Bradycardias

Bradycardia is defined as a heart rate of fewer than 60 beats per minute.

Bradycardia can be normal in some people, but can also lead to haemodynamic compromise, leading to the development of symptoms.

Symptoms

Hypotension (< 90mmHg)

Syncope

Heart failure 

Management

First-line is atropine 500 mcg IV – this can be repeated up to a maximum of 3 mg

If no improvement, use transcutaneous pacing

If persistent, give isoprenaline/adrenaline infusion titrated to response

Long term management involves insertion of a pacemaker 

Atrioventricular heart block

This is an abnormality in the normal electrical conduction of the heart, which leads to delayed transmission of the impulse, resulting in a slow heartbeat.

Atrioventricular heart block results from the interruption of impulse conduction from the atria to the ventricles.

AV blocks are classified according to their electrographic appearance, which gives a clue as to the location. 

1st degree heart block

This is when impulses from the atria are consistently delayed by a certain amount at the AV node.

It can be a normal variant and is often asymptomatic.

Causes

Idiopathic

Digoxin

Coronary artery disease (CAD)

Lyme disease

Symptoms

Often asymptomatic as cardiac output not affected

ECG Features

The PR interval is prolonged (> 0.20 s), but it is the same in each beat.

There are no missed beats. 

2nd degree heart block: Mobitz type I

This is when each successive impulse is delayed more and more by the AV node.

This continues until an impulse fails to be conducted from ventricles, resulting in a missed beat, which then restarts the cycle.

This can also be a normal variant, especially in very fit people at rest.

It is not generally dangerous, but pacing can be considered if it causes symptoms such as dizziness or it is affecting the quality of life.

Causes

Idiopathic

CAD

Inferior MI

Usually temporary due to medication (B-blockers + digoxin)

Symptoms

Often asymptomatic but might show light-headedness or hypotension

ECG Features

PR interval is more and more prolonged each beat, until you miss a beat completely

This is called the Wenckebach phenomenon

2nd degree heart block: Mobitz type II

This is when impulses from the SAN are intermittently not conducted to the ventricles, usually (75%) distal to the bundle of His.

Due to the distal ocation of the conduction defect, the QRS often appears broad.

Mobitz type II requires permanent pacemaker implantation as there is a high risk of progression to complete heart block and sudden cardiac death.

Causes

Severe CAD

Anterior wall MI

Changes in the conduction system (structural damage)

Symptoms

Dyspnea

Fatigue

Light-headedness

Pain

Hypotension

ECG Features

Constant PR interval but many P waves are not followed by QRS

Ratio ranges from 2:1 to 4:1

3rd degree heart block

This is when impulses from SAN are completely blocked from reaching the ventricles.

The site of the block largely determines the ECG characteristics.

An escape rhythm from near to the AV node causes a relatively narrow QRS and higher rate (sometimes above 40 bpm).

A more distal block results in a broader ventricular escape rhythm, a lower heart rate and pacing is even more urgent.

Causes

Inferior wall MI

Congenital

Aortic valve calcification

Digoxin toxicity

Symptoms

Dyspnea

Fatigue

Light-headedness

Pain and hypotension

ECG Features

No relation between P waves and QRS waves, 2 independent rhythms present

Bundle Branch Block (BBB)

In the heart, the two bundles of His conduct electrical activity to the ventricles.

In BBB, either the left or right bundle fails to conduct impulses to the ventricles.

This means that the wave of depolarisation must pass via cell-to-cell conduction, which is much slower than along the specialised bundle branch cells.

This prolongs the contraction rate which results in widening of the QRS complex.

This can slow the heart rate down causing bradycardia, impairing the cardiac output.

It leads to the development of similar symptoms to AV heart block (hypotension, lightheadedness, chest pain etc.).

BBB can occur further down the left bundle, which is divided into an anterior and posterior fasciculus.

A branch block that occurs here is known as a hemiblock. 

Right Bundle Branch Block

This is when the right bundle is blocked.

It is important to know that this can be a normal variant (seen more in the elderly)

Causes

Idiopathic (normal variant if isolated and asymptomatic)

Conditions which increase pressure in right side of heart –  Pulmonary embolism, cor pulmonale

Right ventricular hypertrophy – after a Myocardial infarction, cardiomyopathy

Atrial septal defect

ECG Features

QRS >0.12s

MaRRoW sign on ECG

“RSR” pattern in Lead V1 (M)

“qRS” pattern in V6 (W)

Also may see inverted T waves in V1-3 or V4 and a wide slurred S wave in V6

Left Bundle Branch Block

This is a condition which describes when the left bundle is blocked giving QRS > 0.12s

This never occurs normally and is always considered pathological

Causes

Usually caused by ischaemic heart disease

New onset LBBB may be a sign of myocardial infarction in patients

Conditions which increase pressure in left side of heart – hypertension, aortic stenosis

ECG Features

QRS >0.12s

WiLLiaM sign on ECG

“QRS” pattern in Lead V1 (W)

“RSR” pattern in V6 (M) – WiLLiaM

Bifascicular Block

This is a combination of RBBB and left anterior or posterior hemiblock

Trifascicular Block

This is a combination of bifascicular block accompanied with 1st degree heart block

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