CHEST. 2003;123(3) 

In This Article

Antibiotic Therapy of Ventilator-Associated Pneumonia

Pneumonia occurring during mechanical ventilation, the so-called ventilator-associated pneumonia (VAP), is the major cause of infection in critically ill patients who are receiving mechanical ventilation. Accordingly, lower respiratory tract infection is the major reason for prescribing antibiotics to patients in the ICU.[1] Given the current trends in antimicrobial resistance,[2] the fact that ICUs are the major source and reservoir of resistant strains[3] and inasmuch as resistance is, at least in part, linked to antibiotic use, there is both a strong rationale and an incentive for designing antibiotic policies for the management of infection in the ICU that are aimed at controlling or reducing patient resistance, especially for therapy in patients with respiratory tract infections.[3] Toward this end, several approaches have been advocated, including restrictive formularies, the rotation of antibiotics, and the use of specific diagnostic techniques, which result in the empirical treatment of fewer patients with clinically suspected pneumonia.[4,5]

However, a number of studies[6,7,8,9] have emphasized the importance of adequate early antibiotic therapy in the management of patients who are clinically suspected of infection and have shown that patients who are initially treated inadequately had poorer outcomes than did those receiving adequate therapy at the beginning. One study[10] has even suggested that adequate therapy administered before the clinical suspicion of pneumonia (an objective that obviously is achievable only by chance alone) was associated with a better prognosis. Inadequate empiric antimicrobial therapy can occur because of the presence of unanticipated species that are not covered by the regimen selected but is mostly due to unanticipated resistance.[11] Clinicians, therefore, are faced with the following dilemma: treat early all patients who have a clinical suspicion of VAP, preferably with broad-spectrum combination therapy to cover most likely species and to overcome potential resistance problems, thereby taking the risk of fostering resistance[12]; or make a more conservative, selective, and prudent use of antibiotics that exposes some patients to a potential delay in therapy and to suboptimal management.

Practically speaking, there are actually two challenges for the clinician, as follows: to decide when and which patient should be treated; then to select which antibiotic therapy to prescribe empirically.

One approach to resolve or, to say it more appropriately, to bypass the first challenge has been illustrated by the study of Singh et al.[13] Those investigators tested a strategy in which all patients initially were treated empirically (in this instance with ciprofloxacin), then antibiotic therapy was withdrawn after 3 days in patients who had a low likelihood of pneumonia to start with, as assessed by the clinical pulmonary infection score (CPIS),[14] and who have maintained a low score (ie, < 6) after 3 days of therapy. This approach, while treating all patients who are suspected of pneumonia somewhat indiscriminately, has the merit of limiting the administration of unnecessary antibiotic therapy to some extent.

A more sophisticated approach is to combine the clinical likelihood of pneumonia (based on a variable combination of measurements of purulent tracheal secretions, pulmonary infiltrates, temperature, leukocytosis, and Pao2/fraction of inspired oxygen ratio, and their recent alterations, as is the case with the CPIS) with the results of direct examinations of respiratory tract secretion specimens, such as Gram stains,[15] or the direct examination of, for example, bronchoscopic BAL fluid samples,[16] and the clinical severity of the systemic response to infection17,18 to decide whether or not to initiate antibiotic therapy in a patient having a clinical suspicion of pneumonia.

Once a decision to treat has been made, how should the selection of empiric antibiotic therapy proceed? Although ciprofloxacin has proved to be effective in limiting unnecessary antibiotic use in the specific setting of the study performed by Singh et al,[13] this choice can be questioned. Ciprofloxacin is certainly very effective in most cases against enterobacteriaceae and Haemophilus influenzae, and is usually effective against Staphylococcus aureus (although many methicillin-resistant strains have an associated resistance to fluoroquinolones), but is suboptimal therapy for streptococci, including pneumococci, and for Pseudomonas aeruginosa, at least when used alone.[19] In this issue of CHEST (see page 835), Fowler et al suggest that an anti-Pseudomonas, extended-spectrum penicillin combined with a b lactamase inhibitor (ie, piperacillin-tazobactam) could be the most appropriate empiric therapy for suspected VAP. In this study, the authors analyzed empiric therapy and outcomes in 156 patients with suspected VAP. By Cox regression analysis, they found a lower risk of hospital mortality (odds ratio [OR], 0.41; 95% confidence interval [CI], 0.21 to 0.80) in patients to whom initial empiric therapy with piperacillin-tazobactam was administered by their attending physicians, as well as a strong trend toward reduced mortality (OR, 0.43; 95% CI, 0.16 to 1.11) in patients receiving aminoglycosides, compared to patients receiving other antibiotics. Piperacillintazobactam was the drug most commonly administered in this cohort (63%), followed by fluoroquinolones (57%), vancomycin (47%), cephalosporins (28%), and aminoglycosides (25%). As in most studies of patients with VAP, a majority of patients (54%) actually received combination therapy. Other predictors of hospital mortality that were identified in this study were hepatic failure (OR, 4.6), an immunocompromised status (OR, 2.68), and APACHE (acute physiology and chronic health evaluation) II score (OR, 1.12). The authors were unable to demonstrate improved outcome associated with appropriate vs inappropriate therapy, or with combination therapy vs monotherapy.

As for many studies of this kind, in which several therapeutic approaches are compared within the context of an observational study, the results are difficult to interpret, and such observations do not have the strengths of those derived from randomized controlled trials. Many of the patients studied by Fowler et al received combination therapy, including 45% of those treated with piperacillin-tazobactam, and the authors provide no clue to account for the marked superiority of piperacillin-tazobactam compared to the effects of other antibiotics and its impact on mortality. As noted earlier, this did not appear to be associated with more adequate therapy. However, the very high treatment adequacy rate recorded in this study (92%) and the variety of antibiotics that were examined likely explain why the variable "appropriateness of therapy" was not found as a prognostic factor. It also should be noted that a diagnosis of VAP was microbiologically confirmed with "acceptable specimens" (ie, using tracheal aspirates only) in only 74% of suspected cases, so that the etiology of infection (or even its reality) in the cohort is uncertain, making the judgment on adequacy uncertain and limited to a fraction of the population. The favorable results associated with piperacillintazobactam may have been due to its broad spectrum of activity, encompassing the major microorganisms associated with VAP, including streptococci, staphylococci (non-methicillin-resistant), enterobacteriaceae, and P aeruginosa, as well as anaerobes. Another hypothesis would be that piperacillin-tazobactam therapy resulted in a lower level of resistance emergence and superinfection during or after therapy, as has been suggested in one previous study,[20] but this was not documented by Fowler et al.

Most randomized controlled trials comparing new regimens with established ones in patients with hospital-acquired pneumonia tend to show equivalence of the regimens tested, and none shows an effect on mortality. Several such trials[20,21,22] have compared piperacillin-tazobactam with ceftazidime (both in combination with an aminoglycoside) and have found that, overall, the former was at least as effective and possibly more effective than the latter. The results recorded by Fowler et al, although inordinately favorable, as is often the case in nonrandomized studies, are nevertheless consistent with those from these trials.

In view of these very encouraging results, should piperacillin-tazobactam become the standard therapy for all patients with VAP? The authors adequately caution against this view. In addition to the presence of risk factors that are specific to the underlying disease of patients, it has become clear that there are three important determinants of the etiology of VAP that can be used to guide the selection of initial therapy, as follows: the timing of onset of pneumonia relative to ICU (or preferably hospital) admission; the prior administration of anti-biotics; and unit-specific local epidemiologic factors. The distinction between early pneumonia (ie, after < 5 to 7 days in the ICU) and late-onset pneumonia is a major determinant of the etiology, provided that the patient has not spent some time in the hospital before ICU admission or has had a recent hospitalization. In early-onset cases, the clinician can be fairly confident that "normal flora" (ie, Streptococcus pneumoniae and other streptococci, H influenzae, S aureus, and possibly anaerobes in selected circumstances) are involved, and there is no need for the administration of an antipseudomonal, broad-spectrum penicillin combined with a ß-lactamase inhibitor or of ciprofloxacin. Therapy with a first-generation cephalosporin or a combination of ampicillin with a ß-lactamase inhibitor will suffice. However, if the patient has received antibiotics for some time, especially broad-spectrum ones,[12] then the flora colonizing and infecting the respiratory tract is much more likely to have been modified to include "hospital organisms," such as Klebsiella, Enterobacter, and particularly P aeruginosa, all of which are likely to be resistant to any antibiotics that had been administered previously. The worst-case scenario is for the patient to have received several extended courses of antibiotics, and, when presenting with pneumonia occurring late after hospital/ ICU admission, to be exposed to an environment in which resistant organisms such as Acinetobacter, Pseudomonas, methicillin-resistant S aureus, or vancomycin-resistant Enterococcus are endemic. In this instance, it is very difficult to predict the microorganisms that may be involved and their susceptibility profile, so that a three-agent combination empiric therapy with a ß-lactam, an aminoglycoside, and a glycopeptide is commonly prescribed pending culture results, unless a more targeted approach can be guided by the direct examination of reliable respiratory tract specimens showing a single population. In such settings, piperacillin-tazobactam or any other single drug is unlikely to cover adequately all microorganisms involved.

Again, the high rate of adequate initial empiric therapy (92%), despite the fact that most patients were treated for a suspicion of late-onset pneumonia (after a mean 11 days of mechanical ventilation), suggests that the study was conducted in an environment that was relatively preserved from antimicrobial resistance or that the population was at low risk of resistance to start. This fact again highlights the importance of the local surveillance of the epidemiology of microorganisms in devising antibiotic strategies in a specific ICU.[23]

Nevetheless, the findings by Fowler et al confirm that piperacillin-tazobactam often can provide adequate empiric therapy in many patients who are suspected of late-onset pneumonia, as already has been suggested in the controlled studies[20,21,22] comparing piperacillin-tazobactam with ceftazidime. It should not, however, be considered a magic bullet. In some institutions, this drug cannot be used in confidence, especially in patients having previously received an extended-spectrum penicillin[24] or even fluoroquinolones.[25] While the latter class is more likely to foster the selection of antibiotic-resistant strains in the ICU environment, the routine and systematic use of a drug having marked antianaerobic activity such as piperacillin-tazobactam may put patients at risk for selection of resistance in a favorable environment.[26] There is room for individualized therapy of clinically suspected VAP in many patients, varying the agents prescribed according to the specific condition and risk factors they present with, the antecedent antibiotics, local epidemiologic surveillance, and the microbiological information available at the time of the prescription.

Christian Brun-Buisson, MD, Créteil, France

Dr. Brun-Buisson is associated with the Service de Re´ nimation Medical & Unite´ d'Hygiène et Prevention de l'Infection, Hôpital Henri Mondor, Assistance Publique-Hôpitaux de Paris. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (e-mail: permissions@chestnet.org).

Correspondence to: Christian Brun-Buisson, MD, Service de Réanimation Me´dicale & Unite´ d'Hygie`ne et Pre´vention de l'Infection, Hôpital Henri Mondor, Assistance Publique-Hôpitaux de Paris (AP-HP), 51 Ave du Mare´chal de Lattre de Tassigny, 94010, Créteil, France; E-mail: christian.brun-buisson@hmn.ap-hopparis.fr.

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