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VETERINARY CLINICAL CARDIOLOGY
CARDIOLOGY CONCEPTS
Electrocardiology
Basics of ECG Interpretation
1. What is a P wave? What electrocardiographic event does this represent?

The P wave is the electrocardiographic representation of electrical activation of the atrial myocardium.

2. What is the PR interval? What is the significance of the PR interval?

The PR interval is the electrocardiographic representation of the delay in electrical activation of the ventricles after sino-atrial nodal discharge. This interval is mainly a result of slow conduction through the atrio-ventricular node.

3. What is the significance of the QRS complex? How are a Q, R, S, and QS different?

The QRS complex is the electrocardiographic expression of the electrical activation of the ventricular myocardium.

For a wave to be defined as a Q wave it must fulfill the following criteria:

  1. It must be a negative deflection.
  2. It must be the first deflection of the QRS complex.
  3. It must be followed by an R wave.

Thus there can be only one Q wave per QRS complex.


For a wave to be defined as an R wave it must fulfill the following criteria:

  1. It must be a positive deflection.
  2. All positive deflections are R waves and only R waves (within the QRS complex).

There can be several R waves per QRS complex.


For a wave to be defined as an S wave it must fulfill the following criteria:

  1. It must be a negative deflection.
  2. It must follow an R wave.

There can be several S waves per QRS complex.


For a wave to be defined as a QS wave it must fulfill the following criteria:

  1. It must be a negative deflection.
  2. There must be no R wave in the QRS complex.

Therefore it describes a QRS complex with only one deflection and this deflection is negative.

4. What is a T wave? What is the significance of a T wave?

5. How does a positive vs a negative deflection or wave result on an ECG?

An ECG lead utilizes a combination of electrodes (attached to the patient) to provide a unique perspective of the electrical activity travelling through the heart. Each lead has a positive pole and a negative pole, and the difference in electrical potential between these poles is graphed over time. As a wave of electrical depolarization moves parallel to the direction of a lead, if it moves towards the positive pole of the lead, a positive deflection occurs on the ECG. If it moves toward the negative pole of the lead, a negative deflection occurs on the ECG.

6. What are the ECG leads in the frontal plane and how are they oriented relative to each other?

The frontal, sagittal, and transverse (also called horizontal) planes are illustrated in the cat above. Leads I, II, III, aVR, aVL, and aVF are the leads available in the frontal plane.

In the illustration above, a cross-section of the heart is superimposed on the frontal plane diagram. The arrow heads point toward the positive pole of each lead.

The frontal plane diagram shows the relationship of the 6 leads in the frontal plane. This diagram includes the angles in the frontal plane and the direction of each lead with its negative and positive pole.

The frontal plane corresponds to the radiographic V/D or D/V view of the heart.

7. What are the ECG leads in the transverse plane and how are they oriented relative to each other?

Leads V1, V2, V3, V4, V5, V6, and V10 are horizontal (or transverse) plane leads. V1 to V3 are the most commonly performed in veterinary medicine.

The illustration above shows a transverse section at the 6th intercostal space demonstrating the position of the horizontal plane (or precordial or chest) leads.

8. What is the mean electrical axis (MEA) of the QRS complex?

The mean MEA of the QRS complex (determined in the frontal plane) represents the net or average direction of activation of the entire ventricular myocardium.

There are three common methods of calculating the MEA in the frontal plane: the vector method, the isoelectric method, and the greatest net deflection method. All of these are merely approximations.

The isoelectric method or the greatest net deflection method are easiest and are described below. Examples will be performed in lab.

  1. The Isoelectric Method: Find the lead, of the 6 leads in the frontal plane, that is the most isoelectric (the algebraic sum of all the deflections of the QRS in that lead comes closest to zero) (again omit the T wave). Next, find the lead on the frontal plane diagram that is perpendicular to this isoelectric lead (the MEA will lie along this perpendicular lead). Note the net polarity of the complexes in this perpendicular lead. If the net polarity of these complexes is positive, the MEA is toward the positive pole of the perpendicular lead; if the net polarity of these complexes is negative, the MEA is toward the negative pole of the perpendicular lead.
  2. The Largest Net Deflection Method: Find the lead, of the 6 leads in the frontal plane, that contains the QRS complex with the largest net deflection (the algebraic sum of all the deflections of the QRS in that lead is the largest value [positive or negative]). The MEA will lie along this lead. If this largest net deflection is positive, the MEA is toward the positive pole of this lead with the largest net deflection; if this largest net deflection is negative, the MEA is toward the negative pole of this lead with the largest net deflection.

Determining the MEA is very useful to identify right ventricular enlargement. A MEA of 100 to -75 degrees in the dog or 160 to -75 degrees in the cat suggests right ventricular enlargement.

The MEA also helps determine the nature of aberrant conduction disturbances (so called bundle branch block).

9. What is a sinus rhythm?

A sinus rhythm is a rhythm that originates in and is controlled by the sino-atrial node. This rhythm is the normal cardiac rhythm in all domestic animals. To determine if a rhythm is sinus or not based on the ECG requires the following criteria:

  1. There is a P wave for every QRS complex.
  2. The PR interval is relatively constant (especially if the R wave to R wave interval varies).
  3. The P wave is positive in lead II.

Since the P wave doesn't specifically indicate sino-atrial activation but rather is the result of sino-atrial activation (indicates atrial depolarization), we can only infer the normalcy of sino-atrial activity from the routine ECG.

10. What is a sinus arrhythmia?

A sinus arrhythmia is a rhythm of sino-atrial origin (as defined above) that produces a phasically irregular ventricular rate as a result of a phasically irregular sino-atrial rate. On the ECG, the QRS to QRS interval varies and there is a P wave for every QRS complex.

Example 1: Sinus Arrhythmia in a Dog

(25 mm/sec)

Etiology: Most cases of sinus arrhythmia are phasic and associated with respiration. The rate increases with inspiration and decreases with expiration due to the influence of changes in vagal tone. Individuals with respiratory disease may have an augmented or pronounced sinus arrhythmia.

Consequences: This is a normal finding, and particularly common in fit individuals. The finding of sinus arrhythmia on ECG confirms the presence of an arrhythmia but establishes that this irregularity is not pathologic. No treatment is required.

Treatment: none required

Example 2: Sinus Arrhythmia in a Dog

(25 mm/sec)

11. What is a wandering atrial pacemaker?

ECG Findings:

  • Variable amplitude and/or morphology of the P wave
  • The change in P wave amplitude and/or morphology tends to gradually shift from normalcy
  • It occurs in association with sinus arrhythmia

Example of Wandering Atrial Pacemaker in the Dog - Put cursor over image for more information


(25 mm/sec)

Etiology:

  • Associated with increased vagal tone
  • Due to shifting of the point of origin of depolarization within the complex sino-atrial node structure
  • Usually a normal finding

Consequences:

  • None
  • In the face of a marked sinus arrhythmia, altered P wave morphology may be misinterpreted as atrial premature beats

Treatment: None required

Wandering Atrial Pacemaker - example 2 (highlighted hints)
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