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

Abstract

Introduction: As it becomes clear that mechanical ventilation can exaggerate lung injury, individual titration of ventilator settings is of special interest. Electrical impedance tomography (EIT) has been proposed as a bedside, regional monitoring tool to guide these settings. In the present study we evaluate the use of ventilation distribution change maps (DeltafEIT maps) in intensive care unit (ICU) patients with or without lung disorders during a standardized decremental positive end-expiratory pressure (PEEP) trial.

Methods: Functional EIT (fEIT) images and PaO2/FiO2 ratios were obtained at four PEEP levels (15 to 10 to 5 to 0 cm H2O) in 14 ICU patients with or without lung disorders. Patients were pressure-controlled ventilated with constant driving pressure. fEIT images made before each reduction in PEEP were subtracted from those recorded after each PEEP step to evaluate regional increase/decrease in tidal impedance in each EIT pixel (DeltafEIT maps).

Results: The response of regional tidal impedance to PEEP showed a significant difference from 15 to 10 (P = 0.002) and from 10 to 5 (P = 0.001) between patients with and without lung disorders. Tidal impedance increased only in the non-dependent parts in patients without lung disorders after decreasing PEEP from 15 to 10 cm H2O, whereas it decreased at the other PEEP steps in both groups.

Conclusions: During a decremental PEEP trial in ICU patients, EIT measurements performed just above the diaphragm clearly visualize improvement and loss of ventilation in dependent and non-dependent parts, at the bedside in the individual patient.

Figure 1

Principle of electrical impedance tomography (EIT) and the functional EIT image (fEIT). Electrical excitation currents are applied between pairs of adjacent surface electrodes (1 to 16); the resulting voltages are measured between the other electrodes (U). In the fEIT image, impedance variation induced by the tidal volume is divided into a 32 × 32 matrix. Each pixel contains the individual tidal impedance variation, creating an image of ventilation distribution. The ventral to dorsal oriented ROIs are marked in gray in the right panel.

Figure 2

The effect of a decremental PEEP trial on regional ventilation shown in two representative patients. The functional EIT (fEIT) image at the different PEEP levels (15 to 10 to 5 to 0 cm H₂O) shows the ventilation distribution in a colour-coded matrix in a patient without lung disorders and a patient with lung disorders. The ΔfEIT images are created by subtracting fEIT before the PEEP step from fEIT after each PEEP step. The increase or decrease in regional ventilation between PEEP (ΔfEIT) steps is displayed in a color-coded matrix. Each EIT image represents a thoracic slice with the ventral lung regions at the top and dorsal lung regions at the bottom.

Figure 3

Changes in regional compliance in patients without (left) and patients with (right) lung disorders. During pressure-controlled ventilation with constant driving pressure, the tidal impedance change per pixel can be regarded as regional compliance change per pixel. The open triangle represents the dependent lung region and the open circle represents the non-dependent lung region at the different used PEEP levels. Data are presented as mean and SEM. Significance: * P <0.05; ** P <0.01.

Figure 4

ΔfEIT images in patients without (1 to 6) and with (8 to 14) lung disorders between the PEEP steps used. PaO₂/FiO₂ ratio change (black) and compliance change (red) are presented next to each ΔfEIT image. Images containing a colour-coded 32 × 32 matrix, are generated by subtracting fEIT before the PEEP step from fEIT after each PEEP step. PEEP is decreased stepwise from 15 to 0 cm H₂O. Each EIT image represents a thoracic slice with the ventral lung regions at the top and dorsal lung regions at the bottom.

Figure 5

Response to decremental PEEP steps on tidal impedance change in patients with or without lung disorders. During pressure-controlled ventilation with constant driving pressure, the tidal impedance change per pixel can be regarded as regional compliance change per pixel. The percentage positive of the total Δtidal impedance is calculated from the total increase and total decrease in each Δfunctional EIT image. Data are presented as median and interquartile range.