Animals
A porcine sternotomy wound model was used. Eight domestic landrace pigs with a mean weight of 70 kg were fasted overnight with free access to water. The study was approved by the Ethics Committee for Animal Research, Lund University, Sweden. The investigation complied with the "Guide for the Care and Use of Laboratory Animals" as recommended by the U.S. National Institutes of Health, and published by the National Academies Press (1996).
Anaesthesia and surgery
Premedication was performed with an intramuscular injection of xylazine (Rompun® vet. 20 mg/ml; Bayer AG, Leverkusen, Germany; 2 mg/kg) mixed with ketamine (Ketaminol® vet. 100 mg/ml; Farmaceutici Gellini S.p.A., Aprilia, Italy; 20 mg/kg). Before surgery, a tracheotomy was performed and an endo-tracheal tube was inserted. Anaesthesia was maintained with a continuous infusion of ketamine (Ketaminol® vet. 50 mg/ml; 0.4-0.6 mg/kg/h). Complete neuromuscular blockade was achieved with a continuous infusion of pancuronium bromide (Pavulon; N.V. Organon, Oss, the Netherlands; 0.3-0.5 mg/kg/h). Fluid loss was compensated for by continuous infusion of Ringer's acetate at a rate of 300 ml/kg/h. Mechanical ventilation was established with a Siemens-Elema ventilator (Servo Ventilator 300, Siemens, Solna, Sweden) in the volume-controlled mode (65% nitrous oxide, 35% oxygen). Ventilatory settings were identical for all animals (respiratory rate: 15 breaths/min; minute ventilation: 8 l/min). A positive end-expiratory pressure of 5 cmH2O was applied. A Foley catheter was inserted into the urinary bladder through a suprapubic cystostomy. Upon completion of the experiments, the animals were given a lethal dose (60 mmol) of intravenous potassium chloride.
Wound preparation
A midline sternotomy was performed and the pericardium and the pleurae were opened. Two 6-0 steel wires for use in sternal closure (Syneture, Tyco Healthcare, CT, USA) were secured around the ribs on each side of the sternum, and attached to a custom-made sternal traction device. The purpose of this was to test sternum wound distension when lateral traction was applied to draw apart the edges of the sternotomy (Figure 1). The traction device was connected to a force transducer and a recorder. The wound was treated with NPWT in the presence or absence of a rigid plastic disc, which was inserted between the heart and the sternum. The wound was filled with open-pore polyurethane foam. One layer of foam was placed between the sternal edges. A second layer of foam was placed over the first layer, between the soft tissue wound edges, and secured to the surrounding skin. The wound was sealed with a transparent adhesive drape, and the drain was connected to the vacuum source. The vacuum source was set to deliver negative pressures of -40, -70, -120 or -170 mmHg. The different negative pressures were applied in random order.
Wound contraction
The distance between the lateral wound edges was measured. Measurements were performed before and after the application of negative pressures of -40, -70, -120 and -170 mmHg.
Wound distension
Lateral traction was applied to the sternotomy wound, using the traction device described above, and the distension of the wound was measured. The effects of lateral forces, ranging from 0 to 320 N, were studied on the NPWT treated sternotomy wound, at the negative pressures of -40, -70, -120 and -170 mmHg. This was done to ensure that the sternum is sufficiently stabilized during NPWT to withstand the forces to which the wound is exposed when the patient breathes, coughs or moves.
The protective disc
The protective disc was made out of bio-compatible plastic that could withstand a force of a negative pressure of at least -50 mmHg. The disc was 20 × 8 cm and was then cut to appropriate size to fit between the anterior part of the heart and the posterior part of the sternum. The disc had multiple small perforations all over the disc area to allow drainage. The disc was ridged with flexible edges.
Calculations and statistics
Calculations were performed using GraphPad 5.0 software (San Diego, CA, USA). Statistical analysis was performed using the Mann-Whitney test when comparing two groups and the Kruskal-Wallis test with Dunn's post-test for multiple comparisons when comparing three groups or more. Significance was defined as p < 0.05. Results are presented as the mean of 8 measurements ± the standard error of the mean (S.E.M.).