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Second-degree atrioventricular block (AV block) is a disease of the
electrical conduction system of the heart The cardiac conduction system (CCS) (also called the electrical conduction system of the heart) transmits the signals generated by the sinoatrial node – the heart's pacemaker, to cause the heart muscle to contract, and pump blood through ...
. It is a
conduction block Nerve block or regional nerve blockade is any deliberate interruption of signals traveling along a nerve, often for the purpose of pain relief. Local anesthetic nerve block (sometimes referred to as simply "nerve block") is a short-term block, u ...
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. It is classified as a block of the AV node and is categorized in between first-degree (slowed conduction) and third degree blocks (complete block).


Signs and 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)


Types

There are two non-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 The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG or EKG). It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the ri ...
; but, in Type 1, there are increasing delays in each cycle before the omission, whereas, in Type 2, there is no such pattern. Type 1 second-degree heart block is considered a more benign entity than type 2 second-degree heart block with type 1 not having structural changes found on histology. Both types are named after
Woldemar Mobitz Woldemar Mobitz (31 May 1889 – 11 April 1951) was a Russian-German physician. The forms of second degree AV block are named after him for him. Mobitz was born on May 31, 1889 in St. Petersburg, Russia. He attended the local high school in Mein ...
. Type I is also named for
Karel Frederik Wenckebach Karel Frederik Wenckebach (; March 24, 1864 – November 11, 1940) was a Dutch anatomist who was a native of the Hague. He studied medicine in Utrecht, and in 1901 become a professor of medicine at the University of Groningen. Later he was a profe ...
, and type II is also named for
John Hay John Milton Hay (October 8, 1838July 1, 1905) was an American statesman and official whose career in government stretched over almost half a century. Beginning as a private secretary and assistant to Abraham Lincoln, Hay's highest office was Un ...
.


Type 1 (Mobitz I/Wenckebach)

Type 1 Second-degree AV block, also known as Mobitz I or
Wenckebach Karel Frederik Wenckebach (; March 24, 1864 – November 11, 1940) was a Dutch anatomist who was a native of the Hague. He studied medicine in Utrecht, and in 1901 become a professor of medicine at the University of Groningen. Later he was a profe ...
periodicity, is almost always a disease of the
AV node The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
. Wenckebach published a paper in 1906 on progressively lengthening PR intervals that was later classified as Type I in Mobitz's 1924 paper. Thus, both "Mobitz type I" and "Wenckebach block" refer to the same pattern and pathophysiology. In Wenckebach's 1906 paper, his original observations were from increasing delay in contraction of the atria and ventricles that shortened after a brief pause and this was later observed on ECG after Einthoven's invention in 1901 that became the electrocardiogram (ECG). In modern practice, Mobitz I heart block is characterized by progressive prolongation of the PR interval 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. This grouped beating was described as "Luciani periods" after
Luigi Luciani Luigi Luciani ForMemRS (23 November 1842, in Ascoli Piceno – 23 June 1919) was an Italian neuroscientist. He also contributed to Karel Frederik Wenckebach's work on what is now known as second-degree atrioventricular block Second-degree atri ...
's work in 1873. The result is a lengthening of the R-R intervals as each subsequent P-wave reaches an increasingly refractory AV node until the impulse fails to conduct, which ultimately results in a blocked QRS complex. 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 for the rhythm itself. It can be seen in myocardial ischemia, propranolol use, digitalis use, rheumatic fever, and chronically in ischemic heart disease and other structural diseases (amyloidosis, mitral valve prolapse, aortic valve disease, and atrial septal defect). In symptomatic cases, intravenous
atropine Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given i ...
or
isoproterenol Isoprenaline, or isoproterenol (brand name: Isoprenaline Macure), is a medication used for the treatment of bradycardia (slow heart rate), heart block, and rarely for asthma. It is a non-selective β adrenoceptor agonist that is the isopropyla ...
may transiently improve conduction.


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 Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...
by intermittently non conducted P waves not preceded by PR prolongation and not followed by PR shortening. There is usually a fixed number of non-conducted P waves for every successfully conducted QRS complex, and this ratio is often specified in describing Mobitz II blocks. For example, Mobitz II block in which there are two P waves for every one QRS complex may be referred to as 2:1 Mobitz II block.:181 The medical significance of this type of AV block is that it may progress rapidly to
complete heart block Third-degree atrioventricular block (AV block) is a medical condition in which the electrical impulse generated in the sinoatrial node (SA node) in the atrium of the heart can not propagate to the ventricles. Because the impulse is blocked, an a ...
, in which no
escape rhythm In cardiology, a ventricular escape beat is a self-generated electrical discharge initiated by, and causing contraction of the Ventricle (heart), ventricles of the heart; normally the heart rhythm is begun in the Atrium (heart), atria of the heart ...
may emerge. In this case, the person may experience a Stokes-Adams attack,
cardiac arrest Cardiac arrest is when the heart suddenly and unexpectedly stops beating. It is a medical emergency that, without immediate medical intervention, will result in sudden cardiac death within minutes. Cardiopulmonary resuscitation (CPR) and possib ...
, or
sudden cardiac death Cardiac arrest is when the heart suddenly and unexpectedly stops beating. It is a medical emergency that, without immediate medical intervention, will result in sudden cardiac death within minutes. Cardiopulmonary resuscitation (CPR) and possib ...
. The definitive treatment for this form of AV Block is an
implanted pacemaker An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or pacemaker is a medical device that generates electrical impulses delivered by electrodes to the chambers of the heart eith ...
. The impairment is usually below the AV node. 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''.


P:QRS ratios

Because type I Mobitz block occurs in regular cycles, there is always a fixed ratio between the number of P waves and the number of QRS complexes per cycle. This ratio is often specified when describing the block. For example, a Mobitz type I block which has 4 P waves and 3 QRS complexes per cycle may be referred to as 4:3 Mobitz Type I block.:179 Type II Mobitz block also usually occurs with a fixed P:QRS ratio, with a set number of P waves for every successfully elicited QRS.:179 This ratio is also frequently specified in referring to 3:1, 4:1, 5:1, or higher Mobitz type II block. Higher numbers of P waves for every QRS indicate more severe block:181 and ratios of 3:1 and above are also referred to as "High Grade" AV Blocke60. Of course, because type II Mobitz block is unstable by nature, it is common for the P:QRS ratio in Mobitz type II block to change over time. The P:QRS ratio is always of the form ''X'':(''X'' − 1) in type I Mobitz block and of the form ''X'':1 in type 2 Mobitz block because of the nature of the pattern of each. Thus one may leave out the type and refer to 3:1 Mobitz block or 4:3 Mobitz block, for example, without creating ambiguity, except in the case of 2:1 block.


2:1 AV block

In the case of 2:1 block (2 P waves for every QRS complex) it is impossible to differentiate type I from type II Mobitz block-based solely on the P:QRS ratio or on a pattern of lengthening PR intervals.:182 In this case, a lengthened PR interval with a normal QRS width is most likely indicative of a type I-like pathology, and a normal PR interval with a widened QRS is most likely indicative of a type II-like pathology.:182


See also

* Electrocardiogram (ECG or EKG) *
SA node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
*
AV node The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
*
Atrioventricular block Atrioventricular block (AV block) is a type of heart block that occurs when the electrical signal traveling from the atria, or the upper chambers of the heart, to ventricles, or the lower chambers of the heart, is impaired. Normally, the sinoatr ...
*
First-degree AV block First-degree atrioventricular block (AV block) is a disease of the electrical conduction system of the heart in which electrical impulses conduct from the cardiac atria to the ventricles through the atrioventricular node (AV node) more slowly tha ...
* Third-degree AV block


References


External links

{{Circulatory system pathology Cardiac arrhythmia