Validation of a new WIND classification compared to ICC classification for weaning outcome

All consecutive patients admitted to the medical ICU and requiring MV for more than 24 h between July 2010 and September 2013 were prospectively registered at Samsung Medical Center, a 1989-bed tertiary referral hospital with tertiary-level ICU, in Seoul, South Korea [3, 9, 10]. If a patient was re-admitted to the ICU for MV support during the same hospital admission, only the first weaning episode was included in analysis. Multiple ICU visits during different hospital admissions were enrolled separately. Patients who were transferred from other hospitals after more than 48 h of intubation or were successfully treated by noninvasive ventilation (NIV) were excluded. The Institutional Review Board of Samsung Medical Center approved this study and allowed review and publication of information from patient records. Informed consent was waived because of the study’s observational nature.

Since 2010, the medical ICU of our hospital has utilized a specific protocol-based weaning program according to the recommendations by Boles et al. [1]. Details of our weaning program were described in previous reports [3, 9, 10] and an additional file provided. In short, respiratory care practitioners (RCP), who are registered nurses specializing in respiratory care, screened patients daily for weaning readiness and conducted SBTs according to the protocol. When a patient passed the SBT, extubation proceeded. If a patient failed the SBT, MV was resumed, and the team reviewed possible reversible etiologies for the failure. Again, when a patient proved ready for weaning, the SBT was repeated the following day.

Patients were classified into simple, difficult, or prolonged weaning groups according to ICC classification [1] and Groups 1, 2, 3, or NW according to WIND classification [8]. The three weaning groups by ICC classification were defined as follows: simple weaning, patients who proceed from initiation of weaning to successful extubation (no need to reinstitute ventilator support within 48 h of extubation) on the first attempt without difficulty; difficult weaning, patients who failed initial weaning and required up to three SBTs or as long as 7 days from the first SBT to achieve successful extubation; or prolonged weaning, patients who required more than three SBTs or > 7 days of weaning after the first SBT. To apply the ICC classification, unclassifiable patients were excluded as follows: patients with tracheostomy prior to MV; patients who died, underwent tracheostomy, transferred out, or had unplanned extubation before weaning trial; and patients with unclassifiable weaning after SBT who died or were transferred to another hospital after failure of the first SBT and before the third SBT or 7 days (Fig. 1). The four weaning groups by WIND classification were defined as follows: Group NW, patients who never experienced any separation attempt (SA); Group 1, the first SA resulted in termination of the weaning process within 1 day (successful separation or early death); Group 2, weaning was completed after more than 1 day but in less than 1 week after the first SA (successful separation or death); and Group 3, weaning was not terminated by 7 days after the first SA (by successful separation or death). In WIND classification, SA is defined as SBT or extubation directly performed without SBT (including unplanned extubation) for intubated patients and as ≥ 24 h with spontaneous ventilation through tracheostomy without any mechanical ventilation for tracheostomized patients.

To analyze differences in weaning outcomes among groups according to ICC and WIND classifications, clinical outcomes of MV days, ventilator-free days, tracheostomy rate, successful weaning rate, ICU mortality, LOS in ICU, hospital mortality, and LOS in hospital were investigated. Ventilator-free days were calculated as the number of days without invasive ventilation to day 28. Nonsurvivors were considered as patients with 0 ventilator-free days. Because there is no applicable definition for tracheostomized patients in ICC classification, successful weaning was defined according to WIND definitions as follows: for intubated patients, extubation without death or reintubation within the next 7 days whether postextubation NIV was used or not or ICU discharge without invasive MV within 7 days, whichever comes first; for tracheostomized patients, spontaneous ventilation through tracheostomy without any MV for 7 consecutive days or discharged with spontaneous breathing, whichever comes first. The date of successful weaning was counted to the actual day of extubation or spontaneous ventilation through tracheostomy after the patient had completed 7 days without reintubation or any MV through tracheostomy (or was alive and discharged earlier).

The data are presented as medians and interquartile ranges (IQR) for continuous variables and as numbers and percentages for categorical variables. The Jonckheere–Terpstra test for continuous variables [11] and the Mantel–Haenszel test for categorical variables [12] were used to analyze trends of baseline characteristics and outcomes across weaning groups. The Mann–Whitney U test was used for continuous variables, and Pearson’s Chi-square test was used for categorical variables to identify statistical differences of main weaning outcomes between weaning groups according to the ICC and WIND classifications, respectively. All tests were two-tailed, and a P value < 0.05 was considered significant. The data were analyzed using PASW Statistics 18 (SPSS Inc., Chicago, IL, USA).

During the study period, a total of 1600 patients were eligible after excluding patients who transferred from other hospitals after more than 48 h of intubation (n = 53) or underwent successful NIV (n = 76) (Fig. 1). All eligible patients were classified by the WIND classification as follows: Group NW = 580 (36.3%), Group 1 = 617 (38.6%), Group 2 = 186 (11.6%), and Group 3 = 217 (13.6%). However, only 735 (45.9%) patients could be classified by the ICC classification as follows: simple weaning = 503 (31.4%), difficult weaning = 145 (9.1%), and prolonged weaning = 87 (5.4%).

Baseline characteristics of the total cohort are presented in Table 1. Median age was 65 years, and 68.0% of patients were male. The most common comorbidity was malignant disease (59.5%), and the most common cause of respiratory failure was pneumonia (33.4%), followed by extrapulmonary sepsis (21.6%) and acute respiratory distress syndrome (9.8%).

Agreement of weaning results between ICC and WIND classifications is presented in Table 2. Although most patients in the simple weaning (462/503, 91.8%) or prolonged weaning groups (76/87, 87.4%) were classified as Group 1 or 3, respectively, only 59.3% (86/145) of patients in the difficult weaning group by ICC classification were classified as Group 2 by WIND classification. Of 865 patients whose weaning results could not be classified by ICC, 285 were classifiable to Group 1 (n = 109), 2 (n = 68), or 3 (n = 108) by WIND.

In a comparison of baseline characteristics among weaning groups, there were statistically significant trends with more underlying malignancy and neurologic disorders, longer interval between hospital admission and ICU admission, more pneumonia as a cause of respiratory failure, less pulmonary edema as a cause of respiratory failure, and longer interval between hospital admission and intubation across the ICC classification from simple to prolonged weaning groups (Table 3). In addition to this trend, except for neurologic disorders, there were statistically significant trends with more respiratory disorders and less gastrointestinal and genitourinary disorders as underlying diseases across the WIND classification from Group 1 to Group 3.

Clinical outcomes of the total cohort are listed in Table 4. Median interval between intubation and first SA was 3 days (IQR, 2–6 days), and median MV requirement was 5 days (IQR 2–11 days). Tracheostomy was needed in 416/1580 (26.3%) patients after a median of 11 days (IQR, 6–15 days) of intubation. The successful weaning rate was 51.5%, and ICU and hospital mortality were 41.0% and 53.0%, respectively.

All of these clinical outcomes showed statistically significant trends across the ICC and WIND classifications (Table 5). However, there were no statistical differences in successful weaning rate (96.6% vs. 95.2%, P = 0.416), ICU mortality (5.4% vs. 5.5%, P = 0.944), and hospital mortality (22.5% vs. 25.5%, P = 0.443) between simple and difficult weaning groups by ICC (Fig. 2). Conversely, there were statistically significant differences in successful weaning rate (98.5% vs. 76.9%, P < 0.001), ICU mortality (3.6% vs. 16.7%, P < 0.001), and hospital mortality (21.2% vs. 33.9%, P < 0.001) between Group 1 and Group 2 by WIND. By the WIND classification, only the LOS between Group 1 and Group 2 had no statistically significant difference (median 25 days [IQR 15–51 days] versus median 29 days [IQR 16–52 days], P = 0.300).