Arrhythmias: Bradycardia, Heart block, and Bundle branch block

Question 1

What are the two main types of arrhythmia?

Answer 1

Bradycardia <60bpm

Tachycardia >100bpm

Question 2

Outline the complications of arrhythmias

Answer 2

Palpitations
Chest pain
Dizziness
Syncope
Heart failure
Sudden death

Question 3

Describe the function of the Sinoatrial node

Answer 3

A spindle-shaped structure located in the junction between the SVC and RA.

It is the natural cardiac pacemaker that automatically spontaneously depolarises, faster than other pacemaker cells. This initiates the electrical impulse that results in contraction of myocytes. The rate at which the SAN fires determines the heart rate.

Question 4

Describe the sinoatrial node action potential

Answer 4

1. HCN Na channels open, allowing slow depolarisation (known as the funny current)
2. Upon reaching threshold, L-type Ca channels open, producing an action potential
3. VG K channels open, allowing repolarisation

Question 5

Explain the mechanisms of arrhythmia production

Answer 5

Accelerated automaticity: an area of myocardial cells depolarises faster than the SAN

Triggered activity: myocardial damage release electrical activity (after-depolarisations), that can reach threshold and produce an arrhythmia.

Re-entry: propagating AP travels retrograde to form a re-entry loop, which repeatedly excites the myocardium. This produces the majority of regular paroxysmal tachycardias.

Question 6

Explain how the different types of bradycardia occur

Answer 6

Sinus bradycardia: failure of impulse formation

AV block: failure of impulse conduction from the atria to
the ventricles.

Question 7

Name 4 causes of sinus bradycardia

Answer 7

Extrinsic factors:

• Hypothermia, hypothyroidism
• Cholestatic jaundice
• Raised intracranial pressure
• Drugs: Beta-blockers and other antiarrhythmics
• Neurally mediated syndromes: carotid sinus syndrome, vasovagal syncope, postural tachycardia syndrome

Intrinsic sinus node disease:

• Acute ischaemia and infarction of sinus node
• Chronic degeneration: fibrosis, sick sinus syndrome
• Cardiomyopathies
• Infilitrative disease: sarcoidosis, haemochromatosis

Question 8

Define normal sinus rhythm

Answer 8

Pacemaker impulses arise from sinoatrial node
Regular rhythm at 60-100 bpm
Each QRS complex is preceded by a normal P wave
PR interval is constant
Narrow-complex QRS (<100 ms)

Question 9

Define sick sinus syndrome

Answer 9

Abnormal sinus node function with resultant bradycardia and cardiac insufficiency

Question 10

Define Carotid sinus syndrome

Answer 10

Bradycardia and syncope due to an exaggerated response to carotid sinus baroreceptor stimulation.
Occurs when rotating head.

Question 11

Define Vasovagal syncope

Answer 11

Syncope resulting from autonomic failure due to physical and/or emotional stress. Causes bradycardia and/or vasodilation.

Question 12

Define Postural tachycardia syndrome

Answer 12

Abnormal sudden and significant increases in heart rate produced by sitting or standing up. Failure of peripheral vasoconstriction causes dizziness and syncope.

Question 13

What is the management of vasovagal syncope?

Answer 13

Avoidance of known triggers
Avoid prolonged upright position
Sitting/lying down and applying counter-pressure (pushing palms together, crossing legs) if attack occurs

Increased salt intake, compression hosiery

Fludrocortisone
Disopyramide (negative inotrope)

Question 14

Differentiate the types of heart block

Answer 14

First-degree: PR interval prolongation (>220ms). All atrial activity is conducted to the ventricles.

Second-degree: Some P waves conduct, others don’t.

• Mobitz I: Progressive prolongation of PR interval, until a P wave fails to conduct.
• Mobitz II: Spontaneous dropping of QRS without progressive PR prolongation (usually wide >120ms).
• 2:1 or 3:1 block: Every 2nd/3rd P wave conducts.

Third-degree: All atrial activity fails to conduct to the ventricles.

Question 15

Describe the pathophysiology of third-degree heart block

Answer 15

All atrial activity fails to conduct to the ventricles. Slower pacemaker cells become responsible for initiating ventricular contract to sustain life. These are known as spontaneous escape rhythms, and may be narrow- (originating from bundle of His) or broad-complex (originating from ventricular pacemakers).

Question 16

Define narrow-complex escape rhythms in third-degree heart block

Answer 16

Spontaneous electrical impulses originating in the bundle of His, which produce a QRS complex <120ms. This escape rhythm occurs at 50-60 bpm.

Region of AV block lies more proximal in the AV node.

Question 17

Outline the management of narrow-complex escape rhythms in third-degree heart block

Answer 17

Atropine up to 3mg

Temporary transcutaneous pacing

Question 18

Define broad-complex escape rhythms in third-degree heart block

Answer 18

Spontaneous electrical impulses originating from ventricular pacemaker cells, which produce a QRS complex >120ms. This escape rhythm occurs at 15-40 bpm.

Region of AV block lies more distal in the His-Purkinje system.

Adams-Stokes syndrome: Dizziness and loss of consciousnesses often occur

Question 19

Outline the management of broad-complex escape rhythms in third-degree heart block

Answer 19

Permanent transvenous pacemaker is indicated

If pacemaker is delayed:
Atropine, up to maximum of 3mg
Temporary transcutaneous pacing
Adrenaline infusion: titrated to response

Question 20

List 5 causes of third-degree heart block

Answer 20

Congenital: Maternal SLE, Structural heart disease
Idiopathic fibrosis: Lev’s disease, Lenegre’s disease
IHD: Acute MI, Ischaemic cardiomyopathy
Non-IHD: Calcific aortic stenosis, idiopathic dilated cardiomyopathy, infiltrations (sarcoid, amyloid, cancer)
Cardiac surgery
Iatrogenic: Radiofrequency AVN ablation, pacemaker
Drug: Digoxin, beta-blocks, non-dihydropyridine CCB
Infections: Endocarditis, Lyme disease, Chagas disease
Rheum: SLE, RA
NMD: Duchenne muscular dystrophy

Question 21

Describe the anatomy of the bundle of His

Answer 21

The bundle of His starts from the AVN, and travels down the interventricular septum.
It divides into the main left bundle branch and right bundle branch.
The main LBB divides into an anterior-superior division and a posterior-inferior division.

Question 22

Describe the ECG of complete bundle branch block

Answer 22

Associated with wide QRS (>120ms)

RBBB: Deep S wave in I and V6, Tall late R wave in V1
LBBB: Deep S wave in V1, Tall late R wave in I and V6

Question 23

Describe the ECG of bundle branch conduction delay

Answer 23

Slight widening of QRS (up to 110ms)

Known as incomplete BBB

Question 24

What is hemiblock?

Answer 24

Delay or block in one of the divisions of the LBB.

• Left anterior hemiblock: causes left axis deviation
• Left posterior hemiblock: causes right axis deviation

Question 25

Define Bifascicular block

Answer 25

Combination delay or block of any two of following:

• Right bundle branch
• Left anterior-superior fascicle
• Left posterior-inferior fascicle

Question 26

Define Trifascicular block

Answer 26

Combination delay or block in all three:

• Right bundle branch
• Left anterior-superior fascicle
• Left poster-inferior fascicle

Question 27

What ECG changes are seen in incomplete and complete trifascicular block?

Answer 27

Incomplete: RBBB + LAD + First-degree AV block
Complete: RBBB + LAD + Third-degree AV block

Question 28

What ECG changes are seen in bifascicular block?

Answer 28

RBBB + LAD