The critically ill patient, who is often elderly, immunosuppressed owing to trauma, malignant disease, or following solid-organ transplantation, already infected, or being administered broad-spectrum antibiotics (as treatment or prophylaxis), is a paradigm for the patient at risk of nosocomial infections. Therapeutic invasion of a normally sterile body cavity by virtue of surgery, invasive hemodynamic monitoring, or endotracheal intubation for anesthesia or prolonged ventilatory support also confers risk. The incidence of certain nosocomial infections, notably fungemia and Gram-positive bacteremia, has increased markedly within the past decade ( Ja^rvis

ef a/ 1.991; §311619.93.). The type of intensive care unit (ICU) being studied may influence the observed prevalence of infection. For example, pneumonia has now supplanted surgical wound infection as the most common nosocomial infection in surgical patients in the United States ( Jarvis.etaj, 1991), but catheter-related bacteremia is more common in both medical and pediatric units than in surgical ICUs. The cost of nosocomial infection is enormous in terms of morbidity and expenditure (Pittetetal 1994) for both therapy and the necessary surveillance programs.

An extensive 1-day point-prevalence study (VinceDt. . .. et,M 1995) was conducted to determine the prevalence of and risk factors for ICU-acquired infections, identify the predominant infecting organisms, and evaluate the relationship between ICU-acquired infection and mortality. A total of 1417 ICUs provided 10 038 patient case reports, of whom 4501 (44.8 per cent) were infected; nearly half of these (2064 or 20.6 per cent of the total population) had an ICU-acquired infection. Pneumonia accounted for nearly 50 per cent of the ICU-acquired infections, and bloodstream infections accounted for 12 per cent. The most frequently reported micro-organisms were enteric Gram-negative bacilli (34 per cent), Staphylococcus aureus (30 per cent, but 60 per cent of those strains were methicillin resistant compared with a prevalence of about 25 per cent overall), Pseudomonas aeruginosa (29 per cent), coagulase-negative staphylococci (19 per cent, usually Staphylococcus epidermidis), and fungi (17 per cent). Independent risk factors for ICU-acquired infection were an ICU stay of more than 48 h, mechanical ventilation, trauma, central venous catheterization, and stress ulcer prophylaxis (although the method was unstated).

The impact of nosocomial infection on mortality in a university hospital ICU was assessed by a prospective cohort study using standard definitions for nosocomial infections (BueDorCav.a.Di.llas e.L§L 1994). The crude mortality risk was 2.48 times higher in patients with a nosocomial infection than in non-infected patients. By stepwise logistic regression analysis, the severity-adjusted risk of death in infected patients was 2.1 times greater than in patients without such an infection. Thus nosocomial infections increase the risk of death by a factor of 3 to 3.5.

Patients in the ICU often have symptoms and signs of systemic infection, but have no identifiable source of infection. This constellation is now referred to as the systemic inflammatory response syndrome. The distinction between infection and systemic inflammatory response syndrome, which is the clinical manifestation of the host response to infection or inflammation, can be very difficult to make at the bedside. Identification of bacteria in a specimen may reflect mucosal colonization rather than infection; this is a particular problem in the diagnosis of pneumonia. Moreover, many clinical conditions, including burns, pancreatitis, and aspiration of gastric contents, provoke a major inflammatory response in which the tissue is usually sterile initially but may become infected later. The acceptance of any positive cultures as prima facie evidence of infection may lead to the overuse of antibiotics, which is both costly and leads to morbidity such as allergy, visceral organ dysfunction (e.g. aminoglycoside nephrotoxicity), development of antibiotic-resistant bacterial infections, and disruption of endogenous flora with emergence of opportunistic bacterial and fungal pathogens (e.g. Candida, Enterococcus, Clostridium difficile, and Pseudomonas).

Common sense, cost-consciousness, and concern for the microbial ecology of patients and the facilities in which they are cared for mandates that broad-spectrum antibiotics be reserved for treatment of serious infections. Increasing bacterial resistance to currently available antibiotics is cause for alarm. As in the case of the enterococci, the gene that confers vancomycin resistance has been transferred experimentally to staphylococci. Confirmation of the reported emergence of clinical strains of vancomycin-resistant staphylococci, which many authorities now consider to be purely a matter of time, could render these common and dangerous infections untreatable.

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