Sick sinus syndrome describes
dysfunction of the intrinsic
pacemaker of heart, the
sinoatrial node. As a result, the
cardiac rhythm becomes
abnormal characterized by:
Tachycardia…fast heart rate
alternating slow and fast
Normally, the sinuatrial node produces a steady pace of regular
electrical impulses. In sick sinus syndrome, these signals are
abnormally paced. A person with sick sinus syndrome may have
heart rhythm that are too fast, too slow, punctuated by long pause or
an alternating combination of all of these rhythm problems
TYPES AND CAUSES
Types of sick sinus syndrome and their causes include:
Electrical signals move too slowly through sinus node, causing
abnormally slow heart rate.
The sinus node activity pauses.
The heart rate alternates between abnormally fast and slow
rhythms, often with a long pause (asystole) between heart beats.
It may be brought on by use of drugs like digitalis, calcium channel
blockers, beta blockers and antiarrhythmics.
Sick sinus syndrome usually occurs in people older than 50,in whom
the cause is often a non specific, scar like degeneration of the heart’s
conduction system like amyloidosis,sarcoidosis,chagas disease and
• In children, a common cause of sick sinus syndrome is heart
surgery especially on heart chambers.
• In coronary artery disease, high blood pressure, aortic and
mitral valve diseases may be associated with sick sinus
Interference with the conduction process of the heart causes the
phenomena called heart block or atrioventricular block.
Heart block is classified according to the level of impairment:
First Degree Heart Block
Second Degree Heart Block
Third Degree (complete) Heart Block
FIRST DEGREE HEART BLOCK:
First degree heart block or first degree atrioventricular block is
defined as prolongation of the PR interval on the ECG to more than
200msec.First degree heart block is considered “marked” when the
PR exceeds 300msec.While the conduction is slowed, there are no
With first degree atrioventricular (AV) block every atrial impulse is
transmitted to the ventricles ,resulting in regular ventricular rate.
This type of AV block can arise from delays in the conduction
system in the AV node itself, the His Purkinjie system or a
combination of both. Overall, dysfunction at the AV node is much
more common than dysfunction at the His Purkinjie system.
If the QRS complex is of normal width and morphology on the ECG
than the conduction delay is almost always at the level of the AV
If however, the QRS demonstrates a bundle branch morphology
than the level of the conduction delay is often localized to His
Following are the most common causes of first degree
Intrinsic AV nodal disease.
Enhanced vagal tone.
Acute MI particularly acute inferior wall MI.
Electrolyte disturbances (e.g hypokalemia, hypomagnesemia
Drugs(esp.those drugs that increase the refractory time of the AV
node, there by slowing conduction).
Drugs that most commonly causes first degree AV block include
Class 1c antiarrhythmias (e.g Quinidine, procainamide,
Class2c antiarrhythmias (e.g Flecainide, encainide)
Class3c antiarrhythmias (Beta blockers)
SECOND DEGREE HEART BLOCK
Second degree heart block implies intermittent conduction, some impulses
from the atria are conducted to ventricles whereas others are not.
Inferior wall MI.
Drugs like digitalis, beta blockers, calcium antagonists.
Hyperkalemia in well trained athelets during sleep.
TYPES OF SECOND DEGREE HEART BLOCK:
There are three types of second degree heart block.
Mobitz type 1
Mobitz type 2
MOBITZ TYPE 1
In this condition, there is progressive lengthening of successive PR
intervals followed by a dropped beat (non conducted P). This is also
known as wenckebach’s phenomenon.
In this AV block, there is conduction defect in AV node and AV
conduction time (PR interval) progressively lengthens before
blocked beat pulse is clinically irregular.
Prognosis is good in first degree and in Mobitz type 1,since reliable
alternative pacemaker arise from AV junction below the block, if
complete heart block develops.
Site of block is AV node. QRS complex is normal in morphology
because there is no delay in interventricular depolarization .
MOBITZ TYPE 2
In this condition, the PR interval of the conducted impulses remain
constant but some P waves are not conducted (i.e more P waves
than QRS complexes)
Site of block is infranodal in location and QRS complexes are wide.
Mobitz type 2 AV block is abrupt and is not preceded by
lengthening of AV conduction time. It is usually due to block within
the bundle of His.
Mobitz 2 block is almost always due to organic heart disease, in
case it proceeds to complete heart block.
It may represent as either type for type 2 AV block in which there
are two P waves to each QRS complex and therefore, called 2:1
If PR interval is prolonged and QRS complex is narrow then it is
type1 second degree heart block.
If PR interval is normal then QRS complex is wide, it is called
second degree AV block
THIRD DEGREE(COMPLETE) HEART
Third degree heart block is an advanced form of block. No impulse
from atria reaches the ventricles. Cardiac action is maintained by an
INFECTIONS: Infective endocarditis, Chagas disease, Lyme’s
INFILTRATION: Sarcoidosis, Amyloidosis, Neoplasia
DRUGS: Digioxin, Beta blockers, Amiodaron
TRAUMA: Cardiac surgery.
CONNECTIVE TISSUE DISEASE: SLE,RA
Escape rhythm arising in the bundle of His produces narrow QRS
complex at the rate of 50-60beats/min.Escape rhythm arising below
the His bundle produces broad complexes and at rate of 1540beats/min.
It is more advanced form of heart block due to lesion at the level of
bundle of His or more often distally in Purkinjie system and
associated with bilateral bundle branch block. QRS complex is wide
and ventricular rate is slower. Transmission of atrial pulses through
AV node is completely blocked and a ventricular rate, usually less
In chronic complete heart block, pulse is slow (30-40min) regular
and does not vary with exercise
Episodes of ventricular asystole may occur during periods of
transition from partial to complete heart block lasting several
seconds to minutes. These episodes may cause cardiac syncope
called Stokes-Adams attacks.
These attacks often occur without warning, there is rapid loss of
consciousness and pt. may fall.
Convulsions may occur, if heart does not begin to beat within about
10sec and death will result, if arrest is prolonged
BUNDLE BRANCH BLOCK
In normal heart, each electrical impulses from the atria is
conducted through the AV node to the bundle of His, from which it
is transmitted to the ventricles by the right and left bundle branches.
Left bundle branch block
Right bundle branch block
LEFT BUNDLE BRANCH BLOCK
In LBBB, left bundle branch no longer conducts electrical activity. The
electrical impulse thus enters right bundle branch block and is carried to
right ventricle. From there, it finally spreads to left ventricle. The two
ventricles no longer receive the electrical impulse simultaneously. First, the
Rt. ventricle receives the electrical impulse, then left.
Hypertrophy, dilatation or fibrosis LV myocardium.
Ischemic heart disease.
Advanced valvular heart disease.
Toxic, inflammatory changes.
Degenerative disease of conducting system (Lenegre disease).
TYPES OF LBBB
The LBBB can be divided into two fascicles:
Conduction system is composed of three fascicles:
Right Bundle Fascicle.
Left Anterior Fascicle.
Left Posterior Fascicle.
RIGHT BUNDLE BRANCH BLOCK
RBBB results from a defect in heart’s electrical conducting system.
There is a delay in or failure of transmission of electrical impulses
down the right bundle of heart. As a result , the Rt. Ventricle
depolarizes by means of cell- to- cell conduction. These impulses
spread more slowly than usual from inter ventricular septum to Rt.
Ventricle.This delay in conduction results in characteristic ECG
pattern, which is wide and notched QRS. Although conduction down
the Rt. branch is delayed ,conduction down the Lt. branch is normal.
As a result, the interventricular septum and Lt. ventricle in normal
There are two types.
CAUSES OF RBBB
Following are the causes of RBBB.
After repair of VSD.
Right Venticular hypertrophy
Large ASD or AV cushion defect
Right ventricular dysplasia.
Congenital absence or atrophy of bundle branch.
After CABG and in transplanted heart.
When there is blockage in anterior or posterior division of left
bundle branch, it is called hemiblock.
RBBB plus left anterior or posterior hemiblock.
In bifascicular and trifascicular block there are more chances to
progress to complete heart block.
RBBB + Hemiblock + First degree heart block.