Severe Blunt Muscle Trauma in Rats: Only Marginal Hypoxia in the Injured Area

Background: After severe muscle trauma, hypoxia due to microvascular perfusion failure is generally believed to further increase local injury and to impair healing. However, detailed analysis of hypoxia at the cellular level is missing. Therefore, in the present work, spectroscopic measurements of microvascular blood flow and O-2 supply were combined with immunological detection of hypoxic cells to estimate O-2 conditions within the injured muscle area. Materials and Methods: Severe blunt muscle trauma was induced in the right Musculus gastrocnemius of male Wistar rats by a standardized "weight-drop'' device. Microvascular blood flow, relative hemoglobin amount, and hemoglobin O-2 saturation were determined by laser Doppler and white-light spectroscopy. Hypoxic cells were detected by histologic evaluation of covalent binding of pimonidazole and expression of HIF-1 alpha. Results: Directly after trauma and until the end of experiment (480 minutes), microvascular blood flow and relative hemoglobin amount were clearly increased. In contrast to blood flow and relative hemoglobin amount, there was no immediate but a delayed increase of microvascular hemoglobin O-2 saturation. Pimonidazole immunostaining revealed a hypoxic fraction (percentage area of pimonidazole-labelled muscle cells within the injured area) between 8 to 3%. There was almost no HIF-1 alpha expression detectable in the muscle cells under each condition studied. Conclusions: In the early phase (up to 8 hours) after severe blunt muscle trauma, the overall microvascular perfusion of the injured area and thus its O-2 supply is clearly increased. This increased O-2 supply is obviously sufficient to ensure normoxic (or even hyperoxic) conditions in the vast majority of the cells.