Transcript Defibrelator_ch1

MT414
al Engineering
rs/Cardioversion
Dr. Ali Saad, Biomedical Engineering Dept.
College of applied medical sciences
King Saud University
Dr. Ali Saad, BMT department
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efibrillation
•
•
•
•
used to treat ventricular fibrillation (cardiac arrest) loss
50,000 cardiac arrest cases occur annually in US.
defibrillation involves the application of a strong electr
exact physical mechanism leading to ventricular fibrilla
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ength Duration Curve
defibrillation
occurs
current (amps)
no defibrillation
pulse duration
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ngth Duration Curve (cont.)
• minimum defibrillation energy occurs for pulse durations of 3 - 10 m
• pulse amplitude in tens of amperes (few thousand volts).
• operator selects energy delivered: 50-360 joules, depends on:
– intrinsic characteristics of patient
– patient’s disease
– duration of arrhythmia
– patient’s age
– type of arrhythmia (more energy required for v. fib.)
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l Defibrillators
•
•
•
•
For each minute elapsing between onset of ventricular fibrillation an
defibrillators should be portable, battery operated, small size.
energy in defibrillators usually stored in large capacitors.
total energy stored in capacitor:
1
WC  CVC2
2
Vc = capacitor voltage
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External Defibrillator
standby
power
supply
charge
discharge
gate
patient
switch is under
operator control
energy
storage
timing
circuitry
ECG
monitor
applies shock about 20 ms after
QRS complex, avoids T-wave
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Capacitive Discharge Defibrillator
L
power
supply
2kV
Vc
+
Vc
_
Rlead
Rchest
C
C: 10 mF - 50 mF, takes about 10s to charge
Vc: 4,000 - 9,000 V
up to 40% of energy in C can be dissipated in L and Ri
response is slightly underdamped (depends on chest R)
t
10 ms
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Example
power
supply
Rlead
+
Vc
_
C
+
Vchest
_
Rchest
Rchest = 95W, Rlead = 5W, total energy stored in C is W = 300 J, want
to deliver 90% of W to heart in 8 ms. What value of C should be used?
RL  Rlead  Rchest
VC t   VC 0e  t / RL C , t  0
VC(0) = initial capacitor voltage after charging
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Example (cont.)
Rchest
Vchest t   VC t 
Rchest  Rlead
 VC 0e
 t / RL C
Rchest
 VC 0e t / RLC 
Rchest  Rlead
energy stored in capacitor at t = 0:
0
0


(1)
dVC 

W   VCiC dt   VC  C
 dt
 dt 
0
1
2
 C  VC dVC  CVC 0 (2)
2

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Example (cont.)
energy delivered to Rchest:
tV2
chest
Wchest  t   
0
substituting (1):
0
using (2):
Rchest
tV2
2
C
Wchest t    
t  dt
 2t / R LC
0
e
 
dt
Rchest
0.008 V 2  0e  2 t / R L C
1
0.9  CVC2 0   2  C
dt
2
Rchest
0
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Example (cont.)
 0.9 /   e  2 ( 0.008)/ R L C  1
0.0526  e 2( 0.008)/ R LC
solving for C:
C = 54.3 mF
1
300J  CVC2 0
2
V (0)=3,322.90V
initial voltage across capacitor:
c
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Square Wave Defibrillators
power
supply
n
n
n
+
Vc
_
SCR2
Rlead
+
SCR1
C
timing
circuit
Vchest
_
Rchest
during charging, SCR1 and SCR2 both open
to defibrillate, SCR2 closes, current flows to chest
after a fixed interval, SCR1 closes, shorts out C
Vchest
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Defibrillation Electrodes
n
n
n
metal, 70-100 cm2 surface area
must be coupled to skin using conductive material
(otherwise can burn patient)
two types:
n hand-held: conductive gel must be manually applied,
reusable.
n adhesive: adhesive conducting material holds electrode
to skin, disposable
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Electrode Placement
anterior wall placement
front-to-back placement
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Automatic External Defibrillators (AEDs)
n
n
Two modes of operation:
n Automatic: AED recognizes specific arrhythmias via
signal processing algorithms, applies shock as
needed. No manual control.
n Semi-automatic: operator must confirm shock
advisory from AED to deliver the shock.
Less operator training needed
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(a) Basic circuit diagram for a capacitive–discharge type of
cardiac defibrillator. (b) A typical waveform of the discharge
pulse. The actual waveshape is strongly dependent on the
values of L, C, and the torso resistance RL.
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Electrodes used in cardiac defibrillation (a) A
spoon-shaped internal electrode that is applied
directly to the heart. (b) A paddle-type electrode that
is applied against the anterior chest wall.
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Hand-held transthoracic electrode
(From Tacker Jr. 1980).
Button
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Some examples of the pre-applied adhesive electrodes
(From Tyco/Healthcare Kendall  LTP 2001)
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LIFEPAK 500 Automatic External
Defibrillator (AED) weighs 3 kg and is
portable (Medtronic inc. 2001).
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dioversion
• Cardioversion is also a shock supplied to treat other types of arrhyt
• atrial fibrillation
• ventricular tachycardia (rapid heart rate)
• these types of arrhythmias are not life threatening but do result in re
• it is important that the cardioversion pulse not coincide with a T-wa
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A cardioverter The defibrillation pulse in this
case must be synchronized with the R wave of
the ECG so that it is applied to a patient shortly
after the occurrence of the R wave.
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Implantable cardioverter defibrillator (ICD) (Medtronic inc.,
2001).
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Lead wire
Electrode
Electrode
Electrode
Lead wire
(a)
(b)
(c)
Electrodes for the automatic implantable cardioverter defibrillator
(ICD). (a and b:modified from Tacker Jr. 1994, c: modified from
Owens et al., 1990)
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A defibrillator provides a 5 ms pulse
of 20 A to a 50 W load.
Thus the energy delivered is
E = P = I2Rt = (20 A)2(50 W)(0.005
s)
=
100 J.
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Defibrillator Safety
n
n
n
Shock for treating atrial flutter or atrial fibrillation should
not be applied during T-wave.
Operator should be careful not to touch electrodes during
defibrillation.
Other personnel should remain clear of patient and any
metal objects contacting patient during defibrillation.
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Defibrillator Case Study
CardioServe
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Case Study (cont.)
Instrument type: Portable defibrillator with monitor
Operation modes: Non-synchronized, synchronized, HR
monitoring
Energy levels
External defibrillation: 2, 5, 7, 10, 20, 30, 50, 70, 100, 150, 200,
300, 360 J
Capacitor charging time: To 360J from power line or fully
charged battery typically 8 seconds. To 200J typically 4
seconds.
Waveform: Damped sinusoidal halfwave (Edmark)
Synchronized delay: From R-wave trigger to pulse discharge
approximately 40 ms
Pulse output: Isolated
Safety discharge: Capacitor discharge via internal load
resistance
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Case Study (cont.)
Defibrillation electrodes
Hard paddles: Hard paddles (80 cm2) with charge/shock and printer
buttons. Pediatric adapter (17 cm2).
Pads: Adapter cable for adult and pediatric pads. Buttons for
charge/shock and printer on the defibrillator.
Synchronization
Signal: With ECG signal of either polarity
ECG acquisition: Via hard paddles/pads or ECG patient cable. Via ECG
patient cable up to 7 leads are selectable.
ECG Monitor
Input: ECG via 3 or 5 leadwire cable or defibrillation hard paddles or
pads
Frequency response: 0.5 to 100 Hz (ECG patient cable)
Isolation: Class CF according to IEC. Input protected against high voltage
defibrillator pulses.
Common mode rejection: >110 dB RL referred to ground
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Implantable Defibrillators
n
n
n
n
n
Treat v. fibrillation, tachycardia
Usually incorporated as part of an implanted cardiac
pacemaker (typically VVI).
Defibrillation threshold: 9.8  6.6 J (n = 102), biphasic pulse
Defibrillation electrodes also transvenous.
Must detect arrhythmia prior to defibrillation:
n HR
n PR, RR interval stability
n more sophisticated electrogram analysis (EE 5345)
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References for Defibrillation
n
n
Willis A Tacker, “External Defibrillators,” in The
Biomedical Engineering Handbook, J. Bronzino (ed) CRC
Press, 1995.
M. Neuman, in Webster (ed), Medical Instrumentation:
Application and Design, Houghton Mifflin, 1992.
n
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