The linearity of the volume/pressure response during intracranial pressure "reserve" testing

Simultaneous determination of mechanical properties and physiologic parameters in living rat brain

Mechanical properties were determined in living adult rat brain. Reconstructed magnetic resonance images of the rat brain before and after 2 mm compression with a 4.06 mm diameter vinyl screw showed the total volumetric strain was maximal at the site of indentation. The pressure response to stepwise brain compression showed a linear relationship to the point of respiratory compromise. Instrumented indentation was performed on live brain with intact dura using a 4-mm-diameter flat punch indenter to a maximum depth 1.2 mm at loading-unloading rates not exceeding 0.34 N/min and 1.2 mm/min. The calculated elastic modulus showed no consistent change after death. Creep deformation over 15 s was 7.86+/-1.6% in live brain and 27.0+/-0.24% after death. In constant multicycle indentation, displacement from 1st to 10th cycle increased 8.0+/-1.7% in life and 12.9+/-2.8% after death. The results suggest that elastic properties of rat intracranial contents do not change immediately after death, while changes in the viscous properties are substantial. The process of measuring these properties can alter physiological parameters.

Assessment of cerebrospinal fluid compliance and outflow resistance: analysis of steady-state response to sinusoidal input

Cerebrospinal fluid dynamics have been studied in the past by analyses of responses to bolus, constant rate or constant pressure inputs. In this study, we present a method for analyzing CSF pressure responses to sinusoidal variation in the infusion rate. Infusion of artificial CSF into the cisterna magna of adult rats was modulated sinusoidally between 0 and 30 microliter/min. The resulting sinusoidal variation in intracranial pressure was recorded on a strip chart recorder simultaneously with the infusion rate signal. The two signals were analyzed for peak-to-peak variation, mean value, and phase shift for input frequencies in the range of 0.0015 to 0.01 HZ (0.00942 to 0.0628 radians/sec). The system was analyzed at each mean infusion rate as a parallel resistance and compliance with a first order linear model. The resistance to CSF outflow was determined as the change in mean steady-state pressure divided by the change in mean infusion rate. The compliance was then obtained from the frequency dependent phase shift between input and output using the first-order linear model. Resistance values were lower for higher average infusion rates consistent with our previous work, while compliance remained constant over the measured pressure range.