Heart Pacemakers

Question 1

Are they worth it?

Answer 1

Yes! but effectiveness depends on illness:

very effective for Sick-Sinus Syndrome
less effective for 3rd degree AV block

Question 2

Heart Rhythm Disorders 1

Answer 2

1) Sick Sinus Syndrome: Sinus node dysfunction (–> fainting)
2) Sinus-Tachycardia: pulse > 80/min
3) Sinus-Bradychardia: pulse < 60/min

Question 3

Heart Rhythm Disorders (AV blocks)

Answer 3

1st degree: conduction time increased

2nd degree: conduction time increases periodically up to missing beats
- atrial contraction is passed only every 2nd/3rd/xth beat

3rd degree:

• separate rhythms of atria and ventricles
• symptomatic bradychardia

Question 4

Heart Rhythm Disorders 3

Answer 4

1) Atrial Fibrillation

2) Ventricular fibrillation

Question 5

Current Flow in Pacemaker

Answer 5

for simple pacemaker with one electrode in right ventricle:

current flows from tip of the electrode to the metal pacemaker

Question 6

Stimulation Threshold Urheo

Answer 6

Depolarisation of excitable tissue requires a minimum of energy

Rheobase Urheo: minimal current amplitude that results in the depolarization threshold of the cell membranes being reached (e.g. AP)
bzw. voltage necessary to stimulatea cell

Question 7

Chronaxie time

Answer 7

tchronax: minimum time required for an electric current double the strength of Urheo to stimulate excitable tissue

Question 8

Determinants of threshold

Answer 8

• isolation defects
• infections
• bad connector contact
• bad electrode position

Question 9

Unipolar Electrodes

Answer 9

• current flows through the heart muscle to the pacemaker housing

Question 10

Bipolar Electrodes

pros & cons

Answer 10

• two different conductors inside one electrode

Advantages:

• electrical field for stimulation only intracardiac
• no contraction of pectoralis muscle

Disadvantages:

• thicker electrodes
• more isolation defects
• smaller signals in surface ECG –> more difficult to detect

Question 11

Power Supply

Answer 11

life time 10-14 years

Question 12

Power Demand

Answer 12

total current = idle current of pacemaker electronics + stimulation current * X
= 10-30 uA

Question 13

Table of Pacemaker Codes

Answer 13

Defines:

1) Champer Paced
2) Chamber sensed
3) Response to Sensing
4) Programability
5) Anti-Tachycardia Features

Question 14

Common Type of Pacemaker

Answer 14

DDD: (D= Dual)

1) Atrium and Ventricle paced
2) Atrium and Ventricle sensed
3) response to triggered and inhibited

• -> Detect and Stimulate Atrium
• -> Detect and Stimulate Ventricle
• 2 bipolar electrodes
• for all rhythm disorders
• no limitations

Question 15

Potential Methods for Rate Adaptation

Answer 15

• necessary to adapt to different loads
  1) Open-Loop
  2) synchronized
  3) Closed-Loop

Question 16

Open Loop Heart Rate Adaptation

Answer 16

• measure physical activity with motion sensors
• -> acceleration sensors
  
  • disadvantages: signal may be misleading (e.g. driving in car), pressure sensor may be wrong
    
    • –> maladjustment of heart rate

Question 16

Synchronized Heart Rate Adaptation

Answer 16

• connect to other physiological control loops
  1) respiratory rate
  2) body core temperature
  3) venous O2 saturation (complex electrodes and slow adaptation!)
  4) Minute Ventilation: Bioimpedance Measurements (slow adaptation)

Question 17

Closed-Loop Heart Rate Adaptation

Answer 17

• connect to other cardiac signals
  1) intracardiac impedance
  2) stroke volume & pre-ejection period (need special electrodes)

Question 18

Implantable Cardioverter Defibrillator (ICDs)

Answer 18

• uses electrical pulses or shocks to help control life-threatening arrhythmias
  1) Primary Prevention: before first life-threatening arrhythmia episode
  2) Secondary prevention: for survivors of cardiac arrests –> strongest evidence for benefit

Question 19

Cardiac Resynchronization Therapy (CRT)

Answer 19

triple-chamber pacemaker

• -> resynchronizes the contractions of the right and left ventricle
• 3 leads: right atrium, left & right ventricle

Question 20

Surgical Procedure

Question 21

Hagen-Poisseuille - Why is it proportional to 1/r^4 while in ohmic resistance it is proportional to 1/r²

Answer 21

hydraulic resistance: shear stress distribution

electric resistance: charges flow as block