Implantable cardiac devices

 

(adapted from Dr. Brian Cowie’s tutorial)

 

The basics:

Components

·      Box

·      Leads: through subclavian veins to RA / RV / coronary sinus (for LV)

·      Electrodes to sense electrical activity

·      Leads to deliver pacing impulse

Indications

·      Conduction system doesn’t work (-> PPM)

·      Risk of life threatening arrhythmia (->AICD)

·      Severe heart failure (-> CRT or CRT-D)

Common modes

·      AAI: single lead atrial pacemaker

·      VVI: single lead ventricular pacemaker

·      DDD: dual lead pacemaker (with A-V delay to allow synchronized LV contraction)

·      DDDR: rate-adaptive

 

Pacemaker coding:

1 Pacing

2 Sensing

3 Response to sensing

4 Rate modulation

5 Multi-site pacing

O

O

O

O

O

A

A

I

R

A

V

V

T

 

V

D

D

D

 

D

O = none

A= atrium

V = ventricle

I = inhibited

T = triggered

D = both things

R = rate modulation (many types)

 

Defibrillator coding:

1 Shock

2 Anti-tachycardia

(i.e. overdrive pacing)

3 Detection

4 Anti-bradycardia

(i.e. normal pacing)

O

O

E

O

A

A

H

A

V

V

 

V

D

D

 

D

O, A, V, D = as above

E = electrical

H = haemodynamics

 

Rate responsiveness:

Activity sensors

·      Accelerometer

·      Piezoelectric crystal

·      Note problems with a) oxygen debt b) isometric exercise c)coughing/laughing

Metabolic sensors

·      QT interval

·      Minute ventilation (transthoracic impedance)

·      Peak endocardial acceleration

·      SaO2

·      Venous pH

·      Venous temp

·      Stroke volume

·      Impedance based on myocardial contractility

·      Combinations of the above

 

Electromagnetic interference:

Effect of diathermy

·      PPM: interprets as native electrical activity -> inappropriate inhibition -> flatline

·      AICD: interprets as tachyarrhythmia -> shock or overdrive pacing

Effect of magnet

·      PPM: asynchronous 80-100bpm with fixed A-V delay

·      AICD: shock disabled, anti-tachy disabled, anti-brady (PPM-like) no effect
(inappropriate inhibition of pacing can occur)

·      PPM-AICD: shock disabled, anti-tachy disabled, PPM no effect
(inappropriate inhibition of pacing can occur)

General preventive measures

·      Be very careful if supra-umbilical surgery

·      Use bipolar

·      Place pad far away

·      Use short bursts

Reprogramming

·      Need appropriate staff and equipment at the beginning and the end

·      Important if:

o  Magnet can’t be applied (e.g. thoracic surgery, prone or lateral position)

o  Magnet won’t work (e.g. morbidly obese)

o  Magnet will cause unwanted effects

§  PPM-AICD plus pacemaker -dependent (need asynchronous pacing)

§  BiV-PPM due to severe LV impairment (small change in A-V delay matters)

The most annoying problem

·      BiV-PPM-AICD (CRT-D) in a pacemaker-dependent patient undergoing thoracic surgery in the lateral position requiring monopolar diathermy

o  A magnet would fall off

o  High risk of inappropriate inhibition of pacing

o  Need tech to a) switch off shock and anti-tachy b) make pacemaker asynchronous c) preserve A-V delay

 

Mechanical interference:

CVC insertion

·      Jiggle AICD lead -> shock

Pushed around

·      Inappropriate triggering of rate response function

·      e.g. sternal sawing, pulling a specimen out of a laparoscopy port site

 

What to do?

Assessment

·      What kind of device is it? (be very sure – patient may be mistaken)

·      Formal check within last 6-12 months?

·      What was the indication for the device?

·      What is the underlying rhythm?

·      Is the patient pacemaker dependent? (assume yes if 12-lead ECG is covered in spikes)

·      Is there likely to be interference?

·      What should be done to mitigate? Nothing vs magnet vs reprogramme

Intra-op

·      Pacemaker: nothing vs magnet vs reprogramme

·      Monitoring: pulse ox or art line to make sure there is output!!

·      Diathermy: bipolar vs mono with distant plate

·      Inserting CVC: magnet on AICD so no shock when line is jiggled

·      If actually wanting shock: remove the magnet

 

Pacemakers and MRI:

Problems

·      Mechanical injury

·      Thermal injury

·      Inappropriate pacing or no pacing or shock

·      Imaging artifact

Likely a problem if

·      Older device <2010

·      Strong magnet >1.5T

·      Thoracic imaging

Recommend

·      Programming needed before and after the MRI

 

X ray appearance:

Single chamber

Dual chamber

Bi-V PPM

1: RA

2: ?

3: RV

4: coronary sinus

AICD

 

 

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