Cardio 7 – Identifying Some Basic Disturbances in Rhythm
Anil Chopra
1. Describe a systematic approach to ECG interpretation.
(1) Is the reading correct – e.g. age, sex, etc
(2) Identify the leads – where does one lead switch to another.
(3) Check calibration and speed of paper – standard 25mm/s and 1mVolt
produces 10mm vertical deflection.
(4) Identify the rhythm – regular or irregular.
(5) Look at the QRS axis.
(6) Look at P wave.
(7) Look at PR interval
(8) Look at QRS complex.
(9) Determine the position of ST segment.
(10) Calculate QT interval.
(11) Look at T wave.
(12) Check again.
P wave
2. Know the normal duration and amplitude of the components of the ECG
Waveform.
Duration < 0.11s; Amplitude < 2.5mm in lead II
PR
interval
QRS
complex
0.12-0.20 s
Q wave
Duration < 0.04 s; Amplitude: < 25% of total
QRS complex amplitude
QT
interval
ST
segment
T wave
0.38-0.42 s
Duration < 0.12 sec; Amplitude: R wave in V6
< 25mm; Axis -30 to +90 degrees
Should be ‘isoelectric’
May be inverted in III, aVR, V1 & V2 without
being abnormal
3, 4 & 5. Recognise normal sinus rhythm on the ECG, recognise common
abnormalities of cardiac rhythm on the ECG, recognise common pattern of acute
myocardial infarction on the ECG.
(1) Is the reading correct
- Check that the name, age, gender and date are correct.
(2) Identify the leads.
- Check that the leads are in the standard format on the sheet with the rhythm
strip at the bottom.
(3) Check calibration and speed of paper.
- The standard is 25mm/s and 1mVolt produces 10mm vertical deflection.
- May wish to study parts in more detail so print out changed.
(4) Identify the rhythm.
- Divide 300 by number of large squares between QRS complexes to get the
heart rate.
- Normal heart rate is 75 bpm.
- If it is below 60bpm it is bradycardia.
- If it is above 100bpm, it is tachycardia.
- Check if the rhythm is a sinus rhythm i.e. the P wave followed by QRS wave
every time.
- Rhythm may be irregular due to:
Ectopic beat
- Irregular or extra QRS complex
Sinus Arrhythmia
- Slight increase in heart rate in inspiration
Atrial Fibrillation
- No P wave!
- Fast atrial rate
- Irregular ventricular rate
- Fast ventricular rate.
Atrial Flutter
- Saw-toothed baseline.
- Fast atrial rate (250-350)
- Regular fast ventricular rhythm
Ventricular Fibrillation
Ventricular Tachycardia
Pre-excitation Syndrome
- Bundle of His is bypassed so ventricular diastole occurs early.
(5) Look at the QRS axis.
- QRS axis determined by orientation in the chest and the thickness of the ventricular
walls. Also known as the MFPA.
- Usually between -30° or +90° – tends to be more vertical in thinner patients and
more horizontal in fat patients.
- Cane have right axis deviation ( in right ventricular hypertrophy) and left axis
deviation ( in left ventricular hypertrophy)
- Can also be indicative of diseases of the conducting tissue.
(6) Look at P wave.
- Look for duration (<0.11s) and amplitude (<2.5mm)
- If they are greater than normal  atrial hypertrophy
- Higher amplitude suggests right atrial hypertrophy (possibly due to pulmonary
hypertension)
- Long P-waves suggest left atrial hypertrophy especially if there are 2 peaks.
(Possibly caused by mitral stenosis)
(7) Look at PR interval
- Should be between 0.12 and 0.20 seconds.
- If it is SHORT then it is indicative of Wolf – Parkinson – White syndrome. This
is because something is causing the nerve impulse to bypass the delay of the AV
node.
Secondary degree heart block
First degree heart
Type I – PR interval gets longer
block – longer PR
until one is not followed by a
interval due to
QRS complex and then the cycle
delayed conduction
starts again.
through the AV
Secondary degree
node.
heart block Type II –
PR interval is
Complete heart block
constant but
– no relation
occasionally misses
between P and QRS
out a QRS complex.
waves. The atria and
ventricles are
contracting
independently.
(8) Look at QRS complex.
- Amplitude should be around 25mm in V6
- High amplitude indicative of ventricular hypertrophy.
- Low amplitude indicative of obesity, COPD, pericardial effusion, hyper-inflated
lung, hypothyroidism.
- Length should be nor more than 120 milliseconds.
- Longer time indicative of bundle branch block. The different branches of the bundle
of His become blocked. This is because if a bundle branch is blocked, it will take
longer time for the electrical signal to pass through.
Right Bundle Branch Block
Has “rabbit ears” in V1 lead, and
odd V2 shape as they overlie right
ventricle.
Left Bundle Branch Block
Wide QRS complexes.
(9) Determine the position of ST segment.
- ST should be on the isoelectric line.
- If below the isoelectric line, then it is “ST depression”. This can be due to drugs,
myocardial ischaemia or ventricular hypertrophy. (exercise is used to test for if it is
myocardial ischaemia or ventricular hypertrophy by testing pain when exerted on a
treadmill).
- Elevated ST segments may be indicative of acute myocardial infarction or
aneurysm.
- Myocardial infarction.
 Starts if with abnormal elevation of ST segment.
 Then T wave inversion.
 Development of Q-waves.
(10) Calculate QT interval.
- Normally between 0.38 and 0.42 seconds.
- Will be shorter with increasing heart rate.
- Can be prolonged with drugs, hypocalcaemia and congenital heart diseases (which
result in sudden death).
(11) Look at T wave.
- Check its amplitude and whether it is upright or inverted. Duration should also be
checked.
- Inversion can be a sign of previous infection or can indicate damage to underlying
heart muscle (monocarditis or hypertrophy).