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PACEMAKERS 2
Pacemakers: Nomenclature
P
Pacers
use a 5-letter
5 l tt code:
d first
fi t 3 lletters
tt mostt iimportant
t t



First Letter: Chamber Paced
A= Atrium
At i
V= Ventricle
D= Dual (A+V)
2nd Letter: Chamber Sensed
A= Atrium
V= Ventricle
D Dual
D=
D l (A+V)
O= None
3rd Letter: Response after Sensing:
I = Pacing Inhibited
T= Pacing Triggered
D= Dual (I+T)
O None
O=
N

4th Letter: Programmability
P = Rate & Output
M = Multiprogramable
C = Communicating
R = Rate adaptive
O = None

5th Letter: Arrhythmia Control
P = pacing
S= shock
D= Dual (P+S)
O = None
Pacemaker Terminology




Rate: the heart rate at which the pacemaker will pace.
Standby rate: the lowest rate at which the pacemaker
will pace.
Capture: depolarization
C
d
l i i and
d resultant
l
contraction
i off the
h
myocardium in response to a pacemaker generated
electrical stimulus.
Sensing: Dependent on the amplitude,
amplitude slew rate and
signal frequency, it describes the pacemakers ability to
recognize
i a native
ti electrical
l t i l signal.
i
l
Pacemaker Terminology


SSensitivity: the
h minimum intracardiac
d signaling
l required
d
by the pacemaker to initiate a pacemaker response.
Threshold: minimum quantity, of either amplitude
(milliamperes, volts) pulse duration (milliseconds),
charge
h
((coulombs)
l b ) or energy (Joules)
( J l ) produced
d d by
b
the pacemaker that persistently produces an action
potential and myocardial contraction.
contraction
Pacemaker Terminology..





Mode: indicates pacemaker capabilities: fixed rate or
demand.
Fixed rate: pacemaker is one that fires at a specific preset
rate, regardless of the patient's own heart rate.
Demand: that is, only when the patient's heart rate falls
below a preset value.
Pulse interval: the total time of the AV and VA intervals.
number of p
pulses per
p minute.
Hysteresis: an intentional prolonged pulse interval in order
to allow the generation of a spontaneous
spontaneous-intrinsic
intrinsic electrical
depolarization event.
Pacemaker Terminology…


Atrioventricular (AV) interval (ie AV delay): described
for dual-chamber pacemakers. The equivalent to a
native PR interval. Represents the time (msec) between
an atrial event and a paced ventricular event. Time
that the pacemaker discerns whether or not to pace
dependant upon sensing a native R wave. Allows the
ventricle time to fill following an atrial contraction.
Ventriculoatrial (VA) interval: described for dual
dualchamber pacemakers. Represents the time (msec)
between a ventricular event and a paced atrial event.
event
Ventricular Synchronous Demand
Pacemaker
5-100 Hz,
Centered at 30 Hz
Detection sensitivity :1 – 2 mV
Cardiac Signal Range: 1
1-30
30 mV
Pulse Generator
Generator: functions – Pacing and Sensing
 T1: limits pulse delivery rate in presence of EMI and prevents retriggering of astable MV
 Free running MV – provides fixed rate mode with an interval of T2 via o/p circuit
 O/P – pulses of length T3 synchronous with i/p signals that falls outside of sensing
refractory period T1 are delivered to stimulating electrodes
Commercial pacemakers

U Defibrillation
Use
D fib ill ti protection
t ti circuit
i it
One diode or 2 back-to-back placed diodes (symmetrical)
 Symmetrical diodes minimize high-level,
high-level high-frequency
high-frequency,
pulsed EMI artifacts – raise noise detection threshold


Output of pacemaker

Constant current


Constant Voltage


8-10 mA with 1.0 to 1.2ms
5V with 500 – 600 ms
Pulse rates
70-90 bpm
 Refractory period: 400 – 500 ms


Unipolar
Unipolar or Bipolar electrodes are used
Bipolar
Programmable pacemaker
External
External
Pacemaker
Internal
Pulsating
Electromagnet
Rate and width
adjustable
Reed Switch
Pulse Generator

Methods of transmitting information
 Magnetic
 Radio-frequency
waves
 Acoustic-ultrasonic pressure waves
Heart
Programmable pacemaker…

Main Requirement: immune to accidental
programming
 Information
to be sent is coded – pacemaker decodes
after receiving

Consists of 3 systems
 System
1 – Main timing function
 System 2 – Sensing and signal discriminating function
 System 3 – Programmable control
Timing Control Circuit
Rate Limiter
Output Circuit
System 1
V i function
Vario
f ti
Battery test
Reed Switch
Stimulate
Data validate
System 3
Crystal
Width Rate
RP
HYST
Amplitude
Programmable Control Circuit
Sensitivity
Electrode
Sense
System 2
Comparator
Amplifier
R.F. Filter
Indifferent Electrode
Detection – System 2

Signal from electrode
 High
frequency filtered by RF filter
 Selectively amplified – depending on sensitivity level
 Resultant signal
g compared
p
with preset
p
value at
comparator
 Abs.
Magnitude
g
greater
g
than preset
p
– enable an i/p
/p signal
g
to be fed to timing control circuit
 Signals below preset value – ignore
Operation – System 1

N
Normal
l Operating
O
i C
Conditions
di i

Timing control circuit periodically triggers o/p circuit to send
stimulation pulses of programmed pulse width and amplitude

Period b/w 2 pulses checked by rate limiter

Any component failure – rate is limited to <180 bpm

If inhibit signal
g from system
y
2 – timing
g controls checks with
programmed refractory period

Within refractory period – ignore

Outside refractory period

0 hysteresis - Reset timing control – inhibit o/p circuit

Hysteresis – reset timing control with period of escape before next
stimulation = programmed basic level + hysteresis period
Programming – System 3

Reed switch receives programming signals

Feeds these signals to data validate






Data validate – checks if reed switch was closed for a minimum period of
300 ms
Checks for speed of arrival of signals and executes code validation check
When all checks are satisfied, new code is stored in programmable
control circuit
Parameters programmed: rate, pulse width and amplitude, sensitivity,
refractory period and hysteresis
Measurements taken at 37C with 500 resistance
Program contained in 20-bit command code – in addition to parameters
contains, mode of operation, identification and check codes
Rete responsive Pacemakers


When SA node is diseased – HR cannot be increased in
response to metabolic demands
Synchronous pacemakers cannot replicate functions of
heart during
g stressful activities like exercise.
Sensor
pH,
Respiratory
p
y rate,,
Vibration/ motion
Blood temperature
QT interval
Controller
circuit
C
Control
l Algorithm
Al i h
Pulse
generator
Lead Wire
and electrode
system
Power sources

Mercury Batteries
 Used
by first American
Pacemaker
 Mercury zinc oxide with
1200mAh
 Produces 1.35 V


Failure due to dendritic
mercury growth, zinc
oxide migration, leaky
separators and corroded
wells
Biological Power Sources
 Ga
Galvanic
va c
Cells
Ce s using
us g body fluids
u ds
 Eventually become permanently electrically isolated


Nuclear Batteries
Lithium-iodine Cells
Nuclear Batteries



Used Plutonium 238 with half-life of 87 years
Energy
gy liberated byy decayy of 1g
g of Pu238 with a
power density of 0.56 W/g is 780kWh
With 1% efficiency
efficiency, for only pulse power,
power 20 mg of
Pu will be required.
Lithium-iodine
Lithium
iodine Cells
Most commonly used because it offers long battery life
 It is solid-state device consisting of metallic Lithium
anode and molecular iodine bonded in complex form o
an organic carrier as cathode
 2Li + I2 = 2LiI + e Lithium has highest
g
electrochemical
equivalent of alkali metal - Most energetic
anode material
 Ideal for use in high energy density batteries
 Develops 2.8 V which can be stepped up to
5V

Cardiac pacemaker electrodes
(a) Bipolar intraluminal electrode. (b) Intramyocardial electrode
((formerlyy used).)
Active and passive fixation mechanisms of various types for endocardial
and epicardial pacing leads
Unipolar and bipolar implementations of both J-shaped and nonpreshaped leads. All models have distal cathode. Bipolar designs
typically have a ring anode proximal 10–15
10 15 mm on the lead.
lead
Pacing voltage loss at the myocardium–electrode interface is reduced by
implementing a porous as opposed to relatively smooth tipped electrode. Decrease
in voltage
g loss is largely
g y contributed to byy decreased electrode polarization
p
associated with increased active surface area.
Electrode body
Porous, platinum
coated titanium tip
Silicon rubber
plug
l (impregnated
(i
d
with DSP steroid)
Cross-sectional view of a steroid-eluting intracardiac electrode
(Medtronic CapSure® electrode,
electrode model 4003)
4003). Note silicone
rubber plug with impregnated steroid DSP. Steroid elutes
through the porous tip into surrounding tissue, thus reducing
i fl
inflammation.
ti