If the R and S waves are of equal size it means depolarisation is travelling at exactly 90° to that lead. If the S wave is greater than the R waves it suggests depolarisation is moving away from that lead. If the R wave is greater than the S wave it suggests depolarisation is moving towards that lead. The lead with the most positive deflection is most aligned with the direction the heart’s electrical activity is travelling. the higher the deflection, the greater the amount of electrical activity flowing towards the lead). The height of the deflection represents the amount of electrical activity flowing in that direction (i.e. wave) on the ECG represents the average direction of electrical travel (which is calculated using mathematical formulae by the ECG machine). In reality, electrical activity in the heart flows in many directions simultaneously.Įach deflection (a.k.a. When the electrical activity within the heart travels away from a lead you get a negative deflection. When the electrical activity within the heart travels towards a lead you get a positive deflection. This is because each lead is recording the electrical activity of the heart from a different direction (a.k.a viewpoint).
Lead I: lateral view (calculated by analysing activity between the RA and LA electrodes).Black (RL): on the medial or lateral malleolus of the right legĪn ECG lead is a graphical representation of the heart’s electrical activity which is calculated by analysing data from several ECG electrodes.Green (LL): on the medial or lateral malleolus of the left leg.Yellow (LA): on the ulnar styloid process of the left arm.Red (RA): on the ulnar styloid process of the right arm.Chest electrode positions Limb electrodes You might also be interested in our OSCE Flashcard Collection which contains over 2000 flashcards that cover clinical examination, procedures, communication skills and data interpretation. V6:left mid-axillary line at the same horizontal level as V4 and V5.V5:left anterior axillary line at the same horizontal level as V4.V4:5th intercostal space in the midclavicular line.V3:midway between the V2 and V4 electrodes.V2:4th intercostal space at the left sternal edge.V1:4th intercostal space at the right sternal edge.The electrodes used to generate a 12 lead ECG are described below. lead I is calculated using data from the electrodes on both the right and left arm). The data gathered from these electrodes allows the 12 leads of the ECG to be calculated (e.g. Only 10 physical electrodes are attached to the patient, to generate the 12 leads.
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It is important to understand the difference between an ECG electrode and an ECG lead.Īn ECG electrode is a conductive pad that is attached to the skin to record electrical activity.Īn ECG lead is a graphical representation of the heart’s electrical activity which is calculated by analysing data from several ECG electrodes.Ī 12-lead ECG records 12 leads, producing 12 separate graphs on a piece of ECG paper. Each large square represents 0.2 seconds.Each small square represents 0.04 seconds.The paper used to record ECGs is standardised across most hospitals and has the following characteristics:
It represents the time taken for the ventricles to depolarise and then repolarise. The QT interval begins at the start of the QRS complex and finishes at the end of the T wave. It represents the time between two QRS complexes. The RR interval begins at the peak of one R wave and ends at the peak of the next R wave. It appears as a small wave after the QRS complex. The T wave represents ventricular repolarisation.
The ST segment is an isoelectric line that represents the time between depolarisation and repolarisation of the ventricles (i.e. The ST segment starts at the end of the S wave and ends at the beginning of the T wave. It appears as three closely related waves on the ECG (the Q, R and S wave). The QRS complex represents depolarisation of the ventricles. It represents the time taken for electrical activity to move between the atria and the ventricles. The PR interval begins at the star t of the P wave and ends at the beginning of the Q wave. In healthy individuals, there should be a P wave preceding each QRS complex. You may also be interested in our ECG flashcard deck which can be purchased as part of our collection of 2000+ OSCE flashcards.