Changes in Dielectric Properties (800-1900 MHz) of Pig Brain Tissue in the Transition from Life to Death

We have reported preliminary results of six animal experiments using pigs as test animals to examine possible changes of dielectric properties of brain tissue in the transition from life to death (1, 2). These experiments indicated a noticeable decrease in conductivity after death. This project was extended to collect data on four additional pigs in order to validate the preliminary data. The animals were anaesthetised by a procedure which was designed to have minimum influence on relevant brain tissue characteristics such as water content and blood perfusion (Thiopental only for inducing and Isoflurane for maintaining anaesthesia). To provide access for the probe head of the dielectric property measurement system (HP8722C vector analyser in combination with the HP85070B dielectric probe kit), a trephine hole (diameter: 3 cm) was made in the pigs´ skull about 3 cm off the head midline and about 5 cm behind the eye-to-eye line. The dura mater was removed and the calibrated measurement probe was carefully placed onto the brain cortex (the arachnoidea was left intact). Brain tissue temperature was monitored and recorded during the experiments using the fibre optic temperature measurement system LUXTRON 790. Important vital parameters (aortic blood pressure, venous blood pressure, heart rate, electrocardiogram, rectal temperature, artificial respiration pressure and end tidal CO2 pressure) were monitored and recorded during each experiment by a medical recording system. Dielectric property measurements in the frequency range from 800 to 1,900 MHz were performed periodically about once per minute. After measuring the dielectric properties of the brain tissue for about one hour during stable anaesthesia the pigs were sacrificed using a potassium chloride solution (applied intravenously) which caused death by cardiac arrest within 3 minutes. Continuous measurements of dielectric brain tissue properties, brain tissue temperature and intracranial pressure were made from 1 hour prior to death to about 18 hours after death. The additional animal experiments performed so far confirmed the indication that conductivity decreased after death. At 900 MHz, the mean value of conductivity dropped from 1.29 S/m immediately before sacrificing the animal to 1.07 S/m at 2.5 hours after death. Correcting the measured data for the drop of tissue temperature after death with a temperature coefficients of 1 %/°C, partly compensates the drop in conductivity. However a drop of about 10 % still remains, which probably is due to biochemical processes in the tissue after death.