Arrhythmias- Heart Block

Question 1

Bradycardia (HR <60bpm) can be classified into absolute bradycardia and relative bradycardia; explain the difference

Answer 1

• Absolute; HR <60bpm *CHECK WB SAYS 40bpm
• Relative: HR is inappropriately slow for the haemodynamic state of the pt e.g. HR of 64 in a pt with septic shock

Question 2

Alongside classifying bradycardias as absolute or relative, we can also classify them according to the pacemaker at fault. Which two pacemakers of heart could be at fault?

Answer 2

SA node or AV node

Question 3

What is an arrhythmia?

Answer 3

Abnormal heart rhythm

Question 4

Causes of sick sinus syndrome can be either extrinsic or intrinsic to the heart; state some extrinsic causes of sinus bradycardia

Answer 4

• Drugs e.g. beta blockers
• Hypothyroidism
• Hypothermia
• Cholestatic jaundice
• Raised ICP

*Treat by treating underlying cause

Question 5

Describe how we can classify tachy arrhythmias

Answer 5

Bradyarrhythmias

• Sinus arrhythmia
• Sinus bradycardia
• Heart block

Tachyarrhythmias

• SVTs
  
  • Sinus tachycardia
  • Atrioventricular junction tachycardias (AVNRT, AVRT)
  • Atrial tachycardias (AF, atrial flutter, atrial ectopics)
• VTs
  
  • Ventricular tachycardia
  • Ventricular fibrillation

Question 6

What is sinus arrhythmia?

Who is it common in?

Answer 6

Heart rate increases on inspiration (as on inspiration parasympathetic tone decreases) and heart rate decreases on expiration (as on expiration parasympathetic tone increases).

Normal particularly in children and young adults.

Question 7

Sinus bradycardia can be either extrinsic or intrinsic to the heart; state some intrinsic causes of sinus bradycardia

Answer 7

• Ischaemia & infarction of SA node as complication of MI
• Chronic degenerative changes e.g fibrosis of atrium and SA node in the elderly

Both of the above can lead to sick sinus syndrome: intermittent failure of SA node depolarisation (sinus arrest) or failure of impule to propagate through perinodal tissue to atria (SA block)

Question 8

What is sick sinus syndrome?

Answer 8

• Usually caused by sinus node fibrosis
• Sinus node becomes dysfunctional; in some cases it causes bradycardia and in others it can generate tachyarrhythmias e.g. AF, atrial tachycardia
• Treat the manifestation e.g. bradycardia, AF etc..
• Some pts develop tachy-bradysyndrome suffering from alternating tachycardic & bradcardic rhythms. Difficult to treat. May give pacing for bradycardic episodes combined with rate slowing meds for tachycardic episodes when pt is symptomatic

Question 9

Bradycardias can be classified according to…?

Answer 9

Pacemaker which is at fault:

• SA node (e.g. in sick sinus syndrome)
• AV node (e.g heart block)

Question 10

We have already said that bradycardias can be classified based on the pacemaker at fault (SA or AV node). If the problem is with conduction at the AV node we refer to this as heart block

Define heart block and state the 4 types of heart block

Answer 10

• First degree AV block
• Second degree AV block
  
  • Mobitz type I (Wenckebach)
  • Mobitz type II
• Third degree heart block

*Remember heart block is delay, or failure, of conduction of impulses

Question 11

State some risk factors/causes for developing heart block

Answer 11

• Age
• Chronic heart failure (CHF)
• ACS (e.g. following inferior STEMI. If it is following an anterior MI- infranodal- it is more worrying)
• Hypertension
• Recent cardiac surgery
• Cardiomyopathy
• Medications e.g. beta blocker
• Electrolyte imbalances

Question 12

State some potential causes of heart block

Answer 12

• Calcification & fibrosis of conducting system
• AV nodal blocking medications e.g. beta blockers, CCBs
• Some antiarrhythmics e.g. class I anti-arrhythmics
• Blunt cardiac injury
• Post catheter ablation for arrhythmias

Question 13

If heart block is caused by an MI, will it resolve?

Answer 13

Yes, heart block following ACS event can resolve in hours to days

Question 14

If heart block is caused by hyperkalaemia, how do we treat it?

Answer 14

IV calcium chloride (10ml of 10% solution over 3-5 mins)

Question 15

State some medications which increase risk of heart block

Answer 15

• Beta blockers
• CCBs
• Adenosine
• Digoxin toxicity *COMMON

Question 16

For first degree heart block, state:

• What the problem is
• ECG changes
• Management

Answer 16

• Slowing of conduction at AV node
• Prolonged PR interval >0.2secs/5 small squares
• No specific management but:
  
  • If pt on digoxin, check for toxicity
  • If there are symptoms of dizziness or syncope cardiac monitoring shoudl be considered to identify highter degrees of heart block

Question 17

For second degree heart block, Mobitz type I/Wenckebach type state:

• What the problem is
• ECG changes
• Managment

Answer 17

• Some of atrial depolarisations fail to pass through AV node to ventricles
• Progressive lengthening of PR interval until a QRS is dropped
• No specific management but:
  
  • If pt presents with syncope or dizziness then should investigate for higher degrees of heart block

*NOTE: may occur in young fit patients with high vagal tone or post inferior MI

Question 18

For second degree heart block, Mobitz type II state:

• What the problem is
• ECG changes
• Management

Answer 18

• Some of atrial depolarisations fail to pass through AV node to ventricles
• Constant PR interval followed by sudden drop of a QRS. This may be in a fixed ratio e.g. 3:1 (every 3rd atrail depolarisation is conducted)

Management

• Stop any drugs that may be causing heart block
• In the absence of a recent acute coronary event, permanent pacing should be arranged as Mobitz type II, in comparison to Mobitz type I, indicates more serious involvement of conduction pathways and has a high risk of progression to third degree heart block

Question 19

Which coronary artery supplies the SA node?

Which coronary artery supplies the AV node?

Answer 19

• SA node: sinoatrial nodal artery from RCA (60%) or from LCA (40%)
• AV node: atrioventricular nodal branch of RCA (in over 80% of people)

*This is why heart block more common following inferior STEMI as RCA supplies inferior heart

Question 20

For third degree heart block, state:

• What the problem is
• ECG
• Management

Answer 20

• No conduction from atria to ventricles. Block can occur above the AV node at the His region or beneath the AV node. It can also be intermittent therefore you should look for ECGs with trifasicular and bifasicular block
• No relationship between P waves and QRS complexes

Management of Stable Pt

• Stop any medications which may be causing the heart block
• Urgent pacemaker (should be considered within 24 hours) in all patients except those who have reasonable likelihood of recovery of conduction e.g. those who have heart block following MI

Management of Haemodynamically Unstable Pt

• Try atropine (600mg up to max of 3mg)
• Isoprenaline (at 5ug/min) can also be tried
• Consider temporary transvenous or transcutaneous pacing

Question 21

Do people with first degree heart block often have symptoms?

Answer 21

No, pts with first degree heart block are generally asymptomatic and only find out they have heart block when they have e.g. ECG

Question 22

Do people with Mobitz type I/Wenkebach second degree heart block have any symptoms?

Answer 22

Most people don’t experience symptoms but some may experience:

• Light headedness
• Dizziness
• Fainting

Question 23

Do people with Mobitz type II second degree heart block often experience symtoms?

Answer 23

Likely to experience symptoms of:

• Light headedness
• Dizziness
• Fainting
• Chest pain
• SOB
• Fatigue (particularly on exertion)

Question 24

Do people with third degree heart block experience symptoms? If so, state some.

Answer 24

Yes, will experience symptoms similar to second degree heart block:

• Light headedness
• Dizziness
• Fainting
• Chest pain
• Fatigue (particulary during physical exertion/activity)
• SOB
• Palpitations
• Nausea & vomitting
• Slow HR

Question 25

Describe what you would find on examination of someone with heart block

Answer 25

• Bradycardia
• Hypertension (often) or hypotension (more acute/emergency)
• Other signs of cardiac disease (as cardiac disease is a risk factor for heart block)

Question 26

If someone presents with suspected heart block, state:

• Bedside tests
• Bloods
• Imaging

*For each one, explain why you would request it

Answer 26

Bedside tests

• ECG

Bloods

• FBC
• U&Es (mainly to look at K+ and Ca2+)
• Troponin (check no other cause for chest pain)
• Serum digitalis levels (if pt taking digoxin as digoxin toxicity is a cause of AV block)
• Serum pH (abnormal values can cause AV block)

Imaging

• ECHO (check for structural heart disease. Transthoracic echo also required prior to pacemaekr insertion)
• CXR (rule out any lung disease and check for signs of other cardiac pathology)

Question 27

State some potential complications of heart block

Answer 27

Prognosis if treated with pacemaker (if required) is generally good. Increased risk of AF with type 1 and Mobitz type 1. Risk of asystole if third degree untreated.

Question 28

Discuss the treatment for heart block, include treatment for:

• Stable pts
• Pts who are unstable or at risk of asytole (e.g. Mobitz type II, complete heart blok)
• Pts with high risk of astyole (e.g. Mobitz type II, complete heart block or previous asystole)

Answer 28

Stable

• Observe

Unstable or at risk of astyole

• Atropine 500ug IV

Unstable or risk of asytole

• Atropine 500ug IV repeated (can give up to 6 doses)
• Other inotropes e.g. Noradrenalin
• Transcutaenous cardiac pacing (use a defib)

High risk of astyole

• Temporary transvenous cardiac pacing (use electrode on end of wire that is inserted into a vein and fed through venous system to right atrium or ventricle to simtulate directly)
• Permanent implantable pacemaker

Question 29

Describe the mechanism of action of atropine

State some ADRs

Answer 29

Antimuscarinic which inhibits parasympathetic nervous system; it is a competitive antagonist of muscuranic receptors

ADRS

• Pupil dilation
• Urinary retention
• Dry eyes
• Constipation