Detection of 'best' positive end-expiratory pressure derived from electrical impedance tomography parameters during a decremental positive end-expiratory pressure trial

Abstract

Introduction: This study compares different parameters derived from electrical impedance tomography (EIT) data to define 'best' positive end-expiratory pressure (PEEP) during a decremental PEEP trial in mechanically-ventilated patients. 'Best' PEEP is regarded as minimal lung collapse and overdistention in order to prevent ventilator-induced lung injury.

Methods: A decremental PEEP trial (from 15 to 0 cm H2O PEEP in 4 steps) was performed in 12 post-cardiac surgery patients on the ICU. At each PEEP step, EIT measurements were performed and from this data the following were calculated: tidal impedance variation (TIV), regional compliance, ventilation surface area (VSA), center of ventilation (COV), regional ventilation delay (RVD index), global inhomogeneity (GI index), and intratidal gas distribution. From the latter parameter we developed the ITV index as a new homogeneity parameter. The EIT parameters were compared with dynamic compliance and the PaO2/FiO2 ratio.

Results: Dynamic compliance and the PaO2/FiO2 ratio had the highest value at 10 and 15 cm H2O PEEP, respectively. TIV, regional compliance and VSA had a maximum value at 5 cm H2O PEEP for the non-dependent lung region and a maximal value at 15 cm H2O PEEP for the dependent lung region. GI index showed the lowest value at 10 cm H2O PEEP, whereas for COV and the RVD index this was at 15 cm H2O PEEP. The intratidal gas distribution showed an equal contribution of both lung regions at a specific PEEP level in each patient.

Conclusion: In post-cardiac surgery patients, the ITV index was comparable with dynamic compliance to indicate 'best' PEEP. The ITV index can visualize the PEEP level at which ventilation of the non-dependent region is diminished, indicating overdistention. Additional studies should test whether application of this specific PEEP level leads to better outcome and also confirm these results in patients with acute respiratory distress syndrome.

Figure 1

Example of electrical impedance tomography (EIT) image reconstruction. Shown is the distribution of impedance to the dependent and non-dependent lung regions of one representative patient. The lighter the color, the higher the impedance and the more aerated the lung region. The surface area used in all calculations at every positive end-expiratory pressure (PEEP) step is equal to the largest surface area. The EIT image is divided into two equal regions of interest represented by the white line; all EIT modalities are calculated using this setup. Decreasing the PEEP value resulted in a decrease in aeration of the lungs, especially in the dependent region.

Figure 2

Different electrical impedance tomography (EIT) modalities calculated for the decremental positive end-expiratory pressure (PEEP) trial. Effects of different PEEP levels on regional changes in (A) tidal impedance variation (TIV); (B) regional compliance; (C) ventilation surface area (VSA); (D) center of ventilation (COV); (E) regional ventilation delay (RVD) index; and (F) global inhomogeneity (GI) index. Data are presented as means ± 95% CI. Dashed lines represent the interpolation lines; open circles = non-dependent regions; solid circles = dependent regions; solid squares = entire EIT image.

Figure 3

Changes in partial arterial oxygen pressure (PaO₂)/inspired oxygen fraction (FiO₂) ratio and dynamic compliance during a decremental positive end-expiratory pressure (PEEP) trial. (A) The PaO₂/FiO₂ ratio (mmHg) and (B) dynamic compliance (mL/cm H₂O) are shown for the entire group. The PaO₂/FiO₂ ratio decreased with every PEEP step. The PaO₂/FiO₂ ratio showed a significant decrease at 5 and 0 cm H₂O PEEP compared with 15 cm H₂O PEEP. Dynamic compliance increased after reducing PEEP from 15 to 10 cm H₂O. Thereafter, dynamic compliance decreased with each PEEP step. At 0 cm H₂O PEEP dynamic compliance was significantly reduced compared with 15 cm H₂O PEEP. Solid squares = PaO₂/FiO₂ ratio; solid diamonds = dynamic compliance. *P <0.05.

Figure 4

Intratidal gas distribution at varying positive end-expiratory pressure (PEEP) levels. Mean intratidal gas distribution in eight iso-volume steps during four PEEP levels during the decremental PEEP trial. Decreasing the PEEP level resulted in a higher overall gas distribution to the non-dependent region and, subsequently, a lower gas distribution to the dependent region. During the course of inspiration, gas distribution to the non-dependent region decreased whereas it increased to the dependent region. At a PEEP level of 10 cm H₂O, during inspiration the lines representing gas distribution to both regions crossed each other. Dashed lines represent the interpolation lines; open circles = non-dependent region; solid circles = dependent region.

Figure 5

Intratidal gas distribution (ITV) index at varying positive end-expiratory pressure (PEEP) levels for the individual patients. Mean ITV index is shown as a percentage of 1. Here, the value 100% indicates that both lung regions are equally ventilated. Values >100% indicate that ventilation is predominantly distributed to the non-dependent region whereas values <100% indicate that the dependent lung region is predominantly at that PEEP level.