Mediastinitis 

Mediastinitis is a life-threatening condition with an extremely high mortality rate if recognized late or treated improperly.^{[1, 2, 3]} Although long recognized as a complication of certain infectious diseases, most cases of mediastinitis are associated with cardiac surgery (>300,000 cases per year in the United States).^[3] This complication affects approximately 1-2% of these patients. Although small in proportional terms, the actual number of patients affected by mediastinitis is substantial. This significantly increases mortality and cost. After 10 years of evolution, the optimal therapy for mediastinitis is more clearly understood.

Frequency

In the United States, mediastinitis most commonly occurs in the postoperative setting following coronary bypass grafting.^[3] The incidence rate is 1-2% at most large surgical centers; however, certain subsets of patients, such as patients who have undergone a heart transplant, are at much higher risk.

• Postoperative

  • Most cases of mediastinitis in the United States occur following cardiovascular surgery.
  • Risk factors for the development of mediastinitis include the following:
    • In general, the use of pedicled bilateral internal mammary artery (BITA) grafts carries increased risk for mediastinitis after coronary artery bypass graft (CABG).^[4] This risk is even higher among patients with diabetes, thus rendering many surgeons reluctant in using BITA grafting in this subgroup of patients. However, the use of skeletonized BITA grafts may reduce this risk, patients with and without diabetes could be considered for skeletonized BITA.^[5]
    • Emergency surgery^[6]
    • External cardiac compression (conventional cardiopulmonary resuscitation)
    • Obesity (>20% of ideal body weight)^[7]
    • Postoperative shock, especially when multiple blood transfusions are required^[8]
    • Prolonged bypass and operating room time^{[6, 7]}
    • Reoperation^{[9, 6, 7]}
    • Reexploration following initial surgery
    • Sternal wound dehiscence
    • Surgical technical factors (eg, excessive use of electrocautery, bone wax, paramedian sternotomy)^[6]
• Esophageal perforation^{[10, 3]}
• Trauma, especially blunt trauma to the chest or abdomen
• Tracheobronchial perforation, due to either penetrating or blunt trauma or instrumentation during bronchoscopy
• Descending infection following surgery of the head and neck, great vessels, or vertebrae^[11]
• Progressive odontogenic infection (Ludwig angina)
• Mediastinal extension of lung infection
• Chronic fibrosing mediastinitis due to granulomatous infections^[12]
• Microbiology^[9]

  • See the image below.The frequency of various microbiological pathogens isolated in cases of postoperative mediastinitis.
  • Most mediastinitis cases occur after cardiac surgery and indicate gram-positive cocci,^[6] with Staphylococcus aureus^[13] and Staphylococcus epidermidis accounting for 70-80% of cases. Mixed gram-positive and gram-negative infections account for approximately 40% of cases. Isolated gram-negative infections are rare causes.
  • Fibrosing mediastinitis is most commonly associated with Histoplasma capsulatum and Mycobacterium tuberculosis, although mediastinitis is an extremely rare complication of these infections.^[12]

Infection from either bacterial pathogens or more atypical organisms can inflame any of the mediastinal structures, causing physiological compromise by compression, bleeding, systemic sepsis, or a combination of these.

The origin of infection following open heart operations is not known in most patients. Some believe that the process begins as an isolated area of sternal osteomyelitis that eventually leads to sternal separation. Others hold that sternal instability is the inciting event, and bacteria then migrate into deeper tissues. Inadequate mediastinal drainage in the operating room may also contribute to the development of a deeper chest infection. The patient's own skin flora and the bacteria in the local surgical environment are possible sources of infection. Because some bacterial contamination of surgical wounds is inevitable, host risk factors are likely critical in promoting an active infection.

History

Mediastinitis manifests within a spectrum that ranges from the subacute patient to the fulminate critically ill patient who requires immediate intervention in order to prevent death.

The typical postoperative patient presents with fever, high pulse, and reports suggestive of a sternal wound infection such as sternal instability. Approximately two thirds of patients present within 14 days following surgery. Although a delay of months is occasionally observed, signs or symptoms typically develop within 1 month of the operation. Patients may report sternal pain that has increased since surgery, drainage from the wound site, an audible click due to sternal nonunion, and progressive redness over a variable period.

Physical

Vital signs generally may show tachycardia and fever. In more advanced cases of sepsis, hypotension may be present and the patient may require large volumes of crystalloid or vasopressor medication for support.

The Hamman sign is a crunching sound heard with a stethoscope over the precordium during systole. Its presence should alert the clinician to possible mediastinitis, although its absence does not change the probability of disease.

Direct signs of sternal infection may be among the initial presenting signs or may be delayed until after the diagnosis is already considered. Sternal pain, instability, or click; local cellulitis; and drainage can all be observed.

Distinguishing between a superficial wound infection and a deeper chest infection associated with mediastinitis can be challenging. Systemic signs of sepsis strongly suggest mediastinal involvement. Local wound exploration should be utilized as a mechanism to distinguish a superficial wound infection from a deep sternal wound infection.

Operative exploration includes reopening the previous sternotomy and debridement of necrotic and infected tissue. The sternum is separated from the ventricle bypass grafts and the aorta carefully to not cause bleeding. Cultures are sent to direct antibiotic therapy. Wound closure is usually delayed until reasonable control of infection is achieved; however, some surgeons perform closure with muscle flaps at the initial debridement with good results. Delayed closure is usually accomplished with muscle flaps (pectoralis, rectus) and recently is aided by vacuum-assisted closure.^{[1, 14, 15]}

Sterile sternal dehiscence, which is described as a sternal nonunion, is usually not treated. Occasionally, patients abruptly separate their sternum in close proximity to cardiac surgery, necessitating sternal reclosure. Also, some have extreme pain or cannot tolerate the clicking and discomfort of the nonunion and require sternal reclosure.

The portion of the thorax defined as the mediastinum extends from the posterior aspect of the sternum to the anterior surface of the vertebral bodies and includes the paravertebral sulci when defining the location of specific mediastinal masses. It is limited bilaterally by the mediastinal parietal pleura and extends from the diaphragm inferiorly to the level of the thoracic inlet superiorly.

Traditionally, the mediastinum is artificially subdivided into 3 compartments for better descriptive localization of specific lesions. When the location or origin of specific masses or neoplasms is discussed, the compartments or spaces are most commonly defined as the anterior, middle, and posterior.

The anterior compartment extends from the posterior surface of the sternum to the anterior surface of the pericardium and great vessels. It normally contains the thymus gland, adipose tissue, and lymph nodes. The physiology of the anterior mediastinum includes the lymphatics and thymus gland. The physiology of the middle mediastinum includes the bronchi, the heart and pericardium, the hila of both lungs, the lymph nodes, the phrenic nerves, the great vessels, and the trachea. The physiology of the posterior mediastinum includes the azygos vein, the descending aorta, the esophagus, the lymph nodes, the thoracic duct, and the vagus and sympathetic nerves.

For simple sternal dehiscence (postoperative mediastinitis), take great care to exclude active infection before rewiring the sternum.

Surgery is seldom recommended for cases of chronic fibrosing mediastinitis unless compression of the major mediastinal structures has occurred.

In cases of sternal nonunion, surgery should be deferred except when patients have extreme pain.