Bioautomation (Apr 2006)
Transthoracic Impedance Study with Large Self-adhesive Electrodes
Abstract
The external electrical therapy of the heart requires the application of high voltage electrical pulses via large external electrodes, placed on selected locations on the thorax surface. The position of the electrodes is one of the major determinants of the transthoracic impedance (TTI), which influences the intracardiac current flow during electric shock and therefore affects the defibrillation success. The indefinite nature of the factors affecting TTI raised our interest in clinical study of the TTI behavior during long-term applications of the defibrillation pads in different positions on the patient's chest. The study involved 86 randomly selected patients (39 male and 49 female, age (20-83) years, height (150-190) cm, weight (50-110) kg, chest size (86-130) cm, 67 patients with normal skin, 13 patients with dry skin and 6 patients with greasy skin, 70 patients without and 16 patients with chest pilosity). TTI was measured by passing of a low-amplitude high-frequency (23 kHz) current between the two PADs (active area about 92 cm2). For each patient, the TTI was measured 10 s, 1 min and 5 min after sticking on the electrodes to the skin surface, separately for the two tested electrode positions - Position 1 (sub-clavicular/sub-axillar position) and Position 2 (antero-posterior position). TTI range is comparable for the two tested PAD positions - between 58 Ohm and 152 Ohm for Position 1 and between 55 Ohm and 149 Ohm for Position 2. TTI mean +/- SD value in Position 1 (107,2 +/- 22,3) Ohm is significantly higher than TTI in Position 2 (96,6 +/- 19,2) Ohm. Both the pilosity and the skin type do not change significantly the TTI value, however the patients with chest pilosity presented slightly higher TTI than those without pilosity. The TTI was higher for normal skin, followed by dry and greasy skin. TTI presented weak correlation with both the patient chest size and weight (r<0.5, p<0.05). The mean value of the TTI decreases in time. In time-interval (10s to 1min) after sticking on the electrodes, TTI drops with about (3,8 +/- 4,03) Ohm for Position 1 and (2,44 +/- 3,75) Ohm. for Position 2. The TTI drop (10s-5min) is (7,36 +/- 5,33) Ohm for Position 1 and (5,06 +/- 7,08) Ohm for Position 2.
