Conclusions: Four Steps to Making The Most of the ECG

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Conclusions

Four Steps to Making the Most of the ECG

The theme of this book has been that the ECG is just one way of helping with the management of patients. The ECG is not an end in itself, and must always be seen in the context of the patient from whom it was recorded. To make the most of an ECG you need to think in four steps:

1. Describe it.

2. Interpret it.

3. See how it helps with the diagnosis.

4. Ask how it helps with treatment.

DESCRIPTION

An ECG can be described by anyone with the most basic knowledge, and an accurate description is needed as a basis for the later steps. The description starts with the heart rate and regularity, as measured by the intervals between the QRS complexes. The P waves must be identified; and if there are none, a clear statement of their absence is necessary. The relationship of the P waves to the QRS complexes is the next logical step, and the PR interval must be measured. The shape of the P wave needs to be recorded if it is abnormally peaked or bifid.

The QRS complexes need to be described in terms of their width and height, and also their shape: whether Q waves are present; whether there is more than one R wave in the QRS complex; and whether there are S waves in the leads where they would be expected. If there are Q waves, are they small and narrow, and are they only seen in the lateral leads, where they may be due to septal depolarization? If there are pathological Q waves, in which leads are they present and do they suggest a possible inferior or anterior myocardial infarction? The cardiac axis should be defined.

Elevation or depression of the ST segment must be noted. If the ST segment is elevated, does it follow an S wave, so indicating high take-off? The T waves must be inspected in each lead, and while inversion in VR and V₁ is always normal, inversion in any other leads should be recorded. The QT interval should be measured, and if it appears long, should be corrected for heart rate.

All these features can be identified without any knowledge of the patient, or indeed much knowledge of cardiology. The description of an ECG is reasonably well done by the automatic ‘interpretation’ function built into most modern ECG recorders, but it is important to remember that these are far from perfect. Automatic recorders tend to over-interpret ECGs so that nothing of importance is missed, and their descriptions are not always totally accurate. They can be poor at identifying P waves and they often miss ST segment changes, and sometimes T wave inversion. Therefore, you should never depend solely on a description provided by the ECG recorder itself.

INTERPRETATION

Always establish the cardiac rhythm first, because it may influence your interpretation of the rest of the ECG. For example, ventricular tachycardia, with its broad QRS complexes, will prevent any further interpretation – as will the broad complexes of complete heart block. The rhythm is established from the presence or absence of P waves and their relationship to the QRS complexes, from which arrhythmias and conduction defects can be accurately identified. On the whole, this part of ECG interpretation can be independent of the patient.

Otherwise, the accurate interpretation of an ECG should depend on a knowledge of the patient. If the ECG has been recorded from a healthy subject, or a patient with no clinical suggestion of cardiac disease, then it is essential to remember the range of normality of the ECG. First degree block, and supraventricular or ventricular extrasystoles, are commonly seen in healthy people. P waves can be bifid in healthy people; right axis deviation can be normal in tall thin people; and minor degrees of apparent left axis deviation with a narrow QRS complex can occur in fat people and in pregnancy. An RSR¹ pattern with a normal QRS complex duration in lead V₁ is perfectly normal, and in some perfectly normal people there can be a small dominant R wave in lead V₁. Tall QRS complexes are frequently seen in healthy young people, and do not in themselves indicate left ventricular hypertrophy. Septal Q waves may be present in leads VL and V₅− V₆. Inverted T waves in the anterior chest leads can be normal in black people, while in white people they may be due to hypertrophic cardiomyopathy. Peaked T waves are often of no significance at all, though they can be due to hyperkalaemia.

In a patient with chest pain, however, the interpretation of the same ECG abnormalities can be quite different. T wave inversion in the anterior chest leads may indicate an NSTEMI. Left bundle branch block may be the result of an old or new infarction. A change of cardiac axis to the right may be due to a pulmonary embolism. A dominant R wave in lead V₁ might be due to a posterior myocardial infarction.

In a patient with breathlessness, right axis deviation, a dominant R wave in lead V₁ or T wave inversion in leads V₁− V₃ may indicate multiple pulmonary emboli or idiopathic pulmonary hypertension. A deep S wave in lead V₆ may be due to chronic lung disease or to a pulmonary embolus. In patients complaining of attacks of dizziness, a finding such as first degree block, of little significance in a healthy subject, might indicate transient episodes of higher degrees of block causing a symptomatic bradycardia. A prolonged QT interval might point to episodes of torsade de pointes ventricular tachycardia.

Any described abnormality in an ECG must therefore be interpreted in the context of a knowledge of the patient’s condition; otherwise, ECG changes will support a less focused differential diagnosis.

DIAGNOSIS

The ECG is essential for the diagnosis of problems involving rhythm and conduction, in which the interpretation and the diagnosis are clearly strongly linked. But it is necessary to remember that the identification of a specific arrhythmia does not complete the diagnosis, which should include the cause of the arrhythmia. For example, the cause of atrial fibrillation may be ischaemic or rheumatic heart disease, or alcoholism, or thyrotoxicosis, or a cardiomyopathy, and so on. Heart block may be due to idiopathic His bundle fibrosis, but it also raises the possibility of ischaemic or hypertensive heart disease. Left bundle branch block may be due to aortic stenosis, and right bundle branch block may be associated with an atrial septal defect.

ECG appearances that suggest faults in the recording technique may sometimes point to a clinical diagnosis. For example, artefacts due to movement may suggest a neurological disorder such as Parkinson’s disease. Low-voltage QRS complexes may be due not to poor standardization but to obesity, emphysema, myxoedema or a pericardial effusion.

An ECG cannot diagnose the presence of heart failure, though with a totally normal ECG, heart failure is unlikely. The ECG may, however, help in diagnosing the cause of heart failure, which is often the key to treatment – atrial fibrillation, ventricular hypertrophy or left bundle branch block may suggest valve disease, or there may be evidence of an old myocardial infarction. Similarly, the ECG is not a good way of identifying electrolyte abnormalities, but flat T waves, U waves, and long QT intervals should at least suggest the possibility that there may be an electrolyte problem. A long QT interval, on the other hand, may be due to one of the congenital syndromes or to one of a wide variety of drugs.

The accurate identification of an ECG abnormality is thus only part of the diagnostic process: we still need to determine the underlying cause. The ECG often points the way to appropriate further investigations, such as chest X-rays, echocardiography, blood tests for electrolyte abnormalities, or cardiac catheterization, and the ECG is simply part of the diagnostic process.

TREATMENT

The ECG is obviously paramount in determining the treatment of an arrhythmia or conduction defects. It is also crucial for the proper use of acute interventions in both STEMIs and NSTEMIs. But its limitations must also be understood: in particular, it must be remembered that the ECG can be normal in the early stages of a myocardial infarction, and a normal, or near-normal, ECG is not an adequate reason for sending a patient with chest pain home from an A & E department.

Without an understanding of the ECG, devices such as pacemakers and implanted cardioverter defibrillators (ICDs) could not have been invented. These devices, and the techniques that use them, such as dual chamber pacing and cardiac resynchronization therapy, are the province of the specialist. But as the devices and techniques become increasingly prevalent, they will be encountered more and more by general practitioners and specialists in non-cardiac disciplines. For example, patients with these devices tend to be elderly, and it is the elderly who most frequently experience multiple medical problems – so non-cardiac specialists are bound to come across patients who have problems, but who also have a modern electrical device that is working perfectly normally.

CONCLUSION

The ECG is easy to describe and interpret, but it is often more difficult to appreciate the range of normality, and to remember that a full diagnosis encompasses the cause of any abnormality that may have been identified. The ECG is an essential part of the overall diagnostic process in a wide variety of patients, and in some it influences treatment. The most important thing to remember is that diagnosis and management depend on a full consideration of the individual patient, not just of the ECG.

NOW TEST YOURSELF

150 ECG Problems, a companion to this volume, gives 150 clinical scenarios with full related ECGs, and poses questions about ECG interpretation and the diagnosis and management of patients.

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The ECG In Practice