  certified ECG Training, can be super helpful.

Here, you’ll learn the right skills from experienced professionals. This 1-day ECG course gives you hands-on practice to accurately record and read a 12-lead ECG. 

This will surely boost your confidence in interpreting heart rhythms. Not just for phlebotomists, it’s ideal for any healthcare workers who need to perform ECGs as part of their job.

FAQs

What is a normal ECG reading?

A normal ECG shows a steady heart rate of 60-100 bpm, with each QRS complex following a normal P wave. The PR interval stays the same, and QRS complexes are narrow (less than 100 ms).

Is an ECG painful?

No, an ECG is a painless procedure. You may feel a slight stick when the electrodes are attached to your skin.

What should I wear for an ECG test?

Wear loose-fitting clothing that has easy access to your chest and limbs.

Can I eat or drink before doing an ECG test?

Yes. You can eat and drink moderately before an ECG test unless your doctor has advised otherwise.

How long does an ECG test take?

A typical 12-lead ECG test takes only a few minutes to perform.

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