36. Heart Block and Temporary Pacing

Question 1

Have a look at this ECG. What does it show?

Answer 1

The ECG shows sinus rhythm at a rate of 87 beats per minute. The axis is
normal. The PR interval is prolonged. There are no other abnormalities. The
diagnosis is first-degree heart block.

Question 2

How do you know it is first-degree heart block?

Answer 2

The normal PR interval is 120–210 milliseconds (some books say 200 ms!). The
PR interval is measured from the start of the P wave to the start of the
ventricular complex, whether that is a Q or an R wave. At a standard paper
speed of 25 mm/s, each small square is equivalent to 40 ms. The normal PR
interval is therefore 3–5(ish) small squares.

Question 3

What types of heart block are there?

Answer 3

The term ‘heart block’ usually refers to atrio-ventricular block as distinct from ‘bundle-branch’ block.

Atrio-ventricular heart block is classified into first-, second- or third-degree
depending on the effect on atrio-ventricular conduction.

First-degree
As described above.

Question 4

In Mobitz type I

Answer 4

In Mobitz type I block the PR interval increases successively
with each beat until a QRS complex is ‘dropped’.

When seen
in young, fit people (often nocturnally) with high vagal tone,
it may be benign.

When it cannot be attributed to high vagal
tone it may have a similar prognosis to Mobitz type II.

Question 5

In Mobitz type II

Answer 5

In Mobitz type II block there is intermittent failure of A-V
conduction. This results in 2:1 or 3:1 A-V block, for example..
The block is usually below the A-V node and is associated with
an increased incidence of Stokes-Adams attacks, slow
ventricular rates and sudden death.

Question 6

What are the indications for pacing pre-operatively?

Answer 6

Suggested indications include:

Acute anterior myocardial infarction (MI)

Acute MI with Mobitz type II or third degree block (?also new BBB)

First-degree heart block with bifascicular block

Any symptomatic brady-arrhythmia

Refractory supra-ventricular tachy-arrhythmia

Question 7

How may peri-operative pacing be achieved?

Answer 7

Transthoracic non-invasive

Epicardial

Transvenous

Transoesophageal

Question 8

Transthoracic non-invasive

Answer 8

The anterior patch must be negative and placed just
to the left of the xiphoid process to avoid the
pectoral muscle-mass. The posterior patch is placed
inferior to the left scapula.

Question 9

Epicardial

Answer 9

Epicardial Usually performed by surgeons during cardiac
surgery.

Question 10

Transvenous

Answer 10

Transvenous May or may not be balloon-tipped to help with
insertion. Requires X-ray control to check its position
in the apex of the right ventricle (pointing down). A
pacing wire that is pointing towards the left
shoulder may be in the coronary sinus.

Question 11

Transoesophageal

Answer 11

Transoesophageal Used when slow atrial rhythm. Left atrium lies
anterior to the oesophagus. The pacer is switched on
and advanced until capture occurs (usually at
30–35cm from teeth).