By combining the magnitude of the measured voltage and the direction of the voltage a vector is formed. Each of these leads represents a measured voltage that also has a direction. The limb leads are leads AVR, AVL, AVF, I, II and III. Leads V1 and V2 look at the right ventricle, leads V3 and V4 look at the septum and leads V5 and V6 look at the anterior and lateral walls of the left ventricle. These leads look at the heart in a horizontal plane from the front and left side. The positive pole is the electrode itself and the negative pole is the centre of the heart. These are unipolar leads as they only have one associated electrode. There are six chest leads V1, V2, V3, V4, V5 and V6. The chest leads view the heart from a horizontal plane whilst the limb leads view the heart from a vertical plane The ECG leads are grouped into two electrical planes, horizontal and vertical. The 10 electrodes provide 12 views, hence the term ’12-lead ECG’. It plays no role in the formation of the ECG itself.Īn ECG lead is not actually a physical lead but instead a view of the hearts electrical activity from a particular angle across the body. The lead attached to the right leg is a neutral lead and is solely present to complete the electrical circuit. RL – Right leg (above the ankle and below the torso).LL – Left leg (above the ankle and below the torso).RA – Right arm (between shoulder and elbow).LA – Left arm (between shoulder and elbow).Conduction then spreads out via specialized tissue within the ventricular walls known as Purkinje fibres.įigure 2. The bundle of His divides in the septum between the two ventricles into the left and right bundle branches, which are situated in the left and right ventricular muscle respectively. Here the impulse is briefly slowed before continuing down the conduction pathway to the bundle of His. These electrical impulses stimulate the atria to contract and then travel to the atrioventricular node (AV node), which is located in the inter-atrial septum. The SA node is located in the right atrium and automatically generates an electrical impulse 60-100 times per minute under normal conditions. The sinoatrial node (SA node) is the pacemaker of the heart and is the point of origin of the electrical impulses that are propagated through the heart. The pumping action of the heart is controlled by the cardiac conduction system. The heart is often compared to a pump that is made up of muscle. In order to be able to interpret an ECG, a basic understanding of the heart’s electrical conduction system is required. The Electrical Conduction System of the Heart The ECG is undoubtedly one of the most useful investigations available to us in medicine. If a stepwise, logical approach is taken the ECG should be a simple process and can provide an immense amount of useful information, especially when used in conjunction with a patient’s clinical presentation. Many medical students, and even some more experienced medical professionals, struggle with ECG interpretation. The electrocardiogram, or ECG, is a simple diagnostic test which records the electrical activity of the heart over a set time period via the process of attaching a series of electrodes at particular points on a patient’s body.
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