More than 600,000 patients undergo cardiac procedures each year. Those who have a median sternotomy, or midline chest incision, are at risk for mediastinitis, an uncommon but potentially fatal deep wound infection.
Occurring in about 5% of patients who undergo sternotomy, mediastinitis increases morbidity and lengthens hospital stays. Also called deep sternal wound infection, mediastinitis may progress to osteomyelitis, sternal dehiscence, sepsis, or right ventricular rupture. Up to 47% of patients who develop it die.
Here's what you need to know about the causes of mediastinitis, signs and symptoms, prevention, and treatment options. For anatomic details, see Chest the facts.
Sternal separation
Why does mediastinitis develop? Researchers believe these events open the door to infection:
* A localized area of sternal osteomyelitis extends, separating the sternum.
* Separation of the sternum leads to skin breakdown and bacterial contamination of deep tissues.
* If the mediastinum isn't sufficiently drained after surgery, large amounts of fluids can collect in the retrosternum and contribute to a deep infection.
However, new investigations suggest that mediastinitis also is the result of a systemic inflammatory reaction involving the activation of complement, cytokines such as chemokines and interleukins, and other cell-derived proteins.
Mediastinitis may occur as early as 3 days postoperatively or as late as 3 months, but it typically occurs between postoperative days five and seven. Depending on the causative organism, the wound may or may not look infected.
Patients with chronic diseases and those undergoing lengthy surgeries are at particular risk for mediastinitis. (See Who's at risk?) In some patients, fever, leukocytosis, and a positive blood culture may be the warning signs of an impending wound infection. Minor or superficial infections may occur first; you'll note localized erythema and tenderness. The sternal incision may produce serous drainage, or you may find a localized area of tissue breakdown at the incision site with purulent drainage. The sternum usually is stable.
In some patients, purulent drainage is the first sign of trouble; between 70% and 90% of these patients have an unstable sternum. The patient also may have fever, chills, lethargy, leukocytosis, wound pain, and chest wall pain. (Pain from the infection can be masked by postoperative pain.) Extensive cellulitis may surround the sternal incision.
Suspect a serious sternal wound infection if your patient has chest wall pain or a slow recovery not explained by other conditions, Gram-positive bacteremia, leukocytosis, or fever. The infected area may involve any or all areas of the mediastinum, including the surgical cannulation and anastomosis sites.
Obtain specimens for blood culture if your patient develops a temperature above 100.4 F (38 C) after the first 48 postoperative hours. The initial chest X-ray may appear normal, but serial X-rays may be useful. As the infection develops, X-rays may reveal air in the mediastinum and a "sternal stripe" (a vertical sternal lucency), indicating that air is separating the two halves of the sternum. A computed tomography (CT) scan may reveal the loss of integrity of soft tissue in the retrosternum, bilateral pleural effusions, or bone destruction. It may also show fluid in the normally air-filled retrosternal space. (Interpretive skills are required because it may be difficult to distinguish hematoma formation from an infection.) Needle aspiration of this fluid guided by CT may help with the diagnosis. Radionucleotide scans may also help identify the presence and location of infection.
Up to 50% of sternal wound infections are related to Gram-positive bacteremias. The cause of most mediastinal infections is Staphylococcus aureus, including methicillin-resistant S. aureus; other causative organisms include coagulase-negative staphylococci, Staphylococcus epidermidis, Enterobacter, Enterococcus, Klebsiella, Pseudomonas, and Serratia. When secondary bacteremias occur, they're typically associated with S. aureus. Mixed infections and fungal infections are rare.
Treatment priorities
The first step in treating mediastinitis is to assess the degree of sternal stability and classify the infection by type. (See Defining mediastinitis.)
Sternal preservation is a primary goal of treatment for a deep wound infection such as mediastinitis. If surgery is performed early, before the sternum deteriorates (type I mediastinitis), the procedure may consist of debridement of the sternal edges, exposing fresh bone and cartilage, and rigorous irrigation of the mediastinum with placement of drainage catheters. The sternum is rewired, the incision is closed, and the patient is put on antibiotic therapy.
More serious infections (generally types IV or V) are treated with aggressive sternal debridement and wound packing with moist gauze that's changed frequently. In the worst cases, the sternum is removed.
When the infection is under control, the surgeon (usually a plastic surgeon) will use muscle and skin flaps to close the incision by secondary intention. The bone may be approximated, depending on the degree of sternal resection required. He may use a technique known as the Robicsek procedure to give the weakened sternum additional support.
The newest method of treating mediastinitis involves vacuum-assisted closure. The wound is debrided and a polyurethane foam is applied. An evacuation tube, vacuum pump, and transparent drape complete the dressing. Negative pressure is applied to the wound to drain fluid, inhibit bacterial colonization, stimulate tissue granulation, and reduce the frequency of dressing changes. Delayed primary closure may occur, or the wound may be prepared for secondary closure with flaps.
Patients must be carefully selected for negative-pressure wound therapy: If arteries are lying superficially in the wound bed, negative pressure can cause arterial thrombosis. This method may require more days of therapy to achieve healing and remains under investigation.
What if my patient needs CPR?
When performing cardiopulmonary resuscitation (CPR) on a patient with an unstable sternum, you need to equally distribute the pressure of chest compressions across the entire chest wall, rather than applying pressure directly over the sternum. The best way to do this is to use a fracture bedpan, which can be easily placed on the chest to disperse the pressure of compressions while protecting the sternum. Place the bedpan inside a pillowcase, then place it sideways with the open end over the midsternum and the lipped end on the right side of the chest. The smooth bottom of the pan should be against the patient's chest; place your hands in the pan opening to perform CPR.
Perioperative precautions
What can you do to reduce your patient's risk of mediastinitis? Take these steps before surgery:
* Identify and treat any preexisting infections, such as urinary tract infections or pneumonia.
* For patients undergoing elective surgery, use a nasopharyngeal swab to determine if they're carriers of methicillin-resistant staphylococci. Treat patients who are carriers with mupirocin (Bactroban) nasal preoperatively, as ordered.
* In the operating room (OR), if necessary, the patient's hair at the incision site will be clipped (not shaved) immediately before surgery.
* Provide an antibacterial skin wash the night before surgery. Before surgery, in the OR, the patient's skin will be prepared with a topical antiseptic containing chlorhexidine.
* Administer prophylactic antibiotics 30 to 60 minutes before the time of incision, as ordered. For more details, see "Best-Practice Interventions: Preventing Surgical Site Infections" in the June issue of Nursing2006.
During surgery, these steps reduce infection risks:
* reducing OR traffic
* using laminar airflow ventilation in the OR
* maintaining meticulous surgical technique, including the use of double-glove barrier techniques for the OR team
* using a midline sternotomy incision (as opposed to a "lazy S" incision) with secure sternal closure
* using subcuticular sutures and a topical skin adhesive instead of staples for skin closure
* avoiding the use of bone wax to establish hemostasis and control bleeding. Bone wax provides a culture medium for bacteria, particularly S. aureus, and also may embolize to the lungs.
* minimizing operative time and the use of electrocautery, which can make tissue more susceptible to infection.
After surgery, you can help reduce your patient's risk of infection by:
* maintaining tight glycemic control (blood glucose level of 80 to 110 mg/dL), especially in patients with diabetes. Blood glucose levels over 180 mg/dL are associated with cellular damage. Administer a continuous insulin infusion to avoid the peaks and troughs in blood glucose that can occur with subcutaneous insulin.
* maintaining the sterile surgical dressing for the first 24 to 48 hours postoperatively, unless gross drainage is evident
* avoiding homologous blood transfusions, which may increase the possibility of viral or bacterial infection
* discontinuing prophylactic antibiotics 24 hours after incision closure, in accordance with the Centers for Disease Control and Prevention's guidelines for surgical site infection prevention for open-heart surgery. Continued use of antibiotics following the initial postoperative period should be based upon suspected or confirmed infection.
Staying vigilant
By understanding how to recognize mediastinitis, minimize risks, and respond to problems, you can help your patient avoid serious postoperative complications.
Chest the facts
The mediastinum, the area between the right and left pleural cavities, is bordered by the diaphragm and the thoracic inlet. The middle mediastinum, the area affected by the midline sternotomy incision, includes the heart, the aorta and aortic arch, vena cavae, main pulmonary arteries and veins, phrenic nerves, the tracheal bifurcation and the main bronchus, the hilum of each lung, the esophagus, and lymph nodes. Blood is supplied to the area primarily by the left and right internal thoracic mammary arteries.
Who's at risk?
Patient risk factors for mediastinitis include obesity, diabetes (especially if uncontrolled), chronic obstructive pulmonary disease (COPD), smoking, renal dysfunction, peripheral vascular disease, advanced age, impaired nutritional status, advanced heart failure (New York Heart Association class IV), and preexisting infection. Patients with a tracheostomy, those with a preoperative hospital stay of more than 5 days, and women also are at higher risk for mediastinitis.
Intraoperative risk factors include emergency surgery, repeat surgery, surgery lasting more than 4 hours, cardiopulmonary bypass lasting more than 2 hours, cross-clamping of the aorta, intra-aortic balloon pump therapy, and use of both internal mammary arteries (especially in patients with diabetes).
Postoperative risk factors include excessive mediastinal bleeding, reexploration for bleeding, transfusions of more than five units of blood, prolonged ventilatory support, delayed sternal closure, low cardiac output requiring inotropic support, long ICU stay, postoperative tracheostomy, and infection at another site.
The more risk factors the patient has, the greater the risk of developing mediastinitis. You can estimate his risk using one of two tools. The Pre-operative Estimation of Risk of Cerebrovascular Accident and Mediastinitis, developed by the Northern New England Cardiovascular Disease Study Group, assigns weighted values to patient characteristics, such as diabetes and COPD. The higher the score, the higher the patient's risk of developing mediastinitis.
A second tool is the National Nosocomial Infections Surveillance risk index, which is based on three variables proven to be predictive of surgical site infections: the intrinsic degree of microbial contamination at the surgical site, the duration of the operative procedure, and host susceptibility. Patients who score more than 2 on the 0 (no risk)-to-3 (highest risk) scale are at increased risk for mediastinitis.
Defining mediastinitis
According to the Centers for Disease Control and Prevention, an infection is considered mediastinitis if it meets one or more of the following criteria:
* Organisms are cultured from mediastinal tissue or fluid obtained during a surgical operation or needle aspiration.
* The patient has evidence of mediastinitis seen during surgery of histopathologic examination.
* The patient has at least one of the following signs and symptoms that can't be attributed to another cause: fever (greater than 100.4[degrees] F [38[degrees] C]), chest pain, or sternal instability. The patient also has at least one of the following: purulent discharge from the mediastinal area, organisms cultured from blood or discharge from mediastinal area, or mediastinal widening on X-ray.
Mediastinitis can be classified as follows:
* type I-developing within 2 weeks of surgery in a patient with no risk factors
* type II-developing 2 to 6 weeks after surgery in a patient with no risk factors
* type III-types I and II in patients with risk factors
* type IV-type I, II, or III after one or more failed therapeutic trial
* type V-developing more than 6 weeks after surgery.
At St. Joseph Mercy Oakland Hospital in Pontiac, Mich., Diane M. Bosen is a clinical nurse specialist in critical care and Soundra D. Mackavich is an advanced thoracic cardiac surgery physician extender.
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