12 Lead ECG
Rhythm Awareness
ECG Abnormalities Simply Explained
When the 12 lead ECG prints out, it can feel daunting if you lack confidence in interpreting them. Student paramedics often ask themselves: “what if it is a rhythm I don’t know?” or “What if I miss something?” By structuring your interpretation of the ECG, along with some basic knowledge, you should feel confident enough to interpret the rhythms you’ll ‘need’ to know as a student paramedic.
This page goes through:
Tachycardia
Tachycardia is simply all the characteristics of sinus rhythm but with a heart rate of more than 100bpm, or above the normal range for age in children.
Sometimes, if the rate is too fast, you’ll see a camel hump appearance of the T wave. This is because the P wave is hidden within the T wave before it.
Bradycardia
Bradycardia is all the characteristics of sinus rhythm but with a heart rate of less than 60bpm, or below the normal range for age in children. Absolute bradycardia is a heart rate less than 40 bpm. In the pre-hospital setting in the UK, we treat symptomatic absolute bradycardia with atropine.
It is important to note that asymptomatic bradycardia can be normal in patients who are physically fit, e.g. long distance runners or keen cyclist.
The world record for the slowest heart rate in a healthy person is… 27bpm!
Atrial Fibrillation
Atrial Fibrillation is characterised by:
Irregular rhythm
Absence of P waves
Loss of isoelectric baseline
Fibrillatory waves in between QRS complexes
Mechanism of Atrial Fibrillation (AF)
Atrial fibrillation happens when the atria start firing electrical signals in a chaotic way.
For AF to occur, you usually need two things:
A trigger → something that starts it (like an early beat in the atria, called a PAC)
A suitable environment → something that allows it to continue (like a stretched or enlarged left atrium)
The Two Main Ways AF Can Happen
1. Focal activation (a single trouble spot)
AF starts from one specific area in the atria
This area fires off rapid, abnormal signals
It’s often found in the pulmonary veins
Think of it like one faulty spark plug misfiring repeatedly
2. Multiple wavelets (chaotic electrical activity)
Instead of one source, there are lots of small electrical waves moving randomly around the atria
These waves bounce around and create loops (re-entry circuits), keeping AF going
A larger (dilated) left atrium makes this worse because there’s more space for the waves to travel
Think of it like pinballs bouncing around a big room, constantly hitting things and never stopping
Atrial Flutter
Atrial flutter comes under the umbrella of ‘supraventricular tachycardias (SVT)’ that starts in the right atrium.
It happens because an electrical signal loops around in a circle in the right atrium (called a re-entry circuit).
The size of this loop depends on the size of the atrium, which is why the atrial rate is usually very regular and predictable — around 300 beats per minute (typically between 200–400).
In short: Atrial flutter = a fast, regular rhythm caused by a circular electrical loop in the right atrium.
ECG Characteristics:
Narrow complex tachycardia.
Flutter waves that are regular in shape, size and rate around 300bpm.
Saw tooth morphology of inverted flutter waves in inferior leads.
Ventricular rate is dependent on the AV conduction ratio (the number of flutter waves to a QRS complex).
Do not count Flutter Waves. Instead, look at the ventricular rate!
When working out the AV conduction ratio, look at the ventricular rate and calculate how many flutter waves can fit in between two QRS complexes based on the fact that the atria flutter at a rate of 300bpm.
For example, if the ventricular rate is 150bpm, you know that TWO flutter waves can fit in between, so the AV conduction ratio is 2:1.
Take your learning a step further!
Atrioventricular (AV) Blocks
First Degree AV Block
A first degree AV block is caused by a delay between the SA node and AV node.
It is characterised by a PR interval more than 0.2s (3 small squares).
Second Degree Mobitz Type 1 (Wenckebach)
A second degree mobitz type 1 is caused by malfunctioning AV nodal cells.
It is characterised by a progressive lengthening of the PR interval until a QRS complex drops.
Second Degree Mobitz Type 2 (Hay)
A second degree mobitz type 2 is caused by a failure of conduction below the AV node.
It is characterised by a constant PR interval until a QRS complex drops.
Third Degree / Complete Heart Block
A third degree / complete heart block is a complete block between the SA node and AV node.
It is characterised by bradycardia and complete AV dissociation. So there will be regular P waves, regular QRS complexes, but there is no pattern to be discerned.
A bundle branch blocks occurs when one of the bundle branches become blocked, inhibiting their ability to conduct electricity. This means electricity has to find another route to the ventricle.
A bundle branch block is characterised by a QRS width more than 0.12s (3 small squares). There are differences to look for when ascertaining if it is the left or right bundle branch that is blocked.
Bundle Branch Blocks
Left Bundle Branch Block
Characteristics:
QRS width more than 0.12s (3 small squares).
Mostly negative QRS complex in V1.
Absence of Q waves in lateral leads.
Right Bundle Branch Block
Characteristics:
QRS width more than 0.12s (3 small squares).
Mostly positive QRS in V1.
RSR pattern of the QRS in V1-V3.
Wide and slurred S wave in V6.
Take your learning a step further!