Negative pressure wound therapy in the treatment of surgical site infection in cardiac surgery

Rev Bras Enferm. 2020;73(5): e20190331 http://dx.doi.org/10.1590/0034-7167-2019-0331 7 of ABSTRACT Objectives: to describe the relationship between epidemiological and clinical characteristics of postoperative cardiac surgery patients undergoing negative pressure wound therapy for the treatment of surgical site infection. Methods: an observational, cross-sectional analytical study including a convenience sample consisting of medical records of patients undergoing sternal cardiac surgery with surgical site infection diagnosed in medical records treated by negative pressure wound therapy. Results: medical records of 117 patients, mainly submitted to myocardial revascularization surgery and with deep incisional surgical site infection (88; 75.2%). Negative pressure wound therapy was used on mean for 16 (±9.5) days/patient; 1.7% had complications associated with therapy and 53.8% had discomfort, especially pain (93.6%). The duration of therapy was related to the severity of SSI (p=0.010) and the number of exchanges performed (p=0.045). Conclusions: negative pressure wound therapy has few complications, but with discomfort to patients. Descriptors: Surgical Wound Infection; Negative-Pressure Wound Therapy; Sternotomy; Thoracic Surgical Procedures; Cardiology.


INTRODUCTION
Surgical site infection (SSI) is the most common healthcareassociated infection (HAI) among surgical patients. It accounts for 21.8% of all HAI, associated with a 3% mortality rate, with approximately three quarters of deaths attributable directly to SSI (1)(2) .
The Centers for Disease Control and Prevention (CDC) has developed criteria that define and categorize SSI according to their topography, that is, the compromise by the infectious process of the different tissues opened and manipulated during the surgical procedure. They divide infections into superficial incisional SSI when it involves only skin and subcutaneous tissue; deep incisional SSI when affecting deep soft tissues such as fascia and muscle; and organ/space SSI, when it covers all space and organ manipulated during surgery (3) .
Negative pressure wound therapy (NPWT) is among the many therapeutic options for treating complex wounds with infection. It can be considered as an adjuvant method by local application of controlled subatmospheric pressure to accelerate the healing process by reducing edema, exudation, bacterial colonization, increased perfusion, granulation tissue formation and promotion. patient comfort by reducing daily wound handling (13)(14)(15)(16)(17)(18)(19)(20)(21) .
The World Health Organization (WHO) recommends only prophylactic use of NPWT to prevent SSI in previously closed, high-risk surgical incisions (22) . However, it is observed in clinical practice that the most frequent use of this technology is related to the treatment of SSI, although its use is supported by limited scientific evidence, especially for the treatment of SSI after cardiac surgery (23) . For this reason, its indication should be cautious due to the expensive device-related costs and the low quality of evidence (22) , leading to the need to explore the clinical outcomes of patients undergoing NPWT SSI treatment.

OBJECTIVES
To describe the relationship between epidemiological and clinical characteristics of postoperative cardiac surgery patients undergoing NPWT for the treatment of SSI.

Ethical aspects
This project was approved by the Research Ethics Committee of the University of Sao Paulo Nursing School. It was prepared in accordance with the Regulatory Guidelines and Norms for Research Involving Human Beings, issued by Resolution 466 of 2012 of the National Health Council (CNS, acronym in Portuguese), safeguarding the confidentiality of information regarding the identity of participants.

Design, place of study and period
An observational cross-sectional analytical study, guided by the STROBE tool, conducted in a large hospital specialized in the clinical and surgical treatment of cardiovascular diseases in the state of Sao Paulo. Data extracted are from January 2017 to May 2018.

Population or sample; inclusion and exclusion criteria
The convenience sample consisted of 117 medical records of patients undergoing sternal cardiac surgery with SSI diagnosed in medical records. They met the diagnostic definitions proposed by the CDC and National Sanitary Surveillance Agency (ANVISA, acronym in Portuguese), dealt with by the NPWT (3,6) .
The medical records of patients over 18 years old who underwent cardiac surgery who developed SSI and were treated with NPWT were included. Medical records of patients undergoing cardiac surgery, with or without prosthesis implantation, within 30 days prior to surgery that presented SSI, and those who remained with NPWT for less than 72 hours were excluded.

Study protocol
A list of NPWT installation procedures was obtained through the Medical and Hospital Information Service of the hospital in question, which guided the search for medical records, and found 559 records. After excluding the duplicate records and applying the inclusion and exclusion criteria, 117 medical records were selected. Data collection was performed through a form prepared by the authors, containing information on clinical characteristics, health history, preoperative origin and preparation, characteristics of the anesthetic-surgical procedure, postoperative situation, surgical incision, culture and sensitivity profile, antimicrobial use and NPWT use.
Prior to the actual data collection, a pilot test was performed to adjust the proposed data collection instrument to the established objectives. Three medical records of patients who underwent cardiac surgery prior to the delimited period for this investigation were used and were therefore not included in the sample.
With regard to preoperative preparation, the routine employed by the institution consists of a bath with 2% degenerating chlorhexidine and trichotomy with electric trichotomizer (if necessary) at most two hours before surgery; antibiotic prophylaxis administered in anesthetic induction according to hospital standardization (Cefuroxime); and skin preparation with degerming 2% chlorhexidine and 0.5% alcoholic chlorhexidine before surgical incision. In addition, the sterile dressing is maintained for 24 hours after surgery.

Analysis of results, and statistics
The data obtained were tabulated in a database using Microsoft Excel software and analyzed with the help of a statistical professional using the Statistical Package for the Social Sciences (SPSS) software, 21.0.
Quantitative variables were analyzed by means and standard deviations and categorical variables by frequency distribution. The Kruskal-Wallis test was used to evaluate the relationship between SSI severity and time of therapy use and number of exchanges performed. Fisher's exact test measured the correlation between the severity of SSI and Diabetes Mellitus (DM). Pearson's test was used to verify the association between the of Negative pressure wound therapy in the treatment of surgical site infection in cardiac surgery Oliveira MC, Andrade AYT, Turrini RNT, Poveda VB.
time of therapy use and obesity, as well as the presence of microorganisms detected in the culture. The Wilcoxon-Mann-Whitney test was used to test whether the duration of therapy was related to readmission and discomfort. The significance level adopted was p ≤0.05.

RESULTS
A total of 117 patients who used NPWT as a result of sternal SSI were included. The sample consisted predominantly of women, with a mean age of 59.9 (±13.4) years. Regarding lifestyle habits, 6.8% of the individuals were active smokers, 29.9% former smokers, 21.2% were alcoholics, with a mean Body Mass Index (BMI) of 29.2 (±5.9) kg/m². Most of the population (98.3%) had chronic diseases, the most prevalent being: heart disease, Systemic Arterial Hypertension (SAH), DM and obesity (Table 1).
No statistical association was observed regarding obesity (p = 0.120) or presence of DM (p=0.4504) and severity of SSI.
Most patients were admitted through the inpatient unit (IU), taking a mean of 6.2 (± 8.9) days from admission to cardiac surgery ( Table 1).
The most commonly performed surgical procedure was coronary artery bypass grafting (CABG) (57; 48.7%), followed by valve surgery (37; 31.6%). The use of cardiopulmonary bypass (CPB) during the surgical procedure was performed in 92.3% of the situations; with a mean of 101.2 (±52.1) minutes of CPB (p=0.740) and 78.2 (±36.7) minutes of anoxia (p=0.868), with no statistical significance regarding the type of SSI. The hospital readmission rate for SSI was 48.7% and death equal to 20.5% (Table 1). It was not possible to infer statistical association between length of stay (p=0.383) and readmission time (p=0.201) with SSI classification.
Regarding the classification of SSI, the deep incisional type prevailed (88; 75.2%), followed by organ/space (27; 23.1%) and superficial incisional SSI (2; 1.7%). The characteristics presented by the patients that determine each type of SSI are shown in Table 2.
NPWT was used on mean for 16 (±9.5) days in each patient, with minimum use time equal to five days and maximum time of 64 days, and on mean 2.2 (± 0.8) exchanges. Exudate drained during therapy has often been described as serohematic (58.4%). More than half (53.8%) of the individuals had discomfort related to therapy, the main one being pain (93.6%). Only 1.7% had complications resulting from the use of therapy, which are considered as occurrences requiring immediate invasive intervention by the health team, such as hemorrhage (Table 3).  The duration of use of NPWT was related to the severity of SSI, that is, the more severe the longer the therapy (p=0.010), as well as the number of system changes performed (p=0.045).
In all cases, culture and antibiogram were performed, using as secretion material and surgical wound (SW) fragment and pericardial fluid. Of these, 90.6% had microorganisms (MOs) detected in the sample, and the most commonly found genera were Staphylococcus (59.8%), Klebsiella (21.4%), and Candida (19.7%) ( Table 2). Those who had no positive culture were diagnosed with SSI through clinical signs. Staphylococcus (p=0.050), Klebsiella (p=0.040), Stenotrophomonas (p=<0.001), Proteus (p=0.011), and Enterobacter (p=0.013) were found to be associated with longer NPWT. In addition, the presence of Aspergillus (p=<0.001) was associated with longer hospital stays. The sensitivity and resistance profile of the isolated MOs is shown in Table 4.
The glycopeptide, carbapenems and aminopenicillin drugs were the most commonly used postoperatively (Table 5).

DISCUSSION
The results of the present study demonstrated that the time of use of NPWT and the amount of dressing changes is directly related to the severity of SSI. The therapy was used on mean for 16 (±9.5) days/patient and changed for 2.2 (±0.8) times. In addition, there was a statistical association between longer NPWT use and the occurrence of Staphylococcus, Klebsiella, Stenotrophomonas, Proteus, and Enterobacter-related SSIs, with a mean ICU stay of 14.9 days and a mean total hospital stay of 41.4 days.  ---1  7  2  20  -1  6  2  1  Carbapenems  S  ---1  4  4  10  -3  7  5  3  R  ---1  3  -15  ---1  5  Aminoglycosides  S  58  10  -2  4  4  23  -3  7  6  7  R  9  4  -1  4  -11  --3  1  1  Chloramphenicol  S  --6 -  Interestingly, this result is close to those found in previous studies in which the mean duration of therapy was longer for cases of deep incisional SSI, with treatment means ranging from 13 to 49.4 days (13,17,24) .
Unlike in this study, in which NPWT was reserved for more complicated infectious conditions, the National Institute of Cardiovascular Diseases in Slovakia has mostly used NPWT to treat less complex SSI, especially superficial incisional SSI, in 89% of the evaluated cases, followed by 10% deep and 1% organ/space (25) .
It is noteworthy that a recent integrative review analyzing eight retrospective observational studies comparing the use of NPWT with conventional dressings to treat mediastinitis concluded that NPWT was superior to other technologies. NPWT contributed to increased healing speed and low rates of reinfection, mortality or hospital readmission (23) .
Considering the multifactorial aspects involved in SSI, in general, the presence of associated comorbidities is an aggravating factor in the development and recovery of SSI. These include hypertension, diabetes mellitus, obesity and smoking, which in addition to systemically influencing metabolism, are still unfavorable for healing (25)(26) . Thus, when compared to patients who did not use therapy, the literature showed that the mean BMI and DM prevalence is significantly higher in those who used NPWT (13) .
National research conducted in Minas Gerais also found that the microorganisms most related to infection were Staphylococcus and Klebsiella, an aspect observed in other countries such as Slovakia and Japan (4,17,25,27) . In Poland, the most frequent infections were caused by Gram-negative rods, and the most used antimicrobial therapy was the cephalosporin class, followed by glycopeptides and carbapenems (13) .
Analyzing patients undergoing sternal surgeries, with SSI and not submitted to NPWT, it is observed that, on mean, these patients remained in the ICU 19.1 days and were hospitalized for periods between 33 and 67 days. NPWT is suggested to reduce ICU stay compared to conventional treatment (4,11) . Studies comparing the use of NPWT to another type of intervention found a significantly shorter ICU stay (28)(29) and a tendency to shorter length of stay in those treated by NPWT (28) .
The importance of post-discharge surveillance is emphasized, since the long interval between the date of surgery and the diagnosis of infection suggests that the patient is discharged before the onset of the first infectious signs and symptoms. This justifies the high readmission rates in the present study (48.7%), higher than those found in previous studies (23.8%) (26) . This is possibly also related to the fact that in the current investigation, the time elapsed between surgery and the onset of NPWT was approximately 35.5 days, a result higher than that found in the literature (mean 22 and 29.3 days) (13,17) .   A multicenter study comparing health care costs after cardiac surgery found that SSI patients cost healthcare services five times the amount spent among patients without infection (11) . Similarly, treatment costs for patients using NPWT are significantly higher than those who did not use NPWT (13) .
The prevalence of complications related to NPWT treatment, such as bleeding, was estimated at 1.7% in this study, which may be linked to the fact that the most prevalent therapy-related discomfort was pain, followed by bleeding. However, it is interesting to note that complications such as those found in the present study were considered absent in previous investigations (13,17) . Others are suggested, such as damage to adjacent blood vessels, granulation tissue hypertrophy and long-term use, increased anxiety, decreased quality of life, and malnutrition (30)(31) .
A study analyzing 73 medical records of patients who developed deep incisional SSI after cardiovascular surgery showed a mortality rate of up to 33% for patients using NPWT (27) . Such data may be associated with the severity of the clinical condition due to infection and, consequently, higher risk of death. However, this same study demonstrated that NPWT was associated with a significant reduction in in-hospital mortality caused by deep incisional SSI when compared to patients who did not use the therapy (27) .
Scientific literature indicates antimicrobial-associated NPWT as a first instance treatment for sternal SSIs that do not present with sternal instability (17,32) . There are also those that indicate its use in closed wounds in order to optimize treatment and healing, and prevent infections in those patients who already have risk factors for surgical incision complications (33) . In addition, early treatment and prevention strategies for SSI have been shown to reduce mortality (32) .
Finally, it is emphasized that much remains to be investigated to accurately establish criteria for therapy indication, potential complications, and especially to determine the quality of benefits associated with the use of NPWT in infected complex surgical wounds, such as those from cardiac procedures compared to treatments already employed.

Study limitations
Limitations include the analysis of a convenience sample, limited to a single specialized treatment center. It is believed that the replication of this study, comparing patient profiles across institutions, may provide further insights into the discussion of the therapeutic use of NPWT in complex wounds.

Contributions to nursing, health or public policies
The recognition of the characteristics related to the use of NPWT in this category of patients, especially the mean exchange, microorganisms of greater epidemiological relevance and complications may help nurses to make decisions regarding nursing care to patients during the treatment of SSIs.

CONCLUSIONS
NPWT was used for about 16 (±9.5) days/patient, with a mean of 2.2 (±0.8) exchanges and was mainly applied to the treatment of deep incisional and organ/space SSI. Only 1.7% had complications resulting from the use of therapy, although 53.8% had discomfort, especially pain.
The duration of use of NPWT was related to the severity of SSI and the number of changes made. Staphylococcus, Klebsiella, Stenotrophomonas, Proteus, and Enterobacter were found to be associated with longer NPWT.
Therefore, NPWT has been employed in cases of more severe infections, with few complications associated with therapy, but with discomfort for patients.