Thursday, August 25, 2016
PREFACE – LỜI NÓI ĐẦU
Learning about ECG interpretation from books such as The ECG Made Easy or The ECG in Practice is fine as far as it goes, but it never goes far enough. As with most of medicine there is no substitute for experience, and to make the best use of the ECG there is no substitute for reviewing large numbers of them. ECGs need to be interpreted in the context of the patient from whom they were recorded. You need to learn to appreciate the variations of normality and of the patterns associated with different diseases, and to think about how the ECU can help patient management.
Although no book can be a substitute for practical experience, 150 ECG Problems goes a stage nearer the clinical world than books that simply aim to teach ECG interpretation. It presents 150 clinical problems in the shape of simple case histories, together with the relevant ECG. It then invites the reader to interpret the ECG in the light of the clinical evidence provided, and to decide on a course of action before looking at the answer. Having seen the answers, the reader may feel the need for more information, so each one is cross-referenced to The ECG Made Easy and/or The ECG in Practice.
The ECGs in 150 ECG Problems range from the simple to the complex. About one-third of the problems are of a standard that a medical student should be able to cope with, and should be answered correctly by anyone who has read The ECG Made Easy. A junior doctoi specialist nurse or paramedic should get another third right, if they have read The ECG in Practice. The remainder should challenge the MRCP candidate. As a very rough guide to the level of difficulty of each problem, each answer is graded using stars (see the summary box of each answer): one star represents the easiest records, and three stars the most difficult.
The ECGs are arranged in random ordei not in order of difficulty, to maintain the reader’s interest. Readers are invited to attempt their own interpretation before looking at the star rating - after all, in a real-life situation one never knows which patient will be easy and which will be difficult to diagnose or treat. In this fourth edition there are many new ECGs, mainly to provide examples that reproduce more clearly. However to maintain the “real world” approach, some technically poor records have deliberately been included. The balance between easy, moderately difficult and very difficult records has been maintained.
I am extremely grateful to Alison Gale, my copy-editor and to Rich Cutler of Helius. Their patience, understanding and attention to detail made the preperaration of this new edition an easy and satisfying experience for me.
INTRODUCTION: MAKING THE MOST OF THE ECG
LỜI GIỚI THIỆU: KHAI THÁC TỐI ĐA ECG
Recording and reporting an ECG should never be an end in itself. The ECG is a basic and valuable tool in the investigation of cardiac problems, and it can be helpful in the case of non-cardiac problems too, but it must always be viewed in the context of the patient from whom the record came. The ECG must never be a substitute for taking a proper medical history and carrying out a careful physical examination. Because it is simple, harmless and cheap, the ECG is usually the first investigation in a patient with possible cardiac disease and it may be followed by the plain chest X-ray, the echocardiogram, radionuclide studies, CT and MR imaging, and cardiac catheterization and angiography - but none of these are substitutes. The ECG, a recording of the electrical activity of the heart, gives information that can be obtained in no other way. However even though it is irreplaceable, it is not infallible.
ECGs are recorded from a wide variety of patients, in an attempt to help with a wide variety of possible diagnoses. An ECG is frequently recorded in the course of ‘health screening’, but here it must be regarded with considerable caution. It can not be assumed that individuals who present themselves for screening are asymptomatic - the process may be being used as a substitute for a consultation with a doctor. The ECG itself may cause difficulties of interpretation, for there are a dozen or more normal variants. Minor abnormalities, such as nonspecific ST segment or T wave changes, will have diagnostic and prognostic significance if the individual has symptoms that may be cardiac in origin, but these changes can be of no importance in totally healthy people. It is rare for an ECG to demonstrate anything of importance in a totally healthy individual, although in athletes the detection of abnormalities suggesting asymptomatic hypertrophic cardiomyopathy is important.
In patients with chest pain, the ECG is important but sometimes misleading. It is essential to remember that the ECG can remain normal for some hours after the onset of a myocardial infarction. Too often patients are sent home from an A & E department because their ECG is normal, despite a reasonably convincing story of ischaemic chest pain. Under such circumstances the ECG should be repeated several times to see if changes are appearing, and patient management should depend on the plasma troponin level rather than on the ECG. Nevertheless the ECG is important for deciding treatment in a patient with chest pain, for the management of a patient with myocardial infarction with ST segment elevation is quite different from that of a patient whose ECG shows a non-ST segment elevation infarction.
Patients with intermittent chest pain that could be angina frequently have completely normal ECGs at rest - and then the exercise test can be valuable. The exercise test is to some extent being replaced by myocardial perfusion scanning for the diagnosis of coronary disease because its predictive accuracy depends on the likelihood of the patient having angina, because there can be false negative or false positive results, and because exercise tests are sometimes unreliable in women. Remember that an exercise test is safe, but not totally safe, because arrhythmias (including ventricular fibrillation) may be induced. Nevertheless the exercise test has the great advantage of showing a patient’s exercise tolerance, and also showing what limits his capability.
The ECG also has a role in the investigation of patients with breathlessness, for it can show changes associated with heart disease (e.g. an old myocardial infarction) or with chronic chest disease. Evidence of left ventricular hypertrophy may point to hypertension, mitral regurgitation or aortic stenosis or regurgitation, and right ventricular hypertrophy may be the result of pulmonary emboli or mitral stenosis - However all of these should have been detected during the examination of the patient. The ECG is not a good tool for grading the hypertrophy of the different heart chambers. It is particularly important to remember that the ECG cannot demonstrate heart failure: it may suggest a condition that may cause heart failure,
but is impossible to determine from an ECG whether a patient is in heart failure or not. However in the presence of a completely normal ECG, heart failure is certainly unlikely.
There are characteristic ECG appearances in several conditions that are not primarily cardiac - for example with severe electrolyte derangement. ECG monitoring is not an acceptable way of following electrolyte changes in conditions such as diabetic ketoacidosis, but at least any abnormalities may prompt the appropriate biochemical tests. The ECG has, However become important in the development of new drugs, for any drug that causes QT prolongation - and this is by no means uncommon - may cause sudden death due to ventricular tachycardia.
It is in the investigation and management of patients with possible arrhythmias that the ECG is of paramount importance. Patients may complain of palpitations or dizziness and syncope as a result of rhythm disturbances, and there is no way of identifying these with certainty other than with an ECG. Dizziness and syncope can be the result of rhythms that are either too fast or too slow for an effective cardiac output, or of slow rhythms associated with disorders of conduction. There may be little in the patient’s history to point specifically to a cardiac problem when dizziness or collapse is the main symptom, but an appropriately abnormal ECG may immediately point to the right diagnosis. When a patient complains of palpitations there is a clearly a heart problem of some sort, and it is usually possible to come close to a diagnosis by taking a careful history - the patient with extrasystoles will describe the heart ‘jumping out of the chest’ or something equally unlikely, and the problem will be worse when lying down at night, and after smoking and alcohol. The patient with a true paroxysmal tachycardia will describe the sudden onset (and sometimes the sudden cessation) of the rapid heartbeat, and if the attack is associated with chest pain, dizziness or breathlessness then the presence of a paroxysmal tachycardia becomes highly likely.
Few patients will have their arrhythmia at the time they are seen, but the ECG can still give valuable clues to its nature. A patient whose ECG shows bifascicular block, or first degree atrioventricular block together with left bundle branch block, may have intermittent complete block and Stokes-Adams attacks. A patient whose ECG shows pre-excitation (the Wolff-Parkinson-White or Lown-Ganong-Levine syndromes) is at risk of paroxysmal arrhythmias - though many people with these ECG patterns never have any problems at all. A patient with a prolonged QT syndrome, as a result of either a congenital defect or drug treatment, is at risk of torsade de pointes ventricular tachycardia. Under all these circumstances, ambulatory ECG recording, by one of a variety of techniques, may demonstrate the true nature of the arrhythmia that causes the symptoms - but it must be remembered that many, if not most, arrhythmias will be seen transiently in completely healthy people and only when an abnormal ECG corresponds to symptoms can one be certain that the two are related.
So the way to approach the ECG, and this book - and indeed any medical situation - is to start with the patient. If you cannot make a reasonable diagnosis from the history, and to a lesser extent the examination, the chances of doing so as a result of investigations are not great. The role of the ECG and of more complex investigations is to help differentiate between the various possible diagnoses suggested by talking to, and examining, the patient. The clinical scenarios given with each ECG in this book are of necessity brief, but think about them, ask yourself what the diagnosis might be, and then describe and report on the ECG. That is the way to make the most of the ECG.