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:





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.

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Related posts:

  1. The ECG When the Patient has a Bradycardia
  2. The ECG When the Patient has a Tachycardia
  3. The ECG in Healthy People
  4. The ECG in Patients with Chest Pain

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Jul 26, 2016 | Posted by in CARDIOLOGY | Comments Off on Conclusions: Four Steps to Making The Most of the ECG

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