1. SSC Emergency Medicine Project Sept 2015 Craig Meek (40099752)
How to Interpret an ECG
SSC Emergency Medicine Project
Sept 2015
Craig Meek ( )

2. Overview This short project is split into 3 sections.
The first section covers some general background information - when ECGs are used, normal lead positions and expected traces.
The second section looks at a systematic way of interpreting ECGs.
The final section examines some specific cases I have witnessed during my time in Antrim ED.

3. Background
Electrocardiograms are routinely used in the ED for cases of chest pain, syncope, dizziness and dyspnoea
It is also important to take an ECG in cases of upper abdominal pain
A normal trace contains:
P wave (atrial depolarisation)
QRS complex (ventricular depolarisation)
T wave (ventricular repolarisation)

4. Background – Lead Positions
The ECG measures waveforms in different directions with 6 chest leads (V1-V6) and 3 limb leads (aVR, aVL, aVF)
V1 & V2 focus on the right ventricle
V3 & V4 focus on the intraventricular septum
V5 & V6 focus on the left ventricle

5. Background – Lead Positions
aVR is situated on right arm; aVL on left arm, and aVF on the left foot
The aVR lead is in the opposite direction to the normal direction of travel for the electrical signal of the heart, and will therefore have downward deflection
Similarly, aVL will have a weakly upward deflection, while aVF will have the strongest upward deflection
Limb leads I,II,III are derived from these aVR, aVL and aVF

6. Background – Lead Positions
From these,
Lead I gathers information between aVR and aVL
Lead II gathers information between aVR and aVF
Lead III gathers information between aVL and aVF
Therefore, we can expect some extent of upward deflection for leads I and II, with some variation in lead III. Lead II will have the most positive deflection because it is situated in a similar direction to the normal electrical activity of the heart
Applying knowledge of these lead positions can help determine what area of the heart is being affected in an abnormal ECG
For example, ST elevation in leads aVF, II and III could suggest a STEMI affecting the inferior region of the heart

7. ECG Interpretation 1. Check - name, DOB and time
2. Rate – count number of big squares between two R waves and divide into 300
3. Rhythm – check if the R-R distance is equal between peaks
In this example, the rate is 300/4 = 75, and the rhythm is regular
Note that this only works for the standard paper speed of 25mm/s

8. ECG Interpretation 4. Axis deviation –
Determined by looking at leads I,II and III. Normally, lead II will have the most positive deflection because electrical signals in the heart travel in a similar direction.
In left axis deviation, lead I will have the most positive deflection, while leads II and III will often be negative.
In right axis deviation, lead III will have the most positive deflection, while lead I will often be negative.

9. ECG Interpretation
5. P waves – Often easiest to see in lead II. They should be regular, smooth and associated with a QRS complex. Normal size is < 0.12s width and < 2.5mm tall.
6. PR interval – Should be consistently between 0.12s and 0.2s. Indicates adequate conduction through the AV node.
Abnormal or absent P waves can suggest a problem with atrial conduction
Abnormal PR intervals can suggest a problem with the conduction pathway between atria and ventricles

10. ECG Interpretation
7. QRS complex – Normal width is 0.12s. Narrow complexes are supraventricular in origin; Broad complexes are ventricular in origin.
8. ST segment – Normally should be on the isoelectric line.
ST elevation is commonly caused by myocardial infarction
ST depression is non-specific, and therefore has to be analysed in the context of the patient

11. ECG Interpretation
9. T waves – Should be regular, smooth and have an associated QT interval of < 0.42s.
Inverted T waves are abnormal but non-specific, and so must be taken in context with the patient. Inverted waves are often seen in V1 and V2 in healthy adults.
Tall T-waves are characteristic of hyperkalaemia.

12. Case Example 1

13. Case Example 1 1. Check – Name, DOB and time 2. Rate 3. Rhythm
300/2 = 150. Therefore the patient is displaying tachycardia
3. Rhythm
R-R interval varies between big squares and shows no consistent pattern – therefore rhythm is irregularly irregular

14. Case Example 1
4. Axis Deviation – Peaks of I > II, therefore possible evidence of left axis deviation
5. P waves – Absent. Instead there are many miniature deflections with no obvious association with the QRS complexes

15. Case Example 1
6. PR Interval – Does not exist due to absence of P waves. Therefore, there is inadequate conduction through the AV node.
7. QRS Complex – Normal, but depolarisation is not being driven by P waves and is therefore spontaneous.
8. ST Segment – Normal
9. T waves – Mostly present and normal
Analysis has revealed a tachycardic, irregular rhythm with absence of P waves. This is characteristic of atrial fibrillation.

16. Case Example 2

17. Case Example 2 1. Check – Name, DOB and time 2. Rate 3. Rhythm
300/4 = 75. This is a normal rate.
3. Rhythm
R-R interval is consistent, therefore rhythm is regular.

18. Case Example 2
4. Axis Deviation – Peak sizes and directions of I, II and III look normal.
5. P waves – Present and normal.
6. PR Interval – Normal duration of 0.12s, or three small squares.

19. Case Example 2
7. QRS Complex – Normal width and height. Associated with P waves.
8. ST Segment – Clear ST elevation in leads I, aVL, V5 and V6.

20. Case Example 2 9. T waves – Evidence of broad T waves in V5 and V6.
Analysis suggests a ST Elevation myocardial infarction. As the name indicates, this is characterised by elevation of ST segments. Broad T waves may also be seen in the early stages of a STEMI.
Furthermore, we can localise the STEMI by considering the leads which displayed ST elevation:
Leads I and aVL indicate involvement of the lateral side of the heart
Leads V5 and V6 indicate involvement of the left ventricle

21. Case Example 2
Therefore, this MI was localised to the lateral wall of the left ventricle.
STEMIs are now a much less common ED presentation due to most patients being transferred directly from home to one of the primary PCI centres in either Belfast or Altnagelvin.

22. References
Ballinger, A; Patchett, S (2004) Saunders’ Pocket Essentials of Clinical Medicine. Ch 9: Cardiovascular Disease. pp
Burns, E. (2015). ECG Library and clinical cases. [ONLINE] Available at: [Accessed 23 September 15].
Cline, D.M et al. (2012). Tintinalli’s Emergency Medicine Manual, 7th Edition. Ch 17: Chest Pain: Cardiac or Not? McGraw-Hill.