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* From the Veterans Affairs Medical Center (Dr. Singh), Pittsburgh, PA; and University of Pittsburgh (Dr. Yu), Pittsburgh, PA.
Correspondence to: Victor L. Yu, MD, VA Medical Center, Infectious Disease Section, University Drive C, Pittsburgh, PA 15240; e-mail: vly+{at}pitt.edu
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Abstract |
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 TOP Abstract IntroductionReferences |
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Key Words: antibiotics • ICU • nosocomial pneumonia
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Introduction |
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TOPAbstractIntroductionReferences |
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We present data to show that the ecology of potential pathogens, the predominant types of infections, and specific patient populations at risk for infection are unique to individual ICUs. Antibiotic recommendations based on studies performed at a few selected centers may, therefore, have limited widespread applicability and may not be generalizable to all ICU settings. Concepts, but not practices, can be extrapolated from such studies. We propose that research in individual ICUs is essential in guiding antibiotic prescription practices.
The spectrum of potential pathogens and the predominant bacterial flora can vary considerably in different ICU settings. Pseudomonas aeruginosa has emerged as the predominant pathogen in cases of nosocomial pneumonia in many ICUs. However, the proportion of the cases of pneumonia that are due to P aeruginosa varies widely from 6 to 31% at various institutions.6 9 10 11 12 13 14 Of patients with nosocomial pneumonia in whom a bacteriologic diagnosis was established, Acinetobacter spp accounted for 25% of the episodes in one report11 and none in others.12 15 Indeed, Acinetobacter was the single most frequently isolated bacteria in patients with ventilator-associated pneumonia in the former study.11 Enterobacter cloacae accounting for 26% of all cases, was the most common pathogen in ventilator associated pneumonia in one study.16 Twenty-seven percent and 11%, respectively, of the cases of ventilator-associated pneumonia in two other studies were also due to Enterobacter spp; only P aeruginosa was isolated more often at these institutions.9 12 By contrast, none of the cases of pneumonia in other ICU studies have been due to Enterobacter.11 15
Methicillin-resistant Staphylococcus aureus (MRSA) has become established as an endemic pathogen in many hospitals and has emerged as a significant cause of nosocomial pneumonia in ICUs at these institutions. Thirty-seven percent of the cases of pneumonia occurring in ICU patients in one report were due to MRSA.15 MRSA and P aeruginosa were the foremost pathogens discovered in cases of nosocomial pneumonia occurring in liver transplant recipients who were being cared for in that ICU.17 Legionella has been documented to be an important nosocomial pathogen for immunocompromised patients in selected studies. Two to 9% of the cases of nosocomial pneumonia in various ICU patients have been shown to be due to Legionella.10 11 15 However, the single most important risk factor is simply whether the ICU potable water system is colonized with Legionella. If it is not, then Legionella will be a consideration only in cases of community-acquired pneumonia at that institution.18
The rates of nosocomial infections in similar types of ICUs can vary
10- to 20-fold among hospitals.19
20
21
The frequency and
types of infection also vary among different ICUs within the same
hospital and between subsets of patients within the same ICU. This is
due largely to the unique characteristics predisposing patients to
infections in particular ICU populations. Infection rates are generally
the highest in burn units (
64%) and are the lowest in coronary
care or cardiac surgery ICUs (0.5 to 4.7%).1
22
23
24
Medical and surgical ICUs have an intermediate risk, however,
nosocomial infection rates are higher in surgical ICUs (28 to 31%)
than in medical ICUs (3.2 to 24%).
The relative frequency of predominant infections also varies in different ICUs. Rates of nosocomial pneumonia ranged from 0.5 cases per 100 admissions in the coronary care ICU, to 9 per 100 in the burn unit, with other ICUs ranging between 1.5 and 2.5 cases per 100 admissions in one report.22 In another study, nosocomial pneumonia occurred equally frequently in medical ICU (10%) and surgical ICU (8%) patients, however, all other major nosocomial infections, except pneumonia, occurred significantly more frequently in the surgical ICU.23 There was a strong trend toward a higher incidence of ventilator-associated pneumonia in cardiothoracic surgery patients; ventilator-associated pneumonia occurred in 9% of the medical ICU patients, in 14% of the surgical ICU patients, and in 22% of the cardiothoracic ICU patients.16 The rate of ventilator-associated pneumonia was 5.8 cases per 1,000 ventilator-days in the pediatric ICU, 14.5 cases per 1,000 ventilator-days in the surgical ICU, and 18.3 cases per 1,000 ventilator-days in the neurosurgical ICU.20 It should, however, be noted that a "gold standard" for the diagnosis of ventilator-associated pneumonia does not exist. Thus, variability in the criteria and methods of diagnosis of ventilator-associated pneumonia may account partly for the differences in rates of nosocomial pneumonia at different institutions.
Primary bacteremia comprised 15% of infections in the neonatal ICU, a rate that is 500% higher than those for the other ICUs at that institution.1 The central line-associated bloodstream infection rate varied from a mean of 4.9 cases per 1,000 central line-days in the surgical ICU to 6.1 cases per 1,000 central line-days in the medical ICU, and to 14.6 cases per 1,000 central line-days in the burn ICU.20
Fifty-one percent of the infections in the coronary care unit, but only
1% of those in the neonatal ICU in the same study, were secondary to a
genitourinary source.1
Candida, enteric Gram-negative
bacterial, and Pseudomonas infections were more common in patients in
medical/surgical ICUs than in patients in other ICUs.1
These data have significant bearing on discerning the likelihood,
potential source, and, ultimately, the empiric antimicrobial therapy
for infections in a specific ICU population. The size of the ICU has
been shown to affect the risk of infection; patients on units with
11 beds were at significantly greater risk than those patients on
units with 5 beds (p < 0.05).21
Underlying disease and comorbid illness also influence the susceptibility of patients to specific pathogens. In a study in trauma patients, the predominant pathogen causing nosocomial pneumonia was Haemophilus influenzae.25 Trauma patients also were shown to be significantly more likely to develop pneumonia due to Haemophilus or pneumococcus than other patients in another ICU.15 Haemophilus and pneumococcal carriage is common in healthy patients in the community. Trauma patients, unlike patients with chronic illnesses who are admitted to the ICU, were healthy before admission. A lower incidence of Haemophilus and pneumococcal pneumonia in patients with chronic illnesses (and therefore prior or prolonged hospitalization) may be reflective of the modification of the resident flora of the oropharynx and colonization with more resistant organisms, compared to trauma patients who may still be colonized with Haemophilus or pneumococcus. Neurosurgical patients and patients with craniocephalic trauma were uniquely susceptible to pneumonia due to methicillin-sensitive S aureus in two studies.26 27 MRSA and P aeruginosa, on the other hand, were significant causes of pneumonia in the patients with COPD.26 28 Liver disease and cirrhosis were documented to be risk factors for MRSA pneumonia in the ICU at one institution.15
The time of onset of pneumonia after hospitalization has been shown to
influence the incidence and etiology of pneumonia. Variable criteria
and cutoffs (ie, > 48 h, > 72 h, or 4 days) have
been proposed to define early-onset vs late-onset nosocomial
pneumonia.29
30
The rationale, however, being that
prolonged hospital stays increase the likelihood of pneumonia due to
more resistant microorganisms. In this regard, time after admission to
the hospital may be more relevant than time after intubation as a risk
factor for colonization and acquisition of nosocomial infections. We
recommend that ICUs assess their institutional trends for the time
required for the development of specific types of pneumonia, rather
than utilize an arbitrary time criterion. For example, at one
ICU,15
the mean time to the onset of nosocomial pneumonia
was 4 days for Haemophilus, 36 days for Gram-negative organisms, and 42
days for MRSA (p = 0.003). These data suggest that even though MRSA
was endemic at that institution, vancomycin need not be included
routinely in the empiric antimicrobial regimen of patients developing
nosocomial pneumonia unless they had been hospitalized for least a
month.15
The rationale and arguments forming the basis of our recommendations can be summarized as follows: precise knowledge of the pathogen allows a rational antibiotic selection, an assessment of the likely portal of entry, the normal flora at the site of entry, the clinical setting, and the underlying defect of the host defenses in patients being cared for in that ICU.31 We show that bacterial flora, specific patient population, underlying diseases, and timing of nosocomial pneumonia are sufficiently unique so as to warrant research within individual ICUs to precisely determine the ecology of pathogens specific to that ICU. While therapeutic algorithms from textbooks and consensus approaches to management from expert panels can empower physicians with general concepts, they cannot predict precise antimicrobial choices. Since the adverse impact of inadequate antimicrobial treatment on higher mortality rates in the ICU setting has been amply demonstrated,32 appropriate empiric therapeutic strategies for suspected infections in critically ill patients can contribute to improved outcomes. Recommendations regarding anti-infective regimens based on a computer program that took into consideration epidemiologic information, along with other details relevant to particular patients, led to a significant improvement in the quality of care and to reduced costs.33
While research would be valuable in all ICUs, it is particularly critical for ICUs in tertiary-care centers (where antimicrobial resistance is more likely to be prevalent) and for ICUs caring for subsets of patients with specific underlying illnesses (ie, those likely to have a unique microbial ecology). How then can such research be conducted and should funding be a necessary prelude? We suggest that the implementation of this approach should not require additional institutional resource allocation since infection-control practitioners are already collecting much of the required data. Critical-care providers and intensivists, in conjunction with infection-control practitioners, would, therefore, be the most appropriate personnel to conduct such research in their ICUs. Unfortunately, hospital epidemiology data often are collected haphazardly and in an unfocused fashion that do not enable the ICU clinicians to address the issues listed in Table 1 . Thus, the ICU clinician must work in conjunction with the infection-control practitioner to standardize data collection such that data can be used for critical-care research.
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Footnotes |
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Received for publication August 18, 1999. Accepted for publication December 23, 1999.
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This article has been cited by other articles:
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  L. Tan, X. Sun, X. Zhu, Z. Zhang, J. Li, and Q. Shu Epidemiology of Nosocomial Pneumonia in Infants After Cardiac Surgery Chest, February 1, 2004; 125(2): 410 - 417. [Abstract] [Full Text] [PDF]
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 G. H. KARAM and J. E. HEFFNER Emerging Issues in Antibiotic Resistance in Blood-borne Infections Am. J. Respir. Crit. Care Med., November 1, 2000; 162(5): 1610 - 1616. [Full Text]
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