Is there a need for individualized adjustment of electrode belt position during EIT-guided titration of positive end-expiratory pressure?

Technical Principles and Clinical Applications of Electrical Impedance Tomography in Pulmonary Monitoring

Pulmonary monitoring is crucial for the diagnosis and management of respiratory conditions, especially after the epidemic of coronavirus disease. Electrical impedance tomography (EIT) is an alternative non-radioactive tomographic imaging tool for monitoring pulmonary conditions. This review proffers the current EIT technical principles and applications on pulmonary monitoring, which gives a comprehensive summary of EIT applied on the chest and encourages its extensive usage to clinical physicians. The technical principles involving EIT instrumentations and image reconstruction algorithms are explained in detail, and the conditional selection is recommended based on clinical application scenarios. For applications, specifically, the monitoring of ventilation/perfusion (V/Q) is one of the most developed EIT applications. The matching correlation of V/Q could indicate many pulmonary diseases, e.g., the acute respiratory distress syndrome, pneumothorax, pulmonary embolism, and pulmonary edema. Several recently emerging applications like lung transplantation are also briefly introduced as supplementary applications that have potential and are about to be developed in the future. In addition, the limitations, disadvantages, and developing trends of EIT are discussed, indicating that EIT will still be in a long-term development stage before large-scale clinical applications.

Setting positive end-expiratory pressure by using electrical impedance tomography

PURPOSE OF REVIEW

This review presents the principles and possibilities of setting positive end-expiratory pressure (PEEP) using electrical impedance tomography (EIT). It summarizes the major findings of recent studies where EIT was applied to monitor the effects of PEEP on regional lung function and to guide the selection of individualized PEEP setting.

RECENT FINDINGS

The most frequent approach of utilizing EIT for the assessment of PEEP effects and the PEEP setting during the time period from January 2022 till June 2023 was based on the analysis of pixel tidal impedance variation, typically acquired during stepwise incremental and/or decremental PEEP variation. The most common EIT parameters were the fraction of ventilation in various regions of interest, global inhomogeneity index, center of ventilation, silent spaces, and regional compliance of the respiratory system. The studies focused mainly on the spatial and less on the temporal distribution of ventilation. Contrast-enhanced EIT was applied in a few studies for the estimation of ventilation/perfusion matching.

SUMMARY

The availability of commercial EIT devices resulted in an increase in clinical studies using this bedside imaging technology in neonatal, pediatric and adult critically ill patients. The clinical interest in EIT became evident but the potential of this method in clinical decision-making still needs to be fully exploited.

Distribution of regional lung function in upright healthy subjects determined by electrical impedance tomography in two chest examination planes

Objective. The variation in pulmonary gas content induced by ventilation is not uniformly distributed in the lungs. The aim of our study was to characterize the differences in spatial distribution of ventilation in two transverse sections of the chest using electrical impedance tomography (EIT).Approach. Twenty adult never-smokers, 10 women and 10 men (mean age ± SD, 31 ± 9 years), were examined in a sitting position with the EIT electrodes placed consecutively in a caudal (6th intercostal space) and a cranial (4th intercostal space) chest location. EIT data were acquired during quiet breathing, slow and forced full expiration manoeuvres. Impedance variations representing tidal volume (VT), vital capacity (VC), forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) were calculated at the level of individual image pixels and their spatial distribution was determined using the following EIT measures: the centres of ventilation in ventrodorsal (CoVvd) and right-to-left direction (CoVrl), the dorsal and right fractions of ventilation, the coefficient of variation (CV) and the global inhomogeneity (GI) index.Main results. The sums of pixel ventilation-related impedance variations reproduced reliably the volumetric dissimilarities amongVT, VC, FEV1and FVC, with no significant differences noted between the two examination planes. Significant differences in ventilation distribution were found between the planes during tidal breathing and slow full expiration, mainly regarding the ventrodorsal direction, with higher values of CoVvdand dorsal fraction of ventilation in the caudal plane (p< 0.01). No significant differences in the spatial distribution of FEV1and FVC were detected between the examination planes.Significance. The spatial distribution of ventilation differed between the two examination planes only during the relaxed (quiet breathing and slow VC manoeuvre) but not during the forced ventilation. This effect is attributable to the differences in thoracoabdominal mechanics between these types of ventilation.

Clinical Applicability of Electrical Impedance Tomography in Patient-Tailored Ventilation: A Narrative Review

Electrical Impedance Tomography (EIT) is a non-invasive bedside imaging technique that provides real-time lung ventilation information on critically ill patients. EIT can potentially become a valuable tool for optimising mechanical ventilation, especially in patients with acute respiratory distress syndrome (ARDS). In addition, EIT has been shown to improve the understanding of ventilation distribution and lung aeration, which can help tailor ventilatory strategies according to patient needs. Evidence from critically ill patients shows that EIT can reduce the duration of mechanical ventilation and prevent lung injury due to overdistension or collapse. EIT can also identify the presence of lung collapse or recruitment during a recruitment manoeuvre, which may guide further therapy. Despite its potential benefits, EIT has not yet been widely used in clinical practice. This may, in part, be due to the challenges associated with its implementation, including the need for specialised equipment and trained personnel and further validation of its usefulness in clinical settings. Nevertheless, ongoing research focuses on improving mechanical ventilation and clinical outcomes in critically ill patients.

Individualized PEEP without Recruitment Maneuvers Improves Intraoperative Oxygenation: A Randomized Controlled Study

Individualized positive end-expiratory pressure (PEEP) combined with recruitment maneuvers improves intraoperative oxygenation in individuals undergoing robot-assisted prostatectomy. However, whether electrical impedance tomography (EIT)-guided individualized PEEP without recruitment maneuvers can also improve intraoperative oxygenation is unknown. To test this, fifty-six male patients undergoing elective robot-assisted laparoscopic prostatectomy were randomly assigned to either individualized PEEP (Group PEEPIND, n = 28) or a control with a fixed PEEP of 5 cm H2O (Group PEEP5, n = 28). Individualized PEEP was guided by EIT after placing the patients in the Trendelenburg position and performing intraperitoneal insufflation. Patients in Group PEEPIND maintained individualized PEEP without intermittent recruitment maneuvers, and those in Group PEEP5 maintained a PEEP of 5 cm H2O intraoperatively. Both groups were extubated in a semi-sitting position once the extubation criteria were met. The primary outcome was arterial oxygen partial pressure (PaO2)/inspiratory oxygen fraction (FiO2) prior to extubation. Other outcomes included intraoperative driving pressure, plateau pressure and dynamic, respiratory system compliance, and the incidence of postoperative hypoxemia in the post-operative care unit (PACU). Our results showed that the intraoperative median for PEEPIND was 16 cm H2O (ranging from 12 to 18 cm H2O). EIT-guided PEEPIND was associated with higher PaO2/FiO2 before extubation compared to PEEP5 (71.6 ± 10.7 vs. 56.8 ± 14.1 kPa, p = 0.003). Improved oxygenation extended into the PACU with a lower incidence of postoperative hypoxemia (3.8% vs. 26.9%, p = 0.021). Additionally, PEEPIND was associated with lower driving pressures (12.0 ± 3.0 vs. 15.0 ± 4.4 cm H2O, p = 0.044) and better compliance (44.5 ± 12.8 vs. 33.6 ± 9.1 mL/cm H2O, p = 0.017). Our data indicated that individualized PEEP guided by EIT without intraoperative recruitment maneuvers also improved perioperative oxygenation in patients undergoing robot-assisted laparoscopic radical prostatectomy, which could benefit patients with the risk of intraoperative hemodynamic instability caused by recruitment maneuvers. Trial registration: China Clinical Trial Registration Center Identifier: ChiCTR2100053839. This study was registered on 1 December 2021. The first patient was recruited on 15 December 2021.

A novel method for diaphragm-based electrode belt position of electrical impedance tomography by ultrasound

BACKGROUND

This aim of study was to introduce a diaphragm-based EIT-belt placement method based on diaphragm position by ultrasound, and to evaluate the difference between diaphragm-based EIT-belt placement and conventional EIT-belt placement.

METHOD

The diaphragm position (L0) determined by ultrasound was taken as zero reference level. The direction of headward is defined as positive, and toward feet is negative. For EIT data collection, the electrode belt was placed at 7 different levels, respectively (denoted as L-2 cm, L0, L2cm, L4cm, L6cm, L8cm, L10cm) at supine position in healthy volunteers. The diaphragm-based EIT-belt level (Lxcm) was defined where highest tidal impedance variation (TV) was achieved. Subsequently, EIT measurements were conducted at diaphragm-based EIT-belt levels and traditional EIT-belt level in 50 critically ill patients under mechanical ventilation.

RESULT

The highest TV was achieved at L6cm and the smallest at L-2 cm., so the L6cm were taken as diaphragm-based EIT-belt level by ultrasound in 8 healthy volunteers. In 23 patients, the diaphragm-based EIT-belt plane agreed with the conventional planes (4th-6th ICS), which was defined as the Agreed group. Other patients were classified to the Disagreed group (above 4th ICS). The Disagreed group has a significantly higher BMI and lower global TV at the diaphragm-based EIT-belt plane compared to the Agreed group.

CONCLUSIONS

The diaphragm-based EIT-belt position by ultrasound was feasible and resulted in different belt positions compared to the conventional position in > 50% of the examined subjects, especially in patients with higher BMI. Further study is required to validate the impact on EIT images with this novel method on clinical management.

The influence of gravity on electrical impedance tomography measurements during upper body position change

Objective

The aim of the study was to examine the influence of gravity on regional ventilation measured by electrical impedance tomography (EIT) with the standard electrode belt position at the 5th intercostal space during tilting from supine to sitting positions.

Methods

A total of 30 healthy volunteers were examined prospectively in supine position during quiet tidal breathing. Subsequently, the bed was tilted so that the upper body of the subjects achieved 30, 60 and 90° every 3 min. Regional ventilation distribution and end-expiratory lung impedance (EELI) were monitored with EIT throughout the whole experiment. Absolute tidal volumes were measured with spirometry and the volume-impedance ratio was calculated for each position.

Results

The volume-impedance ratio did not differ statistically between the studied body positions but 11 subjects exhibited a large change in ratio at one of the positions (outside 99.3% coverage). In general, ventilation distribution became more heterogeneous and moved towards dorsal regions as the upper body was tilted to 90-degree position. EELI increased and tidal volume decreased. The lung regions identified at various positions differed significantly.

Conclusion

Gravity has non-negligible influence on EIT data, as the upper body tilted from supine to sitting positions. The standard electrode belt position might be reconsidered if ventilation distribution is to be compared between supine and sitting positions.

Gas distribution by EIT during PEEP inflation: PEEP response and optimal PEEP with lowest trans-pulmonary driving pressure can be determined without esophageal pressure during a rapid PEEP trial in patients with acute respiratory failure

Objective. Protective ventilation should be based onlungmechanics and transpulmonary driving pressure (ΔPTP), as this 'hits' the lung directly.Approach. The change in end-expiratory lung volume (ΔEELV) is determined by the size of the PEEP step and the elastic properties of the lung (EL), ΔEELV/ΔPEEP. Consequently, EL can be determined as ΔPEEP/ΔEELV. By calibration of tidal inspiratory impedance change with ventilator inspiratory tidal volume, end-expiratory lung impedance changes were converted to volume changes and lung P/V curves were obtained during a PEEP trial in ten patients with acute respiratory failure. The PEEP level where ΔPTP was lowest (optimal PEEP) was determined as the steepest point of the lung P/V curve.Main results. Over-all EL ranged between 7.0-23.2 cmH₂O/L. Optimal PEEP was 12.9 cmH₂O (10-16) with ΔPTP of 4.1 cmH₂O (2.8-7.6). Patients with highest EL were PEEP non-responders, where EL increased in non-dependent and dependent lung at high PEEP, indicating over-distension in all lung. Patients with lower EL were PEEP responders with decreasing EL in dependent lung when increasing PEEP.Significance. PEEP non-responders could be identified by regional lung P/V curves derived from ventilator calibrated EIT. Optimal PEEP could be determined from the equation for the lung P/V curve.