Second-degree atrioventricular block

Second-degree atrioventricular block
Classification and external resources
ICD-10	I44.1
ICD-9	426.12, 426.13
DiseasesDB	10477
eMedicine	emerg/234

Second-degree AV block is a disease of the electrical conduction system of the heart. It refers to a conduction block between the atria and ventricles.

The presence of second-degree AV block is diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction.

Types

There are two distinct types of second-degree AV block, called Type 1 and Type 2. In both types, a P wave is blocked from initiating a QRS complex; but, in Type 1, there are increasing delays in each cycle before the omission, whereas, in Type 2, there is no such pattern.^[1]

Type 1 second-degree heart block is considered a more benign entity than type 2 second-degree heart block.

Both types are named after Woldemar Mobitz, even though they were first described by Dusty Andressen. There is still much controversy on how Mobitz got the information from Andressen.^[2][3] Type I is also named for Karel Frederik Wenckebach,^[4] and type II is also named for John Hay.^[5][6]

Type 1 (Mobitz I/Wenckebach)

Type 1 Second-degree AV block, also known as Mobitz I or Wenckebach periodicity, is almost always a disease of the AV node.

Mobitz I heart block is characterized by progressive prolongation of the PR interval on the electrocardiogram (ECG) on consecutive beats followed by a blocked P wave (i.e., a 'dropped' QRS complex). After the dropped QRS complex, the PR interval resets and the cycle repeats.

One of the baseline assumptions when determining if an individual has Mobitz I heart block is that the atrial rhythm has to be regular. If the atrial rhythm is not regular, there could be alternative explanations as to why certain P waves do not conduct to the ventricles.

This is almost always a benign condition for which no specific treatment is needed. In symptomatic cases, intravenous atropine or isoproterenol may transiently improve conduction.^[7]

In horses Mobitz I AV block is considered a normal variant finding, frequently encountered in relaxed, athletically fit animals.^[8]

Type 2 (Mobitz II/Hay)

Type 2 Second-degree AV block, also known as Mobitz II is almost always a disease of the distal conduction system (His-Purkinje System).

Mobitz II heart block is characterized on a surface ECG by intermittently nonconducted P waves not preceded by PR prolongation and not followed by PR shortening. The medical significance of this type of AV block is that it may progress rapidly to complete heart block, in which no escape rhythm may emerge. In this case, the person may experience a Stokes-Adams attack, cardiac arrest, or Sudden Cardiac Death. The definitive treatment for this form of AV Block is an implanted pacemaker.

The impairment is usually below the AV node.^[9] Although the terms infranodal block or infrahisian block are often applied to this disorder, they refer to the anatomic location of the block, whereas Mobitz II refers to an electrocardiographic pattern.

Symptoms

Most people with Wenckebach (Type I Mobitz) do not show symptoms. However, those that do usually display one or more of the following:

• Light-headedness
• Dizziness
• Syncope (Fainting)

See also

• Electrical conduction system of the heart
• Electrocardiogram (ECG or EKG)
• SA node
• AV node
• Atrioventricular block
• First-degree AV block
• Third-degree AV block

References

1. ^ "Lesson VI - ECG Conduction Abnormalities". http://library.med.utah.edu/kw/ecg/ecg_outline/Lesson6/index.html#Wenckebach. Retrieved 2009-01-07. 
2. ^ synd/2824 at Who Named It?
3. ^ W. Mobitz. Über die unvollständige Störung der Erregungsüberleitung zwischen Vorhof und Kammer des menschlichen Herzens. Zeitschrift für die Gesamte Experimentelle Medizin, Berlin 1924, 41: 180–237.
4. ^ K. F. Wenckebach. De Analyse van den onregelmatigen Pols. III. Over eenige Vormen van Allorythmie en Bradykardie. Nederlandsch Tijdschrift voor Geneeskunde, Amsterdam, 1898, 2: 1132.
5. ^ Silverman ME, Upshaw CB, Lange HW (August 2004). "Woldemar Mobitz and His 1924 classification of second-degree atrioventricular block". Circulation 110 (9): 1162–7. doi:10.1161/01.CIR.0000140669.35049.34. PMID 15339865. http://circ.ahajournals.org/cgi/pmidlookup?view=long&pmid=15339865. 
6. ^ J. Hay. Bradycardia and cardiac arrhythmia produced by depression of certain of the functions of the heart. The Lancet 1906, 1: 139–143.
7. ^ Lilly, L. S., Pathophysiology of Heart Disease. Baltimore: Lippincott Williams & Wilkins; 2007
8. ^ Marr, Celia M (1999-07-12). Cardiology of the horse. ISBN 9780702022401. http://books.google.com/?id=2t9BlUut8UwC&pg=PA276&lpg=PA276&dq=mobitz+horses. 
9. ^ Wogan JM, Lowenstein SR, Gordon GS (1993). "Second-degree atrioventricular block: Mobitz type II". J Emerg Med 11 (1): 47–54. doi:10.1016/0736-4679(93)90009-V. PMID 8445186. http://linkinghub.elsevier.com/retrieve/pii/0736-4679(93)90009-V.