The document details a structured approach to interpreting ECGs, emphasizing the importance of understanding heart rhythms, voltages, and complex timings. It covers technical factors, characteristics of different ECG components, and their clinical implications, such as diagnosing myocardial infarctions. Contextual clinical scenarios highlight the necessity of considering patient history and symptoms when analyzing ECG results.

1. A structured method for looking
at ECGs
Dr G Abraham,
Royal Free,
London

2. What is an ECG and why are we
interested?
• A graph of voltage versus time.
• Heart muscle’s depolarization and repolarisation
results in measurable electrical activity on the
skin.
• Electrodes are positioned strategically around the
body to capture the heart’s electrical potential
from various angles or ‘leads’.
• 12 leads on a standard ECG are computed from
10 electrodes giving characteristic tracings.

3. • ECGs give rapid at the bedside diagnostic
information with potentially significant
changes to the management

5. Rate
• What is the rate and rhythm of the above
ECG strip?

6. Technical Factors
• ECG paper are recorded on a special graph
paper graded into 1 𝑚𝑚2
grid boxes.
• The ordinary paper speed is 25 mm/second
• Each small square corresponds to 40 ms and
each large square made up of heavier lines
corresponds to 200 ms.
• The height of the complex corresponds to
voltage with 1 small square equivalent to 0.1
mV.

7. Rhythm
• Is it regular?
• Are there P waves?
• Do the P waves look normal?
• Is each P wave followed by a QRS in a 1:1
relationship?

8. Is it sinus rhythm or something else?
If the P wave
is upright in
lead II and
negative in
aVR, it
suggests
sinus rhythm

9. Cardiac Axis
39
12
6

10. Electrodes, leads and axis –
Einthoven’s Triangle

11. PR interval
• 0.12 – 0.20 ms
• Represents the time taken to depolarise the
atria and traverse the atrioventricular node.

12. QRS complexes
Timing
represents
depolarisation of
the ventricles.
Narrow
complexes less
than 120 ms
represent a slick
conduction
system.

13. Broad QRS
• Supraventricular rhythm with bundle branch
block.
• Ventricular rhythm
• Pre-excitation

14. QRS complexes – other observations
• Usually the left ventricle carries more mass
than the right.
• Positive deflections represent the terminal
vector of electrical current oriented towards
the lead in question.
• Septal depolarisation is from left to right.
• Very tall R waves or very deep S waves suggest
cardiac muscle hypertrophy.

15. • AF, Right bundle branch block
• Delayed right ventricular depolarisation leads to rsR’ in right precordial leads, deep
slurred s wave in lateral leads.

16. Q waves
• Normal in most leads except V1-V3, represent
septal depolarisation left to right.
• Considered pathological if over 1 mm wide
and 2 mm deep, over 25% of the following
QRS or in V1-V3.
• Signify evolving myocardial infarction with
scar or a sign of cardiomyopathy.

17. ST segment
• ST elevation measured at the J point may
localise a coronary artery territory in the
context of myocardial infarction.
• This is the most important cause of ST
segment deviation.

18. T waves
• Represents ventricular depolarisation, upright
in all leads except aVR, V1.
• T wave inversion in lead III and V2 may be a
normal variant.
• Generally <5mm in the limb leads and <15mm
in the precordial leads.

19. T wave abnormalities
• Inversion
• Hyperacute – asymmetrical or symmetric
tented?
• Biphasic
• Main causes to consider are myocardial
ischemia and potassium disorders.

20. QTc
• Electrolyte abnormalities: hypomagnesemia, hypocalcemia,
hypokalemia
• Hypothermia
• Drug causes – antipsychotics, antidepressants, macrolide
antibiotics, anti-arrhythmic drugs
• Inherited Long QT syndromes
Prolonged
if:
• >440
ms men
• >460
ms
women

22. Context is everything

23. 65 year old male smoker with hypertension presents
with ongoing central crushing chest pain which started 2
hours ago, to the heart attack centre.

24. 74 year old woman attends routine cardiology
outpatients. She is asymptomatic. She has a
background of previous anterior MI.

25. 30 year old woman with no risk factors for coronary artery
disease, sharp central chest pain since yesterday evening.

26. 62 year old with cardiac sounding syncope. Recent
viral infection and high fever.

27. Summary
• A systematic approach encompasses:
• Rate
• Rhythm
• Axis
• PR interval
• QRS duration and morphology
• Q waves
• ST segment
• T waves
• QTc
• The clinical context is crucial when
giving a diagnosis.

28. References
• http://lifeinthefastlane.com
• www.uptodate.com
• Roffi M et al. (2016) 2015 ESC Guidelines for the
management of acute coronary syndromes in patients
presenting without persistent ST-segment elevation: Task
Force for the Management of Acute Coronary Syndromes in
Patients Presenting without Persistent ST-Segment
Elevation of the European Society of Cardiology (ESC).
European Heart Journal. 37(3). 267-315.
• Thanks to Dr D Antonaki, Dr M Ahmed for ECGs and other
pictures.