Update: Nosocomial Bacteremia in Critically Ill Patients
Special Feature
Update: Nosocomial Bacteremia in Critically Ill Patients
By Francisco Baigorri, MD, PhD, and Jordi Vallés, MD
Patients hospitalized in intensive care units (ICUs) are at high risk of developing bloodstream infections. The incidence rate of nosocomial bacteremia in the ICU ranges between 1.65 and 4.1 episodes per 100 admissions.1-3 This incidence rate in ICU patients can be up to seven times higher than in general ward patients; in total, 30-45% of all episodes of nosocomial bacteremia occur in critically ill patients.
Moreover, despite important progress in antibiotic therapy and in ICU management of critically ill patients, the fatality rate of ICU patients with nosocomial bacteremia remains high.1 However, few studies have adequately analyzed the infection in this selected population. This essay discusses some specific characteristics of bloodstream infections in ICU patients with reference to the leading sources of bacteremia, leading pathogens, and prognosis.
Sources of Nosocomial Bacteremia in the ICU
The source of bloodstream infections in an ICU can be identified in almost 75% of all episodes of bacteremia.1,3,4 The main sources of bloodstream infections in critically ill patients are catheter-related and lower respiratory tract infections. (See Table 2.)
| Table 2 | |||
| Sources of Nosocomial Bacteremia in the ICU | |||
| Source of bacteremia |
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| Lower respiratory tract |
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| Intravenous catheter |
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|
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| Urinary tract |
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|
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| Surgical wound |
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| Intra-abdominal infection |
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| Other |
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| Unknown |
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| ___________________________________________________________ | |||
Pathogens Responsible for Nosocomial Bacteremia in the ICU
Bloodstream infections caused by gram-positive microorganisms are increasing in number.1-3 Gram-positive microorganisms are now the major pathogens in cases of monomicrobial bacteremia (? 50%). Gram-negative microorganisms are identified in 30-40% of episodes. Fungemia represents less than 10% of the cases of nosocomial bacteremia in the ICU. Polymicrobial episodes are relatively common (10-20%). (See Table 3.)
| Table 3 | ||||||
| Pathogens Responsible for Nosocomial Bacteremia in ICU | ||||||
| Author (Ref. #) |
|
|
Fungi | |||
| Pittet (1) |
|
Coagulase-negative staphylococci
Staphylococcus aureus Enterococci |
39% | Enterobacter spp.
Klebsiella pneumoniae Serratia marcescens |
9% | Candida spp. |
| Rello (4) |
|
Coagulase-negative staphylococci
Staphylococcus aureus Enterococci |
40% | Pseudomonas aeruginosa
Escherichia coli Serratia marcescens |
5% | Candida spp. |
| Vallés (3) |
|
Coagulase-negative staphylococci
Staphylococcus aureus Enterococci |
|
Pseudomonas aeruginosa
Acinetobacter baumannii Klebsiella pneumoniae |
4% | Candida spp. |
| ____________________________________________________________________________________ | ||||||
Prognosis of Nosocomial Bacteremia in the ICU
Bloodstream infections in the ICU are associated with a crude mortality that ranges between 30-80%.1-5 The values of mortality directly associated with the episodes are lower, but still high (19-23%).3,4 In addition, in case-control studies, the mortality rate directly attributable to the bloodstream infection is approximately 30%.1,5 This means that the underlying disease makes a significant contribution to the crude mortality from bloodstream infection. Crude mortality is also associated with the severity of the patient’s condition at the time of bacteremia and the grade of systemic response.3 However, other microbiological and therapeutic variables may influence the outcome of critically ill patients with nosocomial bacteremia (related mortality): the source of the bacteremia, the type of microorganism involved, and the appropriateness of antibiotic treatment.
The Source of Nosocomial Bacteremia as a Prognostic Factor
Studies analyzing the outcome of nosocomial bacteremia consistently show that episodes complicating lower respiratory tract or intra-abdominal infections are more frequently associated with a fatal outcome than episodes of bacteremia secondary to other sites.2,4,6,7 Moreover, the influence on prognosis of the source of bacteremia remains when corrected by the severity of illness (as assessed by the APACHE II score).3,8
It is also well known that the grade of systemic response to the infection is an important prognostic factor of nosocomial bacteremia. The presence of septic shock is associated with higher crude and directly associated mortality. Since catheter-related bacteremia is associated with a significantly lower incidence of septic shock than bloodstream infections of an abdominal source, this may explain the difference in prognosis according to the source of the bacteremia. However, when the episodes of bacteremia with the same grade of systemic response to the infection are studied, related mortality again varies according to the source of the infection.3,8
In fact, the source of the infection is an important prognostic factor even in the absence of bacteremia.9,10 Gross and colleagues9 studied a group of patients who died as a consequence of nosocomial infection, finding that lower respiratory tract infection was involved in 60% of those cases but urinary infection was present in only 17%.
The Type of Microorganism Involved as a Prognostic Factor
Gram-negative bacteremias are associated with higher related mortality.3 This is to be expected, in view of the higher virulence of these microorganisms, which present a higher incidence of severe sepsis and septic shock, and a higher incidence of bacteremias originating from sources other than a catheter.
In addition, the differences in the microorganisms involved according to the source of the bacteremia may explain the prognostic influence of the source of the bacteremia. However, when separated according to type of microorganism isolated, bacteremias secondary to lower respiratory tract infections present the highest mortality, and bacteremias originating from a catheter present lower mortality.3,11
Appropriate Antibiotic Treatment as a Prognostic Factor
Empirical antibiotic treatment has been considered as one of the main factors influencing prognosis of episodes of bacteremia.3,12 In a multicenter study with 590 consecutive episodes of nosocomial bacteremia in ICUs,3 related mortality among patients who received inappropriate empirical antibiotic treatment was twice that in the group who received appropriate empirical treatment. (Empirical therapy for bacteremia was considered inappropriate when the microorganisms isolated in the bloodstream were not susceptible to the antimicrobial agents administered, or if the patient was not receiving empirical antimicrobial agents before the blood culture result was known.)
Also, in this case, even in the group of patients who receive appropriate empirical antibiotic therapy, related mortality differs according to the source of the infection. Bacteremias secondary to lower respiratory tract infections consistently have the highest mortality, and bacteremias originating from a catheter present low mortality.
Practical Remarks
Underlying diseases and the severity of the patient’s condition markedly influence crude mortality among ICU patients with nosocomial bacteremia. Furthermore, immediate prognosis after an episode of nosocomial bacteremia correlated with the grade of systemic response, the type of microorganism involved, and the different sources of bacteremia.
Intravascular catheters and lower respiratory tract infection are the main sources of bacteremia in these patients. Improving the prevention of catheter infection and nosocomial pneumonia is therefore a primary objective in attempts to reduce the incidence of bacteremia in the ICU.
Finally, the management of the systemic response to the infection and the appropriateness of antibiotic treatment are the most important modifiable variables affecting the outcome for patients with nosocomial bacteremia in ICU.
(Dr. Vallés is Staff Intensivist, Hospital Parc Tauli, Sabadell, Univ Autonoma de Barcelona, Spain. He was the leader of the Spanish Collaborative Group for Infections in Intensive Care Units of the Spanish Society of Intensive and Critical Care Medicine, and Coronary Care Units—SEMICYIUC—carrying out a multicenter study evaluating epidemiology and prognosis of nosocomial bacteremia in critically ill patients.3)
References
1. Pittet D, Tarara D, Wenzel RP. Nosocomial bloodstream infection in critically ill patients. Excess length of stay, extra costs, and attributable mortality. JAMA 1994;271:1598-1601.
2. Brun-Buisson C, Doyon F, Carlet J, and the French bacteremia-sepsis study group. Bacteremia and severe sepsis in adults: A multicenter prospective survey in ICUs and wards of 24 hospitals. Am J Respir Crit Care Med 1996;154:617-624.
3. Vallés J, León C, Alvarez-Lerma F. Nosocomial bacteremia in critically ill patients: A multicenter study evaluating epidemiology and prognosis. Clin Infect Dis 1997;24:387-395.
4. Rello J, Ricart M, Mirelis B, et al. Nosocomial bacteremia in a medical-surgical intensive care unit: Epidemiologic characteristics and factors influencing mortality in 111 episodes. Intensive Care Med 1994;20:94-98.
5. Smith RL, Meixler SM, Simberkoff MS. Excess mortality in critically ill patients with nosocomial bloodstream infections. Chest 1991;100:164-167.
6. Bryan CS, Reynolds KL. Bacteremic nosocomial pneumonia. Am Rev Respir Dis 1984;129:668-671.
7. Gatell JM, et al. Nosocomial bacteremia in a large Spanish teaching hospital: Analysis of factors influencing prognosis. Rev Infect Dis 1988;10:203-210.
8. Wong DT, Wagner DP, Knaus WA. Lower ICU mortality in septic shock due to urosepsis compared to non-urosepsis. Anesthesiology 1992;77:A318.
9. Gross PA, et al. Deaths from nosocomial infections: Experience in a university hospital and community hospital. Am J Med 1980;68:219.
10. Bueno Cavanillas A, et al. Influence of nosocomial infection on mortality rate in an intensive care unit. Crit Care Med 1994;22:55-60.
11. Roberts FJ, Geere IW, Coldman A. A three-year study of positive blood cultures, with emphasis on prognosis. Rev Infect Dis 1991;13:34-46.
12. Weinstein MP, et al. The clinical significance of positive blood cultures in the 1990s: A prospective comprehensive evaluation of the microbiology, epidemiology, and outcome of bacteremia and fungemia in adults. Clin Infect Dis 1997;24:584-602.
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