Canis ISSN: 2398-2942

Heart: pacemaker

Contributor(s): Mark Oyama, Mark Rishniw

Overview

  • Artificial pacing (AP) provides an artificial electrical stimulus that elicits cardiac contraction. AP is used to ensure adequate heart rate in animals with bradyarrrhythmias.

Principles of AP

  • There are two main components of a pacemaker, the generator Heart: pacemaker generator and the lead Heart: pacemaker leads.
  • The pacing generator contains a battery and sophisticated circuitry that allows customized programing of the pacemaker's function.
  • The pacing lead delivers the electrical stimulus from the generator to the myocardial tissue.
  • Pacemakers serve multiple functions. Not only do they deliver the electrical impulse (pace) but they also detect native cardiac depolarizations (sense). Based on the activity the pacemaker senses, various responses can be programed.
    • For instance, the pacemaker listens to see if a spontaneous impulse is generated by the myocardium - if so, this is sensed by the pacemaker and the next pacemaker signal is delayed by an appropriate interval.
  • Pacemaker batteries have a lifespan of approximately 4-10 years.
    Reducing the number of pacemaker impulses generated increases the lifespan of the battery - although battery life is not normally an issue in animals.

Classification

  • The nomenclature surrounding pacemakers appears complex but is actually very straightforward.

Generator units

  • Generators are either unipolar or bipolar. These need to be matched to corresponding pacemaker leads.
  • A code comprising 5 letters is used to describe the generator:
    • The 1st letter describes the cardiac chamber paced by the pacemaker either atrium (A), ventricle (V) or dual (D).
    • The 2nd letter describes the cardiac chambers sensed by the pacemaker either atrium (A), ventricle (V) or dual (D).
    • The 3rd describes the response to a sensed signal either triggered (T), inhibited (I) or dual (D).
    • The 4th denotes rate-responsiveness (R) - ability to vary impulse rate with activity.
    • The 5th describes programmability (P).
  • Most human pacemakers are DDD, whereas most pacemakers used in pets are VVI which means that the lead is implanted in the ventricle and if an impulse is sensed from the ventricle the pacemaker pulse is inhibited.
  • The ability to program the pacemaker to respond to patient physical activity and to increase the heart rate of (rate-responsiveness) is available in most pacemakers.

Leads

  • Leads are either unipolar or bipolarUnipolar systems use the generator as part of the electrical circuit (one pole); bipolar systems have both the positive and negative poles within the tip of the lead.
  • Leads are classified as epicardial or endocardial:
    • Epicardial leads: implanted into the ventricle from outside the heart.
    • Endocardial leads: implanted in the right ventricle via a transcutaneous route (usually through the jugular vein).
  • The end of endocardial lead which attaches to the myocardium has either a corkscrew end or a number of plastic hooks (tines) which hook onto trabeculae in the ventricle and push the electrode tip against the endocardium.
    Always make sure that the lead has the correct attachment for connection to the generator and that the lead and generator are of similar polarity as the 2 components are not always compatible. (It is sometimes possible to use a bipolar lead with a unipolar generator - the system functions as unipolar system.)

Programing

  • Programing units can be used to reset pacemakers in situbut these are not normally available to veterinary practitioners and most pacemakers for animals are programed to a pre-set rate before delivery. Most veterinary cardiologists either have programing units, or have access to them as necessary.
  • Pacemaker rates can be set at a predetermined rate which in animals is usually selected to be 80-100 BPM for dogs and >120 BPM for cats. Rate-responsive pacemakers are often programed to provide a range of heart rates from 70-150 BPM in dogs.

Indications

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Complications associated with implantation

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Connection problems

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Other problems

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Long-term problems

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • James R (2007) Use of pacemakers in dogs. In Practice 29 (9), 503-511 VetMedResource.
  • Oyama M, Sisson D D & Lehnmukuhl L B (2001) Practices and outcome of artificial cardiac pacing in 154 dogs. JVIM 15 (3), 229-239 PubMed.
  • Fingeroth J M (1994) Pacemaker therapy for bradycardias. Semin Vet Surg (4), 192-199 PubMed.
  • Wilbanks R (1992) Using the abdominal approach for pacemaker implantation. Vet Med 87, 139.
  • Fox P R, Moise N S, Woodfield J A et al (1991) Techniques and complications of pacemaker implantation in four cats. JAVMA 199 (12), 1742-1753 PubMed.
  • Sisson D D, Thomas W P, Woodfield J et al (1991) Permanent transvenous pacemaker implantation in forty dogs. JVIM (6), 322-331 PubMed.
  • Snyder P S, Atkins C E & Sato T (1991) Syncope in three dogs with cardiac pacemakers. JAAHA 27 (6), 611-16 VetMedResource.
  • Cobb M A, Nolan J, Brownlie S E et al (1990) Use of a programmable, activity-sensing, rate-regulating pacemaker in a dog. JSAP 31 (8), 398-400 VetMedResource.
  • Darke P G G, McAreavey D, Been M (1989) Transvenous cardiac pacing in 19 dogs and one cat. JSAP 30 (9), 491-99 VetMedResource.

Other sources of information

  • Moise N S (1999) Pacemaker Therapy. In: Textbook of Canine & Feline Cardiology. 2nd edn. Eds: Fox, Sisson, Moise. W B Saunders. pp 400-425.


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