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Atrioventricular conduction is influenced by autonomic activity. AV
block can therefore be intermittent and may only be evident when the
conducting tissue is stressed by a rapid atrial rate. Accordingly, atrial
tachyarrhythmias are often associated with AV block .
First degree heart block
In this condition AV conduction is delayed so the PR interval is
prolonged ( > 0.20 s) it rarely causes symptoms.
Second degree heart block B(Intermittent Atrioventricular Conduction)
In this condition dropped beats occur because some impulses from the
atria fail to conduct to the ventricles .
In Mobitz type 1 there is progressive lengthening of successive PR
intervals culminating in a dropped beat. The cycle then repeats itself . this
is known as Wenchebach 's phenomenon and is usually due to impaired
conduction in the AV node itself .it may be physiological and sometimes
observed at rest or during the sleep in athletic young adult with high
vagal tone .
In Mobitz type 2 the PR interval of the conducted impulses remains
constant but some P waves are not conducted .this usually caused by
diseases of the His-Purkinje system and carries a risk of a systole .
In 2:1AV block alternate P waves are conducted , so it is impossible to
distinguish between Mobitz type 1 and Mobitz type 2.
Third degree (complete ) heart block
When AV conduction fails completely , the atria and ventricles beat
independently ( AV dissociation )
Ventricular activity is maintained by an escape rhythm arising in the AV
node or bundle of His ( narrow QRS complexes) or the distal Purkinje
tissues ( broad QRS complexes ) .
Complete heart block produces a slow (25-50/min), regular pulse that,
except in the case of congenital complete heart block, does not vary with
exercise. There is usually a compensatory increase in stroke volume with
a large volume pulse and systolic flow murmurs. Cannon waves may be
visible in the neck and the intensity of the first heart sound varies due to
the loss of AV synchrony.
 Idiopathic fibrosis
 Myocardial infarction/ischaemia
 Inflammation
o Acute (e.g. aortic root abscess in infective
o Chronic (e.g. sarcoidosis, Chagus disease).
.Trauma ( cardiac surgery)
.Drugs ( digoxin , b- blockers )
Stokes-Adams attacks
Episodes of ventricular asystole may complicate complete heart block or
Mobitz type 2 second degree heart block or occur in patient with
sinoatrial disease .this may cause recurrent syncope or Stokes-Adams
Typical episode is characterized by sudden loss of consciousness that
occurs without warning and results in collapse . A brief anoxic seizure
( due to cerebral ischemia ) may occur if there is prolonged asystole.
There is pallor and death like appearance during the attack, but when the
heart starts beating again there is a characteristic flush . Unlike epilepsy ,
recovery is rapid .Sinoatrial disease and neurocardiogenic syncope may
cause similar symptoms.
AV block complicating acute MI
Acute inferior MI is often complicated by transient AV block because the
RCA supplies AV node . there is usually a reliable escape rhythm and if
the patient remains well no treatment is required . symptomatic second
and third degree heart block may respond to atropine (0.6mg to 3 mg ) or
if this fail a temporary pacemaker . In most cases the AV block will
resolve within 7-10 days .
second and third degree heart block may complicate acute anterior MI
indicates extensive ventricular damage involving both bundle branches
and carries a poor prognosis . Asystole may ensue and temporary
pacemaker should be inserted promptly . If the patient presents with
asystole i.v atropine or iv isoprenaline 2mg in 500 ml 5% dextrose
infused at 10-60 ml/hr may help to maintain the circulation until
temporary pacemaker inserted . External (transcutaneous) pacing can
provide effective temporary rhythm support .
Chronic AV block
Patient with symptomatic bradyarrhythmias associated with AV block
should receive a permanent pacemaker . asymptomatic first degree or
Mobitz type 1 second degree heart block does not require treatment but
may be an indication of serious underlying heart disease . permanent
pacemaker is usually indicated in a patient with asymptomatic Mobitz
type 2 second or third degree heart block because of the risk of asystole
and sudden death. Pacing improve the prognosis .
Bundle branch block
With complete bundle branch blocks, the QRS interval is 0.12 s in
duration; with incomplete blocks the QRS interval is between 0.10 and
0.12 s. Conduction block of left bundles can ocurr as a result of many
pathologies , including ischemia , hypertensive heart disease or
cardiomyopathy or aortic valves disease while conduction block in the
right could be normal or from ASD or ischemia . Depolarization proceeds
the slow myocardial route in the affected ventricle rather than through the
rapidly conducting Purkinje tissues that constitute the bundle branches
.this causes delayed conduction into LV or RV , broadens the QRS
complex(>0.12 s) and produce the characteristic alterations in QRS
morphology with RBBB, the terminal QRS vector is oriented to the right
and anteriorly (rSR' in V1 and qRS in V6, typically). Left bundle branch
block alters both early and later phases of ventricular depolarization .
Right bundle branch block can occur in healthy people but the left bundle
branch block often signifies important underlying heart disease .
The left branch of the bundle of His divides into an anterior and a
posterior fascicle. Damage to the conducting tissue at this point
(hemiblock) does not broaden the QRS complex, but alters the mean
direction of ventricular depolarisation (mean QRS axis), causing left axis
deviation in left anterior hemiblock and right axis deviation in left
posterior hemiblock. The combination of right bundle branch and left
anterior or posterior hemiblock is known as bifascicular block.
left anterior hemiblock 1
Sinoatrial disease can occur at any age, but is most common in the
elderly. The underlying pathology is not understood but may involve
fibrosis, degenerative changes and/or ischaemia of the sinoatrial (sinus)
node. The condition is characterised by a variety of arrhythmias (Box
below) and may present with palpitation, dizzy spells or syncope, due to
intermittent tachycardia, bradycardia, or pauses with no atrial or
ventricular activity (sinoatrial block or sinus arrest).
A permanent pacemaker may benefit patients with troublesome
symptoms due to spontaneous bradycardias, or those with symptomatic
bradycardias induced by drugs required to prevent tachyarrhythmias.
Atrial pacing may help to prevent episodes of atrial fibrillation.
Permanent pacing does not improve prognosis and is not indicated in
patients who are asymptomatic.
 Sinus bradycardia
 Sinoatrial block (sinus arrest)
 Paroxysmal supraventricular tachycardia
 Paroxysmal atrial fibrillation
 Atrioventricular block .
Sinus Bradycardia
Rates less than 60 beats per minute are usually described as bradycardia.
In healthy persons, rates of 50 beats per minute are not unusual, however,
and rates of 30 beats per minute may be recorded during sleep. Sinus
bradycardia of clinical significance is usually defined as persistent rates
less than 45 beats per minute while awake. sinus bradycardia can occur in
athletes , hypothyroidism , ischemia , obstructive joundice or secondary
to cardioactive drugs such as β-blockers or calcium-channel blockers.
Correction of the underlying cause
Asymptomatic : only follow up
Symptomatic : either :
Medical : 1- atropine and isoprenaline in the emergency states
2- B2 agonists or thiophylline may be benificial
3- Pacemakers
Sinus Arrest
Sudden disappearance of P waves could be due to either SA exit block or
cessation of sinus node pacemaker function. Treated By pacemaker
Bradycardia-Tachycardia Syndrome
Because SND often represents atrial disease processes (e.g., fibrosis,
degeneration, inflammation), coexistence of atrial tachyarrhythmias with
bradycardia is not surprising. When an atrial tachycardia such as AF is
terminated, the underlying rhythm may reveal sinus bradycardia, SA exit
block, or even complete atrial standstill with an escape rhythm from a
lower pacemaker in the AV junction or the HPS. Treated by permanent