Mechanical Ventilation with or without 7-Day Circuit Changes

Nosocomial pneumonia is the leading cause of death among all hospital-acquired infections [1]. The estimated incidence of nosocomial pneumonia in intensive care units ranges from 10% to 65%; most studies [2-6] show case fatality rates of more than 20%. Ventilator-associated pneumonia specifically refers to nosocomial pneumonia that develops in a mechanically ventilated patient and that was not present at the time of airway intubation [7]. Various clinical risk factors have been associated with an increased incidence of ventilator-associated pneumonia, either because they predispose the patient to bacterial colonization of the oropharynx and stomach (for example, the administration of antacids or histamine-2-receptor antagonists) or because they facilitate aspiration of contaminated contents from these sites (for example, supine positioning) [1, 2, 8, 9]. Craven and colleagues [10] first showed that the frequency of ventilator circuit changes also influences the incidence of ventilator-associated pneumonia. They found that changing circuits every 24 rather than every 48 hours was independently associated with the occurrence of nosocomial pneumonia [10]. This association has been attributed to increased manipulation of the patient, the endotracheal tube, and the ventilator circuit, which results in increased aspiration of contaminated tubing condensate or upper airway secretions [10, 11]. More recently, several groups of investigators have found that ventilator circuits can be used safely for more than 48 hours without increasing the incidence of nosocomial pneumonia [12-16]. However, because of limitations in the design of these studies and the small number of patients prospectively examined, the Centers for Disease Control and Prevention has given no clear recommendation for the maximum length of time that ventilator circuits can safely be left in place during prolonged mechanical ventilation [17]. This has resulted in the development of ambiguous guidelines about the frequency with which ventilator circuits should be changed [18, 19] and in a call for well-designed investigations to resolve this issue [20]. We did a randomized, controlled trial to compare the effect and cost-efficacy of routine and no routine ventilator circuit changes in patients having prolonged mechanical ventilation. Our main goals were to determine 1) the incidence and outcome of ventilator-associated pneumonia in patients receiving scheduled ventilator circuit changes and 2) whether this incidence was increased in patients whose ventilator circuits remained unchanged. Methods Study Location and Patients The study was conducted at two university-affiliated teaching hospitals: Barnes Hospital (900 beds) and Jewish Hospital (450 beds). During a 7-month period (June 1994 to December 1994), all patients receiving mechanical ventilation in the intensive care units of these hospitals (surgical, trauma, medical, cardiothoracic, and neurosurgical units at Barnes Hospital; surgical, medical, and cardiothoracic units at Jewish Hospital) were potentially eligible for this investigation. Patients were entered into the trial if they were older than 18 years and had received mechanical ventilation for more than 5 days. Mechanical ventilation for more than 5 days was predetermined, on the basis of our previous experience at these institutions [2, 21], to be necessary so that a more homogeneous cohort of patients requiring prolonged mechanical ventilation could be accrued. Patients were excluded if they were likely to be extubated within 24 hours of randomization, if they had transferred from other hospitals and had already received mechanical ventilation for more than 24 hours, if they had had lung transplantation, or if they had active hemoptysis. Barnes Hospital and Jewish Hospital share the same respiratory therapy and infection control departments. The study was approved by the Washington University School of Medicine Human Studies Committee and the Institutional Review Board of Jewish Hospital. Both waived the requirement for informed consent because this study was a quality assessment of two low-risk practices already in clinical use. Study Design Patients were randomly assigned to receive no routine ventilator circuit changes or circuit changes every 7 days within 24 hours of meeting eligibility criteria. A schedule of changing ventilator circuits every 7 days was selected on the basis of available clinical data [12-16] and our survey of 16 regional medical centers (DM Baker. Unpublished communication). Stratification according to hospital site was done before randomization to control for differences in patient populations and health care personnel. Randomization within each hospital was done using opaque, sealed envelopes, which were opened at the time each patient was enrolled in the study. For the purposes of this investigation, ventilator circuits were defined to include gas delivery tubing, humidifier water reservoirs, water traps, and medication delivery devices (such as metered-dose inhaler chambers or adapters). Ventilator circuits could be changed at any time, at the discretion of individual care providers (physicians, nurses, and respiratory therapists), secondary to a mechanical failure of the ventilator circuit (such as an air leak) or visible soil (such as that resulting from hemoptysis or aspirated emesis). Scheduled ventilator circuit changes were done during the evening or night shifts to minimize the identification of individual patient group assignments to blinded investigators. All nonscheduled circuit changes were done when an appropriate indication for the circuit change (that is, a mechanical defect or soil) was identified. Patients transferred to the operating room for a surgical procedure (such as tracheotomy) or to diagnostic radiology received the same mechanical ventilator and circuit when they returned to the intensive care unit. The ventilators used for this study included Siemens Servo 900C (Siemens-Elema Ventilator Systems, Schaumburg, Illinois), Puritan-Bennett 700 Series (Puritan-Bennett Corporation, Carlsbad, California), and Bird 8400 Series ventilators (Bird Products Corporation, Palm Springs, California). All ventilators were equipped with wick-type humidifiers (Concha Therm III Plus, Hudson Respiratory Care, Inc., Temecula, California) filled with sterile irrigation water. All ventilator circuits were disposable (Hudson Respiratory Care, Inc., model 1613) and equipped with Y connectors. Each ventilator circuit had an attached trap for the collection of tubing condensate (Marquest Medical Products, Inc., Englewood, Colorado). As per our standard procedure, all ventilator circuits were monitored at least every 2 hours and water traps were emptied when full. Data Collection For all study patients, the following characteristics were prospectively recorded by one of the investigators: age, sex, diagnosis at hospital admission, indication for mechanical ventilation, Premorbid Lifestyle score, the ratio of arterial blood oxygen tension to the concentration of inspired oxygen (Pao 2: Fio 2), severity of illness based on APACHE II (Acute Physiology and Chronic Health Evaluation [22]) scores, the Organ System Failure Index, and the occurrence of a witnessed aspiration event. Specific processes of medical care examined to assess risk factors for ventilator-associated pneumonia were the administration of antacids or histamine-2-receptor antagonists, pharmacologic aerosol treatments during mechanical ventilation (such as bronchodilators, antibiotics, and mucolytics), fiberoptic bronchoscopy, surgical tracheostomy, and the number of ventilator circuit changes done and the indications for those changes (scheduled according to the study protocol, soil, or mechanical defect). Two of the investigators made daily rounds in the intensive care units of each hospital to identify eligible patients. Patients entered into the study were prospectively followed for the occurrence of ventilator-associated pneumonia until they were successfully weaned from mechanical ventilation, were discharged from the hospital, or died. All patients suspected by these investigators of having ventilator-associated pneumonia were prospectively and independently reviewed by another investigator who was blinded to the patient's treatment group assignment. The diagnosis of ventilator-associated pneumonia was strictly based on the predetermined criteria described below. Patients could not be entered into the study more than once during the same hospitalization, and only the first episode of ventilator-associated pneumonia was evaluated. In addition to the occurrence of ventilator-associated pneumonia, secondary outcomes assessed included the length of hospitalization, the duration of mechanical ventilation, hospital mortality, and mortality directly attributed to ventilator-associated pneumonia. All study variables were recorded in data collection books maintained at each of the participating hospitals. Definitions All definitions were selected prospectively as part of the original study design. The Premorbid Lifestyle score was used as previously defined [23]: Zero indicated that the patient was employed without restriction; 1 indicated that the patient was independent, fully ambulatory, not employed, or employed with restriction; 2 indicated that the patient had restricted activities, could live alone and get out of the house to do basic necessities, or had severely limited exercise ability; 3 indicated that the patient was housebound, could not get out of the house unassisted, could not live alone, or could not do heavy chores; and 4 indicated that the patient was bed- or chairbound. We calculated APACHE II scores on the basis of clinical data available from the 24-hour period before study enrollment (day 5 of mechanical ventilation). The Organ System Failure Index was modified from that used by Rubin and coworkers [24]. One point was given for acquire