Ectopic pacemaker

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An ectopic pacemaker or ectopic focus is an excitable group of cells that causes a premature heart beat outside the normally functioning SA node of the human heart. Acute occurrence is usually non-life threatening, but chronic occurrence can progress into tachycardia[1], bradycardia or ventricular fibrillation[2]. In a normal heart beat rhythm, the SA node usually suppresses the ectopic pacemaker activity due to the higher impulse rate of the SA node. However, in the instance of either a malfunctioning SA node or an ectopic foci bearing an intrinsic rate superior to SA node rate, ectopic pacemaker activity may rule over the heart rhythm[3].

Etiology

Ectopic pacemakers can occur within healthy hearts in response to various stimulating events, such as:

They can also occur within unhealthy hearts, caused by:

  • Infection
  • Disease, such as sinus venosis and atrial defects[9][10].
  • SA node dysfunction, (1st degree block) which can cause the rate of impulse to slow.[11].
  • SA node blockage so that impulses never leave the atria.[12]
  • AV node blockage (3rd degree block) prevents normal conduction across ventricles[13].

Physiology

An ectopic pacemaker can reside within a part of the electrical conduction system of the heart, or within the muscle cells of the atria or ventricles. When an ectopic pacemaker initiates a beat, premature contraction occurs. A premature contraction will not follow the normal signal transduction pathway, and can render the heart refractory or incapable of transmitting the normal signal from the SA node. Location of the pacemaker can also change its effect on the SA node and its rhythm. An ectopic pacemaker located in the atria is known as an atrial pacemaker and can cause the atrial contraction to be faster[14]. An ectopic pacemaker situated near the AV node and the septum is known as a junctional pacemaker[15]. The pacemaker that is operating in the ventricles is known as the ventricular [16]. Other such ectopic pacemakers can even lie within the pulmonary vein [17] and thoracic vein[18] walls.

Symptoms

  • Palpitations.
  • Feeling faint.[19]

Diagnosis

On an ECG, the QRS complex will be abnormally shaped when looking at ventricular ectopic activity, often it occurs earlier with an absent P wave. It can be perceived as a skipped beat on both the ECG and through normal pulse-taking[20]. During atrial ectopic activity where the P wave is normally rounded can be inverted or peaked. However the QRS complex and T waves appear relatively normal[21]. Conversely, during junctional ectopic activity the P wave is frequently absent or can be hidden in the QRS complex[22]

References

  1. ↑ Phibbs, B." Paroxysmal Atrial Tachycardia with Block Around the Ectopic Pacemaker." Circulation Vol. 28, Page 949-950. Available at: http://circ.ahajournals.org/cgi/content/abstract/circulationaha;28/5/949.[Accessed 30/01/09]
  2. ↑ Tveito, A. and Lines, G.T. 2008. A condition for setting off ectopic waves in computational models of excitable cells. Mathematical Biosciences, [Online] Vol. 213, Page 141-150. Available at: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VHX-4SBHXCS-2&_user=128860&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000010638&_version=1&_urlVersion=0&_userid=128860&md5=26a20ef17f8abb9445cccd5207b0d4e3. [Accessed 30/01/09]
  3. ↑ Rozanski, G.J. 1991. Atrial ectopic pacemaker escape mediated by phasic vagal nerve activity. American Journal of Physiology, [Online] Vol. 260 (5), Page 1507-1514. Available at: http://ajpheart.physiology.org/cgi/content/abstract/260/5/H1507. [Accessed 30/01/09]
  4. ↑ Rozanski, G.J. 1991. Atrial ectopic pacemaker escape mediated by phasic vagal nerve activity. American Journal of Physiology, [Online] Vol. 260 (5), Page 1507-1514. Available at: http://ajpheart.physiology.org/cgi/content/abstract/260/5/H1507. [Accessed 30/01/09]
  5. ↑ Rozanski, G.J. 1991. Atrial ectopic pacemaker escape mediated by phasic vagal nerve activity. American Journal of Physiology, [Online] Vol. 260 (5), Page 1507-1514. Available at: http://ajpheart.physiology.org/cgi/content/abstract/260/5/H1507. [Accessed 30/01/09]
  6. ↑ Carol, Mattson, Port. Pathophysiology: Concepts of Altered Health States 7th Ed, Pg 588. Lippincott Williams & Wilkins, 2005. ISBN 0-7817-4988-3
  7. ↑ Maupoil, V. 2007. Ectopic activity in the rat pulmonary vein can arise from simultaneous activation of α1- and β1-adrenoceptors. British Journal of Pharmacology, [Online] Vol. 150 (7), Page 899–905. Available at: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2013875. [Accessed 28/01/09]
  8. ↑ Keller-Wood, Maureen. "Electrical Activity in the Heart." Lecture at the University of Florida College of Pharmacy, 23-Oct-2007.
  9. ↑ Hamilton, S.D. et al. 1968. Disturbances in Atrial Rhythm and Conduction Following the Surgical Creation of an Atrial Septal Defect by the Blalock-Hanlon Technique. Circulation, [Online] Vol. 38, Pages 73-81. Available at : circ.ahajournals.org/cgi/reprint/38/1/73.pdf. [Accessed 28/01/09]
  10. ↑ Hoogaars, W.M.H et al. 2007. Tbx3 controls the sinoatrial node gene program and imposes pacemaker function on the atria. Genes and Development, [Online] Vol. 21, Page 1098-1112. Available at: http://genesdev.cshlp.org/content/21/9/1098.full. [Accessed 01/02/09]
  11. ↑ Phibbs, B. 1963. Paroxysmal Atrial Tachycardia with Block Around the Ectopic Pacemaker. Circulation, [Online] Vol. 28, Page 949-950. Available at: http://circ.ahajournals.org/cgi/content/abstract/circulationaha;28/5/949. [Accessed 30/01/09]
  12. ↑ Phibbs, B. 1963. Paroxysmal Atrial Tachycardia with Block Around the Ectopic Pacemaker. Circulation, [Online] Vol. 28, Page 949-950. Available at: http://circ.ahajournals.org/cgi/content/abstract/circulationaha;28/5/949. [Accessed 30/01/09]
  13. ↑ Phibbs, B. 1963. Paroxysmal Atrial Tachycardia with Block Around the Ectopic Pacemaker. Circulation, [Online] Vol. 28, Page 949-950. Available at: http://circ.ahajournals.org/cgi/content/abstract/circulationaha;28/5/949. [Accessed 30/01/09]
  14. ↑ Lippincott. Williams. & Wilkins. 2007. ECG Interpretation: An Incredibly Easy! Pocket Guide. [e-book]. 4th edition. s.l.: Lippincott Williams & Wilkins. Page 116-117. Available at: http://books.google.com/books?id=Q90JP6zlEuQC&pg=PA116&dq=ecg+interpretation+of+atrial+pacemaker#PPA116,M1. [Accessed 02/01/09]
  15. ↑ Lippincott. Williams. & Wilkins. 2007. ECG Interpretation: An Incredibly Easy! Pocket Guide. [e-book]. 4th edition: revised. s.l.: Lippincott Williams & Wilkins. Page 111-113 . Available at: http://books.google.com/books?id=5cfqAR1KWOYC&pg=PT126&dq=ecg+interpretation+of+junctional+pacemaker#PPT127,M1. [Accessed 02/01/09]
  16. ↑ Lippincott. Williams. & Wilkins. 2005. ECG Interpretation: An Incredibly Easy! Pocket Guide. [e-book]. Edition: illustrated. s.l.: Lippincott Williams & Wilkins. Page 96-97+. Available at: http://books.google.com/books?id=krbOcMuLDSAC&printsec=frontcover#PPA97,M1. [Accessed 02/01/09]
  17. ↑ Haghjoo, M. 2007. Efficacy, safety, and role of segmental superior vena cava isolation in the treatment of atrial fibrillation. Journal of Electrocardiology, [Online] Vol. 40 (4), Page 327. Available at: http://www.ncbi.nlm.nih.gov/pubmed/17599474. [Accessed 28/01/09]
  18. ↑  Tan, A.Y. et al. 2008. Ectopic atrial arrhythmias arising from canine thoracic veins during in vivo stellate ganglia stimulation. American Journal of Physiology, [Online] Available at: http://ajpheart.physiology.org/cgi/content/abstract/295/2/H691. [Accessed on 30/01/09]
  19. ↑ Bupa. Atrial Fibrillation- information, symptoms and treatment. [Online] Available at: http://hcd2.bupa.co.uk/fact_sheets/html/atrial_fibrillation.html?print [Accessed 02/01/09]
  20. ↑ Lippincott. Williams. & Wilkins. 2005. ECG Interpretation: An Incredibly Easy! Pocket Guide. [e-book]. Edition: illustrated. s.l.: Lippincott Williams & Wilkins. Page 96-97+. Available at: http://books.google.com/books?id=krbOcMuLDSAC&printsec=frontcover#PPA97,M1. [Accessed 02/01/09]
  21. ↑ Lippincott. Williams. & Wilkins. 2007. ECG Interpretation: An Incredibly Easy! Pocket Guide. [e-book]. 4th edition. s.l.: Lippincott Williams & Wilkins. Page 116-117. Available at: http://books.google.com/books?id=Q90JP6zlEuQC&pg=PA116&dq=ecg+interpretation+of+atrial+pacemaker#PPA116,M1. [Accessed 02/01/09]
  22. ↑ Lippincott. Williams. & Wilkins. 2007. ECG Interpretation: An Incredibly Easy! Pocket Guide. [e-book]. 4th edition: revised. s.l.: Lippincott Williams & Wilkins. Page 111-113 . Available at: http://books.google.com/books?id=5cfqAR1KWOYC&pg=PT126&dq=ecg+interpretation+of+junctional+pacemaker#PPT127,M1. [Accessed 02/01/09]

See also

Cardiac ectopy Clinical cardiac electrophysiology Electrical conduction system of the heart

fr:Foyer ectopique
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