Atrioventricular block

Question 1

what is atrioventricular block

Answer 1

often referred to as ‘heart block’
involves the partial or complete interuption of impulse transmission from the atria to the ventricles

Question 2

common causes of AV block

Answer 2

idiopathic fibrosis and sclerosis of the conduction system

Question 3

what investigations should be done

Answer 3

ECG: to help determine the subtype of AV block
lab investigations: FBCs, U&Es, TSH, troponin: to rule out inderlying causes
echo: to rule out structural heart disease

Question 4

first degree AV block is when

Answer 4

there is consistant prolongation of the PR interval (>0.2 seconds) due to delayed conduction via the AV node
there is no dropped QRS complexes
common and may be an incidental finding

Question 5

causes of first degree heart block

Answer 5

• enhanced vagal tone, aften seen in atheletes and non-pathological
• post myocardial infarction
• lyme disease
• systemic lupus erythematosus
• congenital
• myocarditis
• electrolyte derangements
• drugs: particularly AV blocking drugs such as beta blockers, rate limiting calcium-channel blockers, digoxin and magnesium
• thyroid dysfunction

Question 6

typical ECG findings of first degree heart block

Answer 6

regular rhythm
P wave: every p wave is present and followed by a QRS complex
PR interval: prolonged >0.2 seconds
QRS complex: normal morpholohy and duration <0.12 seconds

Question 7

clinical features of first degree heart block

Answer 7

patients are usually asymptomatic
clinical examination is usually unremarkable

Question 8

management of first degree block

Answer 8

any AV blocking drugs should be ceased
no intervention is usually required if the patient is asymptomatic
if the patient is symptomatic, a pacemaker may be considered

Question 9

complications of first degree heart block

Answer 9

does not usually progress to higher degree AV blocks
those with first degree AV block may be at risk for atrial fibrillation

Question 10

what is second degree AV block type 1

Answer 10

progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped

Question 11

other naames for second degree AV block type 1

Answer 11

mobiitz type 1 AV block or wenckebach phenomenon

Question 12

aetiology of second degree AV block type 1

Answer 12

increased vagal tone: often seen in athletes, not pathological
drugs: beta blockers, calcium chennel blockers, dioxin, amiodarone
infrior myocardial infarction
myocarditis
cardiac surgery (mitral valve repair, tetrology of fallot repair)

Question 13

second degree AV block type 1 ECG findings

Answer 13

irregular rhythm
P wave: every p wave is present, but not all are followed by a QRS complex
PR interval: progressively lengthens before a QRS complex is dropped
QRS complex: normal morphology and duration (<0.12 seconds), but are repeatedly dropped

Question 14

clinical features of second degree AV block type 1

Answer 14

patients are usually asymptomatic, but some can develop symptomatic bradycardia and present with symptoms such as pre-syncope and syncope
O/E: irregular pulse and bradycardia

Question 15

management of second degree AV block type 1

Answer 15

AV blocking drugs should be stopped
usually benign and rarely causes haemodynamic compromise
usually, no intervantion is required if the patient is asymptomatic
if the patient is symptomatic a pacemaker may be considered

Question 16

complications of second degree AV block type 1

Answer 16

the patient may become haemodynaamically compromised but this is rare

Question 17

what is second degree AV block type 2

Answer 17

also called Mobitz type 2 AV block
consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction
dropping of QRS complexes usually follows a repeating cycle

Question 18

when is mobitz type 2 block pathological

Answer 18

always, with the block typically occuring at the bundle of His or the bundle branches

Question 19

causes of second degree AV block type 2

Answer 19

• myocardial infarction
• idiopathic fibrosis of the conducting system
• cardiac surgery
• inflammatory conditions (rhuematic fever, myocarditis, lyme disease
• autoimmune (SLE, systemic sclerosis)
• infiltrative myocardial disease (amyloidosis, haemochromotosis, sarcoidosis)
• hyperkalaemia
• drugs eg. beta blockers, calcium channel blockers, digoxin, amiodarone)
• thyroid dysfunction

Question 20

ECG findings of second degree type 2 heart block

Answer 20

rhythm: irregular, may be regularly irregular
p wave: present but there are more p waves than QRS complexes
PR interval: consistant normal PR interval duration with intermittantly dropped QRS complexes
QRS complex: normal (<0.12 seconds) or borad (>0.12 seconds)
the QRS complex will be broad if the conduction failure is located distal to the bundle of His

Question 21

clinical features of mobitz type 2 heart block

Answer 21

palpitations
pre-syncope
syncope
O/E: may detect a regularly irregular pulse, where there is a pattern of how many atrial depolarisation (p waves) lead to ventricular depolarisation (QRS waves) such as 3:1 block

Question 22

management of mobitz type 2 heart block

Answer 22

due to risk of progression to complete heart block, patients should be places on a cardiac monitor as soon as possible
underlying cause should be investigated
temporary pacing or isoprenaline may be required is the patient is haemodynamically compromised due to bradycardia
a permanenet pacemaker is usually inserted if there are no reversible causes identified

Question 23

complications of mobitz type 2 heart block

Answer 23

patients are at risk of progressing to complete ysmptomatic AV block, in which the escape rhythm is likely to be ventricular and thus too slow to maintain adequate systemic perfusion
patients are also at risk of developing asystole

Question 24

what is third degree (complete) AV block

Answer 24

there is no electrical communication between the atria and ventricles due to complete failure of conduction
presence of p waves and QRS complexes that have no association with each other due to the atria and the ventricles perfoming independantly

Question 25

narrow complex escape rhythms in third degree heart block

Answer 25

originate above the bifurcation of the bundle of His
a typical heart raate woul dbe above 40bpm

Question 26

broad complex escape rhythms in third degree heart block

Answer 26

originate from below the bifurcation of the bundle of His
these escape rhythms produce slower, less reliable heart rates and more significant clinical features eg. heart failure, syncope

Question 27

aetiology of third degree heart block

Answer 27

congenital: structural heart disease eg. transposition of the great vessels, autoimmune eg. maternal SLE
idiopathic fibrosis
ischameic heaart disease: myocaardial infarction, ischaemic cardiomyopathy
non-ischaemic heart disease
iatrogenic: post ablative therapies and pacemaker implantation, post-cardiac surgeries
drug related: digoxin, beta blockers, calcium channel blockers, amiodarone
infections
thyroid dysfunction

Question 28

ECG findings of third degree heart block

Answer 28

variable rhythm
p wave: present but not associated with QRS complexes
P interval: absent
QRS complex: narrow or broad depending on the site of the escape rhythm

Question 29

clinical features of third degree heart block

Answer 29

Typical symptoms of third-degree heart block may include:

Palpitations
Pre-syncope/syncope
Confusion
Shortness of breath (due to heart failure)
Chest pain
Sudden cardiac death

O/E: irregular ppulse, profound bradycardia, haemodynamic compromise eg. prolonged cap refil time and hypotension

Question 30

management of third degree heart block

Answer 30

Patients should be placed on a cardiac monitor.

Transcutaneous pacing/temporary pacing wire or isoprenaline infusion may be required. Some rhythms (particularly narrow-complex escape rhythms) may respond to atropine.

A permanent pacemaker is usually required.

Question 31

complications of third degree heart block

Answer 31

the main complication is sudden cardiac death due to ventricular arrhythmias