Patients
This study for human subjects was approved by The Institutional Research and Ethics Committee of the Peking Union Medical College Hospital (JS-1896). Written informed consent was obtained from all patients before enrollment.
Adult critically ill patients with respiratory failure who received early mobilization were enrolled in this study. Inclusion criteria were the following: (1) presentation of respiratory failure-Partial pressure of oxygen/Fraction of inspiration oxygen(PaO2/FiO2)<300, oxygen saturation(SpO2)< 92%, or respiration rate(RR) > 25 bpm. (2) Weaned from mechanical ventilation>24h. (3) need to carry out early mobilization according to patient's condition (e.g. ICU duration≥1 week, high risk of respiratory complication). Patients were excluded from the study if they were aged < 18 years, were pregnant, had a body mass index (BMI) over 50 kg/m2, had ribcage malformation, had any contraindication against using EIT monitoring (automatic implantable cardioverter defibrillator, chest skin injury, etc.), or had any contraindication against early mobilization (hemodynamic instability, FiO2>60%, arrhythmia, etc).
Physiological measurements
We collected baseline data at enrollment, including age, sex, ICU duration, days of intubation, type of surgery. Respiratory parameters like RR, SpO2, FiO2, Tidal volume, and hemodynamic parameters, including heart rate (HR), mean arterial pressure (MAP) were obtained at different time points.
Experimental protocol
Participants went through early mobilization as sitting on the wheel chair. The patient’s condition before early mobilization was defined as baseline, with supine position on the bed. Then patients were moved from bed to wheelchair and sitting for 1 hour. After that, patients will return to supine position on the bed. During this movement, patients had EIT belt fixed on their chest and monitoring regional lung ventilation at 4 time points (Tbase: baseline, supine position before early mobilization, T30min: sitting position on the wheelchair for 30min, T60min: sitting position on the wheelchair for 60min, Treturn: return to supine position on the bed after early mobilization.) Moreover, patients were spontaneous breathing with fixed FiO2 during the procedure.
EIT measurements
EIT measurements were performed with PulmoVista500 (Dräger Medical, Lübeck, Germany). During the protocol, a silicone EIT belt with 16 electrodes were attached on the surface of patient’s chest in one transverse plane corresponding to the 4th intercostal parasternal space and was then connected to the EIT monitor for bedside visualization. At different positions, we make sure the 16-electrode array was attached to the same level on the chest surface, so that the lung ventilation is comparable. EIT evaluates lung ventilation by collecting potential differences and the known excitation currents in a 32*32 pixel matrix [17]. Each scan pixel demonstrates the instantaneous relative local impedance change compared with a reference state of local impedance. The stimulation frequency and amplitude were adjusted automatically by the EIT device to minimize the influence of background noise. EIT measurements were continuously performed at 20 Hz. In addition, the data were digitally filtered using a low-pass filter with a cutoff frequency of 0.67 Hz to eliminate cardiac-related impedance changes. EIT data were analyzed by offline computer program.
Analysis of EIT data
The EIT data of 2 minutes at baseline and under different time points were acquired. Tidal images were calculated as the difference images between end-inspiration and end-expiration. An average of 2-min tidal images was calculated at each position to minimize the influence of spontaneous breathing. Mean tidal images were divided into four symmetrical, non-overlapping ventral-to-dorsal horizontal regions of interest (ROIs), ranging from the gravity-independent area to the gravity-dependent area, namely, the ventral (ROI1), mid-ventral (ROI2), mid-dorsal (ROI3) and dorsal (ROI4) regions.
The global inhomogeneity (GI) index can indicate the overall change in inhomogeneity of ventilation and the status of local lung distribution [18]. In brief, the sum of differences between individual pixels to average value was calculated. This sum value was normalized to the amplitude of impedance tidal variation. A lower GI index value is related to a better ventilation homogeneity.
The center of ventilation (CoV) describes the weighted geometrical center of the ventilation distribution [19-21]. When most of the tidal ventilation distributes to the dependent lung region, this results in a higher CoV value.
Patients were divided into two groups: (1) Dependent region recruited group(DR group): ventilation distribution of ΔROI(3+4)/ROI(3+4)Tbaseline≥ 15%, (2) Dependent region not-recruitment group(Non-DR group): ventilation distribution ΔROI(3+4)/ROI(3+4)Tbaseline< 15%, (ΔROI(3+4)= ROI(3+4)T60min- ROI(3+4)Tbaseline)
Statistical analysis
Statistical analysis was measured with SPSS 23.0 (IBM, Armonk, NY) and Prism 7 (GraphPad Software, San Diego, California, USA).
Normal distribution was assessed with Kolmogorov-Smirnov normality test. Normally distributed results are presented as mean ± SD whereas non-normally distributed results are presented as median (25th–75th percentile). The Mann–Whitney test was used for comparisons of groups (recruitment group VS not-recruitment group). Comparisons of the trends of the related parameters according to the different positions were performed using a General Linear Model Repeated Measures (GLMRM)[22]. This model is an extension of the classical ANOVA, which allows handling both fixed effect (different position) and random effect (patient). GLMRM takes into account the correlation between multiple measurements on one patient and thus the estimated marginal means were adjusted for the covariates and the trends of related EIT parameters corresponding to the different position. When Mauchly’s test of sphericity is not demanded (p <0.05), Epsilon (Greenhouse-Geisser) was used for the corrected test. All statistics were two-tailed, and a p value of less than 0.05 was considered to be significant.