Chest Impedance: Characteristics of Local Patients

F Lateef, SH Lim, V Anantharaman, CS Lim
Correspondence: Dr Fatimah Lateef

ABSTRACT
The Pre-hospital Defibrillation Program in Singapore has in some cases demonstrated a lower amplitude of Ventricular Fibrillation (VF) than considered the norm. The Electrode Skin Impedance (ESI) refers to the skin impedance determined between two electrodes placed at specific positions on the body surface. The objective of this prospective study was to measure the ESI of patients at 5 Hz and 2 kHz frequencies, and assess its change with time from the application of electrodes, the difference between the ESI at two different sets of electrode placement positions and correlation with patient factors. Patients who were 25 years or older and not critically ill had their ESI measured with a modified Heart-Save 911 defibrillator, using signal frequencies at 5 Hz and 2 kHz, at 10 seconds, 1 and 2 minutes after electrodes application. Two sets of positions were used; Position 1 where an electrode is placed in the right infra-clavicular region and another just lateral to the apex beat on the left and Position 2, which represents the mirror image of Position 1. 36 each of male and female patients were studied. The mean age and weight were 59.9 +/- 13.5 years and 56.8 +/- 24.1 kg respectively. There was no significant correlation between the ESI and patients' body weight or sex. However, there was a significant decrease in the ESI with time from application of electrodes at both Positions (p < 0.05) with the two different frequencies. The ESI was lower when measured at lower frequencies and higher when taken at higher frequencies, but there was no statistically significant difference between the two mirror-image positions used. Thus, with lower frequency, the ECG amplitude of VF recorded on the automated external defibrillator could be enhanced.

Keywords: ventricular fibrillation, skin impedance, ECG amplitude
Singapore Med J 2000; 41(7): 331-334