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How to Interpret an ECG

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

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How to Interpret an ECG

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

  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 anabnormal ECG • For example, ST elevation in leadsaVF, II and III could suggest a STEMIaffecting 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 standardpaper 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 300/2 = 150. Therefore the patientis displaying tachycardia 3. Rhythm R-R interval varies between 1.25-2 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 • 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 indicateinvolvement 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. 367-385 • Burns, E. (2015). ECG Library and clinical cases. [ONLINE] Available at: http://lifeinthefastlane.com/ecg-library/. [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.

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