How to Read ECG

💻 How to read an ECG ECG Leads Basics: How to read an ECG 1 II, III, aVF = Inferior = Right coronary artery (also supplies AV node (+-heart block) and inferior wall (+-V1-4 ST depression)) V1, V2, V3, V4 = Anterior = Left anterior descending artery I, aVL, V5, V6 = Lateral = Left circumflex artery Note how V1 the R wave is always a negative deflection, but slowly becomes a positive deflection in V6. Note aVR should always be negative deflection. 1) Rate Do: 300/(number of large boxes in between R-R) or Number of QRS complexes in the rhythm strip (10 seconds) x 6 60-100 BPM is normal. 2) Rhythm Measure the distance between R-R and see if it is regular. Irregular rhythms are either: Regularly irregular Irregularly irregular 3) Axis Deviation How to read an ECG 2 Normal Cardiac Axis Right Axis Deviation Limb Lead I has negative deflection Limb Lead II has positive deflection (Right for Returning) ↙️ Indicates Right ventricular hypertrophy usually Left Axis Deviation Limb Lead I has positive deflection Limb Lead II has negative deflection (Left for Leaving) How to read an ECG 3 ↗️ Indicates heart conduction abnormalities 4) P Waves Are there P waves? Do QRS complexes follow P waves? If yes then it is sinus (ventricular contraction comes from SA node). Do P waves look normal? 2 small boxes wide Upright 5) PR Interval Is PR interval normal length? (3-5 small boxes) Prolonged if more than 1 large box (>0.2s) How to read an ECG 4 🛑 Indicates Atrioventricular delay (AV block) 1st Degree Heart Block FIXED prolonged PR interval. 2nd Degree Heart Block Mobitz Type 1 second degree Progressive prolongation of PR interval until a QRS is dropped. It is predictable. Mobitz Type 2 second degree Consistently prolonged PR intervals but randomly dropped QRS. Not predictable. How to read an ECG 5 3rd Degree Heart Block Complete heart block, where there is no conduction between atria (P waves) and ventricles (QRS complex). 🔌 Cardiac function is maintained by a junctional or ventricular pacemaker. 6) QRS Complex Width QRS should be no longer than 3 small boxes. If it is broad, then you CANNOT comment on ST changes. Broad QRS complexes indicate an ectopic depolarisation originating from the Ventricles (not sinus) How to read an ECG 6 ⛔ They indicate Left Bundle Branch Block (LBBB) or Right Bundle Branch Block (RBBB) Use the Mnemonic “WiLLiaM MaRRoW” LBBB = W in V1 and M in V6 RBBB = M in V1 and W in V6 If wide QRS and positive deflection in V1 (always abnormal) then it must be RBBB. Height Tall complexes imply Ventricular hypertrophy, or just tall and slim build. Morphology Q Waves Pathological Q waves in an entire area of the heart can show previous Myocardial Infarction. They are Q waves >25% of the R wave following the Q. R and S Waves How to read an ECG 7 R wave should be negative deflection in V1 but becomes positive deflection in V6. Abnormalities in this may suggest previous MI. J Point Segment J point is where S wave joins ST segment. This point can be elevated resulting in a raise ST segment, but this is not the same as an elevated ST! This is called “High take-off” or “Benign early repolarisation”. Usually under 50s J point is raised with widespread ST elevation in multiple leads, making ischaemia unlikely. T waves also raised (in contrast to normal T wave with ST elevation in STEMI) ST elevation in benign early repolarisations does not progress over time. STEMI’s ST elevation has evolving ST changes over time. 7) ST Segment It should be an isoelectric line in healthy people. ST Elevation Indicates Acute Myocardial Infarctions if in at least 2 leads. ST Depression Indicates Myocardial Ischaemia How to read an ECG 8 8) T Waves They represent ventricular repolarisation. Tall T Waves They are associated with: Hyperkalaemia Hyperacute STEMI Inverted T Waves T waves are normally inverted in V1 and sometimes Lead III. They are very common. Inversion in other leads is non-specific: Ischaemia Bundle branch blocks PE Hypertrophic cardiomyopathy How to read an ECG 9 Extra stuff If all waves are small, then consider a Pericardial Effusion. In atrial flutter, there should be a Sawtooth sign: How to read an ECG 10