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BMT414Pacemakers

Dr. Ali Saad, Biomedical Engineering Dept. College of applied medical sciences

King Saud University

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Cardiac Electrophysiologic Assist Devices

• Pacemaker • Defibrillator • Cardioverter • These are used to treat arrhythmias:

– AV block (pacemaker) – A or V fibrillation (defibrillator, cardioverter) – tachycardia (defibrillator) – bradycardia (pacemaker)

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Short History of Pacemakers

• The basic approach to cardiac pacing is to supply an electrical shock to the heart, resulting in a ventricular contraction.

• Early pacemakers utilized skin electrodes with large surface areas or subcutaneous needle electrodes (1950’s).

• Electrodes placed on the surface of the heart were then introduced via an opening in the chest wall (thoracotomy).

• Modern pacemakers use catheter electrodes introduced into the right ventricle via the cephalic or sub-clavian vein.

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Cardiac Conduction System

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Cardiac depolarization

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Representative electric activity from various regions of the heart. The bottom trace is a scalar ECG, which has a typical QRS amplitude of 1-3 mV. (© Copyright 1969 CIBA Pharmaceutical Company, Division of

CIBAGEIGY Corp. Reproduced, with permission, from The Ciba Collection of Medical Illustrations, by Frank H. Netter, M. D. All rights

reserved.)

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Atrioventricular block (a) Complete heart block. Cells in

the AV node are dead and activity cannot pass from atria to ventricles. Atria and ventricles beat independently, ventricles

being driven by an ectopic (other-than-normal) pacemaker. (B) AV block wherein the node is diseased (examples include

rheumatic heart disease and viral infections of the heart).

Although each wave from the atria reaches the ventricles, the

AV nodal delay is greatly increased. This is first-degree heart block. (Adapted from

Brendan Phibbs, The Human Heart, 3rd ed., St. Louis: The C.

V. Mosby Company, 1975.)

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Normal ECG followed by an ectopic beat An irritable focus, or ectopic pacemaker, within the ventricle or specialized conduction system may discharge, producing an extra beat, or extrasystole, that interrupts the normal rhythm. This

extrasystole is also referred to as a premature ventricular contraction (PVC). (Adapted from Brendan Phibbs, The Human Heart, 3rd ed., St. Louis: The C. V.

Mosby Company, 1975.)

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(a) Paroxysmal tachycardia. An ectopic focus may repetitively

discharge at a rapid regular rate for

minutes, hours, or even

days. (B) Atrial flutter. The atria begin a very rapid, perfectly

regular "flapping" movement, beating at

rates of 200 to 300 beats/min. (Adapted

from Brendan Phibbs, The Human Heart, 3rd ed., St. Louis: The C. V. Mosby Company,

1975.)

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(a) Atrial fibrillation. The atria stop their regular beat and begin a feeble,

uncoordinated twitching. Concomitantly, low-amplitude, irregular waves appear in the ECG, as shown. This type of recording can be clearly distinguished from

the very regular ECG waveform containing atrial flutter. (b) Ventricular fibrillation. Mechanically the ventricles twitch in a feeble, uncoordinated fashion

with no blood being pumped from the heart. The ECG is likewise very uncoordinated, as shown (Adapted from Brendan Phibbs, The Human Heart, 3rd

ed., St. Louis: The C. V. Mosby Company, 1975.)

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11 Modern Pacemakers

power supply

timing circuit

pulse output circuit

lead wires & electrodes

hermetically sealed stainless steel or titanium package

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Block diagram of an asynchronous cardiac

pacemaker

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A demand-type synchronous pacemaker Electrodes serve as a means of both applying the stimulus pulse and

detecting the electric signal from spontaneously occurring ventricular contractions that are used to inhibit the

pacemaker's timing circuit.

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Demand Synchronous Pacing (cont.)

after each stimulus, timing circuit resets, and waits a certain time interval, T (1 sec).

if amplifier detects naturally occurring R-wave during this interval, timing circuit reset again.

timing circuit keeps resetting with each naturally occurring beat as long as it occurs within T seconds of previous beat.

if no naturally occurring beat occurs after T seconds, output circuit stimulates.

useful for bradycardia (slow HR).

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An atrial-synchronous cardiac pacemaker, which detects electric signals corresponding to the contraction of the atria and

uses appropriate delays to activate a stimulus pulse to the ventricles. Figure 13.5 shows the waveforms corresponding to

the voltages noted.

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Atrial Synchronous Pacing (cont.)

120 ms 120 ms

2 ms 2 ms

500 ms 500 ms

atrial pulsesv1

v2

v3

v4

t

t

t

t

triggers stimulus

gate input

AV node delay

ventricular signal can be detected at atrium, gating insures that v. signal is not confused with an atrial signal

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Block diagram of a rate-responsive pacemaker

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Power Supply• Lithium Iodide battery (most common):

– cathode reaction: – anode reaction: – 2.8V open circuit voltage – lifetime of 15 years (big improvement over earlier

batteries) • Experimental Sources:

– transcutaneous induction – mechanical generators, based on movement in

heart and large vessels. – electrochemical, using ions found in body. – plutonium

Li Li e

I e I2 2 2

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Piezoelectric element bonded to the inside of the pacemaker can. Body motion causes pressure fluctuations which cause the can to deflect which bends the sensor to produce a voltage. The leads from the piezoelectric sensor are connected to the pacemaker

electronics. This is one possible layout for the pacemaker components.

 

 

 

RibsBack can edge facing inside of body

Skin

Piezo-electric sensor

View from right shoulder

Ribs

Electronics

Battery

View from above patient

View from front of patient's chest

Sensor

Skin

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The three-letter pacemaker coding system was recommended by ICHD in 1974 and became the first widely adopted pacemaker code. It was simple and easy to use and it only contained three letters. The first letter designates the chamber(s) paced: ventricle (V), atrium (A), or both (D for double). The second letter designates the chamber(s) sensed. The third letter designates the mode of response(s): T = triggered, I = inhibited, D = double, O = none. The code was revised in 1981 to accommodate new functionalities of pacemakers.

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A logical diagram of relationship between rhythm disturbances and therapeutic pacing modes for selecting the proper pacing

mode. From Schaldach, M. M. 1992. Electrotherapy of the heart.

Berlin: Springer–Verlag.

 

 

 Normal Sinus

Function

Atrial Arrhythmias Often/Chronic Rate Adaptation

Indicated

N Y Y Y

NN

N

DDD+ Dual Demand

VVI

VVIR

DDI

VVI

Y

Y

Chronotropic Incompetence

Atrial Synch. Indicated

Temp. P-wave Trig. Pacing

DDDR

DDIR

Y Y Y

N

AV-Conduction DDD

Y N

N

N

N

VVIR

N

Atrial Synch. Indicated

Rate Adaptation Indicated

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DDD Pacemaker

combines: •demand synchronous •atrial synchronous •ventricular synchronous

atrial sensor detects natural or stimulated atrial contraction, then triggers ventricles if no naturally occurring ventricular pulse is detected within TAV = 120 ms.

ventricular sensor detects natural or stimulated ventricular contraction, then triggers atria if no naturally occurring atrial pulse is detected within TVA = 700 ms.

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Evolution of implantable pacemaker technologyOriginal Current

Asynchronous fixed-rateoscillator

Pacing on demand; rhythmanalysis and defibrillation

Discrete components Hybrid integrated circuitsEpoxy with silastic coating Laser-welded titaniumMechanical adjustments Bi-directional telemetrySutured endocardial electrodes Intravenous catheter electrodesMercury batteries (2-year life) Lithium batteries (8-year life)

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Implanted pacemaker

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Dual Chamber Pacemaker

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Bipolar electrode configuration current pathway. Current flows from one electrode to another, the bottom electrode is in contact with cardiac muscle.

E lectrodeLead w ire

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Bipolar Pacemaker Electrodes

band electrodes •stainless steel •platinum •titanium alloy

Si rubber

Electrode usually located inside heart (intratuminal), via cephalic vein.

Si rubber hookslead wire coil

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Unipolar electrode configuration current pathway. Only the cathode electrode is in contact with myocardium with unipolar stimulation, the other (anode) electrode often is the case of the pulse generator, which is some distance from the heart

PaceG enerator

E lectrode

Lead

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Unipolar Pacemaker Electrodes

Pt electrode

Si rubber

•implanted on surface of heart (epicardial) •reference electrode implanted away from heart

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Two of the more commonly applied cardiac pacemaker electrodes

(a) Bipolar intraluminal electrode. (b) Intramyocardial electrode.

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Active and passive fixation mechanisms of various types for endocardial and epicardial pacing leads (From Ellenbogen,

1996).

 

 

  

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Tip Electrode

Si or polyurethane rubber

Si rubber hooks (tines): entangle in trabeculae (net-like lining)

wire coil

tip electrode

porous, platinized tip for steroid elution, reduces inflammation

steroid

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Cross-sectional view of a steroid-eluting intracardiac electrode (Medtronic CapSure® electrode, model 4003). Note silicone

rubber plug with impregnated steroid DSP. Steroid elutes through the porous tip into surrounding tissue, thus reducing

inflammation. From Mond, H., and Stokes, K. B. 1991. The electrode–tissue interface: the revolutionary role of steroid

elution. PACE, 15: 95–107.

 

 

 

Electrode body

Silicon rubber plug (impregnated with DSP steroid)

Porous, platinum coated titanium tip

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Threshold evolution after implantation. (a) Once an electrode is placed against or within sensitive tissue, local reaction causes enlargement of its surface area as the virtual electrode is formed. As chronicity is reached, the virtual electrode is smaller than early after implant and the threshold decreases and is stabilized. (b) Steroid-eluting electrodes have produced a distinct reduction in stimulation threshold acutely and chronically. Sensing characteristics have also improved. This figure compares similar solid tips, without steroid and steroid-eluting electrodes. The increase in stimulation threshold for the steroid electrode early after implant is much reduced and the long-term stable threshold for both is characteristic (Modified from Furman et al., 1993).

Stero id e lectrode

Solid e lectrode

M onthsW eeks

87654324320 1

% C hange ofthreshold

700

600

500

400

300

200

100

Acutethreshold

C hronic threshold

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Stim ulatingC urrent

Pulse W idth

1

VA

V: Ventric le

A : A trium

10

100

0.1

m A

0.1 1 100.01 m s100

Q = constant

I = constant

The current strength (I)–duration (d) curve: for canine muscle: A = atrium, V = ventricle (modified from Geddes 1984).

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A pacemaker provides a 1 mA pulse with a duration of 1 ms so the total charge for one

pulse is 1 C. The number of pulses per

year at one per second is 60 60 24

365 = 31,536,000. Over the 10 year life of the pacemaker, the charge drawn from the

battery is 1 C 31,536,000 10 = 315 C = 315 As = 0.087 Ah. This is a small portion of the total battery life of 2 Ah,

most of which supplies the electric circuits.

 

 

 

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Block diagram of pacemaker programming and telemetry interface.

 

 

 

Debouncer

Driver

Amplifier Driver

Amplifier

Decoder

Encoder

PacemakerProgrammer

Reed switch

Decoder

Encoder

Pacemaker logic

Control and error detection

Programmer microprocessor

Control and error detection

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Timing and Output Circuits

Asynchronous: runs at a fixed pacing rate, set by technician (70-90 BPM): these are no longer used since if a stimulus is applied during the T-wave of a normal beat, can get v. fibrillation.

Synchronous: uses feedback from ECG and/or other sources to determine pacing rate (60-150 BPM).

output circuit: constant current pulses: 8-10 mA, 1-1.2 ms duration constant voltage pulses: 5-5.5 V, 500-600 s duration

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Lead Wires and Electrodes

Must be able to withstand constant bending due to beating of the heart (35 million beats per year).

Must be biocompatable, tissues can be very corrosive

The two above criteria are satisfied via interwound helical coils embedded in silicon or polyurethane rubber.

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Pacemaker Placement

RV

trans-venous sub-clavian or cephalic vein, much less traumatic

epicardial requires thoracotomy

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Monostable Multivibrator (MSMV):

rising edge trigger input

vi

vi

vo

Gate:

vi

vi = H, switch open vi = L, switch closed

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Comparator

_

+Vout

V+

V_

VV V V

V V Voutr

r

,

,

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Monostable Multivibrator (cont)

_

+

R1

R2

R

C

Vtrig

Vout

0 V

-AV

t0

Vc

+

_

A > 0 V

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Monostable Multivibrator (cont.)

when switch is open, circuit is in stable state:

V Vout r

V Vc 0 7. V

V VR

R RVr r

2

1 2

0 7 V.

at t = t0, switch is momentarily closed:V 0 7. V

V A V < 0.7 V

this immediately causes V Vout rV Vr

(momentarily)

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Monostable Multivibrator (cont.)

when switch is open, circuit is in stable state:

V Vout r

V Vc 0 7. V

V VR

R RVr r

2

1 2

0 7 V.

at t = t0, switch is momentarily closed:V 0 7. V

V A V < 0.7 V

this immediately causes V Vout rV Vr

(momentarily)

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Monostable Multivibrator (cont.)

diode now has a negative voltage across it, capacitor no longer clamped at 0.7 V. capacitor begins to charge up to a negative voltage with time constant, RC at the instant that capacitor voltage becomes more negative than , comparator output switches back to: multivibrator is now in stable state again. the interval during which comparator output is is called an astable state.

V Vr

V Vout r

V Vout r

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Monostable Multivibrator (cont.)

Vr

Vr

0.7V

Vout

Vc

Vr

tt0

0

RC

V Vrln/1

11

V1 = diode forward bias voltage (0.7 V)

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Triggering Circuit

want to trigger monostable multivibrator on leading edge of a negative going pulse:

0 V

-5 V

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Astable Multivibrator (Square Wave Generator)

_

+

R1

R2

R

C

Vout

Vc

+

_

no stable state

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Astable Multivibrator (cont.)

_

+

R1

R2

C

Vout

at t = t0: S1 opens S2 closes

R

S1 S2

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Astable Multivibrator (cont.)

for t < t0, assume Vout = Vr:

for t > t0: capacitor begins to charge through R with time

constant RC when capacitor voltage Vc exceeds V+ = Vr , Vout = -Vr

capacitor then begins to charge towards -Vr with time constant RC

when capacitor voltage Vc becomes more negative than

V+ = -Vr , Vout = Vr

V V V Vout r 0 (stable state)

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Astable Multivibrator (cont.)

Vr

Vr

Vout

Vc

Vr

tt0

0

Vr

can be used along with MSMV

for asynchronous pacing

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RC ln

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Operational Amplifiers (Op Amps)

_

+Vout

V

V

inputs: output:

I = 0

I = 0

V V

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Constant Current Source

_

+

+

_

R R

R R

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Constant Current Source (cont.)

_

+

+

_

R R

R R

or

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Constant Current Source (cont.)

_

+

+

_

R R

R R

node b

node analysis at node b gives:

or:

IL is independent of load resistor RL

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Constant Voltage Source

Most modern pacemakers use constant voltage output circuit:

+

_2.8V

charging

C1 C2 C1

C2

Rheart

discharging

5.6V

+

_

use capacitors to increase stimulus voltage:

amplitude: 0.8 - 5V pulse duration: 0.01-1.5 ms

+_+_

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Constant Voltage Source (cont.)

5.6V

_

+

voltage follower prevents loading by high impedance loads

RR