[Mechanical impedance: a new, noninvasive method for measuring tissue pressure in tibial compartment syndrome. I. Physical principles and results of an animal experiment].

The basic factor involved in the development of compartmental syndrome is increased tissue pressure, which leads to impairment of microcirculation, the ultimate cause of muscle cell necrosis, In practice, however, the technical side of pressure measurement has its problems since the current invasive methods using intracompartmental catheters are always at risk of infection. It would be preferable to do the assessment in a noninvasive way from the exterior the pressure that prevails inside. An apparatus was developed to measure the mechanical impedance in the soft tissue layer above the anterior tibial compartment: an impedance head is applied to the overlying tissue with a defined initial pressure. The impedance head contains a driving probe that touches the skin, a receptor measuring force, and acceleration. The ratio of force and acceleration describe and record the mechanical impedance [Z = kg/s]. The accuracy of the appliance was tested in a test using the hindlegs of 21 rabbits with compartmental syndrome, which was induced experimentally by gel instillation. The mechanical impedance was measured under increasing tissue pressures in the anterolateral muscle. We performed a total of 100 gel instillations and 121 times the mechanical impedance was measured at a frequency of 20 Hz. The graphs of all 21 measuring series showed an average rise of 0.0243 kg/s per cm H2O. In general, all graphs showed a linear rise in the mechanical impedance that was proportional to the intracompartmentally measured tissue pressures.