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The Basics of ECG Interpretation
- 1. student led tutorials Interpreting anECG Student led tutorial 1 Tim Jackson t.jackson1@uni.bsms.ac.uk Chris Pavitt c.pavitt1@uni.bsms.ac.uk
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- 3. student led tutorials Depolarisation ofthe heart Relating the ECG wave form to electrical activity within the heart
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- 9. student led tutorials Normal rangesfor ECG intervals PR interval QRS QT interval
- 10. student led tutorials Remember thesevalues! Interval Normal range (per ms) Pathology P-R 120-200 Long – heart block (drugs, electrolytes) QRS 80-120 Long – conduction abnormalities e.g. bundle branch block QT (varies with heart rate) >450 can lead to ventricular tachycardia Long – repolarisation abnormalities, ion channelopathies e.g. long QT syndrome
- 11. student led tutorials Working outyour intervals LEARN THIS! TASK – Work out the PR interval in this ECG
- 12. student led tutorials Remember thesevalues! Interval Normal range (per ms) Pathology P-R 120-200 Long – heart block (drugs, electrolytes) QRS 80-120 Long – conduction abnormalities e.g. bundle branch block QT (varies with heart rate) >450 can lead to ventricular tachycardia Long – repolarisation abnormalities, ion channelopathies e.g. long QT syndrome
- 13. student led tutorials Rate Normal range:60 – 100 bpm
- 14. student led tutorials Working outthe rate (bpm)
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- 16. student led tutorials Rhythm The heartpumps blood at a fixed rate (feel your own pulse) This maintains a constant cardiac output Recall that each P-QRS-T cycle = systole (one beat)
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- 20. Heart block A blockingof the depolarisation wave spreading through the heart Heart block Mobitz type 2 Some P waves not conducted, PR interval fixed Type 3 / Complete P wave and QRS dissociationType 2 Mobitz type 1/ Wenkebach Progressive PR prolongation, then dropped beat Type 1 PR interval > 0.2 seconds Advanced Block Fixed ratio of P: QRS complex
- 21. student led tutorials Interpreting anECG Student led tutorial 1 Tim Jackson t.jackson1@uni.bsms.ac.uk Chris Pavitt c.pavitt1@uni.bsms.ac.uk
- 22. student led tutorials Appendix 1– Rate calculations 22 The simple method: 1. Find a QRS complex 2. Find the next QRS complex along 3. Find the R wave in both 4. Count how many big boxes there are in between each R wave 5. With a calculator do: • 300 / (number of big boxes) 6. You can make this exact as you like ie 3.4 boxes
- 23. student led tutorials Appendix 2– Rhythm 23 Method: 1. Grab a piece of paper 2. Line it up horizontally across ECG lead 2 tracing 3. Make sure you can see the tips of each R wave 4. Make a vertical mark on the paper for the first 2 R waves 5. Line up 1st mark with 2nd R wave 6. Does the 2nd mark line up with 3rd R wave? 7. Carry on repeating this. Interpretation If the heart rate is regular then the distance between each QRS should be regular, and thus your marks will line up If the heart rate is irregular, then the distance between each QRS will vary, and your marks will not always line up
- 24. student led tutorials Appendix 3– Clinical information heart block Aetiology: Congential Acquired 1. Idiopathic fibrosis 2. Myocardial infarction 3. Inflammatory process 1. Acute 2. Chronic 4. Drugs Clinical features Type 1 – Rarely symptoms Type 2 – • Wenkebach usually not a problem and may be normal in athletes/ asleep • Mobitz 2 usually a sign of severe disease and syncope may occur. Treated with a pace maker Type 3/complete – Syncope or stokes-adams attacks may occur. Also treated with pacemaker
Editor's Notes
- #4 Do chris’s ECG etc
- #5 **** Electrical Signals – Need to talk about the atria contracting before the ventricles. The heart controls itself. The SA node is where is begins. Fibres carry the impulse across both atria and then end up at the AV node. Atria and ventricle are insulated from each other electrically. The impulse passes through the bundle of His into the ventricles. It then passes down the septum to the apex. The impulse passes laterally and away from the apex towards the base via the purkinje fibres. Then if you imagine the heart is now altered electrically it has to return to normal (charges). A relaxation force flows backward across the ventricles to ‘neutralise’ it almost.
- #6 Look at how the electrical signals move through the heart in comparison to the heart. The P wave in when the impulse moves across the atria and how the wave is made with direction of flow.
- #7 The Q wave is the movement of the impulse through the AV node and the Bundle of His and across the septum.
- #8 The R wave is the movement of the impulse down to the apex
- #9 The S wave is moving away from apex and to base along purkinje fibres.
- #11 PLEASE LEARN THESE VALUES!
- #13 PLEASE LEARN THESE VALUES!
- #14 BPM = beats per minute
- #18 So in Summary: This schematic is quite a good way of working out what is going on. For each example given it is useful to think, with respect to the physiology, why it is happening. For example: Irregularly irregular pulses are commonly felt in elderly patients who are in AF. Why? The atria are not contracting as they are fibrillating (uncoordinated, very rapid attempts at contraction: jittery-like). The impulses are conducted, via the AV node, to the ventricles in a chaotic, IRREGULAR fashion and thus the ventricles receive the impulses in an IRREGULAR fashion; hence the irregular QRS pattern on the ECG Regularly irregular pulses can be felt in very fit or young patients when they are breathing (we showed you an example of this in the tutorial). Pulses like this would feel like : bum – bum – pause – bum – bum – pause - bum – bum – pause. Tap this out and you will see that the pulse is not regular, but there is a pattern to it, hence the name regularly irregular.
- #21 In summary Type 1 is simple. One big box is 5mm or 0.2 seconds. If the peak of the P wave and R wave is more than or equal to 1 box they have type 1 heart block. Type 2 is divided into Mobitz type 1 (the same as wenkebach) and mobitz type 2. In mobitz 1the PR interval is prolonged and with each successive cycle prolongs even more until finally there is a dropped QRS complex with an isolated P-wave. The cycle then resets and starts over again. Thus you might have 4 P-waves for every 3 QRS complexes. In mobitz type 2 you have a fixed PR interval and a dropped QRS complex in the same manner as type 1/Wenkebach. In advanced block a P wave is only associated with a QRS complex intermittently, and this occurs in a fixed ratio. There may be 2 P waves for every QRS (2:1 ratio), for example. This can occur in various ratios 3:1, 4:1, 5:1. Your AV node is not conducting impulses in type 3 AV block and thus there is complete dissociation between the P waves and QRS complexes. These patients often have a bradycardia (slow) heart rate. You can work out the atrial rate and ventricular rate separately and this might come up in the exam. To do this you would use the method outlined previously: count the number of boxes between each R-R wave for ventricular rate and P-P wave for atrial rate.
- #25 You can think about this logically. Remember that it is the AV node that is causing the delays and heart blocks. Thus any diseases affecting the AV node can lead to heart blocks. Acquired causes are common: for example after a myocardial infarction affecting the septum the AV node may be damaged and thus malfunction, leading to a type of heart block.