Atrioventricular Block (AV Block) – Symptoms And Treatment
A disturbance in conduction between the sinus impulse (atrial impulse) and its associated ventricular response is called atrioventricular block. (AV block) .The conduction may be abnormally slowed or completely blocked. The AV block results from a functional or pathologic defect in the AV node, bundle of His or bundle branches. Cardiac ischemia and infarction are frequently associated with conduction blocks and delays.
Three categories of AV blocks have been traditionally described
- First-degree block
- Second-degree block(which includes types I and II)
- Third-degree block( complete)
First-degree block . It is generally identified on the ECG by a prolonged PR interval(more than 0,20 second). The rhythm remains regular and each P wave is associated with a QRS complex.
First-degree block is a common finding and may occur in the absence of organic heart disease.Drugs and organic heart disorders(myocardial ischemia, congenital heard defects) may be at the root of first-degree block. First-degree block is generally monitored but is not actively managed except to alleviate the underlying cause if possible.
Second -degree block
It is diagnosed when some of the atrial impulses are not conducted to the ventricles.
Two types os second-degree block are identified by the pattern of non-conducted impulses:
- Type I(also known as – Mobitz type I, Wenckenbach)
It is associated with progressive lengthening PR intervals until one P wave is not conducted(dropped beat).The pattern repeats, causing the QRS complexes to occur in groups. This means: the PP intervals are constant, whereas the RR intervals vary.
Type I second-degree block is usually due to reversible ischemia of the AV node, often associated with acute MI. The ischemic node is slow to recover after each depolarisation, resulting, thus, in a longer and and longer nodal delay until one impulse is not conducted. This phenomenon( Wenkenbach phenomenon) gives the AV node time to recover, and the next atrial impulse is conducted more quickly with a nearly normal PR interval, beginning the cycle again.Treatment is rarely required.If the block progresses to type II block, a pacemaker may be required.
- Type II
Is identified by the presence of non-conducted P waves(dropped beats) with a consistent PR interval.The QRS complex is usually but not always, wide( 0.12 second or greater)
Type II block is usually associated with pathologic lesion of the bundle of His, the right bundle branch or both.It is the bundle branch block that causes abnormally enlarged QRS complexes. Type II is less common than type I but it is more serious. The pathology associated with this type of block is usually anterior septal MI or fibrosis of the conduction system. The evolution of this block might be complete heart block with slow ventricular escape rhythm and poor cardiac output. Type II block may also result in severe bradycardia (SLOW HEART BEAT RATE) due to the number of dropped beats. Symptomatic type II block may require implantation of a pacemaker.
Third- degree block
This block may occur as a result of pathological lesion of the AV node, bundle of His, or bundle branches.
No impulses are conducted from the atria to the ventricles and a jonctional or an escape rhythm is evident.
The ECG shows regularly occurring P waves that are totally independent of the ventricular rhythm.
If the QRS is narrow, the block is most likely in the AV node, proximal to the bundle of His
A prolonged QRS interval(more than 0,12 second) indicates pathology distal to the bundle of His within the bundle branches.
The severity of symptoms is determined primarily by the heart rate, with slower rhythms being more serious.Apacemaker is usually required.
First-degree AV block has no symptoms
Second-degree AV block of type I– is characterised by the temporary absence of a heart beat described by the patient as a break.
Second-degree AV block of type II-the patient might have bradycardia, a decrease in the ability to adapt to exertion, rarely syncope.
Third-degree AV block -can have one of the following clinical manifestations:
- Severe bradycardia
- Cardiac failure symptoms
- Rare, regular arterial pulse
- Medication – alters the properties of ion movement across cardiac membranes and affect automaticity as well as the rate and duration of depolarisation and repolarisation.
Class I ( blocks the ions of sodium)
- Procainamide, Quinidine
- Lidocaine, Tocainide
- Flecainide, Propafenone
Class II (indirectly blocks the ions of calcium)
- Atenolol, Metoprolol,Propanolol
Class III ( blocks the ions of potassium)
- Amiodarone, Sotalol, Ibutilide
Class IV ( blocks the ions of calcium)
- Diltiazem, Verapamil
- Digitalis (enhances vagal tone, slows heart rate and AV conduction
- Pacemaker implantation – should be performed in any patient with symptomatic bradycardia and irreversible second or third degree AV block regardless of the cause or level of block in the conducting system.
- Implantable defibrillators – that detect lethal rhythms and apply electric shocks to convert to normal sinusal rhythm( for those at high risk)
Treatment for this tpe of dysrhithmias ( AV blocks) centers on maintaining adequate cardiac output, providing antiarhytmic drugs when needed and managing the underlying pathological process.