This was a prospective observational study performed in an intensive care unit (ICU) of a teaching hospital from 2016 to 2018. The study protocol was approved by local institutional review board. As the observational nature of this study, the informed consent was waived. Patients who admitted to ICU for NIV as a first-line intervention were enrolled. However, patients less than 18 years old were excluded.
NIV (BiPAP Vision or V60; Philips Respironics, Carlsbad, CA) was initiated by the attending physicians based on the indications described previously [16, 17]. In patients with chronic obstructive pulmonary disease (COPD), the indications for NIV were respiratory rate more than 25 breaths/min, PaCO2 more than 45 mmHg, pH less than 7.35, PaO2/FiO2 less than 200 mmHg, and vigorous activity of accessory respiratory muscles. In patients with hypoxemic respiratory failure, the indications for NIV were clinical presentation of respiratory distress at rest (such as active contraction of the accessory inspiratory muscles or paradoxical abdominal motion), and PaO2/FiO2 less than 300 mmHg.
Face mask (ZS-MZ-A Face Mask, Shanghai Zhongshan Medical Technology Co., Shanghai, China) was the first choice to connect the patient and the ventilator. The size of the mask was fitted to the patients’ face type. The S/T mode was used in COPD patients or other patients with labored breathing. The initial inspiratory positive airway pressure was 8–10 cmH2O and then it was gradually increased to reach the tidal volume of 6–8 ml/kg or the maximal tolerated level. The initial expiratory positive airway pressure was 4 cmH2O. And then we gradually increased to counterbalance intrinsic positive end expiratory pressure in COPD patients. In hypoxemic respiratory failure patients, it was increased to maintain the alveoli patency and elevate the end expiratory lung volume. In patients with heart failure, the continuous positive airway pressure was used. The fractional concentration of oxygen was set to achieve peripheral oxygen saturation of > 92%.
At the beginning of treatment, continuous use of NIV was encouraged. Once the patient recovered from respiratory failure, liberation from NIV was considered according to our hospital protocols [18]. However, the intubation was indicated if the respiratory failure progressively deteriorated. The criteria of intubation were listed as follows: respiratory or cardiac arrest, failure to maintain PaO2/FiO2 more than 100, development of conditions necessitating intubation to protect the airway (coma or seizure disorders) or to manage copious tracheal secretions, inability to correct dyspnea, lack of improvement of signs of respiratory muscle fatigue, and hemodynamic instability without response to fluids and vasoactive agents [16, 17]. NIV failure was defined as requirement of intubation.
Delirium was screened using Confusion Assessment Method for the ICU (CAM-ICU) every day [9]. The delirium was assessed based on the following 4 features: (1) fluctuation in mental status, (2) inattention, (3) disorganized thinking, and (4) altered levels of consciousness. Delirium was diagnosed in the presence of features 1 and 2 and either feature 3 or 4. The level of consciousness was assessed by Richmond Agitation-Sedation Scale (RASS) [19]. RASS is a 10-point scale ranging from unarousable (− 5 points) or calm (0 points) to combative (4 points). Three subtypes of delirium were defined as follows [20]. Hyperactive delirium was defined as present in patients with all positive daily RASS scores (range, 1–4 points) associated with every positive CAM-ICU assessment. Hypoactive delirium was defined as present in patients with all neutral or negative daily RASS scores (range, 0 to − 3 points) associated with every positive CAM-ICU assessment. Mixed delirium was defined as present in patients with daily RASS scores that included both positive values (range, 1–4 points) and neutral or negative values (range, 0 to − 3 points) associated with every positive CAM-ICU assessment.
Statistical analysis
Statistical software (SPSS 17.0; IBM Corp., Armonk, NY) was used to analyze the data. Continuous variables were presented as means with standard deviations or medians and interquartiles (25–75%) when appropriate. Differences between groups were analyzed using Student’s t test or Mann–Whitney U test as appropriate. Categorical variables were reported as frequencies and percentages. Differences between groups were analyzed using Chi-squared test or Fisher’s exact test. Multivariate logistic regression analysis was used to identify the independent risk factors associated with NIV failure, ICU mortality and hospital mortality. The probability of patients remaining on NIV and in the ICU was analyzed using Kaplan-Meier curves (log-rank test). Statistical significance was considered if a p value was less than 0.05.