Methods
Experimental Animals
Six domestic pigs of both genders, with a median weight of 60 kg, were used. The animals fasted overnight, but were given free access to water. The study design was approved by the ethical committee on animal experiments in Region Skane, Sweden. The study comply with the "Animal Research: Reporting In Vivo Experiments" ARRIVE guidelines.
Anesthesia
All animals were premedicated with intramuscular ketamine (30 mg/kg) before being brought into the laboratory. Sodium thiopental (5 mg/kg), atropine (0.02 mg/kg), and pancuronium (0.5 mg/kg) were given intravenously immediately before surgery. A Portex endotracheal tube (7.5 mm internal diameter, Medcompare, South San Francisco, CA) was used for intubation. A servo-ventilator (Siemens Elema 300A, Stockholm, Sweden) was used for mechanical ventilation throughout the experiments. The ventilator settings used were: minute volume = 100 ml/kg, FiO2 = 0.5, breathing frequency = 16 breaths/minute, and positive end expiratory pressure = 5 cmH2O. Anesthesia and muscular paralysis were maintained by continuous intravenous infusion of 8-10 mg/kg/hour propofol (Diprivan, AstraZeneca, Sweden), 0.15 mg/kg/hour fentanyl (Leptanal, Lilly, France), and 0.6 mg/kg/hour pancuronium (Pavulon, Organon Teknika, Boxtel, the Netherlands).
Data Acquisition
Heart frequency and ventilator parameters were recorded throughout the experiments.
Surgical Procedure
A 30-cm midline incision was made on each pig. The V.A.C.® Granu Foam™ abdominal dressing system (KCI, San Antonio, TX), was used. The visceral protective layer was cut to an appropriate size, extending into the paracolic gutters on both sides (about 30 cm wide and 35 cm long). A layer of polyurethane Granu Foam was placed on top of the visceral protective layer between the edges of the wound. The wound was covered with self-adhesive polyethylene drape, a track pad was inserted through the drape (both from V.A.C., KCI, San Antonio, TX), and then connected to a continuous vacuum source.
Pressure transduction was measured using a custom-made pressure gauge with saline-filled catheters. Pressure transduction probes were sutured to two intestinal ileal loops, one of which was sutured to the inner surface of the dressing, and the other at the anterior abdominal wall. Probe location was confirmed upon completion of the experiments.
A chest tube was inserted through the abdominal wall into the Pouch of Douglas, and 500 ml albumin solution was infused into the Pouch of Douglas to mimic the fluid in an open abdomen. NPWT was applied at pressures of −50, −70, and −120 mmHg with and without a protective disc between the intestines and the vacuum source. The amount of fluid evacuated into a canister was measured according to a scale. The abdomen was completely drained between each pressure setting before another 500 ml albumin solution was infused.
The Protective Disc
The protective perforated plastic disc placed between the dressing and the intestines was soft, flexible, and approximately 3 mm thick.
Calculations and Statistics
Calculations and statistical analysis 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 test for multiple comparisons when comparing three groups or more. Significance was defined as, p < 0.05 (*), p < 0.01 (**), p < 0.001 (***), and p > 0.05 (not significant, n.s.).Given values are means and SEMs.