The P wave is the electrocardiographic representation of electrical activation of the atrial myocardium.
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.
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:
Thus there can be only one Q wave per QRS complex.
There can be several R waves per QRS complex.
There can be several S waves per QRS complex.
Therefore it describes a QRS complex with only one deflection and this deflection is negative.
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.
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.
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.
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.
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).
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:
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.
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
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
Example of Wandering Atrial Pacemaker in the Dog - Put cursor over image for more information
Treatment: None requiredWandering Atrial Pacemaker - example 2 (highlighted hints)