Microbiological surveillance and parenteral antibiotic use: DISCUSSION
ICUs are often the target of surveillance programs and antimicrobial restrictions due to increased rates of bacterial resistance. Although it is difficult to make direct comparisons with other ICUs without detailed demographic information, the results of our study demonstrated a lower incidence of antimicrobial resistance among bacterial isolates of patients receiving parenteral antibiotics relative to reports from other American and Canadian sites. A similar study of resistance in Gram-negative bacilli in a 16-bed ICU in a Vancouver, British Columbia hospital demonstrated a much higher incidence of Gram-negative infections, 35% of which were associated with beta-lactamase-mediated resistance. This study, however, included burn patients, which may have accounted for these results. Another study of 1939 Gram- negative isolates from 20 Canadian ICUs also demonstrated a higher incidence of resistance, particularly to beta-lactam antibiotics and ciprofloxacin. Our CrCU appeared to have a fewer number of more difficult to treat organisms such as pseudomonal isolates (13.5%), compared with most ICUs where Pseudomonas species have been the primary pathogen, accounting for up to 28% of isolates. In addition, we also had a very low incidence of MRSA isolates compared with other sites. The findings in this study were reassuring in that the incidence of bacterial resistance among patients receiving antibiotics has remained low. Also, there were no outbreaks with multiply resistant organisms during the study, although the CrCU has experienced previous outbreaks, possibly due to the excessive use of broad-spectrum antibiotics.
The only study that has quantified microbial resistance rates relative to antibiotic use collected data on several antimicrobial-organism pairs from eight American hospitals (ICARE Project). Data from the ICUs studied demonstrated a strong correlation between the use of ceftazidime and both ceftazidime-resistant E cloacae and P aeruginosa. Surprisingly, there was no relationship between resistance in E cloacae and other third-generation cephalosporins. Furthermore, these relationships were not found in the non-ICU areas of the hospitals.
Several factors may have accounted for the low incidence of antimicrobial resistance in our CrCU. Seventy per cent of our population were surgical or trauma patients who tended to have fewer comorbid conditions and a shorter length of hospital stay. The average length of stay in all patients admitted to our CrCU during the study was only 4.7 days. These patients would, therefore, have had less opportunity to be exposed to the hospital environment and to other critically ill patients colonized with resistant organisms. Results of other studies have also demonstrated a reduction in morbidity and mortality in patients in surgical intensive care units relative to medical units.
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The prudent use of antibiotics in the unit might have been another factor that contributed to the low level of antimicrobial resistance in our unit. Only 32% of patients admitted to the CrCU received antibiotics; in other centres, 83% of patients received antimicrobial agents. More important, third- generation cephalosporins (ceftriaxone 255 g, ceftazidime 265 g) and ciprofloxacin (49 g) were not used very often and no imipenem was prescribed during the study (Table 2). The moderate use of third-generation cephalosporins in the unit may have accounted for the lack of Gram-negative bacilli with ES6L during the period of surveillance. This is in contrast with the ICARE Project, which demonstrated higher use of third- generation cephalosporins and higher microbial resistance rates. The presence of ES6L in our CrCU might also have been underestimated because routine testing for these beta- lactamases was only performed on cultures from sterile sites such as blood and CSF. At the time of this study, there were no guidelines for the detection of ES6L and only the E-test was used for screening. The absence of resistance to imipenem and the low rates of quinolone resistance were also likely a reflection of the limited use of these drugs.