![]() The ST segment can be normal, elevated or depressed.The QRS can also be tall in young, fit people (especially if thin).PR interval: 0.12 – 0.2 secs (3-5 small squares).PR interval: Represents the time taken for excitation to spread from the sino-atrial (SA) node across the atrium and down to the ventricular muscle via the bundle of His.What do the segments of the ECG represent? ![]() All boxes are based on the assumption that the paper speed is running at 25mm/sec, therefore 1 large square is equivalent to 0.2 secs and a small square to 0.04 secs. For the purpose of this we will look at lead II (see Figure 4). The ECG can be broken down into the individual components. aVF = inferior territory (remember ‘F’ for ‘feet’).The areas represented on the ECG are summarized below: ![]() The electrical activity on an ECG (EKG).įigure 3. This can often be seen in V4 (see Figure 3).įigure 2. If it is at 90 degrees then the complex is ‘isoelectric’ i.e. When the activity travels away from the lead the deflection is net negative. When electrical activity (or depolarisation) travels towards a lead, the deflection is net positive. These comprise 4 limb electrodes and 6 chest electrodes.įigure 1. The leads can be thought of as taking a picture of the heart’s electrical activity from 12 different positions using information picked up by the 10 electrodes. The 12-lead ECG misleadingly only has 10 electrodes (sometimes also called leads but to avoid confusion we will refer to them as electrodes). To start with we will cover the basics of the ECG, how it is recorded and the basic physiology. Having a good system will avoid making errors. The location of the conduction disturbance is at the level of the AV node and therefore the QRS complex will be narrow.As with all investigations the most important things are your findings on history, examination and basic observations. Escape beats (atrial, junctional, or ventricular) may occasionally occur during the pause in the ventricular rhythm, and may obscure the diagnosis because they interrupt the group beating pattern ( Figure 8-22). The overall appearance of the rhythm demonstrates group beating (groups of beats separated by pauses) and is a distinguishing characteristic of Mobitz I. After each dropped beat the cycle repeats itself. ![]() The missing QRS complex (dropped beat) causes the ventricular rhythm to be irregular. This rhythm is reflected on the ECG by P waves that occur at regular intervals across the rhythm strip and PR intervals that progressively lengthen from beat to beat until a P wave appears that is not followed by a QRS complex, but instead by a pause. In Mobitz I, the sinus impulse is normally conducted to the AV node, but each successive impulse has increasing difficulty passing through the AV node, until finally an impulse does not pass through (isn’t conducted). This rhythm ( Figures 8-20, 8-21, 8-22 and 8-23 and Box 8-6) is characterized by a failure of some of the sinus impulses to be conducted to the ventricles. Second-degree AV block, type I is commonly known as Mobitz I or Wenckebach (for the early 20th century physician who discovered it). PJCs are less common than PACs or premature ventricular contractions (PVCs) (discussed in Chapter 9). Figure 8-4 shows a PJC with the P wave before the QRS complex Figure 8-5 shows a PJC with the P wave after the QRS complex and in Figure 8-6 the P wave is hidden within the QRS. The PR interval will be short (0.10 second or less). The inverted P waves will occur immediately before or after the QRS, or will be hidden within the QRS complex. Because atrial depolarization occurs in a retrograde fashion with the PJC, the P wave associated with the premature beat will be negative in lead II (a positive lead). Some differences exist, however, between the two premature beats. Like the premature atrial contraction (PAC), the premature junctional beat is characterized by a premature, abnormal P wave and a premature QRS complex that’s identical or similar to the QRS complex of the normally conducted beats, and is followed by a pause that is usually noncompensatory. A premature junctional contraction (PJC) ( Figures 8-3, 8-4, 8-5, 8-6, 8-7 and 8-8 and Box 8-1) is an early beat that originates in an ectopic pacemaker site in the AV junction.
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