The time effect of pressure on tissue viability: investigation using an experimental rat model.

An experimental rat model was used to investigate the time-pressure effect on tissue viability. External loading equivalent to 13.3 kPa (100 mm Hg) of pressure was applied to the greater trochanter and tibialis area of Sprague-Dawley rats using pneumatic indentors for duration of 6 hrs each day for 1 to 4 days. It was observed that postocclusive hyperemic responses were gradually increased at the trochanter throughout the 4 days of loading, whereas for the tibia there was a significant increase (P = 0.04) in postocclusive hyperemic flow between Days 2 and 3. In histologic evaluations, cutaneous tissue damage was observed at the trochanter area but not at the tibialis area after 2 consecutive days of load application. In contrast, degeneration of muscle cells characterized by numerous increases of nuclei occupying the central of the muscle fibers was observed after 2 days of load application at the tibialis. The situation was found to progress with time (P = 0.17). The presence of other histologic signs, including the internalization of peripherally located nuclei, replacement of muscle cells by fibrosis and adipose tissues, and the presence of pyknotic nuclei as well as karyorrhexis, confirmed that the affected tissues were damaged. These findings suggest that postocclusive hyperemia and the distress of tissues under loading could be closely related.