It is important to develop a routine when reading an ECG; one is suggested below.

(1) Calculate the heart rate.
  • Count the number of complexes over 6s and multiply by 10.
  • Count both the P rate and the QRS rate if these differ.
  • Remember to note the paper speed before marking a 6-s period.
(2) Analyse the rhythm.
  • Note whether all complexes are complete, i.e. there is a P wave for every QRS and vice versa.
  • If there are QRS complexes without an associated P wave, are the QRS
  • complexes:
    • normal or abnormal in shape (bizarre, prolonged, inverted);
    • early (premature) or very late (escapes). If early, they occur before the next QRS complex is due and therefore have a shorter R-R interval than normal complexes. If late, they occur well after a QRS was due.
(3) Measure the complex amplitudes and intervals (Fig. 3.5 ).
(4) Mean electrical axis (MEA).
  • This is of debatable value in animals, as the vector in the frontal plane is not always representative of the true direction of the vector in three dimensions.
  • MEA is mainly used to assist in the assessment of ventricular enlargement and in the recognition
  • of intraventricular conduction defects.

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