Download Cardiac Pacemakers

Cardiac Pacemakers
Ratko Magjarević
University of Zagreb
Faculty of Electrical Engineering and Computing
Zagreb, CROATIA
E-mail: [email protected]
Electric Stimulators
• One of the most successful applications of
electronics and technology of materials in
medicine
• 600.000 implantations/year worldwide
• First pacemaker implantation in 1958
• Nowadays stimulators stimulate, sense, they are
multiprogrammable
• Based on microcomputers (computational power
app. like PC)
• Adaptable to physiological needs (can adjust
heart rate)
• Due to software support can make decisions
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The First Pacemaker
• Rune Elmquist developed
the first implantable
pacemaker in 1958
• pulse amplitude - 2 volts
• pulse width - 1.5 ms
• constant rate of 70-80
impulses a minute
• 180 g
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Cardiac Stimulators
• Basics of heart anatomy and physiology
• Electrophysiology and patology of heart
function
• Electrotherapy of heart
• Performance of cardiac stimulators
• Implantable cardiac stimulators
ways of functioning, components, frequencyadjustable stimulators
• Implantable cardioverters and defibrillators
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Heart Disease Therapy
The aim, coordinated functioning of heart in
accordance with physiological needs is achieved
by:
• repeating heart contractions sufficient to keep
patients alive,
• reducing syndroms caused by irregular heart
function,
• protecting patients from possible complications,
• improving life quality.
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Heart Disease Therapy
The therapy comprises of:
• medications
• surgical procedures,
• electrotherapy:
- acute (e.g. defibrillation)
- temporary – extra corporal stimulator connected to:
– surface electrodes or
– intravenously inserted catheter, or
– esophageal lead
- permanent
– implanted stimulators (pacemakers
– implanted cardioverter/ defibrillators
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Implantable Cardiac Stimulators
Dual chamber pacemaker: Position of electrodes in
right atrium and ventricle
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Components of Cardiac Stimulator
2
3
1
4
6
1. Package - case 4. Lead
5. Electrode
2. Power supply
6. Electronic circuit
3. Connector
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Cardiac Stimulators Operation
• Asynchronous (competitive)
• On demand (noncompetitive):
- synchronous
- at R wave
- at P wave
- R wave inhibited
• Rate responsive (physiological) = frequency
adjustable according to the physiological
activity
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Function Block Diagram of Stimulator
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Unipolar Pacing System
• Contains a Lead with Only
One Electrode Within the
Heart
• The pulse:
– Flows through the tip
electrode (cathode)
– Stimulates the heart
– Returns through body fluid
and tissue to anode
+
Anode
Cathode
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Bipolar Pacing System
• Contains a Lead with
Two Electrodes Within
the Heart
• The pulse:
– Flows through the tip
electrode located at
the end of the lead
wire
– Stimulates the heart
– Returns to the ring
electrode above the
lead tip
Anode
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Cathode
12
TIME DIAGRAM – one pacing cycle
START
(QRS or
STIMULUS)
T
STIMULUS
TN
TR
TA
Within one stimulation cycle three periods are
defined:
TR – refractory period
TN - noise sampling period
TA - alert period
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TIME DIAGRAM
T
STIMULUS
P QRS
T
STIMULUS
TN
TR
TA
Appearance of QRS complex during the noise sampling
period analysis does not reset stimulation cycle so the
stimulator generates a stimulus at its end
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TIME DIAGRAM
T
STIMULUS
STIMULUS
EM interference
TN
The appearance of electromagnetic interference during the
refractory period disables signal analysis within the period
so the stimulator generates a stimulus at its end despite
possible appearance of heart’s own activity (QRS)
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Time Diagram of an R-inhibited Stimulator
T
QRS
STIMULUS
TA
The appearance of QRS complex during the alert
period resets stimulation cycle and inhibits the
output amplifier of the stimulator initiating a new
cycle (T)
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Block Diagram of a Multiprogramable Stimulator
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Pacemaker Programming
Source: SS Barold, Cardiac Pacemakers Step by Step, Blackwell, 2004
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Pacemaker Telemetry
Source: SS Barold, Cardiac Pacemakers Step by Step, Blackwell, 2004
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Electrodes
Setting:
• epicardial (heart
surface)
• intramyocardial – buried
within the heart wall
• endocardial or
intraluminar - pressed
against the inside heart
surface
Performance:
• monopolar (unipolar) - second electrode is
the case of the stimulator
• bipolar – both electrodes on the lead
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Passive Fixation
• The tines become
lodged in the
trabeculae
(fibrous meshwork)
of the heart
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Active Fixation
• The helix (or
screw) extends into
the endocardial
tissue
• Allows for lead
positioning
anywhere in the
heart’s chamber
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Myocardial and Epicardial Leads
• Leads applied
directly to the heart
• Myocardial screw-in
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Electrodes
Materials: platinum and its alloys, titanium and its alloys,
iridium, vitreous (glass+ metal + carbon), stainless steel
Characterisitcs of the material : biocompatibility, inertion,
resistance to corrosion
Etched surface: increase of effective area
Surface porous: makes possible the ejection of steroids
(1 mg stored in the silicone rubber in the top of the
electrode) – reduction of infections
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Lead Maturation Process
• Fibrotic “capsule” develops around the
electrode following lead implantation
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Steroid Eluting Leads
• Steroid eluting
leads reduce the
inflammatory
process and thus
exhibit little to no
acute stimulation
threshold peaking
and low chronic
thresholds
Porous, platinized tip
for steroid elution
Silicone rubber plug
containing steroid
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Tines for
stable
fixation
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Lead placement
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Muscle structure
•Muscles are
the executive elements in
biological systems (actuators)
•The execution (of an action) is
achieved by shortening
the muscle (contraction)
•The immediate cause of
contraction is action potential that
spreads from the neuromuscular
connections along the muscle fibers
•When a muscle is stimulated
by electrical impulses,
individual fibrils shorten and
cause muscle twitch
The frequency of stimulation
•
•
During stimulation, increasing the frequency of stimulation makes
it impossible to distinguish between individual convulsions
caused by stimuli (pulses). We say that tetanic contraction
occurred
Muscle individual twitches are no longer visible, but the muscle
is tight and smooth (notice a little difference in tension while
stimulation pulse frequency is 40 Hz and 80 Hz)
During stimulation, what kind of
a muscle reaction do we really
want?
Excitation modeling
•
For an infinitely long current pulse (t -> ∞), the intensity of electrical
impulses must reach a value of I (t -> ∞) = I0 = VT/R. Current I0 is
called rheobase current. Minimum charge Q0 required to achieve
the limit of stimulation can also be determined:
•
Minimum charge Q0 is achieved with very short pulses, when t -> 0
•
Normalized energy required for the stimulation:
•
Minimum energy required for the stimulation of t = 1.25 τe ,where τe
is chronaxie, can be determined
Intensity-time curve
• or abbreviated I - t curve, normalized to
chronaxie for current, charge and energy
Chronaxie is the Stimulus
Duration that yields a response
when the Stimulus Strength is
set to exactly 2´rheobase.
Empirical model of
excitability
• The terms current reobaze and chronaxie are
derived from the first experimental model of
excitability, which is described with the hyperbolic
function:
Rheobase= I0
Chronaxie = τe (empirical model)
Chronaxie = τe ln2 (exponential formula)
Stimulation Threshold
For the stimulation of myocardium there has to be sufficient
current density J as to Ohm’s law:
J=κE
where κ is the specific conductivity, and E is the strength of
the electric field.
Electrical field inversely proportional with the square of the
distance r from the electrode (considered point source)
E = J / 4Π κ r2
which is why a small (macroscopical) surface of the
electrode is needed
The effective area of pacemaker active electrode is from
10mm2 to 100 mm2
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Threshold Stimulation
The energy necessary for stimulation depends on
the myocardial excitability and on the
impedance of electrode-myocardium interface.
Supposing a three-element- diagram of the
electrode – tissue interface (RS + RF II CH), a
large (microscopically) surface of the electrode
has to be achieved.
Tissue excitability is macroscopically expressed by
the strength - duration equation, usually plotted
as the I - t curve, where I is the magnitude of an
impulse and t is the duration of the simulating
pulse.
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Strength duration equation
Models:
Hyperbolic (experimental)
Ir – rheobase current
tc – chronaxie time constant
 tc 
I = I r 1 + 
t 

Exponential (theoretical)
I=
U
Rm
−
1− e
t
τm
Rm – resistance of the
membrane
τm – membrane time
constant
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IT-curve
I (mA)
4
capture
assured
3
(0.3 ms, 2.5 mA)
2
1
THR
ESH
OLD
no capture
possible
0.5
1.0
1.5
CUR
VE
2.0
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2.5
t (ms)
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Difference between the S-d models
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Difference between the S-d models
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Excitability dependence of the
pulse polarity
• Cathodic vs. anodic stimulation
Excitability dependence of the
pulse waveforms
• Monopolar vs. bipolar pulses, sinusoidal
current
Extrapolation of sinusoidal current
What is the duration of the
half-cycle sine current to achieve
maximum excitability?
Excitability dependence of the
pulse waveforms
• Monopolar vs. bipolar pulses, pulse
duration and the interval between pulses
I – t curves for sensory and
motor responses
Excitative Volume
LEAD
STIMULATING
VOLUME
ELECTRODE
FIBROUS TISSUE
VENTRICLE
WALL
EXCITATIVE
VOLUME
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LEADS
Conductor
• Interwound helical coils of spring wire
• small resistance
• materials: cobalt alloy(35% Co, 35% Ni, 20% Cr, 10% Mo)
with silver core
• strength, flexibility and elasticity (longevity!)
Insulator
• good isolation characteristics in aggressive surrounding
• biocompatibility
• materials: silicon rubber and polyurethane
• strength, flexibility and elasticity (longevity!)
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Leads
Reliability
• testing stretching and flexibility (15%)
• demand: 200 x 106 cycles without the
degradation of characteristics
At average heart rate
70/min
Predictable life cycle of a stimulator
10 years
Number of flexions:
N = 70 x 60 x 24 x 365 x 10 = 367.920.000
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Output Stage
Stimulation Parameters :
• pulse magnitude (typical values)
1mA, 2 mA, 4 mA, 8mA
or
1V, 2V, 4V, 8V
• pulse duration (typical values)
programmable between 0,5 ms and 2 ms
Stimulation threshold is measured at implantation.
Pulse magnitude corresponding to twice stimulation
threshold is set for better reliability
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Unipolar leads
• Unipolar leads may
have a smaller
diameter lead body
than bipolar leads
• Unipolar leads
usually exhibit larger
pacing artifacts on
the surface ECG
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Bipolar leads
• Bipolar leads are less
susceptible to over
sensing non-cardiac
signals (myopotentials
and EMI)
• Diameter 4-5 F
(1French = 0,33mm)
Coaxial Lead
Design
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Power Supply
Battery: primary cells based on lithium:
lithium (-) / iodine - poli-2-vinilpiridin (+)
Large energy density
Open circuit voltage 2,8V, permanent during the
use; serial connection two-to three cells
High capacity - 1Ah to 3Ah
Replacement of the stimulator when the battery
capacity falls below 0,09Ah
Consummation does not cause formation of
gasses - hermetic closure
Relatively high output resistance
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Power Supply
Small consummation of the control circuits (CMOS
technology) enables long usage of the stimulator
Typical average consummation of modern cardiac
stimulators is less than 20µA.
Output degree spends on average:
ISR = IP x tI x f = 8mA x 1ms x 1Hz = 8µA
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The Lithium Ionide Battery
Source: SS Barold, Cardiac Pacemakers Step by Step, Blackwell, 2004
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Code marking the Stimulator Operation
Mode
Code consists of four letters:
1. cardiac chamber which is stimulated
2. cardiac chamber which is sensed
0 – stimulation excluded
A - atrium
V - ventricle
D – both chambers (D = dual)
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Code marking the Stimulator Operation
Mode
3. modality (in regard to sensing)
0 – sensing excluded (asynchronous)
I – inhibition
T – triggered
D – both (I + T)
4. frequency adjustability
0 – does not exist
R - adaptable
P - programmable
M – multi-programmable
C - communicating
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Programmable stimulator parameters
•
•
•
•
•
•
•
•
•
Output impulse intensity
Impulse duration
Sensitivity of the input amplifier
Repetition rate
Mode of operation
Duration of the refractory period
Duration of the signal analysis period
Duration of the alert period
Algorithm of the frequency adjustability
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Telemetry
•
•
•
•
•
•
•
•
•
Stimulation threshold
Electrode impendancy
Intracardial ECG
Battery voltage
Internal resistance of the battery
Accidents (stored in RAM)
Stimulus number given to the pacient
Accident hystogram
Sensor data of the adaptable stimulators….
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Paced Rhythm Recognition
VVI / 60
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Frequency adjustable stimulators
Adjusting frequency rate of the stimulator to
patient’s activity
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Rate Responsive Pacemaker Block Diagram
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Sensors in Rate Responsive Stimulators
Sensors:
• Accelerator – body motion
• Microphone – breathing – respiration rate
• Electric-impedance plethysmography:
– volume changes, intracardiac
– respiratory rate and/or volume
• Electrodes (intracardial ECG):
– analysis of the QT segment duration
– R wave area
• Blood pressure
• Termistors – blood temperature
• pH
Double sensing – for improved reliability
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Rate Responsive Pacing
• Activity sensors employ
a piezoelectric crystal
that detects mechanical
signals produced by
movement
• The crystal translates
the mechanical signals
into electrical signals
that in turn increase the
rate of the pacemaker
Piezoelectric
crystal
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Rate Responsive Pacing
• Minute ventilation can be measured by
measuring the changes in electrical
impedance across the chest cavity to
calculate changes in lung volume over
time
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Intracardial ECG Processing
Wavelet analysis of an intracardiac signal
S. Haddad et al: The Evolution of Pacemakers, IEEE Magazine, May 2006
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Lead implantation
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Conclusions
• Electrical impulses generated by the heart (SA node) stimulate
the heart muscle in order to contract, pump blood into the body
and lungs.
• In heart disease, electrical impulses may be either too slow
(bradycardia) or too fast (tachycardia) resulting in poor or no
function of the heart (heart failure).
• A pacemaker sends impulses to replace the natural electrical
activity of the heart cells and speeds up a slow heart.
• An intracardiac defibrillator (ICD) sends a shock to slow down a
heart beating to fast.
• Rate responsive pacemakers stimulate the heart by rate most
similar to the natural frequency of the heart.
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References:
• M. Schaldach: Advances in Pacemaker Technology,
New York Univ Press, Monographs in Biomedical
Engineering Series, 1994
• Brown,B.H. et alt., “Medical Physics and Biomedical
Engineering”., IoP Publishing, London, reprinted 2001.
• Webster,J.G. (Ed.), “Medical Instrumentation, Application
and Design.” 2nd ed., J. Wiley & Sons, Inc., New York,
1995.
• Nelson, C.V., Geselowitz D.B., ur.: “The Theoretical
Basis of Electrocardiography”. Claredon Press, 1976.
• Webster,J.G. (Ed.), “Bioinstrumentation”. John Wiley &
Sons, Inc., New York, 2003
For Croatian speaking attendees:
• Šantić, A., “Biomedicinska elektronika”, Školska knjiga,
Zagreb, 1995
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