Pacemakers and Implantable Cardiac Defibrillators

Question 1

Why were PM traditionally changed to asynchronous modes preoperatively

Why did implantable Cardiac Defibrillators (ICD) traditionally have their anti-tachyarrythmia modes deactivated preoperatively

Answer 1

PM changed to asynchronous because of the fear that electromagnetic interference (EMI) from the electro-surgical unit (EXU/diathermy) may cause oversensing and loss of pacing in patient’s who ar PM dependent

ICD anti-tachyarrhythmia modes disabled to prevent inadvertant shocks delivered as a result of misinterpretation of electromagnetic interference as ventricular tachycardia/fibrillation

Question 2

What is the problem with changing pacemakers to asynchronous preoperatively?

What is the problem with disabling the anti-tachyarrhythmia modes on the implantable cardiac defibrillator preoperatively?

Answer 2

The risk of R on T phenomenon in PM set to asynchronous mode

Disabling anti-tachy modes on ICD can lead to undiagnosed and untreated ventricular tachycardia/fibrillation.

Question 3

What are the conditions for use of a magnet on a cardiac implantable electronic device (CIED) intra-operatively?

(CIED includes PMs and ICDs)

Answer 3

Preoperative device interrogation is required with the results of magnet application known to the anaesthetist in advance

Question 4

Describe the coding system for CIEDs (PMs and ICDs)

Answer 4

Position 1: Chamber paced (A or V or D or O)
Position 2: Chamber sensed (A or V or D or O)
Position 3: Response to sensing (T or I or O)

Position 4: Rate modulation (O or R)
Position 5: Multisite pacing (A or V or D or O)

A = Atrial
V = Ventricular
D = Dual (A + V)
O - None

T = Triggered
I = Inhibited

Question 5

What is a DOO,a VOO and a DDD pacemaker

Answer 5

DOO is a dual chamber (atria + ventricles) paced pacemaker. There is no sensing and no response to sensing.

VOO is a ventricular paced pacemaker. There is no sensing and no response to sensing.

DDD is dual paced, dual sensed and response to sensing is also dual in that pacing can either be triggered or inhibited

Question 6

What is the problem with a pacemaker left in DDD and what are some modern solutions to overcome this

Answer 6

Pacemaker left in DDD may oversense electromagnetic interference as a normal heart beat leading to inhibition of the following heart beat leading to periods of asystole in PM - dependent patients when exposed to long bursts of EMIs from ESUs.

Modern pacemakers may contain algorithms that sense EMI and ignore it completely or temporarily change the PM mode to VOO.

Question 7

What steps can be taken to minimize the chance of electromagnetic interference of cardiac implantable electronic devices?

Answer 7

1. Operation site > 15 cm from PM generator or PM leads
2. Place diathermy dispersal pad distal to the operation site to draw the current away from the PM and leads.

Question 8

Under what circumstances might periods of asystole occur in PM dependent individuals who are undergoing surgery below the level of the iliac crest

Answer 8

If the dispersal pad is placed incorrectly.

Question 9

What should the actions of the anaesthetist be if there prolonged periods of asystole are observed during surgery in a PM dependent patient

Answer 9

1. Discontinue diathermy
2. Advance knowledge of effects of magnet application –> apply if magnet changes mode to asynchronous pacing or DOO.

Question 10

What should be done by the anaesthetist of the site of surgery is less than 15 cm from the pacemaker?

Answer 10

Change to asynchronous mode preoperatively

Ideally immediately preop and changed back immediately post op to minimize risk of R on T phenomena.

Question 11

What is R on T phenomena

Answer 11

When a pacing spike is delivered to the patient during the absolute refractory phase of the action potential. This leads to torsade de pointes which may progress to ventricular fibrillation.

Question 12

If the surgical site is within 15cm of the PM/PM leads, when can a ‘PM-on’ strategy be employed

Answer 12

1. Bipolar diathermy (short distance current and no EMI)
2. Plasma blade diathermy (superficial + min generation of EMI)
3. Harmonic scalpel (No EMI - high frequency sound waves)

(4. Short bursts of unipolar diathermy)

Question 13

Why can bipolar diathermy be used within 15 cm of PM

Answer 13

Current travels only a very short distance between the probes 1 - 2 mm. Generalized EMI is not produced

Question 14

Why does a ‘PM-on’ strategy reduce the chance of R on T phenomenon

Answer 14

The PM senses the natural beat that may occur and inhibits the following pacemaker discharge.

Question 15

What is the potential haemodynamic disadvantage of changing a PM from AAI (for example in sick sinus syndrome) to DDD?

Answer 15

in DDD the PM will pace the atrium and then the right ventricle (not via the AV node) so that the current will still need to pass into the left ventricle leading to right and left ventricular dyssynchrony which may impair LV CO by up to 15%

Question 16

What are CRT - P and CRT - D devices?

Answer 16

CRT - P
Cardiac Resynchronisation Therapy (with pacemaker)

These are CIEDs that are implanted in patients with heart failure due to poor LV function and LBBB.

• Atrial lead
• Right ventricle lead
• Lead in the coronary sinus (as far laterally as possible)

The CRT reads the atrial beat and then paces the left and right ventricles at the same time restoring synchronous left and right ventricular contraction. LVEF can improve by 15%.

CRT P (with PM)
CRT - D (with ICD)