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ATRIOVENTRICULAR CONDUCTION BLOCKS (SECOND DEGREE HEART BLOCK [MOBITZ…
ATRIOVENTRICULAR CONDUCTION BLOCKS
FIRST DEGREE HEART BLOCK
ECG
Constant prolonged PR interval (>0.2s 5 small squares) on ECG
sinus rhythm (each P wave is followed by a QRS complex)
Commonly affects patients over the age of 65
AETIOLOGY
Common Causes
inc vagal tone (eg athletes, during sleep)
myocardial ischaemia (RCA, supplies the AVN)
idiopathic degeneration (fibrosis) of the conduction system
Drugs - B-blockers, CCB, digoxin
Other causes
Metabolic disturbances (hypokalaemia, hypomagnesaemia)
Myocarditis
PATHOPHYSIOLOGY
involve AVN itself
structural causes such as fibrosis or damage to nodal inputs will delay impulse conduction
AVN richly innervated by autonomic nervous system - therefore vagal (parasympathetic) activation may also prolong AV conduction time
usually asymptomatic
MANAGEMENT
benign conition
treatment not usually required
PROGNOSIS
normal, although some patients will progress to higher degrees of AV block over time
SECOND DEGREE HEART BLOCK [MOBITZ TYPE I/ WENKEBACH BLOCK]
intermittent failure of AV conduction = result in occasional dropped beats
occurs in 4% of post-inferior MI. more common than Mobitz type II
AETIOLOGY
Common causes
idiopathic fibrosis of the conduction system
drugs [B-blocker, CCB, digoxin, procainamide]
inc vagal tone - athletes, children, during sleep
Other causes
Iatrogenic - transcatheter aortic valve implantation [TAVI]
inferior MI
other causes are similar to those of first degree heart block
PATHOPHYSIOLOGY
can also be vagally mediated as a result of normal physiology or drugs and rarely structural abnormalities
progressive AVN cell fatigue, eventually results in a dropped beat
caused by progressive conduction loss more commonly within the AVN itself (70%)
ECG
Narrow QRS complexes
PR interval longest before the dropped beat and shortest after
progressive prolongation of PR interval until a beat is dropped
MANAGEMENT
IV
atropine
may be used in emergency or severe bradycardia
Permanent pacemaker implantation is indicated in pts with non-resolving symptomatic block. This is shown to have mortality benefits in those over 45y/o
Treatment not usually required unless symptomatc
SECOND DEGREE HEART BLOCK
[MOBITZ TYPE II/NON-WENKEBACH]
AV conduction deficit resulting in intermittent dropped beats without changes in PR interval
AETIOLOGY
Common causes
idiopathic fibrosis of the conduction system
anterior MI
Other causes
infiltrative disease - haemachromatosis, sarcoidosis, amyloidosis
other causes are similar to those in first degree heart block
drugs - B-blockers, CCB, digoxin
PATHOPHYSIOLOGY
the conduction block tends to occur infra-nodally, in the bundles of His (20%) or Purkinje fibres (80%) and is more likely to be caused by structural abnormalities of the conduction system
CLINICAL FEATURES
DIZZINESS AND SYNCOPE
May present with haemodynamic instability and sudden cardiac death in some cases
ECG
constant PR interval
QRS complexes may be broad if the AV block occurs at the Purkinje system
my be assoc with a fixed ratio block (2:1, 3:1, etc)
when the AV block occurs distally in the Purkinje system, there's often a pre-existing BBB => classic wide QRS complexes of BBB as there is a delay in depolarisation of each ventricle
MANAGEMENT treat haemodynamic compromise with
IV isoprenaline infusion may stabilise rhythm in a pt with profound bradycardia
if other measures fail and permanent pacing is not immediately available consider temporary external pacing
IV adrenaline
temporary trans-venous pacing
IV atropine
permanent pacemaker implantation is indicated for all patients (Even asymtpomatic)
PROGNOSIS
COMMONLY PROGRESSES TO THIRD DEGREE
PPM implantation has been shown to improve 5yr survival rates
THIRD DEGREE HEART BLOCK
[COMPLETE HEART BLOCK]
complete failure of AV conduction = loss of communication between the atria and ventricles = beat independently from one another
AETIOLOGY
common causes
ant and inf MI - due to interruption of the blood supply to the AVN. often resolves within 7 days
drugs - B-blockers, CCB, digoxin
idiopathic degeneration of the conduction system (Ageing)
other causes
Iatrogenic - cardiac surgery, cardiac catheterisation
other causes similar to first degree
congenital - maternal systemic lupus erythematosus
PATHOPHYSIOLOGY
complete failure of AV conduction system results in complete AV block
AV dissociation
rhythm often slow
CLINICAL FEATURES
palpitations
intermittent cannon A waves - due to contraction of atria at a time when AV valves are closed - cause regurg of blood into the venae cavae
stokes-adams attacks [syncope +LOC]
cannon A occur when atria contract against a closed tricuspid valve
symptoms of low CO - dizzy, breathless, fatigue
ECG
constant PR and RR intervals but apparent AV dissociation
rate tends to be less than 50bpm
QRS complexes may be narrow (junctional escape rhythm) or wide (subjunctional escape rhythm)
complete AV dissociation
MANAGEMENT
If pt is haemodynamically compromised, emergency IV atropine may be used
however atropine is short acting and IV isoprenaline may be more useful
Correct reversible causes
temporary pacing may be indicated as a bridge to PPM
permanent pace maker implant indicated for all pts to prevent recurrence
PROGNOSIS
pts have 28% mortality if they develop 3rd degree during acute MI