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VETERINARY CLINICAL CARDIOLOGY
CARDIOLOGY CONCEPTS
Electrocardiology
Bradycardias
1. What is bradycardia?

Bradycardia is present when the ventricular rate is too slow. General guidelines are:

  • Dog (< 20 kg) ≤ 70 bpm
  • Dog (> 20 kg) ≤ 60 bpm
  • Cat ≤ 100 bpm
2. What is sinus bradycardia?

This refers to a sinus rhythm wherein the sino-atrial rate is bradycardic.

Example of Sinus Bradycardia in the Dog

Etiology:

  1. Enhanced parasympathetic tone as with:
    • Increased respiratory effort as with respiratory disease.
    • Gastric irritation.
    • Increased CSF pressure.
  2. Hypothyroidism
  3. Hypothermia
  4. Hyperkalemia
  5. Hypoglycemia
  6. Drug therapy

Consequence: profound bradycardia will cause weakness, hypotension, syncope

Therapy:

  1. Treat the underlying cause of the bradycardia
  2. Atropine may abolish the bradycardia
3. What is heart block?

Heart block refers to a group of disorders characterized by variable conduction from the atria to the ventricles.

Heart block can be classified as

  • 1st degree heart block - prolonged conduction across the atrio-ventricular node
  • 2nd degree heart block - failure of SA impulse to activate the ventricular myocardium
  • 3rd degree heart block - no conduction through AV node, and ventricular activation is due to an automatic subsidiary pacemaker focus (escape focus)
4. What is 1st degree Heart Block?

ECG Findings:

  1. Prolonged PR interval
    • Dog - PR > 0.13 sec
    • Cat - PR > 0.09 sec
  2. There is a P wave for every QRS and QRS for every P wave.

Etiology:

  1. This disorder is due to prolonged conduction across the atrio-ventricular node.
  2. Associated with disorders that cause increased parasympathetic tone.
  3. May be normal.
  4. PR may increase with age and lengthen with rapid heart rate.
  5. Drug therapy (digitalis, beta blocker, calcium channel blocker, procainamide, quinidine).
  6. Hyperkalemia.

Consequences: Causes no hemodynamic abnormality

Treatment: None required

Example of First Degree Heart Block in the Dog - Place cursor on the image to see examples of the ECG Findings

(50 mm/sec; 1cm/mV).

Further Examples:

5. What is 2nd degree Heart Block?

Due to failure of the sino-atrial impulse to activate the ventricular myocardium after activating the atrial myocardium.

Depending on the location of the abnormality it may be classified as:

  1. Mobitz type 1 (Wenckebach)
  2. Mobitz type 2

Mobitz type 1 (Wenckebach) Second Degree Heart Block:

ECG Findings:

  1. P wave that is not followed by a QRS complex
  2. QRS complex is usually of normal duration and morphology
  3. The PR interval progressively prolongs just prior to the blocked beat
  4. When comparing the PR interval of the beat immediately preceding the blocked beat with that of the beat immediately following the blocked beat, the PR interval of the preceding beat is longer than the PR interval of the following beat.

Example of Mobitz Type 1 Second Degree Heart Block in a Dog - Place cursor on the image to see examples of the ECG Findings

(25mm/sec; 1cm/mV)

Etiology:

  1. Due to failure of the sino-atrial impulse to propagate through the AV node
  2. Excessive vagal tone can cause this abnormality
  3. May be normal in some dogs
  4. Drug therapy (e.g. digitalis, beta blocker, calcium channel blocker)

Consequences:

  1. Very rarely is the heart rate sufficiently reduced to cause any reduction in cardiac output, so it is often not clinically significant.
  2. This rhythm disturbance is important to distinguish from second degree heart block of the Mobitz type 2 variety which is often more clinically significant.

Treatment:

  1. Often none required
  2. Often atropine responsive

Mobitz Type 2 Second Degree Heart Block:

ECG Findings:

  1. P wave that is not followed by a QRS complex
  2. May have a wide QRS complex (not in example shown)
  3. PR interval of constant duration for the QRS present
  4. May have multiple P waves that are not followed by QRS complexes (i.e. 2:1, 3:1, 4:1, etc. block)

Example of Mobitz Type 2 Second Degree Heart Block in a Dog (with 3:1 conduction)

<

(25 mm/sec; 1 cm/mV)

Etiology:

  1. Due to failure of the sino-atrial impulse to propagate through the AV node, His bundle, or bundle branches
  2. Associated with organic disease of the His bundle or bundle branches. This may be in the form of an ischemic area, scar/fibrosis, inflammation, infection with necrosis, neoplasia or granuloma

Consequences:

  1. Evidence of reduced cardiac output may occur if the ventricular rate is substantially reduced and depending on their activity
  2. May progress to 3rd degree (complete) heart block (see below)

Treatment:

  1. Usually not responsive to atropine or glycopyrrolate
  2. The ventricular rate can usually be increased with intravenous isoproterenol
  3. Unless the ventricular rate is slow and clinical signs present, no treatment may be required
  4. Permanent pacemaker implantation may be indicated in some cases
6. What is 3rd degree Heart Block?

ECG Findings:

  1. The atrial rate and rhythm is independent of a much slower ventricular rhythm.
  2. The atrial rate tends to be fast and regular.
  3. The ventricular rhythm is regular and much slower than the atrial rhythm.
    • Dog - ventricular rate approx. 40 - 60 bpm
    • Cat - ventricular rate approx. 90 - 120 bpm
  4. The resultant ventricular activation is initiated from latent, subsidiary automatic pacemaker foci. The resultant rhythm is referred to as an escape rhythm. If the escape rhythm originates from the AV nodal area, the QRS morphology may look relatively normal. Usually, however, the escape rhythm originates from lower down and the QRS morphology tends to be abnormal (wide and bizarre).

Example of Third Degree Heart Block in the Dog

Etiology:

  1. Due to the same etiologies as for 2nd degree Heart Block Mobitz Type 2

Consequences:

  1. Can cause all the signs of reduced cardiac output

Treatment:

  1. Atropine or glycopyrrolate are usually not effective
  2. IV isoproterenol may increase the ventricular escape rate
  3. Pacemaker therapy is usually the only effective therapy
7. What is Sick Sinus Syndrome?

Sick Sinus Syndrome refers to a group of disorders involving the sino-atrial node usually with further involvement of the atrio-ventricular node, His bundle and/or bundle branches. Frequently, multiple areas of the conduction system are involved.

ECG findings (may include any of the following):

  1. A slow and irregular atrial rate (impaired sino-atrial activity), severe sinus bradycardia, or long pauses of asystole with no escape beats
  2. Evidence of impaired atrio-ventricular conduction such as 2nd degree heart block
  3. The ventricular rate is slow and irregular.
    • Ventricular escape beats are common, though they are often slow to manifest (follow a longer pause than usual)
  4. Paroxysms of supraventricular tachycardia alternating with severe sinus bradycardia

Example of Sick Sinus Syndrome in a Dog

Etiology:

  1. May have SA nodal artery disease - ischemia
  2. Fibrosis of SA node with partial involvement of the rest of conduction system
  3. Possibly inherited in some breeds as Miniature Schnauzer, West Highland White terrier
  4. Usually idiopathic

Consequences:

  1. Periods of asystole are associated with syncope, pre-syncope, weakness
  2. The paroxysms of supraventricular tachycardia may also cause clinical signs

Treatment:

  1. Atropine is usually not effective (or at least there is a suboptimal atropine response), though some anticholinergics or sympathomimetics may accelerate the ventricular rate and provide a temporary response
  2. Permanent pacemaker implantation is the only definitive therapy
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