Effects on drainage of the mediastinum and pleura during negative pressure wound therapy when using a rigid barrier to prevent heart rupture

Abstract

Right ventricular heart rupture is a devastating complication associated with negative pressure wound therapy (NPWT) following cardiac surgery. The use of a rigid disc has been suggested to offer protection against this lethal complication by preventing the heart from being drawn up towards, and damaged by, the sharp sternum edges. The aim of the present study was to compare the wound fluid evacuation from the pericardium and the left pleura when using NPWT with such a disc between the sternal edges and the heart, and when using conventional NPWT. Six pigs underwent median sternotomy followed by NPWT at −120 mmHg, using foam, with or without a rigid plastic disc between the heart and the sternal edges. A 250 ml saline was infused into the pericardium, and the time required for fluid evacuation was measured. A 500 ml saline was infused into the left pleura and the time for fluid evacuation measured. The pericardium was effectively drained of 250 ml fluid in both cases [conventional NPWT: 24 ± 0·7 seconds, NPWT with the disc: 25 ± 1·1 seconds (n.s.)]. The left pleura was effectively drained when using NPWT with the disc, but was not drained at all when using conventional NPWT. The left pleura could be effectively drained of 500 ml fluid when a rigid perforated plastic disc was inserted between the sternal edges and the heart during NPWT. Significantly less drainage of the left pleura was possible when using conventional NPWT without the disc. The pericardium was equally good drained using NPWT with or without the disc.

INTRODUCTION

Poststernotomy mediastinitis is one of the most feared complications in patients undergoing cardiac surgery. The overall incidence of poststernotomy mediastinitis is relatively low, between 1% and 3%; however, this complication is associated with a significant rate of mortality, usually reported to vary between 10% and 25% 1, 2. Conventional forms of treatment usually involve surgical revision with open dressings or closed irrigation, or reconstruction with vascularised soft tissue flaps such as omentum or pectoral muscle 1, 2. Unfortunately, procedure‐related morbidity is relatively frequent when using these conventional treatments, and the long‐term clinical outcome unsatisfactory.

Negative pressure wound therapy (NPWT) is based on the application of local negative pressure to a wound. The application of negative pressure to a sternal wound provides several advantages compared with conventional surgical treatment. Owing to excellent clinical results, and significantly lower mortality and morbidity rates, NWPT has gained acceptance in the treatment of poststernotomy mediastinitis, and is today the standard mode of treatment at many cardiac surgery centres throughout the world 3, 4, 5, 6, 7, 8, 9, 10.

Serious complications associated with the use of NPWT in patients being treated for poststernotomy mediastinitis after cardiac surgery are right ventricle rupture and bypass graft rupture 11, 12. We have previously elucidated the cause of heart rupture in pigs using magnetic resonance imaging 13, 14, 15. 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 the bypass graft on the right coronary artery. However, these events could be prevented by inserting a rigid plastic disc between the anterior part of the heart and the inside of the thoracic wall 13, 14, 15.

A primary goal of wound management using NPWT is the active removal of wound exudate. Patients with poststernotomy mediastinitis have pus and detritus in the mediastinum and the pericardium. If the pleura have been opened during primary surgery, that is, for mammary harvesting for coronary bypass surgery, pus and detritus might drain into the pleura, leading to infected pleura effusion. In the present study we compare the effects on drainage when using a disc between the sternal edges and the heart during NPWT.

MATERIALS AND METHODS

Animals

A porcine sternotomy wound model was used. Six domestic landrace pigs with a mean body 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 experimental protocol for this study was approved by the Ethics Committee for Animal Research, Lund University, Sweden. All animals received humane care in compliance with the European Convention on Animal Care.

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/hour). Complete neuromuscular blockade was achieved by continuous infusion of pancuronium bromide (Pavulon; N.V. Organon, Oss, the Netherlands; 0·3–0·5 mg/ kg/hour). Fluid loss was compensated for by continuous infusion of Ringer's acetate at a rate of 300 ml/kg/hour. 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/minute; minute ventilation: 8 l/minute). A positive end‐expiratory pressure of 5 cm H₂O 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 for NPWT

A midline sternotomy was performed, and the pericardium and the left and right pleura were opened. The wound was treated with NPWT with and without a rigid perforated plastic disc between the heart and the sternum. A polyurethane foam dressing with an open‐pore structure (VAC® black GranuFoam®, KCI, TX, USA) was trimmed so as to be slightly larger than the wound. The first layer was placed between the sternal edges. A second layer of polyurethane foam dressing was placed between the soft tissue wound edges. The wound was sealed with a transparent adhesive drape and connected to a vacuum source set to deliver a continuous negative pressure of −120 mmHg.

Drainage measurements

To determine the drainage from the pericardium, 250 ml saline was infused through the NPWT‐dressing‐sealed sternotomy wound using a chest tube, the tip of the chest tube being placed dorsally in the pericardium. A continuous negative pressure of −120 mmHg was then applied, and the wound fluid was evacuated into a canister placed on a scale. The time was noted for each 50 ml wound fluid evacuated by NPWT, until 250 ml wound fluid had been evacuated.

To determine the drainage from the left pleura, 500 ml saline was infused into the left pleura. The negative pressure was then applied, and the time required for the wound fluid to be evacuated was measured as described above for the pericardium.

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.). All differences referred to in the text have been statistically verified. Values are presented as means ± the standard error on the mean (SEM).

RESULTS

Pericardium

The results of the drainage measurements on the pericardium are shown in Figure 1. The first 50–200 ml of saline was slightly more effectively drained when using NPWT with a rigid disc between the heart and the sternum than when using conventional NPWT without the disc (50 ml: 5 ± 0·2 seconds without the disc, 3 ± 0·3 seconds with the disc, p = 0·007; 100 ml: 8 ± 0·2 seconds without the disc, 5 ± 0·3 seconds with the disc, p = 0·004; 150 ml: 12 ± 0·3 seconds without the disc, 8 ± 0·6 seconds with the disc, p = 0·004; 200 ml: 17 ± 0·3 seconds without the disc, 13 ± 1·2 seconds with the disc, p = 0·034). The total amount of saline (250 ml) was drained similarly well using NPWT with the disc and NPWT without the disc (250 ml: 24 ± 0·7 seconds without the disc, 25 ± 1·1 seconds with the disc, p = 0·807).

Figure 1.

Fluid removal from the pericardium during NPWT. Measurements were performed with conventional NPWT, and with NPWT using a protective disc between the sternal edges and the heart. A 250 ml saline was infused dorsally into the pericardium through a chest tube. A negative pressure of −120 mmHg was then applied and the amount of fluid evacuated into the canister was measured using a scale. The results are shown as means ± the SEM of six experiments. Statistical analysis was performed using the Mann–Whitney test. Significance was defined as p < 0·05 (*), p < 0·01 (**), p < 0·001 (***), and p > 0·05 (not significant, n.s.).

Left pleura

The results of the drainage measurements on the left pleura are shown in Figure 2. The left pleura was drained significantly more effectively when using the rigid disc between the sternum and the heart than when using conventional NPWT. No fluid was drained from the left pleura when using conventional NPWT. NPWT with a disc between the sternum and the heart drained the left pleura effectively (50 ml: 1·6 ± 0·14 minutes, p = 0·001; 100 ml: 3·5 ± 0·23 minutes, p = 0·001; 150 ml: 6·0 ± 0·42 minutes, p = 0·001; 200 ml: 8·7 ± 0·49 minutes, p = 0·001; 250 ml: 12·4 ± 0·51 minutes, p = 0·001; 300 ml: 15·4 ± 0·73 minutes, p = 0·001; 350 ml: 19·2 ± 0·89 minutes, p = 0·001; 400 ml: 23·4 ± 0·86 minutes, p = 0·001; 450 ml: 28·1 ± 0·96 minutes, p = 0·001; 500 ml: 32·5 ± 0·59 minutes, p = 0·001).

Figure 2.

Fluid removal from the left pleura during NPWT. Measurements were performed with conventional NPWT and with NPWT using a protective disc between the sternal edges and the heart. A 500 ml saline was infused into the left pleura through a chest tube. A negative pressure of −120 mmHg was then applied and the amount of fluid evacuated into the canister was measured using a scale. The results are shown as means ± the SEM of six experiments. Statistical analysis was performed using the Mann–Whitney test. Significance was defined as p < 0·05 (*), p < 0·01 (**), p < 0·001 (***), and p > 0·05 (not significant, n.s.). Note that no fluid was drained from the left pleura using conventional NPWT without the disc.

DISCUSSION

Poststernotomy mediastinitis, although infrequent, is a potentially life‐threatening complication of cardiac surgery that continues to have significant rates of morbidity and mortality with traditional forms of treatment 1, 2. The use of NWPT has gained acceptance in the treatment of poststernotomy mediastinitis, and is today the standard mode of treatment at many cardiac surgery centres throughout the world. However, there are increasing numbers of reports of deaths and serious complications associated with the use of NPWT, of which right ventricular heart rupture is the most serious. 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 14, 15.

Fluid evacuation

NWPT uses controlled suction to evacuate wound fluid, decrease bacterial colonisation, stimulate granulation tissue formation, and reduce the need for dressing changes. In the present study, we mimicked a clinical poststernotomy mediastinitis situation with pus and detritus in the mediastinum and the pericardium. If the pleura have been opened during primary surgery, that is, for mammary harvesting for coronary bypass surgery, pus and detritus might drain into the pleura, possibly leading to infected pleural effusion. We therefore compared the draining effects of NPWT with a disc placed between the sternal edges and the heart, and conventional NPWT without the protective disc. The drainage from the mediastinum/pericardium was as good using NPWT with the disc as without the disc. However, no drainage was seen from the left pleura using NPWT without the disc, while NPWT with the disc showed excellent drainage.

The effects of the disc design

The disc, which is embedded in foam, lies beneath the sternum, thereby preventing direct contact between the heart and lungs, and the sharp sternal edges. Theoretically, it could be suspected that the rigid disc might prevent drainage of the fluid in the pericardium and the pleura through the NPWT system, and thus prevent wound healing. However, the results of the present paper indicate that drainage from the pericardium and pleura was superior in the animals treated with NPWT using a disc than those treated without the disc. The reason for this is probably that the disc protrudes laterally in the pleural cavity (anterior to the lung), and thereby comes into contact with the fluid which would otherwise have been isolated and not drained.

Clinical implications

As the pericardium and the pleura are often connected in patients with poststernotomy mediastinitis, these spaces should be regarded as one single space. If one of them is infected, the other is likely to be infected in the majority of cases. Therefore, effective drainage of both these spaces is important in patients with poststernotomy mediastinitis. Furthermore, the disc also acts as a barrier, protecting the heart and lungs from damage because of direct contact with the sharp sternal edges.

CONCLUSIONS

The use of a perforated rigid disc in NPWT not only protects the heart and lungs from direct contact with sharp edges of the sternal bone, but it also improves drainage, which may lead to more rapid healing of the infected thoracic cavity.