Monitoring Lung Function with Electrical Impedance Tomography in the Intensive Care Unit

Ventilation monitoring
The dynamic image (Figure 4) displays real-time variations of air distribution during ventilation using colors ranging from dark blue (least ventilated) to white (most ventilated) to represent regional changes. Gray areas indicate no variation in ventilation. The dynamic images allow quick identification of differences in intrapulmonary time constants and the presence of paradoxical patterns. It is important to note that areas with limited air variation during a respiratory cycle may result from overdistension or collapsed areas.

The "ventilation map" (Figure 4) illustrates how air volume distributes across a defined cross-section during breath cycles. Bright blue indicates lung regions that receive most of the tidal volume, which is proportional to the change of impedance signal between inspiration and expiration. Conversely, dark blue represents areas with low volume variation. The ventilation map allows the assessment of regional ventilation distribution within the lungs. The lungs are divided into anterior/posterior and right/left regions, allowing for detailed assessment and the display of plethysmographs in specific regions on the screen⁴.

The plethysmogram thorax impedance variation curve (Figure 4) represents the wave amplitude corresponding to tidal volume, with the baseline equivalent to pulmonary aeration or Functional Residual Capacity (FRC) or End-Expiratory Lung Volume (EELV). Aeration information can estimate relative changes in total intrathoracic air volume.

Airway parameters on the right side of the screen (Figure 4) are captured by the flow sensor and displayed as waveform graphs and numbers. Parameters such as driving pressure, auto PEEP, alveolar plateau pressure, compliance, and resistance (in the numerical column on the right) are calculated during controlled cycles. The parameters PEEP, peak pressure, tidal volume, and respiratory rate will be displayed in all cycles. Using the proximal flow sensor allows the integration of ventilation and impedance data on the same screen, regardless of the mechanical ventilator brand or model.

PEEP titration tool (Figure 5)
The patient should be synchronized with the ventilator avoiding spontaneous breathing effort and movement that may affect PEEP titration. This can be reached with adequate sedation, and if necessary with paralytic agents. The flow sensor and ventilator tubing should be free from any obstructions, such as liquid and secretions, to maintain accurate monitoring.

EIT detects changes in regional ventilation and, when integrated with a flow meter is capable of estimating regional respiratory mechanics, including airway pressure, tidal volume, and flow. It presents results as percentages of collapsed and hyperdistended areas at different PEEP levels by calculating regional compliance changes. Some authors proposed to titrate PEEP to the crossing point between the percentage of overdistension (white curve in Figure 5 and white area in Figure 6) and the percentage of collapse (blue curve de Figure 5 and blue area in Figure 6). At this PEEP level, there is a minimum occurrence of both hyperdistended and collapsed areas (orange curve in Figure 5) and lung function. Ongoing studies are investigating whether the PEEP set at the crossing point between hyperdistension and collapse is clinically advantageous.

Figure 5: The PEEP titration tool on the EIT screen. The orange curve represents compliance, the white curve represents hyperdistension, and the blue curve represents collapse. Abbreviations: EIT = electrical impedance tomography; PEEP = positive-end expiratory pressure. Please click here to view a larger version of this figure.

Figure 6: Display of the percentages of hyperdistension (white) and collapse (blue), and compliance for different PEEP values on the EIT screen. Abbreviations: EIT = electrical impedance tomography; PEEP = positive-end expiratory pressure. Please click here to view a larger version of this figure.

Assessment of pulmonary perfusion with EIT: a guide for healthcare providers
Electrical Impedance Tomography (EIT) has recently been recognized to be a valuable monitoring tool for lung ventilation by measuring changes in electrical conductivity. While EIT primarily focuses on assessing air distribution within the lungs, it can also provide valuable insights into pulmonary perfusion through innovative techniques.

Changes in impendence from the movement of blood in the thorax are of much smaller amplitude than those related to ventilation. Thus, EIT has not traditionally been used to measure perfusion. However, certain methods involving the intravenous injection of a hypertonic saline solution in combination with a breath-hold maneuver can isolate and amplify impedance changes related to blood flow. As this solution travels through the blood vessels, it alters the electrical properties of the blood, which EIT can detect. EIT can indirectly infer perfusion patterns by observing the impedance changes caused by this solution as it circulates through the pulmonary vasculature. This approach enables us to gain a deeper understanding of both ventilation and perfusion within the lungs simultaneously¹⁰. This tool is for research purposes only in the US and/or according to local hospitals' regulations and/or other nations' approval by legal-bodies regulators.

Visualizing pulmonary perfusion
The intravenous injection of a solution with high electrical conductivity, such as hypertonic saline or sodium bicarbonate, aids in visualizing blood flow within the pulmonary vasculature^11,12,13. Areas with higher perfusion exhibit different impedance patterns compared to less perfused regions. This innovative application of EIT allows for a relative assessment of perfusion alongside ventilation imaging, providing a comprehensive view of lung function, which helps to differentiate hypoxemia caused by perfusion defects, usually treated with therapies that modulate lung perfusion, from hypoxemia caused by ventilatory disturbances, often addressed with ventilation strategies or position changes. This application also allows monitoring of the changes in regional pulmonary perfusion in response to the established treatment (such as inhaled nitric oxide, anticoagulants, and thrombolytic drugs).

Perfusion tool
The perfusion tool within EIT is specifically designed to visualize pulmonary blood flow during controlled mechanical ventilation. It involves the injection of a hypertonic saline solution into a vein during a brief apnea period. The resulting image displays the distribution of pulmonary perfusion, with colors ranging from yellow (indicating higher perfusion) to dark red (indicating lower perfusion) in the chest's cross-section (see Figure 7).

Figure 7: Variations in the percentage of distribution of perfusion to different regions of the chest. Shown are variations in perfusion to anterior, posterior, right, and left, with colors ranging from yellow (higher perfusion) to dark red (lower perfusion) in the chest's cross-section. It is also possible to run the processed video online showing the contrast flowing through the heart in blue color after to the lungs in red colors. Abbreviations: A = anterior; P = posterior; R = right; L = left. Please click here to view a larger version of this figure.

Online and offline analysis
EIT continuously measures plethysmograms and the distribution of air throughout the lungs. The impedance variation reflects tidal volume changes, enabling regional evaluation of the lungs. The plethysmogram graphically represents lung volume changes during inspiration and expiration (Figure 8). The variation of the air can be measured in different parts of the lungs. This is one of the most advantageous measurements from EIT, as it assesses regional ventilation.

The EIT device creates a 32 x 32 matrix to map the entire lung area. This matrix is conveyed into a grid covering the entire lungs. Each tiny square within the grid, known as a pixel, is assigned a value of resistivity or impedance. Changes in the impedance values correspond to changes to the lung volume in the specific part of the lung.

Using dedicated software, EIT takes these changes in impedance values and generates an image. This image helps us to understand the magnitude of the variation in volume, represented on a color scale. Bright blue signifies high volume, and dark blue indicates low volume. No variation in impedance or no change in tidal volume is represented in grey color (Figure 8). Essentially, it functions as a map, pinpointing precisely where these changes occurred within the lung.

Figure 8: The Ventilation Dynamic Image illustrating each pixel in a matrix 32 x 32, totaling 1,024 pixels. The amplitude of the ventilation is represented by the amplitude of the wave and intensity of the color, with gray indicating no volume and transitioning from bright blue to dark blue representing high to low volume, respectively. Please click here to view a larger version of this figure.

There are numerous clinical situations where EIT can be beneficial. For example, in early identification of complications and conditions that may lead to lung injury, such as atelectasis, overdistension, and pneumothorax. Atelectasis is one of the most common pathologies in hospitalized patients. It involves the partial or complete collapse of the lung tissue, reducing lung volumes and impairing gas exchange. Atelectasis could be detected by EIT as shown in Figure 9A. Figure 9A and Figure 9B are the Ventilation Map Images from the same patient, less than 13 min apart. In Figure 9A, only 23% of the impedance changes occur in the posterior region, which can also be seen by a reduction in bright blue and dark blue areas observed in this region. Following an increase in PEEP from 4 to 10 cmH₂O, Figure 9B reveals increased ventilation in the posterior lung which increased from 23% to 43%. Compared to Figure 9A, the patient exhibits an increase in compliance from 18.8 to 27.6 mL/cmH₂O. Notably, this gain occurs in the bilateral posterior region, which is evident from the increased light and dark blue areas in the posterior part (Figure 9B). Furthermore, there is a reduction in driving pressure, indicating that further increases in tidal volume and PEEP do not impose additional stress on the lungs^14,15.

Figure 9: Differences in ventilation at different PEEP values. (A) At PEEP 4 cmH₂O, the image shows a difference in ventilation between the anterior (more ventilated) and posterior (less ventilated) regions. (B) Following an increase in PEEP from 4 to 10 cmH₂O, improved ventilation in the posterior region is evident. Abbreviation: PEEP = positive-end expiratory pressure. Please click here to view a larger version of this figure.

Overdistension refers to the overexpansion or stretching of lung tissue beyond its physiologic capacity, leading to potential damage to the alveoli and surrounding structures. Overdistension could occur when the pressure applied from a mechanical ventilator to inflate the lungs is too high. Monitoring regional lung impedance during ventilatory procedures avoids overdistension and lung injury¹⁶. In Figure 10A, the patient is on PEEP of 22 cmH₂O, whereas in Figure 10B, the PEEP is reduced to 12 cmH₂O. In Figure 10B, the Ventilation Dynamic Image from EIT displays an increase in light and dark blue areas in the anterior lung, indicating increased ventilation. Simultaneously, there is a reduction in light and dark blue areas in the posterior lung (from 67% to 43%), suggesting a relief of overdistension associated with the higher PEEP of 22 cmH₂O in Figure 10A. This example shows the ability of EIT to identify overdistension and promote lung protective ventilation across the lung⁹.

Figure 10: Changes in PEEP. (A) PEEP of 22 cmH₂O; (B) PEEP of 12 cmH₂O. Please click here to view a larger version of this figure.

Pneumothorax is a condition characterized by the presence of air in the pleural cavity, the space between the lung and the chest wall. This accumulation of air can lead to lung collapse, mediastinal shift, and hemodynamic collapse. With EIT, changes in the thorax impedance could be observed in real time, as depicted in the Ventilation Dynamic Image^17,18,19. There is one sign in the Ventilation Dynamic Image showing the suspicion of pneumothorax, called the "out of phase" sign. The "out of phase" sign refers to a visual indication where the impedance changes in the lung do not align correctly with the respiratory cycle. In a normal respiratory cycle, impedance changes in the lung should be synchronized with the inhalation and expiration phases. When pneumothorax occurs, the Ventilation Dynamic Image will demonstrate a deviation from the expected pattern as the impedance changes are not synchronized with the normal inhalation and expiration phases. Additionally, an elevation in the baseline of the plethysmograph meaning an increase in end-expiratory lung impedance (EELI), despite PEEP reduction, may further indicate the presence of a pneumothorax (Figure 11).

Figure 11: The "out of phase" sign in a ventilation map. Simultaneously, the plethysmograph exhibits an elevation of baseline, despite a reduction in PEEP. Both findings strongly support and confirm the presence of pneumothorax. Please click here to view a larger version of this figure.