Modern Approaches to Lung Visualization During Mechanical Ventilation

“Protective” ventilatory strategies have been designed to try to optimize mechanical ventilation and minimize its potential damages. These strategies are mainly based on the application of low tidal volumes and maneuvers designed to increase the functional residual capacity, trying to reduce cyclic alveolar collapse and overdistension of the lung.

 

Global parameters such as pressure-volume curves or measurement of the respiratory system compliance do not reliably translate what is actually happening in the lung, especially regarding to regional distribution of the tidal volume administrated. EIT images of the regional distribution of ventilation and changes in lung volume in real time highlight the relationship between the dependent and non-dependent lung regions. This dynamic evaluation makes EIT a helpful tool to optimize ventilator parameters on an individualized basis, as EIT may assist in defining mechanical ventilation settings, assess distribution of tidal volume and of end-expiratory lung volume (EELV) and contribute to titrate positive end-expiratory pressure (PEEP)/tidal volume combinations.

 

In addition, visualization of the lungs during mechanical ventilation is a critically important aspect of treating respiratory complications arising from combat injuries. Mechanical ventilation is often necessary for soldiers who have suffered acute respiratory failure or significant lung injuries, such as pulmonary contusions, which are commonly sustained during military operations. These lung injuries can lead to serious complications and long-term respiratory problems, highlighting the importance of effective imaging techniques for diagnosis and treatment planning in such acute care cases.

 

Methods of lung visualization during mechanical ventilation

 

Among the methods of lung imaging, computed tomography (CT) is considered the gold standard. CT images allow clinicians to assess patient condition, response to PEEP titrations, alveoli recruitment/distension and gas distributions. However, CT for ICU patients is costly, requires transfer to the radiology department or a portable CT machine and further exposes the patient to radiation. Hence, it is not a regular care-monitoring tool.

 

Portable X-rays remain the most common radiographic examination as they can be carried out at the bedside and subject the patient to a smaller radiation dose. However, portable radiographs have lower and more variable quality and commonly only show one two-dimensional, frontal plane, compared to images in multiple planes with varying slice thickness available in CT, helical CT or other three-dimensional techniques. Finally, and most importantly, imaging methods that use ionising radiation are not suitable for continuous or semi-continuous monitoring of lung condition. 

 

An emerging form of lung imaging is electrical impedance tomography (EIT).

 

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Electrical impedance tomography (EIT) is a non-invasive radiation-free technique under fast development, which allows monitoring the regional distribution of lung ventilation and perfusion at the bedside. EIT has been proposed as a valid means to identify the patient's optimal PEEP through various possible techniques. And it is based on the measurement of lung tissue impedance by injection of small currents and voltage measurements, using electrodes on the skin surface. 

 

EIT images are determined by the distribution of intrathoracic bioimpedance. Bioimpedance is defined by the specific composition of tissues, taking into account water, electrolytes, fat, etc. High content of extracellular water, high concentration of electrolytes, large cells and a high number of cell connections reduce impedance, whilst fat accumulation, bone and air increase impedance. Therefore, pathological changes of the tissue composition (such as pleural effusion, lung fibrosis, alveolar fluid, etc.) entail a change in regional bioimpedance. Lung imaging is therefore an ideal application of EIT; large changes in lung volume (and therefore in conductivity) can be easily detected due to the closeness of the lungs to the surface.

 

Hence, obtaining functional EIT, we are able to visualize global and local ventilation activity over a period of time (selected by the user), picturing a map of ventilation distribution. Functional EIT produces EIT measures, defined as values derived from an EIT data acquisition. 

 

 

EIT image obtained with the help of Elisa 800 R VIT

 

EIT devices use the relative approach and thus calculate difference impedance images in relation to a reference. Medium and Elisa 800 R VIT from Löwenstein Medical SE & Co, which combines the versatile options of a modern intensive care ventilator in the top market segment with ventilator-integrated tomography. As the world’s first ventilation system with integrated electrical impedance tomography, elisa 800VIT provides radiation-free bedside imaging and ventilation under visual control. Ventilation effects can be continuously assessed in real time in order to detect complications. The latest textile sensor technology minimizes the skin injuries caused by older systems and can be used in prone positions. The functional imaging display, specifically designed for use in everyday clinical situations, is easy to interpret.

 

Regional electrical bioimpedance measurements require an electrode belt of electrodes placed around the chest wall, including a reference electrode connected to a central body, preferably to the abdomen (the reference electrode ensures that all measurements of the different electrode pairs are related to the same electrical potential).

 

 

Noteworthy, EIT has several other possible clinical applications for treating patients undergoing mechanical ventilation:

• guiding the diagnostic process in unstable patients that could not be safely moved to the radiological suite 

• detecting pneumothorax, pleural effusion, atelectasis and ventilatory dyssynchrony, 

• assessing the risk of weaning failure in high risk critically ill patients 

• evaluating the benefits of treatments, such as chest physiotherapy techniques 

• monitoring outpatients requiring monitored anesthesia care.

 

EIT has shown:

• good correlation with CT, such that it has been proposed and recently validated to guide ventilation therapy. 

• to be useful in the detection of pneumothoraces, quantification of pulmonary edema and comparison of distribution of ventilation between different modes of ventilation and may offer superior individual titration of PEEP and other ventilator parameters compared with existing approaches.

 

Studies performed in the last decade have demonstrated that EIT-guided ventilation results in improved respiratory mechanics, improved gas exchange and reduced histologic evidence of ventilator-induced lung injury in an animal model. Careful titration of PEEP following maximal lung recruitment effectively reverses existing alveolar collapse and prevents further alveolar closure. EIT-based algorithms that estimate recruitable alveolar collapse, avoiding hyperdistension, have been one of the most interesting topics over the last years. Lung EIT has been used to assess and quantify the global and regional changes in lung impedance at the end of exhalation, quantifying gains (recruitment) and losses (overdistention or de-recruitment), granting a more realistic evaluation of different ventilator modes or recruitment maneuvers, and helping in the identification of responders and non-responders to such maneuvers.

 

Contraindications to artificial lung ventilation with electrical impedance tomography (EIT) include situations where the EIT device itself cannot be used, or where the patient's condition makes EIT monitoring unreliable or potentially harmful. In particular, pacemakers, automatic implantable cardioverter-defibrillators (AICDs), and implantable pumps are contraindications for endotracheal implantation due to potential interference with device function. Other contraindications include severe hemodynamic instability, undrained pneumothorax, and severe neuromuscular diseases. In addition, conditions such as intracranial hypertension or acute coronary syndrome, in which hypercapnia occurs, may also be contraindications.

 

References:

1. Bedside measurement of changes in lung impedance to monitor alveolar ventilation in dependent and non-dependent parts by electrical impedance tomography during a positive end-expiratory pressure trial in mechanically ventilated intensive care unit patients / I. G. Bikker et al. Critical Care. 2010. Vol. 14, no. 3. P. R100. URL: https://doi.org/10.1186/cc9036 (date of access: 09.07.2025).

2. Biomedical engineer’s guide to the clinical aspects of intensive care mechanical ventilation / V. J. Major et al. BioMedical Engineering OnLine. 2018. Vol. 17, no. 1. URL: https://doi.org/10.1186/s12938-018-0599-9 (date of access: 09.07.2025).

3. Effect of EIT-guided PEEP titration on prognosis of patients with moderate to severe ARDS: study protocol for a multicenter randomized controlled trial / X. Yuan et al. Trials. 2023. Vol. 24, no. 1. URL: https://doi.org/10.1186/s13063-023-07280-6 (date of access: 09.07.2025).

4. Electrical Impedance Tomography: A Compass for The Safe Route to Optimal PEEP / N. Sella et al. Respiratory Medicine. 2021. P. 106555. URL: https://doi.org/10.1016/j.rmed.2021.106555 (date of access: 09.07.2025).

5. Electrical impedance tomography / B. Lobo et al. Annals of Translational Medicine. 2018. Vol. 6, no. 2. P. 26. URL: https://doi.org/10.21037/atm.2017.12.06 (date of access: 09.07.2025).

6. elisa 800 R-VIT. Beatmungskompetenz. Aus einer Hand. | Löwenstein. URL: https://loewensteinmedical.com/en/intensive-care-ventilation/elisa-800-r-vit/ (date of access: 09.07.2025).

7. Walsh BK, Smallwood CD. Electrical Impedance Tomography During Mechanical Ventilation. Respir Care. 2016 Oct;61(10):1417-24. doi: 10.4187/respcare.04914. PMID: 27682815.