ECG Analysis

A systematic analysis of the surface ECG is crucial in the diagnostic process (Fig. 1). A good description allows comprehension of the trace and definition of the problem [1-4]. Paper speed and dimensions of grids on ECG paper have to be checked. The rhythm should be precisely defined. If it is not sinus rhythm the following criteria are useful to aid rhythm determination. The heart rate is usually calculated during determination of the rhythm. A small square represents 40 ms. When heart rate is below 60 bpm it is a bradycardia and when it is over 100 bpm it is a tachycardia. The duration, the axis and the morphology of the P wave have to be carefully checked. The P-R interval is the time required for completion of atrial depolarisation; conduction through the A-V node, His bundle and bundle branches; and arrival at the ventricular myocardial cells. Its value is between 120 and 200 ms. The duration, the axis, the morphology and the presence of Q waves should be evaluated. Abnormal Q waves have a duration > 40 ms and an amplitude of at least 25% of the QRS complex. Position of the ST segment should be checked to detect elevation or depression. Non specific changes should be placed in the clinical context. The T wave can be positive, negative or flat. It must also be analyzed within the clinical context. Conduction abnormalities will also modify the T wave and we should not forget that when depolarisation is abnormal, repolarisation is also abnormal. It is measured from the beginning of the QRS complex to the end of the T wave. QT interval duration varies with heart rate and shortens with tachycardia. Its correct value can be calculated with the Bazett formula.

Keywords: Cardiac rhythm, heart rate, sinus rhythm, P wave, T wave, QRS complex, PQ interval, QRS interval, QRS complex duration, QRS complex amplitude.