Accurate interpretation of the ECG is an essential skill for all health professionals. Using a unique self-assessment format, this book presents a comprehensive, incremental approach to ECG interpretation, progressing from basic to advanced concepts in electrocardiography. Amply illustrated with electrocardiograms both in the main text and the self-assessments, ECG Interpretation is a must-have practical guide that features:
- An appealing, user-friendly format that will help with exam preparation
- Clearly defined learning objectives to guide readers efficiently through the intricacies of ECG interpretation
- Numerous practical examples of ECG strips to illustrate important concepts, including clean ECG strips to practice skills
- Multiple-choice questions to consolidate learning and emphasize pertinent facts
Author by:
Zainul Abedin, MD, FRCP (C), FHRS
Robert Conner, RN
Product Details
* Paperback: 240 pages
* Publisher: Wiley-Blackwell; 2 edition (November 28, 2007)
* Language: English
* ISBN-10: 1405167491
* ISBN-13: 978-1405167499
Contents
- Complexes and intervals
- Mean QRS axis determination
- The normal electrocardiogram : Self-Assessment Test One
- Intraventricular conduction defects
- Myocardial ischemia and infarction: Self-Assessment Test Two
- Chamber enlargement and hypertrophy
- Acute pericarditis
- Sinus rhythm and its discontents: Self-Assessment Test Three
- Atrioventricular block
- Atrial arrhythmias: Self-Assessment Test Four
- Supraventricular re-entrant tachycardia
- The Wolff–Parkinson–White syndrome: Self-Assessment Test Five
- Junctional arrhythmias
- Ventricular arrhythmias
- The channelopathies
- Electronic pacing: Self-Assessment Test Six
Index
CHAPTER 1
Complexes and intervals
An electrocardiogram (ECG) is a recording of cardiac electrical activity made from the body surface and displayed on graph paper scored horizontally and vertically in 1 millimeter (mm) increments. Each millimeter on the horizontal axis represents 40 milliseconds (0.04 second) of elapsed time and each millimeter on the vertical axis represents 0.1 millivolt (mV) of electrical force. Each 5 millimeter mark on the paper is scored with a heavier line representing 200 milliseconds (msec) or 0.20 seconds on the horizontal axis or time line and 0.5 millivolt on the vertical axis or amplitude line. Recordings of electrical activity made from within the cardiac chambers are called intracardiac electrograms.
Paper used for routine cardiac monitoring is marked across the top by small vertical lines placed at 3-second intervals. Heart rate per minute can be rapidly estimated by counting the number of beats in a 6-second recording and multiplying that number by 10, or can be precisely calculated by counting the number of small squares between complexes and dividing that number into 1500. All monitoring systems currently marketed display the heart rate both on screen and on paper recordings.
The complexes
An electrocardiogram consists of only two elements: complexes and intervals. The normal complexes are (1) the P wave, (2) QRS complex, (3) T wave, and (4) U wave (Figure 1.1).
The P wave represents depolarization of the atrial myocardium. Normal P waves are rounded, do not exceed 0.25 mV (2.5 mm) in amplitude in any lead or exceed 110 milliseconds (0.11 second) in duration. Normal P wave axis is +15 to +75 degrees in the frontal plane leads. The amplitude of the P wave is measured from the baseline or isoelectric line to the top of the waveform. Because the right atrium is depolarized slightly before the left atrium, the first half of the P wave represents right atrial depolarization and the last half left atrial depolarization, but normally these events overlap, producing a single deflection.
Figure 1.2 correlates the features of the surface ECG with cardiac electrical events. It is essential to note that sinus node discharge (1) is electrocardiographically silent on surface tracings, as is conduction through the atrioventricular node (4), the bundle of His and bundle branches (5).
The recovery sequence can be divided into three phases: (1) the absolute refractory period (7), during which the conduction structures are unresponsive to any stimulus; the supernormal period (8), and the relative refractory period (9), during which the conduction tissues will transmit an impulse, but typically at a slower rate than is normally observed. Refractory periods shorten and lengthen incrementally as the heart rate accelerates or slows, i.e. as the cycle length changes. Therefore the exact length of the refractory periods will vary according to the heart rate and the health of the conduction system.
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