W ith unrelieved pressure, tissue ischemia develops, and metabolic wastes accumulate in the interstitial tissue, resulting in anoxia and cellular death.¹ This pressure-induced ischemia also leads to excessive tissue hypoxia, further promoting bacterial proliferation and tissue destruction.² In clinical settings, some pressure-induced ischemic wounds are caused by support surfaces, and some lower-limb diabetic pressure ulcers are caused by footwear. Infection of these wounds considerably impairs the healing process. For example, with pressure ulcers, the prevalence of infection is 1.4 per 1,000 ulcer days³ or 27% of studied pressure ulcers.⁴ Moreover, the mortality rate for bacteremia due to pressure ulcer infection has been reported at 15.4%.⁵ In another study, 48% of patients with pressure ulcers died of sepsis.⁶ The results of the aforementioned studies suggest that if a bacterial infection develops in a pressure-induced ischemic wound, medical conditions can easily deteriorate and possibly become life threatening. Although the relationship between bacteria and acute wounds using animal models has been closely examined at the postoperative stage,^7–13 no such animal model has been studied to examine the relationship between bacteria and pressure-induced ischemic wounds.¹
The purpose of this study was to develop an animal model of pressure-induced ischemic wounds and to clarify the effects of bacteria on the healing process of such wounds.

Methods

Subjects. Fifty-nine male Wistar rats were used. Unlike humans, rats are loose-skinned animals. The properties of loose skin allow wound contraction to play a significant role in closing rat skin wounds.¹⁴ While the rat may not be the ideal model of pressure-induced ischemic wounds in humans, the authors decided to use this species, since it has been used previously for many wound models.¹⁴
Protocol. Wound healing was compared macroscopically and histologically between a group of rats with pressure-induced ischemic wounds inoculated with bacteria (bacterial-inoculation group) and a group of rats with pressure-induced ischemic wounds without bacterial inoculation (control group). To ensure reproducibility, wounds were experimentally created on the flank regions where vertical pressure can be applied for a set period of time with a degree of stability.
Instrumentation. The authors’ original experimental device was constructed from a pressure applicator and an air compressor to artificially create pressure-induced ischemic wounds (Figure 1). Compressive pressure was applied by lowering the cylinder. A pressure sensor (strain gauge) was placed between the cylinder and the pressure applicator, and the results were displayed using a digital indicator (CSD-701, Minebea Co. Ltd., Japan). The position of the cylinder could be adjusted manually to vary the amount of pressure. The end of the pressure applicator was circular in shape with an area of 3 cm². A small air compressor (PA400B, Hitachi, Japan) was used to lower the pressure applicator.
Preparation of ischemic wounds. Using the experimental device, 8 kg of pressure was applied for 6 hours. Because previous studies used different animals and experimental devices, the authors conducted a pilot study involving 24 rats to clarify the necessary amount and duration of pressure to prepare pressure-induced ischemic wounds using an experimental device. The intensity and duration of pressure used here were decided based on a pressure-time curve obtained from the authors’ pilot study.
Bacterial inoculation. Bacterium. A clinical isolate of Staphylococcus aureus was used, because this bacterium is often detected in human chronic wounds.¹⁵ The bacterial strain used in the present study was subcultured every week using special agar media.