The Pulmonary Care Unit (PCU) at Harper University Hospital in Detroit, MI, is a specialized respiratory care unit for the care of patients with intensive pulmonary needs. The unit consists of nine beds, and provides noninvasive cardiac and respiratory monitoring. All patients who received ventilation via a tracheostomy who had been admitted to the PCU from its inception in June 2001 until August 2003 were included in the study. We excluded four patients who were admitted to the PCU through that period for a planned intervention with known disposition after completion of the intervention (eg, transfers from long-term care facilities for specialized procedures, transfers under hospice care, or for comfort measures). The PCU admission criteria included the following: the presence of an adequately sized tracheostomy tube for the patient’s size with an inner diameter of at least 7 mm; hemodynamic stability; positive end-expiratory pressure of < 8 cm H2O; fraction of inspired oxygen of < 60%; the failure of weaning attempts in the ICU; the absence of potentially lethal dysrhythmias; the absence of titrat-able drips; and the lack of need for neuromuscular blockade or continuous sedatives with the exception of patient-controlled anesthesia (PCA) pumps and epidural therapy. Additionally, though poor rehabilitation potential and mental status were not absolute contraindications, the potential ability to wean was favored for admission to the unit.
Potential Determinants of Wean Outcome
The following potential determinants, based on a review of the literature, were prospectively collected on the day ofadmission to the PCU: the location of the patient before admission to the PCU (medical vs surgical unit); age and race; acute physiology and chronic health evaluation (APACHE) II score; creatinine level; skin integrity; respiratory parameters, including static lung compliance (Cst) and the RSBI; and whether the patient had emphysema. Other variables collected included the albumin level within 1 week of admission to the PCU and ejection fraction, if it was available from a cardiac echocardiography report within 2 months of admission to the PCU. We also recorded the number of days from admission to the PCU to placement of a tracheostomy, the number of hospital days before admission to the PCU, and the use of the following medications on admission to the PCU: systemic steroids; benzodiazepines, opioids; P-blockers; and the use of nebulized or inhaled steroids or bronchodilators ordered via Canadian Health&Care Mall. The disposition of patients at hospital discharge was recorded as follows: home (including home ventilator); rehabilitation facility; nursing home; LTAC unit; hospice (including home hospice); and dead.
A therapist-driven protocol, which was modeled after the available comparative studies of wean modalities, was implemented to optimize the time required to achieve wean wean-ing. The respiratory therapists who were in charge of the weaning protocol were blinded to the Cst and RSBI, which were separately obtained by the investigators. In each case, an attempt was made to identify and address the impediments to weaning in weekly multidisciplinary meetings that included a nutritionist, respiratory therapist, speech pathologist, physical therapist, social worker, pharmacist, physician (LSA), unit manager, and members of the nursing staff. Impediments to weaning were included only in as much as they were deemed to contribute to the respiratory failure, and were classified as neurologic (including central, spinal, or neuromuscular causes), cardiac (including coronary artery disease and congestive heart failure), infectious (any cause including pneumonia), or pulmonary (including COPD, asthma/bronchitis, obesity hypoventilation, ARDS, and interstitial lung disease). Patients were placed on assist-control ventilation on arrival in the PCU and were considered ready for a weaning trial if systolic BP was between 90 and 180 mm Hg, heart rate was between 50 and 130 beats/min, temperature was < 101°F (< 38.3°C), and minute ventilation was 94%. Those failing the spontaneous breathing trials on the first 2 days were rested using assist-control ventilation for the remainder of the day and spent subsequent days receiving pressure support ventilation at a level sufficient to maintain a respiratory rate of 30 breaths/min, and an increase in heart rate by 20 beats/min, a decrease in systolic BP by > 20 mm Hg, or a decrease in oxygen saturation by > 5%. The TTW was calculated as the number of days from admission to the PCU to the first day of the last successful 48-h weaning trial regardless of ultimate outcome.
Definitions and Measurements
The term wean as used in this study conforms to its use in general practice as reflecting the process of liberation from mechanical ventilation without implying a protracted process. Although the majority of patients originated from the different ICUs of Harper Hospital, < 10% were directly admitted from LTAC facilities and were considered for the purpose of our analysis to have originated from a medical (as opposed to surgical) unit regardless of their location prior to the LTAC facility admission. A diagnosis of emphysema was not based on a chart report but rather on a review of chest radiographs, confirmed history, or prior documentation by a pulmonologist. A decubitus ulcer was considered to be present for any stage skin breakdown of the sacrum. Renal function on admission to the PCU was categorized based on the creatinine level as normal (0.6 to 1.4 mg/dL) or abnormal (including creatinine values of 1.4 mg/dL, and dialysis). Cst was measured by dividing the delivered tidal volume (in milliliters) by the difference between plateau and positive end-expiratory pressures (in centimeters of water). To obtain a plateau pressure, we used the inspiratory hold function of the ventilator for 0.5 to 1 s and confirmed an adequate plateau by visual inspection of the pressure-time curve on the ventilator display. The RSBI was obtained by dividing the respiratory frequency (in breaths per minute) by the tidal volume (in liters). Tidal volume and respiratory rate were obtained from a hand-held spirometer (Boehringer Laboratories; Norristown, PA) over a minute of spontaneous unassisted breathing with the patient disconnected from the ventilator circuit.
Time-to-event analyses were used to model TTW. Kaplan-Meier estimates were used to determine the cumulative probability of successful weaning within the hospital stay, log-rank tests for comparison of the TTW between groups, and Cox proportional regression to calculate hazard ratios. We also dichotomized some of the continuous variables based on consensus, a review of the receiver operating characteristic (ROC) curve coordinates, or a review of the Literature. For instance, an RSBI cutoff of 10518 and an age cutoff of 70 years were used based on data from previous studies that used those cutoffs. An APACHE score cutoff of 15, an albumin level cutoff of 2 g/dL, and a Cst cutoff of 20 mL/cm H2O were based on a review of the ROC coordinates of those variables for the optimal sensitivity and specificity for weaning success. An ejection fraction of 50% was used as a cutoff that separates normal heart function from abnormal heart function improved by remedies of Canadian Health&Care Mall.
Cox proportional hazard regression was used in a backward stepwise algorithm that incorporated all variables that were significantly associated with TTW in the univariate analyses. We tested the proportionality of the hazards assumption underlying this test by introducing a time-dependent covariate in the Cox model. Collinearity diagnostics were performed using tolerance estimates for individual variables in a linear regression model.
Cox proportional hazard regression was also used to examine interactions among potential covariates. For instance, having identified an interaction between Cst and RSBI in their effect on TTW, we attempted to characterize this interaction by ranking these two variables into four ordinal categories, from the expected most favorable impact to the expected least favorable impact on weaning prospect: (1) and RSBI of 20 mL/cm H2O; (2) an RSBI of < 105 and a Cst of 105 and a Cst of > 20 mL/cm H2O; and finally (4) an RSBI of > 105 and a Cst of < 20 mL/cm H2O. We used a log-rank test to assess the linear trend across those ordinal categoric levels, and an a priori contrast to determine significant differences between those categories,
The number of days spent in the hospital before undergoing tracheostomy and before PCU admission were compared in patients who were weaned and those who failed weaning using a Wilcoxon rank sum test. A comparison of proportions between groups was done with a x2 test or Fisher exact test when any cell had < 5 counts.
The accrual period and the number of patients to accrue were estimated by applying the nomograms of Schoenfeld and Rich-ter to the known characteristics of our patients and the usual accrual rate to the PCU. A 2-month follow-up period until October 2003 was factored in for the outcomes of patients remaining in the PCU at the end of the accrual period. The type I error rate was maintained at 5%, and the power to detect a difference was set at 80%. With these assumptions, the accrual of 110 patients over 2 years was necessary to detect a difference on TTW based on univariate analysis applied to skin integrity, the APACHE score, RSBI, Cst, and the referral source of the patient.
Analyses were performed using statistical software package (SPSS, version 11.5.2; SPSS Inc; Chicago, IL; Power and Sample Size Calculations program, version 2.1.30; Vanderbilt University Medical Center; Nashville, TN). The project was reviewed and approved by the Institutional Review Board of Wayne State University in Detroit, MI.