Electrical Impedance Tomography in Critical Children

Electrical impedance tomography (EIT) provides a non-invasive, bedside, radiation-free method for monitoring regional ventilation and pulmonary perfusion in critically ill children.

Introduction

Electrical impedance tomography (EIT) is a non-invasive, radiation-free imaging tool that enables real-time monitoring of regional lung aeration at the patient’s bedside (Bachmann et al. 2018; Costa et al. 2008; Walsh and Smallwood 2016).

EIT uses electrodes arranged around the patient’s thorax to deliver small electrical currents during ventilation. A belt composed of 16–32 electrodes is positioned between the fourth and fifth intercostal spaces, in direct contact with the patient’s skin, thereby avoiding the use of interposed bandages.

The resulting electrical impedance changes are converted into a real-time 2D image (corresponding to the real-time representation of the impedance values of each pixel) and a continuous plethysmogram registry (Becher et al. 2022; Franchineau et al. 2024; Nascimento et al. 2022). EIT also enables monitoring of respiratory mechanics simultaneously, as a flow sensor can be placed next to the patient’s endotracheal tube or non-invasive ventilation mask. Additionally, EIT enables lung perfusion monitoring, based on changes in electrical impedance due to regional pulmonary blood distribution (Fossali et al. 2022).

As a significant number of children admitted to the Paediatric Intensive Care Unit (PICU) require mechanical ventilation (MV) (Miller and Scott 2022), EIT may play an essential role in understanding how regional air distribution happens during ventilation, allowing for optimising paediatric respiratory strategies (Inany et al. 2020) (Figure 1).

Main Concepts

EIT generates a 2D real-time image that follows the same spatial distribution pattern as the CT, with the left side of the thorax represented on the right part of the image, and when in supine position, dorsal regions located in the lower part of the image (Figure 2).

The 2D image shows regional lung aeration in different areas, coded by a scale of blue colours (the lighter the blue, the more air is going in and out during ventilation, reaching a white colour, which may indicate overdistension). Grey areas are regions where there are no changes in aeration.

A plethysmogram is a graphical representation of the variation in electrical impedance over time due to regional aeration changes. Each oscillation corresponds to a single breath. The oscillation amplitude, known as Delta Z (ΔZ), is considered a surrogate of tidal volume. End Expiratory Lung Impedance (EELI) corresponds to the baseline of the plethysmogram curve and is regarded as a surrogate of the residual lung volume.

In addition, EIT can assess ventilation segmented into regions of interest (ROIs). Frequently used ROIs include ventral vs. dorsal, as well as right vs. left quadrants: ventral left, ventral right, dorsal left, and dorsal right. Layers include ventral, mid-ventral, mid-dorsal, and dorsal.

Lastly, regional perfusion (Figure 3) can be assessed through the electrical impedance changes caused by pulmonary blood flow, which are detected by EIT. Depending on the EIT device, ventilation and perfusion may be evaluated simultaneously and continuously or intermittently; an inspiratory hold must be set to evaluate perfusion after a normal saline IV bolus.

Main Functionalities

Several functionalities have been described:

• Prompt detection of respiratory complications during mechanical ventilation: (Figure 4) atelectasis, pneumothorax, accidental extubation or selective intubation.
• Evaluation of the effect of body positioning on regional ventilation: it is advantageous in determining the duration of prone-supine postural changes in ARDS (Figure 5).
• PEEP titration during IMV (Figure 6): decremental PEEP test evaluates both regional collapse and overdistension. Ideal PEEP consists of the level with the best balance between both standards.
• V/Q mismatch screening: regional ventilation can be compared with regional pulmonary perfusion to evaluate V/Q mismatch. Pulmonary embolism or asthma are some conditions that could benefit from this.

Research on the Use of EIT in Children

EIT-based research has seen an increasing interest in the scientific community in recent years, mainly focused on paediatrics, on understanding ventilation in critical care children.

One of the main addressed topics has been how body positioning (BP) impacts regional aeration in ventilated children. Evidence suggests that adults exhibit a gravity-dependent ventilation phenomenon, with increased ventilation towards the gravity-dependent lung (e.g., Frerichs et al., 1996). In non-ventilated healthy children between 6 months and 9 years, Lupton Smith et al. (2014) found an absence of consistent gravity-dependent phenomena, with high interpatient variability in the predominant lung aeration (dependent vs non-dependent lung). On the other hand, in a cohort of more than 200 ventilated neonates and infants monitored with EIT, Becher et al. (2022) reported that preterm neonates followed the classical adult pattern; however, a challenging finding was observed in non-preterm neonates and infants, with predominant ventilation towards the gravity-dependent lung and regions. More studies are required to better understand the possible gravity dependence phenomenon in children. Howventilationmodeinfluencesventilationrepresentsanotherchallengingpoint. Whencomparing controlled vs supported ventilation modes, Inany et al. (2020) and Nascimiento et al. (2022) documented a decrease of the dorsal contribution to global aeration in children under controlled modes of ventilation versus spontaneous supported modes, which may be related to a loss of diaphragm contribution.

EIT has also assessed the effect of chest physiotherapy (CPT) on children under MV. Davies et al. (2019) first proposed EIT as a possible method to monitor CPT. McAlinden et al. (2020) compared, in an EIT-based study of MV patients admitted to the PICU, the use of CPT plus periodic suctioning versus suctioning alone, finding the CPT-suctioning combination to be significantly more effective.

Although it seems that this tool is helping to improve the understanding of physiology in mechanically ventilated patients, unfortunately, studies showing improved outcomes after the implementation of this technology are still lacking.

Its use is spreading in the paediatric community, so we expect the proper studies to rapidly increase the level of evidence.

Experience in PICU of a Quaternary Children’s Hospital

a) EIT- guided respiratory therapy in children

EIT has been applied to children admitted to SJD Children’s Hospital PICU since 2020. In our unit, 45 and 80 patients annually are admitted due to respiratory conditions requiring invasive and non-invasive mechanical ventilation, respectively. The main limitation of EIT implementation that we have encountered, as will probably have happened to other units, is the availability of only one EIT monitor. In addition, a significant investment in fungible is required, as 5-10 different sizes of electrode belts are needed for children between 0 and 18 years old, which may limit its widespread use, especially in public PICUs.

EIT has been found to be especially useful at the bedside, particularly for PEEP titration purposes. Severe ARDS patients or children with severe tracheobronchial Malacia benefit from an EIT-based PEEP election, as improvements in lung aeration and balance between the percentage of lung with hyperinflation and collapse can be assessed in real time at the bedside.

Body positioning has been another key point addressed with EIT. Prone positioning to improve dorsal aeration is a widely implemented tool in respiratory patients in the PICU, but there is no clear evidence on its ideal duration. Our PICU usually implements a 20:4 hours of prone: supine approach. Since the EIT acquisition, prone duration has been titrated based on the aeration of the lungs’ dorsal and ventral regions, demonstrating that some patients need to switch to supine after 2 hours of prone positioning and others after 3 days. Thus, an individualised approach of prone-supine can be achieved with EIT monitoring.

Finally, another key point to consider with EIT is assessing atelectasis response to respiratory manoeuvres, such as bronchoscopy, changes in body positioning, or suction of mucus plugs. This approach avoids unnecessary additional suctioning, PEEP increase or clearance manoeuvres, allowing detection in real-time of the resolution of atelectasis.

b) EIT-based research in children

Assessing a possible gravity-dependent ventilation phenomenon in mechanically ventilated children is probably the main challenging research question we have addressed in recent years. The paediatric population, as reflected above, lacks evidence in this matter. We are currently running a 3-year clinical trial to assess through body positioning changes how gravity affects children’s aeration and ventilation while under MV.

Conflict of Interest

None.