First Attempt at Using Electrical Impedance Tomography to Predict High Flow Nasal Cannula Therapy Outcomes at an Early Phase

Abstract

Objective: Spatial and temporal ventilation distributions in patients with acute respiratory failure during high flow nasal cannula (HFNC) therapy were previously studied with electrical impedance tomography (EIT). The aim of the study was to explore the possibility of predicting HFNC failure based on various EIT-derived parameters. Methods: High flow nasal cannula failure was defined reintubation within 48 h after HFNC. EIT was performed with the patients spontaneously breathing in the supine position at the start of HFNC. EIT-based indices (comprising the global inhomogeneity index, center of ventilation, ventilation delay, rapid shallow breathing index, minute volume, and inspiration to expiration time) were explored and evaluated at three time points (prior to HFNC, T1; 30 min after HFNC started, T2; and 1 h after, T3). Results: A total of 46 subjects were included in the final analysis. Eleven subjects had failed HFNC. The time to failure was 27.8 ± 12.4 h. The ROX index (defined as SpO₂/FiO₂/respiratory rate) for HFNC success patients was 8.3 ± 2.7 and for HFNC failure patients, 6.2 ± 1.8 (p = 0.23). None of the investigated EIT-based parameters showed significant differences between subjects with HFNC failure and success. Further subgroup analysis indicated that a significant difference in ventilation inhomogeneity was found between ARDS and non-ARDS [0.54 (0.37) vs. 0.46 (0.28) as evaluated with GI, p < 0.01]. Ventilation homogeneity significantly improved in ARDS after 60-min HFNC treatment [0.59 (0.20) vs 0.57 (0.19), T1 vs. T3, p < 0.05]. Conclusion: Spatial and temporal ventilation distributions were slightly but insignificantly different between the HFNC success and failure groups. HFNC failure could not be predicted by changes in EIT temporal and spatial indexes of ventilation distribution within the first hour. Further studies are required to predict the outcomes of HFNC.

Keywords: acute respiratory failure; electrical impedance tomography; high flow nasal cannula; outcome prediction; ventilation distribution.

Figure 1

Illustration of electrical impedance tomography (EIT)-derived parameters at different time points between high flow nasal cannula (HFNC) success and failure groups. GI, the global inhomogeneity index; CoV, center of ventilation; I:E, inspiration to the expiration time. T1, before HFNC; T2, 30 min after HFNC started; T3, 1 h after HFNC started.

Figure 2

Change of conventional EIT-based parameters at different time points. T1, before HFNC; T2, 30 min after HFNC started; T3, 1 h after HFNC started. All values were normalized to that at T1. The boxes mark the quartiles with median marked red, while the whiskers extend from the box out to the most extreme data value within 1.5^* the interquartile range of the sample. The red crosses are outliers. Green dashed lines marked the value of 0. GI, the global inhomogeneity index. Lower than 0 means the ventilation becomes more homogeneous. CoV, the center of ventilation. Higher than 0 means that the ventilation distribution moves toward dorsal regions. RVD, regional ventilation delay. Higher than 0 means the delay is getting worse.

Figure 3

Change of EIT-based parameters for spontaneous breathing at different time points. T1, before HFNC; T2, 30 min after HFNC started; T3, 1 h after HFNC started. All values were normalized to that at T1. The boxes mark the quartiles with median marked red, while the whiskers extend from the box out to the most extreme data value within 1.5^* the interquartile range of the sample. The red crosses are outliers. Green dashed lines marked the value of 0. RSBI, rapid shallow breathing index. Higher than 0 means the subject is breathing more rapidly or shallowly. MV, minute volume. Lower than 0 means the minute volume becomes less at T3. I:E, inspiration to the expiration time. Lower than 0 means the inspiration time is getting shorter.