Second degree heart block
Encyclopedia
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.
; 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.
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. Type I is also named for Karel Frederik Wenckebach
, and type II is also named for John Hay
.
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.
In horses Mobitz I AV block is considered a normal variant finding, frequently encountered in relaxed, athletically fit animals.
).
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. 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.
Electrical conduction system of the heart
The normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...
of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
. It refers to a conduction block between the atria and ventricles
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
.
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 complexQRS complex
The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...
; 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.
Both types are named after Woldemar Mobitz
Woldemar Mobitz
Woldemar Mobitz 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 Meiningen from which he graduated in 1908...
, even though they were first described by Dusty Andressen. There is still much controversy on how Mobitz got the information from Andressen. Type I is also named for Karel Frederik Wenckebach
Karel Frederik Wenckebach
Karel Frederik Wenckebach 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 professor at the Universities of Strasbourg and Vienna .Wenckebach is primarily remembered for...
, and type II is also named for John Hay
John Hay (cardiologist)
John Hay was a British cardiologist.He was born in Birkenhead, Lancashire, the son of a Scottish architect and educated at the Liverpool Institute and the Victoria University of Manchester, qualifying M.B. in 1896....
.
Type 1 (Mobitz I/Wenckebach)
Type 1 Second-degree AV block, also known as Mobitz I or WenckebachKarel Frederik Wenckebach
Karel Frederik Wenckebach 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 professor at the Universities of Strasbourg and Vienna .Wenckebach is primarily remembered for...
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
Electrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
(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
Atropine
Atropine is a naturally occurring tropane alkaloid extracted from deadly nightshade , Jimson weed , mandrake and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects...
or isoproterenol
Isoproterenol
Isoprenaline or isoproterenol is a medication used for the treatment of bradycardia , heart block, and rarely for asthma...
may transiently improve conduction.
In horses Mobitz I AV block is considered a normal variant finding, frequently encountered in relaxed, athletically fit animals.
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 SystemElectrical conduction system of the heart
The normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...
).
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
Stokes-Adams attack
Stokes–Adams syndrome refers to a sudden, transient episode of syncope, occasionally featuring seizures. It is named after two Irish physicians, Robert Adams and William Stokes .-Signs and symptoms:...
, cardiac arrest
Cardiac arrest
Cardiac arrest, is the cessation of normal circulation of the blood due to failure of the heart to contract effectively...
, or Sudden Cardiac Death
Sudden Cardiac Death
Sudden cardiac death is natural death from cardiac causes, heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms. Other forms of sudden death may be noncardiac in origin...
. The definitive treatment for this form of AV Block is an implanted pacemaker.
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.
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
- SyncopeSyncope (medicine)Syncope , the medical term for fainting, is precisely defined as a transient loss of consciousness and postural tone characterized by rapid onset, short duration, and spontaneous recovery due to global cerebral hypoperfusion that most often results from hypotension.Many forms of syncope are...
(Fainting)
See also
- Electrical conduction system of the heartElectrical conduction system of the heartThe normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...
- ElectrocardiogramElectrocardiogramElectrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
(ECG or EKG) - SA nodeSinoatrial nodeThe sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...
- AV nodeAtrioventricular nodeThe atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...
- Atrioventricular blockAtrioventricular blockAn atrioventricular block involves the impairment of the conduction between the atria and ventricles of the heart.The causes of pathological AV block are varied and include ischaemia, infarction, fibrosis or drugs. Certain AV blocks can also be found as normal variants, such as in athletes or...
- First-degree AV block
- Third-degree AV block