Evolution of ischemic tissue injury in a random pattern flap: A new mouse model using intravital microscopy

Background. Dissection of random pattern flaps may cause microcirculatory dysfunction and ischemia, which jeopardize wound healing due to impaired tissue viability. The aim of this study was to develop an in vivo model that enables continuous monitoring of the interplay between microcirculatory dysfunction, ischemia, and tissue injury by intravital microscopy. Materials and methods. A laterally based random pattern skin flap (15 x 11 mm) including the panniculus carnosus was raised in the back of mice and fixed into a dorsal skinfold chamber (n = 10). Arteriolar blood flow, functional capillary density, number of apoptotic cells, and area of tissue necrosis were analyzed by intravital fluorescence microscopy in the proximal, middle, and distal part of the flap at day 1, 3, 5, and 7 after surgery. Chamber preparations without flap harvesting served as controls (n = 6). Results. At day 1, the distal part of the flap showed a decreased arteriolar blood flow (266 +/- 124 pl/s versus controls: 1418 +/- 351 pl/s; P < 0.05), which resulted in severe alteration of functional capillary density (43 11 cm/cm(2) versus 270 +/- 7 cm/cm(2); P < 0.001). The impaired microcirculation was associated with apoptotic cell death (277 +/- 50 cells/mm(2) versus 50 +/- 5 cells/ mm(2); P < 0.05). Microcirculatory dysfunction persisted over 7 days, and, finally, resulted in 49 +/- 3% flap necrosis. Conclusions. This new model enables repetitive and simultaneous in vivo microscopic evaluation of microvascular hypoperfusion, apoptosis, and tissue necrosis in a random pattern flap. By the use of gene-targeted mice, it bears great potential to analyze distinct mechanisms of flap failure. It further represents an ideal tool to study novel protective strategies, including induction of angiogenesis, heat shock proteins, and HIF-1alpha. (C) 2004 Elsevier Inc. All rights reserved.