Biomedical Engineering Reference
In-Depth Information
120
Threshold
60
0
90
80
70
Reference impedance
60 0
40
80
120
160
200
(a)
Days after implant
90
Reference baseline
80
70
Impedance
reduction
60
Duration of impedance reduction
28
21
14
7
0
(b)
Days before hospitalization
Figure 4.1-14 (a) Fluid index and impedance over 6 months; (b) example of impedance reduction before heart failure hospitalization
(arrow) for fluid overload and impedance increase during intensive diuresis during hospitalization. Source: Reproduced with permission:
Yu et al. (2005).
distance (car ignition system), radar 9 GHz E-field < 1.2
kV/m. Typical pacemaker data are: pulse amplitude 5mA,
impedance monopolar electrode system 1 k U , load volt-
age 5 V, lithium battery 6.4 V with capacity 1800 mAh.
The stimulus electrodes are AC coupled in the pace-
maker's output stage, so that no DC can pass and unduly
polarize the electrodes. The electrodes are made of noble
metals to be biocompatible, and consequently they are
highly polarizable. The monopolar electrode system im-
pedance is not very dependent on faradic impedance be-
cause the admittance of the double layer capacitance is
large at the frequencies used.
Pacemaker implant and the use of electrosurgery are
treated in Section 4.1.13.
10
4.1.12 Defibrillation
and electroshock
1
4.1.12.1 Defibrillator
Defibrillator shocks are the largest electric shocks used in
clinical medicine, up to 50 A is applied for some milli-
seconds through the thorax, driven by approximately
5 kV. The electrode system is usually bipolar with two
equal electrodes of surface ca. 50 cm 2 (adult, defl-
brillation of children is rare). They are positioned so that
as much as possible of the current is passing the heart
0.1
0.01
0.1
1
10
t (ms)
100
Figure 4.1-15 Current-time curves for heart pacing with a square
wave pulse delivered during diastole with intracardial catheter
electrode.
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