| About 50% of war injuries are blast injuries, which are characterized by complicate injury condition, a myriad of foreign bodies, and high susceptibility to infection. The principle of military medical support for blast injuries is damage control which includes hemostasis, radical debridement, irrigation, and delayed closure for wounds that are severely contaminated or caused by land mines. These therapies are, as far as infection control is concerned, not satisfied.It has been documented that vacuum-assisted closure (V.A.C.) decreased the infection rate of open bone fracture and decrease the bacteria load in acute or chronic infectious wound. Whether or not is it capable to decrease the infection rate of blast injuries and control the infection when they are infected? To answer the question, animal model for soft tissue blast injury was established in pigs and V.A.C. was used to treat the blast wounds. The outcome of V.A.C. was evaluated from four aspects: time need to heal, infection control, granulation tissue formation, and inflammation. The five parts of the experiment are described as follows:Part 1. Establishment of animal model for soft tissue blast injury in pigs12 wounds were created by explosion of Φ12 electric detonators, designed and manufactured by 213th Institute (Xi'an, China), which were fixed 1 cm over the skin of the shoulders and hips of 3 small white domestic pigs. At 6 hours, 24 hours, 2 days, and 3 days after injury, specimens were harvested from the tissue at 0, 0.5, 1.0, 2.0, and 3.0cm from the wound rim. Every time after specimens were harvested, wounds underwent debridement and irrigation and were packaged with dressing.6 hours after injury, the mean wound area was (7.34±0.986) cm2 and the mean wound depth was (0.85+0.122) cm; both were increased at 24 hours, 2 days, and 3 days after injury when necrosis of skin and muscle was observed, especially at 24 hours when about 3-5 mm skin and muscle underwent necrosis. The bacteria load of the wound was under 105 clone forming unit (CFU)/g during the 3 days period after injury.6 hours after injury, about 5-mm-thick tissue underwent necrosis. In tissue 1.0 cm from the wound edge, severe hemorrage and edema, along with infiltration of inflammatory cells, were observed under light microscope (LM), and much cell fragments, cell edema, and obstruction of capillary vessels were observed through electric microscope (EM). In tissue 2.0 cm from wound edge, infiltration of inflammatory cells and cell edema were observed under LM, and few cell fragments, cell edema, and congestion were observed under EM. Tissue 3.0 cm from wound rim was normal observed by LM, but minor impairment was observed under EM.After debridement 6 hours after injury, about 3 to 5-mm-thick tissue underwent necrosis 24 hours after injury. In tissue at 1.0 cm from the wound edge, infiltration of inflammatory cells and edema was observed under LM, and tissue 2.0 cm from the wound edge was normal.2 day and 3 days after injury, there was only a slim layer of necrotic tissue on the wound surface, under which was granulation tissue. Tissue 0.5 and 1.0 cm from wound surface contained infiltration of inflammatory cells 2 days after injury and granulation tissue 3 days after injury.Part 2. Temporal change of infected blast wound area and depth under treatment of V.A.C.Forty blast wounds, established by explosion of Φ12 electric detonators which were fixed at 1 cm over the skin of the shoulders and hips of 10 small white domestic pigs, were divided into five groups randomly: Group A which was treated with conventional gauze changing; Group B, C, D, and E were treated with V.A.C. set pressure at -10kPa, -15kPa, -20kPa, and -25kP respectively. All wounds were left untreated until the third day after explosion and subsequently were infected. Wound area and wound depth were measured at different time points.Group A, B, C, D, and E healed on (32.8±1.6), (27.1±1.5), (25.8±1.0),(26.4±1.1), and (27.8±1.4) days after injury respectively. The wound area of Group A increased from (10.76±0.42) cm2 before treatment to (13.30±0.43) cm2 3 days after treatment, then decreased gradually; that of other Groups decreased from 1 day after treatment. From 1 to 24 days after treatment, the wound area of Group A was larger than those of other groups.The wound depth of Group A increased from (1.65±0.25) cm before treatment to (2.42±0.20) cm 3 days after treatment, then decreased gradually; that of other Groups decreased from 1 day after treatment. Wound cavity was filled by granulation tissue on 19 days after treatment in Group A, on 9 days in Group C, on 14 days on other 3 groups. From 1 to 24 days after treatment, the wound depth of Group A was deeper than those of other groups.Part 3. Effect of V.A.C. on the bacteria load and the ratio of G+ bacteria in pigs' infected soft tissue blast injurySpecimens were harvested at different time points from the wounds established in Part 2, weighted and homogenized, then the homogenate was diluted and cultured for 24 hours in Petri dish covering agar and goat blood. The bacteria colonies was counted, part of which were subjected for Gram staining.3 days after injury, the bacteria load of all groups was up to 3 ×107 CFU/g; it decreased to 106 CFU/g level 1 days after treatment in Group B, C, D, and E, and to 105 CFU/g level 3 days; it decreased more slowly in Group A to 106 CFU/g level 6 days after treatment and to 105 CFU/g level 14 days after treatment. From 1 to 19 days after treatment, Group A contained more bacteria than other groups.3 days after injury, the proportion of G+ bacteria was 33-35%; 6 days after treatment, it increased to (51.30±5.48)% in Group B, to (55.58±2.98)% in Group C, to (61.26±2.09)% in Group D, and to (71.30±8.68)% in Group E, all of them maintained at high level thereafter; while that of Group A always fluctuated between 30% and 40%. In Group B, C, D, and E, the proportion of G+ bacteria was inversely proportional to the pressure of V.A.C. therapy.Part 4. Effect of V.A.C. on the granulation tissue formation of infected soft tissue blast injury in pigsSpecimens were harvested at different time points from the wounds of Group A and Group C established in Part 2, processed for HE staining, and forimmunohistochemistry of Factor VIII related antigen and Ki67 .3 days after injury, there was pale fibrotic tissue on the surface of muscle in infected blast wounds, in which cells were sparce and inflammatory cells were scarce. In Group A, the number of inflammatory cells (ICs) increased slightly from 1 to 3 days after treatment, then it increased significantly 6 days after treatment when scattered pink granulation tissue was observed with naked eye which contained dense cellularity under LM. In Group C, the number of inflammatory cells increased significantly 1 day after treatment when granulation tissue was observed under LM; 3 days after treatment, healthy granulation tissue was observed by naked eye.Vascular endothelial cells (VCs) were scarce before treatment, the number of which increased 9 days after treatment in Group A, then decreased. In Group C, the number of VCs increased boomingly 1 day after treatment, then decreased gradually from 6 days after treatment. From 1 to 19 days after treatment, there was significant statistical difference between the two groups (P<0.05), with Group C higher than Group A from 1 to 6 days and with Group A higher from 9 to 19 days after treatment.Proliferative cells (PCs) were scarce before treatment, the number of which in both groups increased 1 day after treatment and peaked 3 days after treatment then decreased. From 1 to 3 days, the number of proliferating cells of Group C was higher than that of Group A (P<0.05), while that of Group C was less than Group A from 6 to 9 days after treatment.The distribution of ICs, VCs, and PCs was heterogeneous in granulation tissue. In Group C, the superficial layer of the granulation tissue had contained high density of the three kinds of cells since 1 day after treatment. In superficial layer of granulation tissue of Group A, the situation was similar but in a way much delayed: PCs had maintained at high density since 3 days aftertreatment, ICs since 6 days, and VCs since 9 days. These three types of cells were sparse in deeper layer of tissue of Group A and Group C.Part 5. Effect of V.A.C. on the inflammation of infected soft tissue blast injury in pigsSpecimens were harvested at different time points from the wounds of Group A and Group C established in Part 2, part of which were processed for immunohistochemistry of intercellular adhesion molecule-1 (ICAM-1) and...
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