Transcript Pacemaker_ch2

BMT414
Pacemakers
Dr. Ali Saad, Biomedical Engineering Dept.
College of applied medical sciences
King Saud University
1
2
ysiologic Assist Devices
•
•
•
•
Pacemaker
Defibrillator
Cardioverter
These are used to treat arrhythmias:
– AV block (pacemaker)
– A or V fibrillation (defibrillator, cardioverter)
– tachycardia (defibrillator)
– bradycardia (pacemaker)
2
ry of Pacemakers
• The basic approach to cardiac pacing is to supply an electrica
• Early pacemakers utilized skin electrodes with large surface a
• Electrodes placed on the surface of the heart were then introd
• Modern pacemakers use catheter electrodes introduced into
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Cardiac Conduction System
4
Cardiac depolarization
5
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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n Pacemakers
11
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.)
n
n
n
n
n
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.)
atrial pulses
v1
t
120 ms
120 ms
v2
AV node delay
t
2 ms
2 ms
v3
triggers stimulus
t
500 ms
v4
500 ms
gate input
t
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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wer Supply
• Lithium Iodide battery (most common):
– cathode reaction:
Li  Li   e 
– anode reaction: I  2e   2 I 
2
– 2.8V open circuit voltage
– lifetime of 15 years (big improvement over earlier batterie
• Experimental Sources:
– transcutaneous induction
– mechanical generators, based on movement in heart and
– electrochemical, using ions found in body.
– plutonium
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View from
right shoulder
View from
above patient
View from front
of patient's
chest
Ribs
Sensor
Skin
Piezoelectric
sensor
Skin
Ribs
Back can edge
Electronics
facing inside
of body
Battery
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 19
components.
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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N
Normal Sinus
Function
Y
Atrial
Arrhythmias
Y
Y
Often/Chronic
N
Y
VVIR
Rate Adaptation
Indicated
N
N
VVI
Y
DDD+
Dual Demand
Rate Adaptation
Indicated
N
Y
Atrial Synch.
Indicated
DDI
N
Y
Chronotropic
Incompetence
Y
Atrial Synch.
Indicated
N
VVI
Y
Temp. P-wave
Trig. Pacing
N
N
DDDR
DDIR
VVIR
N
AV-Conduction
DDD
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.
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DDD Pacemaker
n
n
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 technology
Original
Asynch ronous fixed-rate
oscillator
Discrete compon ents
Epoxy w ith silastic coating
Mechanical adjustments
Sutured endoc ardial electrodes
Mercury ba tteries (2-year life)
Current
Pacing on d emand; rhythm
analysis and d efibrillation
Hyb rid integrated circuits
Laser-welded titanium
Bi-directional telemetry
Intravenous catheter electrodes
Lithium b atteries (8-year life)
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Implanted
pacemaker
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Dual Chamber
Pacemaker
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Lead wire
Electrode
Bipolar electrode configuration current pathway. Current flows from one electrode to another, the
bottom electrode is in contact with cardiac muscle.
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Bipolar Pacemaker Electrodes
Si rubber
lead wire coil
Si rubber hooks
band electrodes
•stainless steel
•platinum
•titanium alloy
Electrode usually located inside heart (intratuminal), via cephalic vein.
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Pace
Generator
Lead
Electrode
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
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Unipolar Pacemaker Electrodes
•implanted on surface of heart (epicardial)
•reference electrode implanted away from heart
Pt electrode
Si rubber
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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
tip electrode
wire coil
steroid
Si rubber hooks (tines):
entangle in trabeculae (net-like lining)
porous, platinized tip
for steroid elution,
reduces inflammation
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Electrode body
Porous, platinum
coated titanium tip
Silicon rubber
plug (impregnated
with DSP steroid)
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
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elution. PACE, 15: 95–107.
700
600
500
% Change of
threshold
400
Solid electrode
300
200
Steroid electrode
100
Acute
threshold
Chronic threshold
0
1
2
3
Weeks
4
2
3
4
5
6
7
8
Months
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 34
Furman et al., 1993).
mA
Q = constant
100
V: Ventricle
A: Atrium
10
Stimulating
Current
V
A
1
0.1
I = constant
0.01
0.1
10
1
Pulse Width
100
ms
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 As = 0.087 Ah. This is a small
portion of the total battery life of 2 Ah,
most of which supplies the electric circuits.
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Programmer
Pacemaker
Debouncer
Programmer
microprocessor
Control
and error
detection
Reed switch
Driver
Control
and error
detection
Amplifier
Encoder
Decoder
Decoder
Encoder
Amplifier
Pacemaker
logic
Driver
Block diagram of pacemaker programming
and telemetry interface.
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Timing and Output Circuits
n
n
n
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:
n constant current pulses: 8-10 mA, 1-1.2 ms duration
n constant voltage pulses: 5-5.5 V, 500-600 s duration
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Lead Wires and Electrodes
n
n
n
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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39
Pacemaker Placement
RV
epicardial
requires thoracotomy
trans-venous
sub-clavian or cephalic vein,
much less traumatic
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Monostable Multivibrator (MSMV):
t
rising edge
trigger input
vo
vi
vi
t
Gate:
vi = H, switch open
vi = L, switch closed
vi
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Comparator
V_
V+
Vout
_
Vout
+
 Vr , V  V

 Vr , V  V
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Monostable Multivibrator (cont)
R
+
Vc
_
C
_
Vout
t0
+
R1
Vtrig
0V
-AV
A>0V
R2
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Monostable Multivibrator (cont.)
when switch is open, circuit is in stable state:
Vout  Vr
Vc  V  0.7 V
R2
V  Vr
 Vr   0.7 V
R1  R2
at t = t0, switch is momentarily closed:
V  0.7 V
V   A V < 0.7 V (momentarily)
this immediately causes Vout  Vr
V  Vr 
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Monostable Multivibrator (cont.)
when switch is open, circuit is in stable state:
Vout  Vr
Vc  V  0.7 V
R2
V  Vr
 Vr   0.7 V
R1  R2
at t = t0, switch is momentarily closed:
V  0.7 V
V   A V < 0.7 V (momentarily)
this immediately causes Vout  Vr
V  Vr 
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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 V  Vr  , comparator output switches back to:
multivibrator is now in stable state again.
Vout  Vr
the interval during which comparator output is
is called an astable state.
Vout  Vr
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Monostable Multivibrator (cont.)
Vout
Vr
t0
Vc
t
0
Vr 
0.7V
Vr
t
1  V1 / Vr 
t  RC ln
1 
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:
0V
-5 V
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Astable Multivibrator (Square Wave Generator)
+
Vc
_
R
C
_
Vout
+
R1
R2
no stable state
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Astable Multivibrator (cont.)
R
S1
C
S2
_
Vout
+
at t = t0:
S1 opens
S2 closes
R1
R2
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Astable Multivibrator (cont.)
n
n
n
n
n
for t < t0, assume Vout = Vr:
V  V  0  Vout  Vr
(stable state)
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
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Astable Multivibrator (cont.)
Vr
Vout
t
Vr 
Vc
0
t
t0
Vr 
Vr
t
can be used along with MSMV
for asynchronous pacing
1 
t  2 RC ln
1 
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Operational Amplifiers (Op Amps)
output:
inputs:
V
V
I=0
I=0
_
Vout
+
V  V
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Constant Current Source
R
R
_
+
R
R
+
_
54
Constant Current Source (cont.)
R
R
_
+
R
R
+
_
or
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Constant Current Source (cont.)
R
R
node analysis at node b
gives:
_
node b
+
R
or:
R
+
_
IL is independent of load resistor RL
56
Constant Voltage Source
Most modern pacemakers use constant voltage output
circuit:
use capacitors to increase stimulus voltage:
+
2.8V
C1
C2
C1
C2
_
charging
+ +
_
+ 5.6V
_ _
Rheart
discharging
amplitude: 0.8 - 5V
pulse duration: 0.01-1.5 ms
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Constant Voltage Source (cont.)
voltage follower prevents
loading by high impedance loads
5.6V
+
_
R
R
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