Several indices can be assessed by the EIT. These indices include the heterogeneity index or global inhomogeneity index, tidal variation of impedance, regional compliance of the respiratory system, regional ventilation delay, center of ventilation, and end expiratory lung impedance. Table 1 summarizes the definitions, usefulness, and associated formulas of all these indices.

Respiratory physiotherapy techniques aim to expand the thorax to promote lung recruitment and prevent the retention of bronchial secretions and atelectasis.13

Respiratory physiotherapy can promote airway permeabilization.14 It also includes techniques such as:

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  Postural drainage (positioning subject in lateral decubitus to facilitate drainage of bronchial secretion using the force of gravity12);

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  Manual lung hyperinflation (using a manual rebreathing bag connected to a flow of oxygen. A pressure is applied to the bag that is capable of generating pressure peaks of up to 40 cmH2O12);

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  Ventilator lung hyperinflation (this maneuver is performed in controlled mechanical ventilation, doing six breaths per minute with an inspiratory slow flow of 20 L/min, then an inspiratory pause of 2 s, followed by hyperinflation by increasing tidal volume by 200 mL until peak airway pressure of 30 cmH2O12);

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  Percussion (percussing with palm of hand rhythmically on the affected lung segment12);

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  Vibro-compression (fast rhythmic movements with high intensity to cause vibration at the bronchial level. Performed by hand open on the thorax applying pressure);

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  Compression techniques should achieve a difference higher than 33 L/min between peak expiratory flow and peak inspiratory flow (PEF–PIF)15); and

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  Aspiration techniques (direct aspiration of the secretion into the endotracheal tube).

The evidence on the use of respiratory physiotherapy techniques in subject with ARDS is still controversial.4 In this sense, EIT could result in a guide for the application of such maneuvers in subjects with high risk and alveolar instability. Recent evidence has shown that EIT – looking at center of ventilation and end-expiratory lung impedance (EELI) – is a tool that can guide physiotherapy at the bedside of the critically ill subject, not only to compare lung aeration pre versus post therapy, but also to obtain a real-time guide to alveolar gas gain and distribution.13,16

In spontaneously ventilated subjects with ARDS, there is a potential risk for spontaneous ventilation-induced lung injury (P-SILI).17 This risk could be exacerbated by lung expansion maneuvers performed by the physiotherapist. By performing a simple calibration consisting of executing 10 breaths inside a calibration balloon and using the tidal variation of impedance differences, it is possible to determine with EIT the tidal volume mobilized by the subject, even during non-invasive respiratory support or respiratory physiotherapy maneuvers in spontaneously breathing subjects. This could help prevent P-SILI and ensure a safer process of respiratory physiotherapy.18

The body position of critically ill subjects affects ventilation and can lead to decreased lung volume and hypoxemia.19 Physical therapy can help mitigate the negative effects associated with bed rest.20 For example, the application of HFOT in postoperative abdominal surgery subjects placed in a semi-sitting position in bed – inclined at 45° – has been shown to increase EELI as evaluated by EIT, compared to conventional oxygen therapy.25 In addition, in subjects undergoing mechanical ventilation with mild to moderate ARDS, higher positioning such as sitting on the edge of the bed, bipedal standing, walking, and other functional activities have been found to increase EELI and oxygenation without the need to increase PEEP or FiO2,5 as shown in Fig. 2.

Figure 2.

Effects of physical therapy and body position on lung impedance. Legend: position 1: bed rest position. First average image of EELI. Position 2: stand up and walk beside bed. Last average image of EELI.

(0.13MB).

Alteration in pulmonary aeration is easily recognized by EIT through three indexes: the heterogeneity index,21 the variation of tidal impedance22 and pulmonary impedance at the end of expiration.23

NIV, which in the last 20 years has played a crucial role in the management of ventilatory failure in the emergency room and in the ICU, is considered the support of choice in 80% of exacerbations associated with chronic cardiac and respiratory failure and the first line technique to relief subject’s symptoms in 40% of cases,24,25 thus providing a positive pressure that increases EELI. Specifically, a clinical trial found that survivors increased EELI of 50% as compared with non-survivors, where the EELI increased only by 24%.26

The use of HFOT can be helpful to prevent intubation and invasive MV in subjects with hypoxemia and a mild to moderate increase of work of breathing.27 In healthy young people, without increased ventilatory work, an increase in EELI is generated when comparing ventilation in high flow nasal cannula device as compared with room air. Several groups of researchers seek to find a predictor index that determines the need for intubation when using HFOT, through evaluation with EIT, without finding differences between subjects who presented a favorable clinical response to high-flow oxygen therapy compared to those who did not.28 Recently, an asymmetric cannula, designed with a smaller diameter tip and a larger diameter tip, has been approved for clinical use, resulting in an overall 30%–40% increase in total cross-sectional area compared to a conventional interface of the same size.29 This clinical trial used EIT to compare the effects on EELI of conventional nasal cannula versus asymmetric cannula.

Recent evidence reports the possibility of visualizing trapped air though EIT – which would constitute intrinsic PEEP – during acute obstructive events in users with exacerbated chronic obstructive pulmonary disease under invasive mechanical ventilation. This by means of visualization regional end-expiratory flow. Indeed, it is possible to diagnose air trapping by EIT, as well as to guide clinical therapeutics such as the execution of aerosol therapy.30 In that way EIT could visualize effective nebulization to provide efficient inhalation delivery of the drug, increasing, if necessary, the effective dose to reach the distal airways.31

The transition from controlled ventilation, under sedation, to spontaneous or partially controlled ventilatory modalities is a challenge that the clinician faces, especially in situations where connection to mechanical ventilation is associated with respiratory failure. In this sense, EIT allows observing how in injured lungs, a “pendelluft” phenomenon can be generated; movement of air within the lung from non-dependent to dependent regions, without any change in tidal volume, generating dangerous mechanical tension in the dorsal areas, all this caused by spontaneous breathing during mechanical ventilation, at the wrong time.32

In this sense, in patients with ARDS, “pendelluft” phenomenon associated with the onset of spontaneous ventilation has been demonstrated. This phenomenon presented an independent association with the increase of specific inflammatory biomarkers related to lung injury induced by mechanical ventilation, such as interleukin (IL)-8, IL-18, and Caspase-1.33

The process of liberation from MV, known as weaning, is a critical event that involves removing the endotracheal tube and invasive MV support. The success or failure of weaning from MV can greatly impact the morbidity and mortality of critically ill subjects. The spontaneous breathing trial (SBT) is a testing process to determine if a subject can ventilate without support. However, there is currently no consensus on the best technique to define the success or failure of extubation. Multi-modal monitoring is crucial in this pathway to aid in clinical decision-making.34

In some cases, respiratory fatigue, indicated by a loss of EELI at EIT, has been observed within the first five minutes of SBT. Patients who experience this exhaustion phenomenon also tend to have a higher rate of heterogeneity.34

If the heterogeneity index is greater than 41.5, it is not recommended to extubate the subject as it is associated with a failure rate of 87.5%, with a sensitivity of 60.9%. For subjects with an initial heterogeneity index of >41.5, the spontaneous ventilation test should be avoided to prevent failures in 87.5% of cases, with a sensitivity of 60.9%. This method could be used as a detection method to support decision-making.35 EIT seems more precise in predicting successful weaning compared to functional parameters of gas exchange or respiratory mechanics.36 Indeed, during spontaneous breathing, the variation of tidal impedance is significantly reduced, indicating increased respiratory fatigue.34

Alveolar recruitment maneuvers (ARM) are a procedure used in MV in selected cases in subjects with acute respiratory failure or ARDS, where alveolar instability is observed. Many techniques of respiratory physiotherapy can promote alveolar recruitment, and protocolized physiotherapy sessions often include ARMs. The goal of ARM is to decrease lung collapse, improve oxygenation, lung compliance, and increase functional residual capacity.37

It is crucial to determine the lung’s recruitment capacity to avoid overdistension and potential VILI.38 In this context, EIT plays an important role in providing real-time information on the behavior of lung ventilation and its homogeneity during the maneuver. This information can be used to optimize the application of PEEP and tailor the strategy to each subject’s characteristics.

A recent study published by Jonkman et al.39 describes in a cohort of subjects diagnosed with moderate to severe ARDS by COVID-19, the use of EIT to define alveolar recruitment potential and PEEP titration. The study analyzed optimal PEEP, regional distribution of collapse and overdistension, regional distribution of ventilation, recruitment/inflation ratio (R/I), respiratory mechanics, hemodynamics, gas exchange, and recruitment capacity. The authors concluded that EIT is important and feasible in establishing individualized PEEP values, optimizing protective mechanical ventilation The authors conclude the importance and feasibility of EIT to establish an individualized PEEP value according to the characteristics of distensibility and alveolar recruitment of each subject, thus optimizing protective mechanical ventilation.39

Recently, the combination of EIT and esophageal balloon has been used to provide individualized mechanical ventilation support and protection in subjects on mechanical ventilation.35 In addition, a recent systematic review has concluded that the use of EIT is a useful imaging technique that provides safety when performing a PEEP titration maneuver.40