INFECTIONS IN THE ICU
Infections are a leading cause of death in ICUs. Seri-ous infections may be “community acquired” or sub-sequent to hospital admission for an unrelated illness. The term nosocomial infection describes hospital-acquired infections that develop at least 48 h follow-ing admission. The reported incidence of nosocomial infections in ICU patients has ranged between 10% and 50%, but with recent attention to aseptic place-ment of central venous catheters and earlier removal of bladder catheters the incidence of bloodstream infections has markedly declined. Nearly universal elevation of the head of bed has also led to a marked reduction in ventilator-associated pneumonia.
Strains of bacteria resistant to commonly used antibiotics are often responsible for infections in patients with critical illness. Host immunity plays an important role in determining not only the course of an infection but also the types of organ-isms that can cause infection. Thus, organisms that normally do not cause serious infections in immu-nocompetent patients can produce life-threatening infections in those who are immunocompromised (Table 57–11).
Critically ill patients frequently have abnormal host defenses from advanced age, malnutrition, drug therapy, loss of integrity of mucosal and skin barriers,and underlying diseases. Thus, age greater than 70 years, corticosteroid therapy, chemotherapy of malignancy, prolonged use of invasive devices, respiratory failure, kidney failure, head trauma, and burns are established risk factors for nosocomial infections. Patients with burns involving more than 40% of body surface area have signifi-cantly increased risk of mortality from infections. Topical antibiotics delay but do not prevent wound infections. After burns, early removal of the necrotic eschar followed by skin grafting and wound closure appears to reverse immunological defects and reduce infections.
Most nosocomial infections arise from the patient’s endogenous bacterial flora. Furthermore, many critically ill patients eventually become colo-nized with resistant bacterial strains. Infections of the urinary tract account for many nosocomial infections. Urinary infections are usually due to gram-negative organisms and are typically asso-ciated with the indwelling catheters or urinary obstruction. Community-acquired and ventilator-associated pneumonias are problems in the ICU. Intravascular catheter-related infections are now relatively rare causes of ICU infections. Surgical site and other wound infections are, however, seen.
Nosocomial pneumonias are usually caused by gram-negative organisms. Gastrointestinal bacterial overgrowth with translocation into the portal cir-culation and retrograde colonization of the upper airway from the gastrointestinal tract as a result of aspiration are possible mechanisms of entry for these bacteria. Preservation of gastric acidity inhibits over-growth of gram-negative organisms in the stomach and their subsequent migration into the orophar-ynx. Tracheal intubation does not provide effec-tive protection because patients commonly aspirate gastric fluid containing bacteria despite a properly functioning cuff; nebulizers and humidifiers can also be sources of infection. Selective decontamination of the gut with nonabsorbable antibiotics may reduce the incidence of infection but does not change out-come. Elevating the head of the bed more than 30o reduces the likelihood of ventilator-associated pneu-monia. Enteral nutrition reduces bacterial translo-cation across the gut and reduces the likelihood of sepsis .
Wounds are common sources of sepsis in post-operative and trauma patients; restricting antibiotic prophylaxis to the immediate perioperative time appears to decrease the incidence of postoperative infections in some groups of patients. Although more commonly seen in postoperative patients, intraabdominal infections due to perforated ulcer, diverticulitis, appendicitis, and acalculous chole-cystitis can also develop in critically ill nonsurgical patients. Intravascular catheter-related infections are most commonly caused by Staphylococcus epi-dermidis, Staphylococcus aureus, streptococci, Can-dida species, and gram-negative rods. Bacterialsinusitis may be an unrecognized source of sepsis in patients ventilated through nasotracheal tubes. The diagnosis is suspected from purulent drainage and confirmed by imaging and cultures.
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