1 bmt414 pacemakers dr. ali saad, biomedical engineering dept. college of applied medical sciences...
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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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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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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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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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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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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
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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 (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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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
+_+_