Sternum wound contraction and distension during negative pressure wound therapy when using a rigid disc to prevent heart and lung rupture

Lindstedt et al. Journal of Cardiothoracic Surgery 2011, 6:42 http://www.cardiothoracicsurgery.org/content/6/1/42 RESEARCH ARTICLE Open Access Sternum wound contraction and distension during negative pressure wound therapy when using a rigid disc to prevent heart and lung rupture Sandra Lindstedt1*, Richard Ingemansson1 and Malin Malmsjö2 Abstract Background: There are increasing reports of deaths and serious complications associated with the use of negative pressure wound therapy (NPWT), of which right ventricular heart rupture is the most devastating. The use of a rigid barrier has been suggested to offer protection against this lethal complication by preventing the heart from being drawn up against the sharp edges of the sternum. The aim of the present study was to determine whether a rigid barrier can be safely inserted over the heart with regard to the sternum wound edge movement. Methods: Sternotomy wounds were created in eight pigs. The wounds were treated with NPWT at -40, -70, -120 and -170 mmHg in the presence and absence of a rigid barrier between the heart and the edges of the sternum. Wound contraction upon NPWT application, and wound distension under mechanical traction to draw apart the edges of the sternotomy were evaluated. Results: Wound contraction resulting from NPWT was similar with and without the rigid barrier. When mechanical traction was applied to a NPWT treated sternum wound, the sternal edges were pulled apart. Wound distension upon traction was similar in the presence and absence of a the rigid barrier during NPWT. Conclusions: A rigid barrier can safely be inserted between the heart and the edges of the sternum to protect the heart and lungs from rupture during NPWT. The sternum wound edge is stabilized equally well with as without the rigid barrier during NPWT. Introduction The use of negative pressure wound therapy (NPWT) for the treatment of deep sternal wound infections has been shown to have remarkable effects on healing [1]. There are, however, increasing numbers of reports of deaths and serious complications associated with the use of NPWT due to heart rupture, lung rupture, bypass graft bleeding and death; the incidence being 4 to 7% of all patients treated for poststernotomy mediastinitis with NPWT after cardiac surgery [2-4]. In November 2009, the FDA filed an alert, and the importance of protecting exposed organs during NPWT and * Correspondence: 1 Department of Cardiothoracic Surgery, Lund University and Skåne University Hospital, Lund, Sweden Full list of author information is available at the end of the article this issue has also been emphasized in the international scientific literature [5-8]. We have previously elucidated the cause of heart rupture in pigs using magnetic resonance imaging [9,10]. The heart was shown to be drawn up towards the thoracic wall, the right ventricle bulged into the space between the sternal edges, and the sharp edges of the sternum protruded into the anterior surface of the heart, in some cases resulting in damage to the left ventricle of the heart or damage to a bypass graft to the right coronary artery [10]. Multiple layers of paraffin gauze over the anterior portion of the heart did not prevent the heart from being deformed. These events could, however, be prevented by inserting a rigid plastic disc between the anterior part of the heart and the inside of the thoracic wall [10]. Heart and lung ruptures © 2011 Lindstedt et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Lindstedt et al. Journal of Cardiothoracic Surgery 2011, 6:42 http://www.cardiothoracicsurgery.org/content/6/1/42 similar to those seen in patients were observed in this experimental set-up without the rigid discs, while no damage to the heart or lungs was seen when the discs were used [10]. Several important aspects must be taken into consideration when treating a sternotomy wound with NPWT. The edges of the sternum move when the patient breaths, coughs and moves. Therefore, the sternum wound must be contracted and stabilized in order to allow adequate respiration and mobilization [5,11]. The aim of the present study was to investigate sternum wound contraction and distension in the presence and absence of a rigid barrier, inserted between the heart and the edges of the sternum, to protect the heart and lungs from damage and rupture during NPWT. Wound contractions were measured before and after negative pressures ranging from -40 to -170 mmHg were applied. Sternum wound distension during mechanical traction to pull apart the edges of the sternotomy, was evaluated using forces up to 320 N. Material and methods 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). Page 2 of 6 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 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., (...truncated)