Effect of circulatory factors on the mechanical properties of the brain

Abstract

The in vivo study of the mechanical properties of the brain may be valuable in clarifying the mechanisms of some physiological and pathological processes which occur in it, in particular, brain edema [3]. This possibility encouraged us in 1974 to undertake the in vivo study of some mechanical properties of the brain in order to explain their dependence on various factors. The aim of the present work was to study the effect of circulatory factors, specifically, changes in the volume and pressure of blood ha the cranial vessels, on the mechanical properties of the brain during life. The study was carried out on 27 adult rabbits of both sexes weighing 2.5-3.5 kg which were anesthetized using intravenous hexobarbital (~30 mg/kg) and paralyzed using diplocene (10-15 mg/kg animal mass). An AID-1 artificial respirator was used for lung ventilation. A "thoracic cage-head" preparation was used in which blood was circulated only in the thoracic cage (including lungs, thoracic cage walls, and heart), neck, and head of the experimental animal [4] in order to obtain reliable control and the desired arbitrary changes in the systemic circulation parameters studied, viz., the total arterial and total venous pressures (TAP and TVP). Two independent systems of pressure reservoirs filled with a blood substitute,using polyethylene catheters attached to the abdominal aorta and caudal vena cava, permitted us to alter or maintain both the TAP and TVP. The levels of the mean TAP and TVP were recorded using Elema-Shenander Mingograf-81 electromanometers (produced in Sweden) connected by catheters with the common carotid artery and jugular vein, respectively. The system for recording changes in brain volume consisted of a pressure detector with one end attached to a stereotactic apparatus. The other end of the pressure detector was connected to a rod ending in a ball 5-7 mm in diameter which was in contact with the brain surface in the parietal region. The detector was introduced into a bridge circuit, the signal from which was amplified by EMT-12 Minograf-81 preamplifiers. The detectors were selected such that the strength of the attachment of the ball to the brain surface was greater than the elastic forces arising in the detector upon deformation resulting from change in brain volume. Thus, the error introduced by the elasticity of the detector in measuring the brain level could be neglected. The brain level detectors were read on a dial-type indicator to _+0.01 mm. The error was a factor of the line thickness of the recorder and was not less than +0.05 mm for the amplification used. Detectors with hysteresis greater than the line thickness were discarded. The instruments were calibrated prior to each experiment, which provided for the quantitative recording of the brain level in the trepanation aperture. The brain bulging from the approximately round trepanation aperture may be considered to be approximately round. The volume of such a segment is 2 1 V=~