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Incidence of and Risk Factors for Ventilator-Associated Pneumonia in Critically Ill Patients

Deborah J. Cook, MD; Stephen D. Walter, PhD; Richard J. Cook, PhD; Lauren E. Griffith, MSc; Gordon H. Guyatt, MD; David Leasa, MD; Roman Z. Jaeschke, MD; and Christian Brun-Buisson, MD
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For the Canadian Critical Care Trials Group From McMaster University, Hamilton, University of Waterloo, Waterloo, and University of Western Ontario, London, Ontario, Canada; and Hopital Henri Mondor, Universite Paris-Val de Marne, Creteil, France. Acknowledgments: The authors thank members of the Canadian Critical Care Trials Group, particularly the Pneumonia Adjudication Committee, for their support of this study. They also thank the research nurses who collected the data and Ms. Peggy Austin for help with coordinating the study. Grant Support: By the Medical Research Council of Canada and Hoechst Marion Roussel, Inc. Dr. D. Cook is a Career Scientist of the Ontario Ministry of Health. Dr. Walter is a National Health Scientist, National Health Research and Development Program, Health Canada. Dr. R. Cook is a Scholar of the Medical Research Council of Canada. Requests for Reprints: Deborah J. Cook, MD, Department of Medicine, St. Joseph's Hospital, 50 Charlton Avenue East, Hamilton, Ontario L8N 4A6, Canada; e-mail, debcook@fhs.csu.mcmaster.ca. Current Author Addresses: Drs. D. Cook and Jaeschke: Department of Medicine, St. Joseph's Hospital, 50 Charlton Avenue East, Hamilton, Ontario L8N 4A6, Canada.

Copyright ©2004 by the American College of Physicians

Ann Intern Med. 1998;129(6):433-440. doi:10.7326/0003-4819-129-6-199809150-00002
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Background: Understanding the risk factors for ventilator-associated pneumonia can help to assess prognosis and devise and test preventive strategies.

Objective: To examine the baseline and time-dependent risk factors for ventilator-associated pneumonia and to determine the conditional probability and cumulative risk over the duration of stay in the intensive care unit.

Design: Prospective cohort study.

Setting: 16 intensive care units in Canada.

Patients: 1014 mechanically ventilated patients.

Measurements: Demographic and time-dependent variables reflecting illness severity, ventilation, nutrition, and drug exposure. Pneumonia was classified by using five methods: adjudication committee, bedside clinician's diagnosis, Centers for Disease Control and Prevention definition, Clinical Pulmonary Infection score, and positive culture from bronchoalveolar lavage or protected specimen brush.

Results: 177 of 1014 patients (17.5%) developed ventilator-associated pneumonia 9.0 ± 5.9 days (median, 7 days [interquartile range, 5 to 10 days]) after admission to the intensive care unit. Although the cumulative risk increased over time, the daily hazard rate decreased after day 5 (3.3% at day 5, 2.3% at day 10, and 1.3% at day 15). Independent predictors of ventilator-associated pneumonia in multivariable analysis were a primary admitting diagnosis of burns (risk ratio, 5.09 [95% CI, 1.52 to 17.03]), trauma (risk ratio, 5.00 [CI, 1.91 to 13.11]), central nervous system disease (risk ratio, 3.40 [CI, 1.31 to 8.81]), respiratory disease (risk ratio, 2.79 [CI, 1.04 to 7.51]), cardiac disease (risk ratio, 2.72 [CI, 1.05 to 7.01]), mechanical ventilation in the previous 24 hours (risk ratio, 2.28 [CI, 1.11 to 4.68]), witnessed aspiration (risk ratio, 3.25 [CI, 1.62 to 6.50]), and paralytic agents (risk ratio, 1.57 [CI, 1.03 to 2.39]). Exposure to antibiotics conferred protection (risk ratio, 0.37 [CI, 0.27 to 0.51]). Independent risk factors were the same regardless of the pneumonia definition used.

Conclusions: The daily risk for pneumonia decreases with increasing duration of stay in the intensive care unit. Witnessed aspiration and exposure to paralytic agents are potentially modifiable independent risk factors. Exposure to antibiotics was associated with low rates of early ventilator-associated pneumonia, but this effect attenuates over time.


Grahic Jump Location
Figure 1.
Proportion of patients free of pneumonia during their stay in the intensive care unit.
Grahic Jump Location
Grahic Jump Location
Figure 2.
Hazard rate for ventilator-associated pneumonia during the stay in the intensive care unit.

The hazard function presents the conditional probability of ventilator-associated pneumonia in the next day, given that a patient is event free. Estimation of the hazard function shows the event rate per day over the duration of ventilation.

Grahic Jump Location




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