Lesson 6, Topic 3
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3rd Degree

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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).
Third degree heart block in the dog. Slide to add or remove annotation of ECG findings


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


  1. Can cause all the signs of reduced cardiac output


  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