The time taken for the regional distribution of ventilation to stabilise: an investigation using electrical impedance tomography

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.

Lung Imaging Acquisition with Electrical Impedance Tomography: Tackling Common Pitfalls

Electrical impedance tomography is a powerful tool for lung imaging that can be employed at the bedside in multiple clinical scenarios. Diagnosing and preventing interpretation pitfalls will ensure reliable data and allow for appropriate clinical decision-making.

Effect of randomization of nasal high flow rate in preterm infants

OBJECTIVE

To assess the effect of nasal high flow (NHF) cannula on end-expiratory level (EEL), continuous distending pressure (CDP) and regional ventilation distribution in preterm infants.

DESIGN

A prospective observational clinical study with randomly applied NHF rates.

PATIENTS AND SETTING

Preterm infants requiring continuous positive airway pressure (CPAP) respiratory support in a Neonatal Intensive Care Unit.

INTERVENTIONS

Infants were measured on randomly applied flow rates at 2, 4, and 6 L/min of NHF and compared with bubble CPAP.

MEASUREMENTS AND RESULTS

Regional ventilation distribution and EEL were measured using electrical impedance tomography (EIT) and respiratory inductance plethysmography (RIP) in 24 preterm infants (31.19 ± 1.17 weeks corrected age). Changes in CDP were measured from the esophagus via the nasogastric tube. Physiological variables were also recorded. There were no differences in ventilation distribution, EEL or CDP between CPAP and NHF (P > .05). However, the physiological variables of FiO₂ (P = .01) and SpO₂ /FiO₂ (P < .01) were improved on CPAP compared with NHF.

CONCLUSION

NHF applied in random order with flow rates between 2 to 6 L/min was equally as good as CPAP in maintaining EEL and ventilation distribution in stable preterm infants. Overall oxygenation was better on CPAP compared to NHF.

A Calibration Technique for the Estimation of Lung Volumes in Nonintubated Subjects by Electrical Impedance Tomography

BACKGROUND

Electrical impedance tomography (EIT) is a bedside monitoring technique of the respiratory system that measures impedance changes within the thorax. The close correlation between variations in impedance (ΔZ) and lung volumes (Vt) is known. Unless Vt is measured by an external reference (e.g., spirometry), its absolute value (in milliliters) cannot be determined; however, measurement of Vt would be useful in nonintubated subjects.

OBJECTIVE

To validate a simplified and feasible calibration method of EIT, which allows estimation of Vt in nonintubated subjects.

MATERIALS AND METHODS

We performed a prospective study on 13 healthy volunteers. Subjects breathed 10 times in a nonexpandable "calibration balloon" with a known volume while wearing the EIT belt. The relationship between ΔZ and the balloon volume was calculated (ΔZ/Vt). Subsequently, subjects were connected to a mechanical ventilator by a mouthpiece under different settings. Vt was calculated from EIT measurements (VtEIT) by means of the ΔZ/Vt coefficient and compared with the value obtained from the ventilator (Vtflow).

RESULTS

There was a close correlation between Vtflow and VtEIT (r2 = 0.89). The fit equation was VtEIT = 0.9 × Vtflow +10.1. The highest correlation was found at positive endexpiratory pressure (PEEP) 0 (mean: VtEIT = 0.93 × Vtflow) versus PEEP 8 (mean: VtEIT = 0.8 × Vtflow), p = 0.01. No differences in the fit equation were found between pressure support ventilation (PSV) 0 and PSV 8, p = 0.50. Further analysis showed no statistically significant differences between sex, height, and BMI.

CONCLUSION

A simple and fast EIT calibration technique enables reliable, noninvasive monitoring of Vt in nonintubated subjects.

Lung aeration and ventilation after percutaneous tracheotomy measured by electrical impedance tomography in non-hypoxemic critically ill patients: a prospective observational study

Background

Percutaneous dilatational tracheotomy (PDT) may lead to transient impairment of pulmonary function due to suboptimal ventilation, loss of positive end-expiratory pressure (PEEP) and repetitive suction maneuvers during the procedure. Possible changes in regional lung aeration were investigated using electrical impedance tomography (EIT), an increasingly implied instrument for bedside monitoring of pulmonary aeration.

Methods

With local ethics committee approval, after obtaining written informed consent 29 patients scheduled for elective PDT under bronchoscopic control were studied during mechanical ventilation in supine position. Anesthetized patients were monitored with a 16-electrode EIT monitor for 2 min at four time points: (a) before and (b) after initiation of neuromuscular blockade (NMB), (c) after dilatational tracheostomy (PDT) and (d) after a standardized recruitment maneuver (RM) following surgery, respectively. Possible changes in lung aeration were detected by changes in end-expiratory lung impedance (Δ EELI). Global and regional ventilation was characterized by analysis of tidal impedance variation.

Results

While NMB had no detectable effect on EELI, PDT led to significantly reduced EELI in dorsal lung regions as compared to baseline, suggesting reduced regional aeration. This effect could be reversed by a standardized RM. Mean delta EELI from baseline (SE) was: NMB − 47 ± 62; PDT − 490 ± 180; RM − 89 ± 176, values shown as arbitrary units (a.u.). Analysis of regional tidal impedance variation, a robust measure of regional ventilation, did not show significant changes in ventilation distribution.

Conclusion

Though changes of EELI might suggest temporary loss of aeration in dorsal lung regions, PDT does not lead to significant changes in either regional ventilation distribution or oxygenation.

Global tidal variations, regional distribution of ventilation, and the regional onset of filling determined by electrical impedance tomography: reproducibility

The reproducibility of the regional distribution of ventilation and the timing of onset of regional filling as measured by electrical impedance tomography lacks evidence. This study investigated whether electrical impedance tomography measurements in healthy males were reproducible when electrodes were replaced between measurements. Part 1: Recordings of five volunteers lying supine were made using electrical impedance tomography and a pneumotachometer. Measurements were repeated at least three hours later. Skin marking ensured accurate replacement of electrodes. No stabilisation period was allowed. Part 2: Electrical impedance tomography recordings of ten volunteers; a 15 minute stabilisation period, extra skin markings, and time-averaging were incorporated to improve the reproducibility. Reproducibility was determined using the Bland-Altman method. To judge the transferability of the limits of agreement, a Pearson correlation was used for electrical impedance tomography tidal variation and tidal volume. Tidal variation was judged to be reproducible due to the significant correlation between tidal variation and tidal volume (r² = 0.93). The ventilation distribution was not reproducible. A stabilisation period, extra skin markings and time-averaging did not improve the outcome. The timing of regional onset of filling was reproducible and could prove clinically valuable. The reproducibility of the tidal variation indicates that non-reproducibility of the ventilation distribution was probably a biological difference and not measurement error. Other causes of variability such as electrode placement variability or lack of stabilisation when accounted for did not improve the reproducibility of the ventilation distribution.

Effect of Electrode Belt and Body Positions on Regional Pulmonary Ventilation- and Perfusion-Related Impedance Changes Measured by Electric Impedance Tomography

Ventilator-induced or ventilator-associated lung injury (VILI/VALI) is common and there is an increasing demand for a tool that can optimize ventilator settings. Electrical impedance tomography (EIT) can detect changes in impedance caused by pulmonary ventilation and perfusion, but the effect of changes in the position of the body and in the placing of the electrode belt on the impedance signal have not to our knowledge been thoroughly evaluated. We therefore studied ventilation-related and perfusion-related changes in impedance during spontaneous breathing in 10 healthy subjects in five different body positions and with the electrode belt placed at three different thoracic positions using a 32-electrode EIT system. We found differences between regions of interest that could be attributed to changes in the position of the body, and differences in impedance amplitudes when the position of the electrode belt was changed. Ventilation-related changes in impedance could therefore be related to changes in the position of both the body and the electrode belt. Perfusion-related changes in impedance were probably related to the interference of major vessels. While these findings give us some insight into the sources of variation in impedance signals as a result of changes in the positions of both the body and the electrode belt, further studies on the origin of the perfusion-related impedance signal are needed to improve EIT further as a tool for the monitoring of pulmonary ventilation and perfusion.

Speaking valves in tracheostomised ICU patients weaning off mechanical ventilation--do they facilitate lung recruitment?

BACKGROUND

Patients who require positive pressure ventilation through a tracheostomy are unable to phonate due to the inflated tracheostomy cuff. Whilst a speaking valve (SV) can be used on a tracheostomy tube, its use in ventilated ICU patients has been inhibited by concerns regarding potential deleterious effects to recovering lungs. The objective of this study was to assess end expiratory lung impedance (EELI) and standard bedside respiratory parameters before, during and after SV use in tracheostomised patients weaning from mechanical ventilation.

METHODS

A prospective observational study was conducted in a cardio-thoracic adult ICU. 20 consecutive tracheostomised patients weaning from mechanical ventilation and using a SV were recruited. Electrical Impedance Tomography (EIT) was used to monitor patients' EELI. Changes in lung impedance and standard bedside respiratory data were analysed pre, during and post SV use.

RESULTS

Use of in-line SVs resulted in significant increase of EELI. This effect grew and was maintained for at least 15 minutes after removal of the SV (p < 0.001). EtCO2 showed a significant drop during SV use (p = 0.01) whilst SpO2 remained unchanged. Respiratory rate (RR (breaths per minute)) decreased whilst the SV was in situ (p <0.001), and heart rate (HR (beats per minute)) was unchanged. All results were similar regardless of the patients' respiratory requirements at time of recruitment.

CONCLUSIONS

In this cohort of critically ill ventilated patients, SVs did not cause derecruitment of the lungs when used in the ventilator weaning period. Deflating the tracheostomy cuff and restoring the airflow via the upper airway with a one-way valve may facilitate lung recruitment during and after SV use, as indicated by increased EELI.

TRIAL REGISTRATION

Anna-Liisa Sutt, Australian New Zealand Clinical Trials Registry (ANZCTR).

ACTRN

ACTRN12615000589583. 4/6/2015.