Contributors: Lisa Moore, Jill Sammarco, Simon Swift

 Species: Canine   |   Classification: Miscellaneous


  • The interpretation should be based on a good ECG recording ECG: overview.
  • Four features should be examined on every ECG:
    • Heart rate.
    • Heart rhythm.
    • Complexes, dimensions and intervals.
    • Mean electrical axis (MEA).
  • The largest positive (upward) deflections are usually the R waves.
  • The Q and S waves are usually negative waves immediately preceding or following the R wave respectively.
  • Q and S waves may not always be present in normal individuals.
  • The T wave is the deflection following the S or R waves after a short pause.
  • The P wave preceeds the Q wave and is usually positive ECG sinus rhythm.

Heart rate

  • Calculated from the lead 2 rhythm strip by counting the number of R waves in 6 seconds and multiplying by 10 ECG sinus rhythm.
  • This gives the ventricular rate and can be used if the heart rate is regular or irregular.
  • If P waves do not precede every QRS complex, a separate atrial rate should be calculated using a similar method with the P waves.
  • If the rate is regular, a heart rate calculator ruler can be used.
  • Alternatively, the number of small boxes on the ECG paper between two R waves can be counted and divided into 3,000 (for a paper speed of 50 mm/s) or 1500 (25 mm/s).
  • This gives the heart rate in beats per minute (bpm).
  • If the heart rate is rapid, it is called tachycardia ECG: sinus tachycardia.
  • If it is slow, the term bradycardia ECG sinus bradycardia can be applied.

Heart rhythm

  • Examine the ECG systematically and have a good working knowledge of the normal anatomical and physiological properties of the impulse-forming and conduction systems in the heart.

General inspection

  • The overall rhythm should be determined which should include whether the heart is in normal sinus rhythm ECG sinus rhythm or an arrhythmia Heart: dysrhythmia is present.
  • An arrhythmia is a deviation in the normal origin of the heart beat or in the conduction of the impulse across the heart. If an arrhythmia is present, it should be noted whether it is occasional, frequent or continuous.
  • Also, the arrhythmia may be regular, or irregular and repetitive, or multiform. If an arrhythmia is detected, a long lead II rhythm strip should be taken.

Measure complexes and intervals

  • Normal values have been established for these parameters:
    • P wave width: 0.04 seconds.
    • P wave height: 0.4 mV.
    • P-R interval: 0.07-0.14 seconds.
    • QRS width: 0.06 seconds.
    • QRS height: 3.0 (large) mV.
    • QRS height: 2.5 (small) mV.
    • Q-T interval: 0.15-0.25 seconds depending on heart rate.
    • S-T segment: +/- 0.2 mV.
    • MEA: +40-+100.

P waves

  • A positive rounded P wave indicates it is generated in the sino-atrial (SA) node in lead II.
  • Variation from this shape can indicate that the P wave is generated elsewhere in the atrium (ectopic P wave) and a negative P wave suggests that the P wave originated in or near the atrioventricular (AV) node ECG wandering pacemaker.
  • Absence of P waves indicates:
    • Atrial fibrillation ECG atrial fibrillation.
    • Persistent atrial standstill ECG persistent atrial standstill.
    • Low voltage atrial activity.
    • P waves buried in QRS complexes as in AV junctional rhythms ECG junctional rhythm.

Interpretation of P waves

  • Tall P waves: right atrial enlargement, P pulmonale.
  • Wide P waves: left atrial enlargement ECG LA enlargement , P mitrale.
  • The P wave may be biphasic.
  • The P wave may have varying appearance on the same ECG tracing, wandering pacemaker.

QRS complexes

  • The configuration, uniformity and regularity of the complexes should be noted.
  • If the QRS is a normal width and configuration, it is likely to be supraventricular (from above the bundle of His) in origin ECG sinus rhythm.
  • If the QRS is wide and bizarre, there may be ectopic ventricular activity ECG ventricular premature complexes or abnormal intraventricular conduction.

Interpretation of QRS complexes

  • Tall or wide: left ventricular enlargement.
  • Wide complexes also occur in left bundle branch block Heart: right and left bundle branch block ECG left bundle branch block 01.
  • Deep S waves in leads 1, 2, 3 and AVF: right ventricular enlargement or right bundle branch block ECG right bundle branch block 01.

Relationship of P wave to QRS wave

  • The P-R interval (the time from the start of the P wave to the start of the QRS wave) is a measure of atrial conduction, AV conduction and AV node conduction.
  • Short P-R intervals occur if there is an accessory pathway, (eg Wolff-Parkinson-White Syndrome Heart: Wolff-Parkinson-White syndrome Heart: WPW - anatomic arrangement and electrical pathway ) or in AV junctional rhythms where the P wave is close to the QRS.
  • Long P-R intervals indicate first degree heart block and can be seen in digoxin toxicity Digoxin assay Digoxin toxicity.
  • When every QRS is not preceded by a P wave, 2nd degree heart block may have occurred ECG second degree AV block.
  • If the P-R interval gradually lengthens until an unconducted P wave occurs, this is called Mobitz type 1 2nd degree AV block or Wenckebach phenomenon.
  • If the P-R interval is constant until an unconducted P wave occurs, the block is called Mobitz type 2 2nd degree AV block.
  • These cases may progress to 3rd degree heart block ECG 3rd degree heart block.
  • When there is no relationship between the P waves and QRS waves, complete (3rd degree) heart block has occurred ECG 3rd degree heart block.
  • These cases are usually symptomatic with severe bradycardia and require pacemaker implantation Heart: pacemaker.
  • It can be useful to plot the P waves on a piece of card and slide it along the tracing. That way, P waves can be seen to merge in and out of the QRS complexes.
  • In complete heart block, P waves continue to be generated by the SA node.
  • The QRS complex, however, is generated by an ectopic pacemaker in the ventricular conduction tissue or myocardium Heart: 3rd degree atrioventricular block.

S-T segment

  • This may be elevated or depressed and is significant if >0.1 mV.
  • ST segment variations usually represent myocardial hypoxia ECG S-T segment abnormality.

Q-T interval

  • This is measured ECG sinus rhythm from the start of the QRS complex to the end of the T wave and hence represents total electrical systole.
  • It varies markedly with heart rate and changes are said to occur with calcium Hypercalcemia: overview or potassium Blood biochemistry: potassium imbalances.
  • Digoxin Digoxin and quinidine Quinidine may also cause changes.

T wave

  • The T wave is a reflection of the previous QRS complex ECG sinus rhythm.
  • Hence an abnormal QRS will cause an abnormal T wave.
  • The T wave should not exceed 30% height of the R wave but as many variables affect the T wave, the significance of changes is debatable.
  • Tall T waves may represent myocardial hypoxia, intraventricular conduction defects, electrolyte imbalances, drug toxicities and metabolic disease.
  • Hyperkalemia is the most important and moderate increases in potassium will cause tall spiked T waves.
    'No pot, no tea'.

Small complexes

  • These are typical of effusions such as pericardial or pleural.
  • Small complexes alternating with normal complexes, electrical alternans; is often seen with pericardial effusion.
  • Occasionally small complexes are seen in obese patients.

Mean electrical axis (MEA)

  • The MEA is the average direction of ventricular depolarisation and is calculated from a vector of the summation of the QRS complexes in the frontal plane.
  • It can be used to indicate which ventricular chamber predominates and can be calculated several ways.
  • It is also affected by intraventricular conduction.

Isoelectric lead method

  • The isoelectric lead is chosen, ie the one with the smallest R wave or in which the sum of the QRS waves is lowest.
  • Using the hexaxial lead diagram, the lead at right angles to this lead is identified.
  • If the R wave is positive, then the MEA is in this direction, if it is negative the lead at 180° is chosen.

Plotting leads 1 and 3

  • The QRS complexes are summated for leads 1 and 3.
  • Using the hexaxial lead diagram, the resulting vectors are plotted.
  • So if lead 1 added up to 2 and lead 3 added up to 3, points would be plotted 2 and 3 units along the respective lines.
  • Lines are then drawn at right angles to these points and the point where they cross connected by a straight line to the center of the hexaxial lead system.
  • The angle of this to 0° is measured.

Right axis deviation

  • Dogs: +100 to +180.
    Indicates right ventricular hypertrophy or right bundle branch block

Left axis deviation

  • Dogs: +40 to -40.
    Indicates left ventricular hypertrophy.