Prone Positioning in COVID-19 ARDS: Comment

The mechanisms underlying oxygenation improvement after prone positioning in COVID-19 acute respiratory distress syndrome have not been fully elucidated yet. The authors hypothesized that the oxygenation increase with prone positioning is secondary to the improvement of ventilation-perfusion matching. In a series of consecutive intubated COVID-19 acute respiratory distress syndrome patients receiving volume-controlled ventilation, the authors prospectively assessed the percent variation of ventilation-perfusion matching by electrical impedance tomography before and 90 min after the first cycle of prone positioning (primary endpoint). The authors also assessed changes in the distribution and homogeneity of lung ventilation and perfusion, lung overdistention and collapse, respiratory system compliance, driving pressure, optimal positive end-expiratory pressure, as assessed by electrical impedance tomography, and the ratio of partial pressure to fraction of inspired oxygen (Pao2/Fio2; secondary endpoints). Data are reported as medians [25th to 75th] or percentages. The authors enrolled 30 consecutive patients, all analyzed without missing data. Compared to the supine position, prone positioning overall improved ventilation-perfusion matching from 58% [43 to 69%] to 68% [56 to 75%] (P = 0.042), with a median difference of 8.0% (95% CI, 0.1 to 16.0%). Dorsal ventilation increased from 39% [31 to 43%] to 52% [44 to 62%] (P < 0.001), while dorsal perfusion did not significantly vary. Prone positioning also reduced lung overdistension from 9% [4 to 11%] to 4% [2 to 6%] (P = 0.025), while it did not significantly affect ventilation and perfusion homogeneity, lung collapse, static respiratory system compliance, driving pressure, and optimal positive end-expiratory pressure. Pao2/Fio2 overall improved from 141 [104 to 182] mmHg to 235 [164 to 267] mmHg (P = 0.019). However, 9 (30%) patients were nonresponders, experiencing an increase in Pao2/Fio2 less than 20% with respect to baseline. In COVID-19 acute respiratory distress syndrome patients, prone positioning overall produced an early increase in ventilation-perfusion matching and dorsal ventilation. These effects were, however, heterogeneous among patients.

PurposeThe purpose of this study was to evaluate the ability of global inhomogeneity index (GI) and left-right asymmetry index (AI) based on electrical impedance tomography (EIT) to be used in assessing cerebral perfusion heterogeneity. The diagnostic value of these two indices in identifying abnormalities in the degree of cerebral perfusion heterogeneity was also explored.MethodsIn this study, Transcranial Doppler (TCD) was used as a control, and unilateral carotid artery was compressed to change the degree of heterogeneity of cerebral perfusion in 15 healthy volunteers. The control group consisted of an additional 15 volunteers without any intervention. EIT perfusion images were obtained by calculating the impedance difference between at the beginning and end of cerebral vasodilation. Subsequently, GI and AI were calculated based on the pixel values of intracranial regions.ResultsThe GI and AI values in the non-carotid artery compression (NCAC) group were significantly lower than those in the unilateral carotid artery compression (UCAC) group (P < 0.001), whereas there was no significant difference between the left carotid artery compression (LCAC) and right carotid artery compression (RCAC) groups. ROC analysis showed that the area under the curve (AUC), specificity and sensitivity of GI in distinguishing between NCAC and UCAC were 0.94, 0.90 and 0.87, respectively. The AUC, specificity and sensitivity of AI in distinguishing between NCAC and UCAC were 0.86, 0.87 and 0.73, respectively.ConclusionThe results demonstrated that the GI and AI effectively quantify the distribution of intracranial perfusion, demonstrating excellent validity and interindividual comparability, and the ability to detect abnormal cerebral perfusion heterogeneity.

BackgroundThe goal of this study was to determine whether an awake prone position (aPP) reduces the global inhomogeneity (GI) index of ventilation measured by electrical impedance tomography (EIT) in COVID-19 patients with acute respiratory failure (ARF).MethodsThis prospective crossover study included COVID-19 patients with COVID-19 and ARF defined by arterial oxygen tension:inspiratory oxygen fraction (PaO2:FIO2) of 100–300 mmHg. After baseline evaluation and 30-min EIT recording in the supine position (SP), patients were randomised into one of two sequences: SP-aPP or aPP-SP. At the end of each 2-h step, oxygenation, respiratory rate, Borg scale and 30-min EIT were recorded.Results10 patients were randomised in each group. The GI index did not change in the SP-aPP group (baseline 74±20%, end of SP 78±23% and end of aPP 72±20%, p=0.85) or in the aPP-SP group (baseline 59±14%, end of aPP 59±15% and end of SP 54±13%, p=0.67). In the whole cohort, PaO2:FIO2 increased from 133±44 mmHg at baseline to 183±66 mmHg in aPP (p=0.003) and decreased to 129±49 mmHg in SP (p=0.03).ConclusionIn spontaneously breathing nonintubated COVID-19 patients with ARF, aPP was not associated with a decrease of lung ventilation inhomogeneity assessed by EIT, despite an improvement in oxygenation.

BackgroundThe effect of positive end-expiratory pressure (PEEP) on intracranial pressure (ICP) dynamics in patients with acute brain injury (ABI) remains controversial. PEEP can benefit oxygenation by promoting alveolar recruitment, but its influence on ICP is complex. The primary aims of this study were to investigate 1) how lung recruitability influences oxygenation and 2) how lung recruitability and regional gas distribution, measured via recruitment-to-inflation (RI) ratio and electrical impedance tomography (EIT), affect ICP in response to PEEP changes in critically ill patients in their early phase of ABI.MethodsTen mechanically ventilated ABI patients were included. Pressure reactivity index (PRx) was estimated. Using RI manoeuvre and EIT, lung recruitability and gas distribution were assessed in response to a standardised PEEP change (from high to low levels, with a delta of 10 cmH2O). Changes in ICP (ΔICP) were calculated between high and low PEEP. Lung inhomogeneity indices (global inhomogeneity index [GI] and local inhomogeneity index [LI]) were derived from EIT. Correlations between ventilatory variables and ICP were analysed.ResultsBlood oxygenation significantly decreased, going from high (14 [IQR: 12–15] cmH₂O) to low (4 [IQR: 2–5] cmH₂O) PEEP. Reducing PEEP significantly increased ICP (from 9 [IQR: 5–13] to 12 [IQR: 8–16] mmHg, p < 0.01), while cerebral perfusion pressure (CPP) improved (from 71 [IQR:67–83] to 75 [IQR: 70–84] mmHg, p = 0.03) and mean arterial pressure (MAP) increased (from 79 [IQR: 69–95] to 84 [IQR: 76–99] mmHg, p < 0.01). The RI ratio correlated significantly with ΔICP (rho = 0.87, p < 0.01), as did Vrec% (proportion of recruited volume, rho = 0.65) and GI (rho = 0.5). LI did not correlate with ΔICP. PRx was 0.30 [IQR: 0.12–0.42], indicating a deranged cerebral autoregulation.ConclusionsPatients with a higher potential for lung recruitability had a more beneficial effect of PEEP on oxygenation. These effects should be interpreted cautiously, given that lung recruitability and global inhomogeneity of gas distribution significantly influenced the intracranial response to PEEP in ABI patients. As indicated by MAP and CPP, PEEP may impact systemic haemodynamics and cerebral perfusion when cerebral autoregulation is deranged. These findings underscore the importance of multimodal (i.e. respiratory, cerebral and haemodynamics) monitoring for optimising ventilation strategies in ABI patients and provide a framework for future research. Trial registration Registration number: NCT05363085, Date of registration: May 2022

Atelectasis and hypoxemia may occur in invasively ventilated patients undergoing bronchoscopy. This study aimed to assess the respiratory effects of recruitment maneuver (RM) in these patients and explore the population likely to benefit from RM after bronchoscopy. This single-center, prospective, observational study included subjects undergoing bronchoscopy who were invasively ventilated. RM was performed 5 min after bronchoscopy by maintaining breaths at a pressure control of 20 cm H2O and a PEEP of 20 cm H2O for 1 min. Lung aeration, monitored using electrical impedance tomography (EIT), was assessed throughout the study. High recruitability was defined as an increase in end-expiratory lung impedance (EELI) greater than 20% relative to its value before RM (△EELI/EELI), along with a decrease in the global inhomogeneity (GI) index due to RM. Of the 80 subjects who completed the study, 32 met the predefined criteria for high recruitability. Subjects' respiratory-system compliance (CRS) before bronchoscopy (odds ratio [OR] 0.921 [0.854-0.993], P = .032) and the decrease in the ratio of PaO2/FIO2 due to bronchoscopy (OR 0.097 [0.020-0.457], P = .003) were significantly associated with high recruitability. In subjects with high recruitability, significant improvements were observed in CRS in the dorsal region (15.3 ± 6.3 vs 13.5 ± 6.0, P = .002), center of ventilation (43.8 ± 4.8 vs 43.0 ± 4.8, P = .01), GI index (0.39 ± 0.07 vs 0.44 ± 0.08, P < .001), and PaO2/FIO2 (257.1 ± 123.5 vs 212 ± 106.6, P < .001) due to RM after fiberoptic bronchoscopy, while no such benefits were observed in subjects exhibiting low recruitability. In subjects with lower CRS before bronchoscopy and a decreased PaO2/FIO2 due to bronchoscopy, RM after bronchoscopy improvedCRS in the dorsal regions, promotedmore uniform ventilation, and improvedoxygenation. These patientsare likely to benefit more from post-bronchoscopy RM.

PurposeTo investigate the difference in the positive end-expiratory pressure (PEEP) selected with chest electrical impedance tomography (EIT) and with global dynamic respiratory system compliance (Crs) in moderate-to-severe pediatric acute respiratory distress syndrome (pARDS).MethodsPatients with moderate-to-severe pARDS (PaO2/FiO2 < 200 mmHg) were retrospectively included. On the day of pARDS diagnosis, two PEEP levels were determined during the decremental PEEP titration for each individual using the best compliance (PEEPC) and EIT-based regional compliance (PEEPEIT) methods. The differences of global and regional compliance (for both gravity-dependent and non-dependent regions) under the two PEEP conditions were compared. In addition, the EIT-based global inhomogeneity index (GI), the center of ventilation (CoV), and standard deviation of regional delayed ventilation (RVDSD) were also calculated and compared.ResultsA total of 12 children with pARDS (5 with severe and 7 with moderate pARDS) were included. PEEPC and PEEPEIT were identical in 6 patients. In others, the differences were only ± 2 cm H2O (one PEEP step). There were no statistical differences in global compliance at PEEPC and PEEPEIT [28.7 (2.84–33.15) vs. 29.74 (2.84–33.47) ml/cm H2O median (IQR), p = 0.028 (the significant level after adjusted for multiple comparison was 0.017)]. Furthermore, no differences were found in regional compliances and other EIT-based parameters measuring spatial and temporal ventilation distributions.ConclusionAlthough EIT provided information on ventilation distribution, PEEP selected with the best Crs might be non-inferior to EIT-guided regional ventilation in moderate-to-severe pARDS. Further study with a large sample size is required to confirm the finding.

Both Electrical Impedance Tomography (EIT) and Computed Tomography (CT) allow the estimation of the lung area. We compared two algorithms for the detection of the lung area per quadrant from the EIT images with the lung areas derived from the CT images. 39 outpatients who were scheduled for an elective CT scan of the thorax were included in the study. For each patient we recorded EIT images immediately before the CT scan. The lung area per quadrant was estimated from both CT and EIT data using two different algorithms for the EIT data. Data showed considerable variation during spontaneous breathing of the patients. Overall correlation between EIT and CT was poor (0.58-0.77), the correlation between the two EIT algorithms was better (0.90-0.92). Bland-Altmann analysis revealed absence of bias, but wide limits of agreement. Lung area estimation from CT and EIT differs significantly, most probably because of the fundamental difference in image generation.

Objective. Wearable electrical impedance tomography (EIT) can be used to monitor regional lung ventilation and perfusion at the bedside. Due to its special system architecture, the amplitude of the injected current is usually limited compared to stationary EIT system. This study aims to evaluate the performance of current injection patterns with various low-amplitude currents in healthy volunteers. Approach. A total of 96 test sets of EIT measurement was recorded in 12 healthy subjects by employing adjacent and opposite current injection patterns with four amplitudes of small current (i.e. 1 mA, 500 uA, 250 uA and 125 uA). The performance of the two injection patterns with various currents was evaluated in terms of signal-to-noise ratio (SNR) of thorax impedance, EIT image metrics and EIT-based clinical parameters. Main results. Compared with adjacent injection, opposite injection had higher SNR (p < 0.01), less inverse artifacts (p < 0.01), and less boundary artifacts (p < 0.01) with the same current amplitude. In addition, opposite injection exhibited more stable EIT-based clinical parameters (p < 0.01) across the current range. For adjacent injection, significant differences were found for three EIT image metrics (p < 0.05) and four EIT-based clinical parameters (p < 0.01) between the group of 125 uA and the other groups. Significance. For better performance of wearable pulmonary EIT, currents greater than 250 uA should be used in opposite injection, 500 uA in adjacent one, to ensure a high level of SNR, a high quality of reconstructed image as well as a high reliability of clinical parameters.

Real-time monitoring of upper airway collapse during sleep could be instrumental for studies in biomechanics of obstructive sleep apnea (OSA) and selecting individualized treatment modalities. Although some imaging techniques are used under sedated sleep, none are available during the entire natural sleep process. We hypothesized that electrical impedance tomography (EIT) can be used for noninvasive continuous imaging of the upper airway during natural sleep and quantifying upper airway collapse in terms of its size. After determining surface landmarks to attach the electrodes for monitoring the retroglossal airway, EIT was conducted in 10 healthy participants. As a feasibility test of EIT in detecting upper airway collapse, transient airway closure was induced by the swallowing maneuver. These EIT images were confirmed by simultaneous magnetic resonance imaging (MRI) scans. Subsequently, EIT scans were conducted in 7 healthy participants and 10 patients with OSA under nonsedated sleep to determine whether it could identify upper airway narrowing or collapse. Respiratory events were identified by concurrent polysomnography (PSG). Swallowing-induced airway closure was identified successfully in all 10 participants on simultaneous EIT and MRI scans. Sizes and positions of the upper airway closures in reconstructed EIT images were well correlated with those in magnetic resonance images. Obstructive hypopnea and apnea were detected successfully by EIT in 10 patients with OSA, and no significant changes in EIT data were observed in 7 healthy participants during concurrent EIT and PSG tests. Additionally, conductivity changes in the airway were greater during obstructive apnea than during hypopnea (64.3% versus 26.3%, respectively; P < .001) compared with those during baseline respiration. EIT could be a useful real-time monitoring device for detecting upper airway narrowing or collapse during natural sleep in patients with OSA. Currently, changes in the upper airway size can be estimated with good accuracy, but shape estimation needs future improvements in the EIT image quality.

Clearance of secretions remains a challenge in ventilated patients. Despite high-frequency percussive ventilation (HFPV) showing benefits in patients with cystic fibrosis and neuromuscular disorders, very little is known about its effects on other patient categories. Therefore, we designed a physiological pilot study investigating the effects on lung aeration and gas exchange of short HFPV cycles in tracheostomized patients undergoing mechanical ventilation. Electrical impedance tomography (EIT) was recorded at baseline (T0) by a belt wrapped around the patient's chest, followed by the HFPV cycle lasting 10 min. EIT data was collected again after the HFPV cycle (T1) as well as after 1 h (T2) and 3 h (T3) from T0. Variation from baseline of end-expiratory lung impedance (∆EELI), tidal variation (TIV) and global inhomogeneity index (GI) were computed. Arterial blood was also taken for gas analysis. HFPV cycle significantly improved the ∆EELI at T1, T2 and T3 when compared to baseline (p < 0.05 for all comparisons). The ratio between arterial partial pressure and inspired fraction of oxygen (PaO2/FiO2) also increased after the treatment (p < 0.001 for all comparison) whereas TIV (p = 0.132) and GI (p = 0.114) remained unchanged. Short cycles of HFPV superimposed to mechanical ventilation promoted alveolar recruitment, as suggested by improved ∆EELI, and improved oxygenation in tracheostomized patients with high load of secretion.Trial Registration Prospectively registered on www.clinicaltrials.gov (NCT05200507; dated 6th January 2022).

Low-volume lung injury encompasses local concentration of stresses in the vicinity of collapsed regions in heterogeneously ventilated lungs. We aimed to study the effects on ventilation and perfusion distributions of a sequential lateral positioning (30°) strategy using electrical impedance tomography imaging in a porcine experimental model of early acute respiratory distress syndrome (ARDS). We hypothesized that such strategy, including a real-time individualization of positive end-expiratory pressure (PEEP) whenever in lateral positioning, would provide attenuation of collapse in the dependent lung regions. A two-hit injury acute respiratory distress syndrome experimental model was established by lung lavages followed by injurious mechanical ventilation. Then, all animals were studied in five body positions in a sequential order, 15 min each: Supine 1; Lateral Left; Supine 2; Lateral Right; Supine 3. The following functional images were analyzed by electrical impedance tomography: ventilation distributions and regional lung volumes, and perfusion distributions. The induction of the acute respiratory distress syndrome model resulted in a marked fall in oxygenation along with low regional ventilation and compliance of the dorsal half of the lung (gravitational-dependent in supine position). Both the regional ventilation and compliance of the dorsal half of the lung greatly increased along of the sequential lateral positioning strategy, and maximally at its end. In addition, a corresponding improvement of oxygenation occurred. In conclusion, our sequential lateral positioning strategy, with sufficient positive end-expiratory pressure to prevent collapse of the dependent lung units during lateral positioning, provided a relevant diminution of collapse in the dorsal lung in a porcine experimental model of early acute respiratory distress syndrome.

Investigating the effect of inhaled nitric oxide combined with prone position ventilation on ventilation/perfusion matching in patients with moderate-to-severe acute respiratory distress syndrome.

Electrical impedance tomography (EIT) is the process of spatially mapping the conductivity distribution of a domain based on voltage-current relationships observed at the boundary of the domain. Due to its low cost, ease of implementation, and benign nature (i.e. non-ionizing), EIT has received much attention for potential application to damage, strain, and saturation mapping in civil, mechanical, and aerospace venues that employ conductive materials (e.g. nanofiller-modified cements and polymers, carbon fiber composites, etc.). Mathematically, however, EIT is an ill-posed inverse problem predicated on reconstructing the conductivity distribution on a model of the domain. This process is prone to nonconvergence due to factors such as experiment-to-model domain shape discrepancies and electrode misplacement. Time difference imaging is commonly used to overcome this challenge–boundary measurements are collected when the material is in a pristine state and another set of boundary measurements are collected after some conductivity-changing perturbation such that experiment-to-model discrepancies largely subtract out. But this process of time-difference imaging is problematic for several reasons: one, a pristine state is not available for existent or already in-service structures. Two, pristine-state data can be lost over time. And three, the pristine-state data may no longer be applicable if there are significant shape changes, electrodes are reapplied, etc. Thus, it is desirable to have a baseline-free technique for EIT. To that end, we herein present the results of a preliminary investigation on damage detection via frequency-difference imaging (fdEIT). In this approach, we leverage the fact that materials generally exhibit frequency-dependent conductivity such that an EIT image can be formed from the difference between two different frequencies. Experimental measurements of AC conductivity up to 10 MHz were collected from a carbon nanofiber (CNF)-modified glass fiber/epoxy laminate. These measurements were then used in a computational simulation to explore the potential of fdEIT for damage detection and localization. The results of this study suggest that fdEIT may indeed be a viable method of eliminating baseline measurements for EIT thereby providing an important advance towards making EIT a practical inspection modality.

This paper introduces a new method of simultaneous acquisition of electrocardiogram (ECG) signals and electrical impedance tomography (EIT) measurements on active electrodes, i.e. electrodes that are applying current, in an EIT system. Howland noise reduction followed by an integrate-and-dump filter and adaptive filtering are used to extract the ECG signal without loss of amplitude. Results of simultaneous ECG signals and EIT reconstructions on human subjects are shown. It is also demonstrated that the recovered ECG waveforms from 32 electrodes can be used to reconstruct the heart current source vectors during a cardiac cycle, yielding an approximate solution to the inverse problem of electrocardiography. Acquiring ECG signals from all electrodes attached to the body simultaneously with EIT data acquisition eliminates the need for a separate system to collect ECG signals. Time-aligned ECG signals are helpful in interpreting pulsatile perfusion images by providing exact timing of each image in relation to the cardiac cycle. Additionally, collecting ECG signals from many or all of the EIT electrodes enables reconstruction of the heart's moving dipole moment at the same time as the EIT images.

A real-time EIT imaging system based on the split augmented Lagrangian shrinkage algorithm