Individualization of PEEP and tidal volume in ARDS patients with electrical impedance tomography: a pilot feasibility study

Background In mechanically ventilated patients with acute respiratory distress syndrome (ARDS), electrical impedance tomography (EIT) provides information on alveolar cycling and overdistension as well as assessment of recruitability at the bedside. We developed a protocol for individualization of positive end-expiratory pressure (PEEP) and tidal volume (VT) utilizing EIT-derived information on recruitability, overdistension and alveolar cycling. The aim of this study was to assess whether the EIT-based protocol allows individualization of ventilator settings without causing lung overdistension, and to evaluate its effects on respiratory system compliance, oxygenation and alveolar cycling. Methods 20 patients with ARDS were included. Initially, patients were ventilated according to the recommendations of the ARDS Network with a VT of 6 ml per kg predicted body weight and PEEP adjusted according to the lower PEEP/FiO2 table. Subsequently, ventilator settings were adjusted according to the EIT-based protocol once every 30 min for a duration of 4 h. To assess global overdistension, we determined whether lung stress and strain remained below 27 mbar and 2.0, respectively. Results Prospective optimization of mechanical ventilation with EIT led to higher PEEP levels (16.5 [14–18] mbar vs. 10 [8–10] mbar before optimization; p = 0.0001) and similar VT (5.7 ± 0.92 ml/kg vs. 5.8 ± 0.47 ml/kg before optimization; p = 0.96). Global lung stress remained below 27 mbar in all patients and global strain below 2.0 in 19 out of 20 patients. Compliance remained similar, while oxygenation was significantly improved and alveolar cycling was reduced after EIT-based optimization. Conclusions Adjustment of PEEP and VT using the EIT-based protocol led to individualization of ventilator settings with improved oxygenation and reduced alveolar cycling without promoting global overdistension. Trial registrationThis study was registered at clinicaltrials.gov (NCT02703012) on March 9, 2016 before including the first patient. Supplementary Information The online version contains supplementary material available at 10.1186/s13613-021-00877-7.


Protocol for adjustment of tidal volume and PEEP with EIT
After initial assessment of ventilation delay, stress and strain, ventilator settings were optimized according to the EIT-based algorithm (cf. main manuscript figure 1). Initially, an arterial blood gas (ABG) sample was collected for assessment of arterial partial pressures of oxygen (PaO2), carbon dioxide (PaCO2) and pH. Respiratory rate (RR) was increased if pH was < 7.30, provided no auto-peep was present. Auto-PEEP was assessed by visual analysis of the flow curve and by performing an endexpiratory occlusion maneuver for direct measurement of air trapping. If auto-PEEP was present or pH >7.40 or arterial partial pressure of carbon dioxide (PaCO2) < 35 mmHg, respiratory rate was decreased.
If RR could not be increased due to the presence of auto-PEEP and pH was below 7.20, VT was increased by 0.5 to1 ml per kg predicted body weight (PBW).
Recruitablity was assessed using a sustained-inflation recruitment maneuver (RM) with airway pressure (Paw) of 40 mbar followed by a PEEP increase of 3 mbarRegional Crs was assessed by dividing the EIT image in four horizontal regions of interest (ROIs) and by multiplying global Crs with the relative tidal impedance change in each of the ROIs. An increase in regional Crs of more than 5% (normalized to global Crs) was interpreted as recruitment. If recruitable lung tissue was identified following the RM, the higher PEEP level was kept. Tidal recruitment and overdistension were analyzed with a short reduction in VT that was achieved by halving the inspiratory pressure difference (∆P) during pressure-controlled-ventilation. Changes in regional Crs during this maneuver were assessed as follows: Regional increases and decreases in Crs were summed up separately and normalized to global Crs with previously set VT. Any regional increase in Crs with reduced VT was interpreted as indicative of overdistension with the previously applied VT, whereas any regional decrease in Crs with reduced VT was interpreted as indicator for tidal recruitment. If tidal recruitment was identified, PEEP was elevated by 3 mbar. If overdistension was identified, VT was decreased by 1 ml / kg PBW provided this did not lead to severe acidosis (pH <7.20). PEEP was reduced by 2 mbar if no recruitability and no tidal recruitment had been identified during the last two hours. After any PEEP-reduction, an assessment of derecruiment was performed and the PEEP-reduction was reversed if relevant derecruitment was detected.
Protocol rules and explanations (cf. main manuscript figure 1)

1) Criteria for VT reduction:
VT can be reduced by 0

5) Recruitment
RM shall be classified as successful, if after 1 min − regional Crs in any ROI increased by ≥ 3% of global Crs (measured before RM) OR − if global Crs increased by ≥ 10% in all other cased RM is classified as "unsuccessful"

6) Derecruitment
Derecruitment is present if 5 minutes after a PEEP decrease − regional Crs in any ROI decreased by > 3% of global Crs (measured before PEEP decrease) OR − global Crs decreased by > 10%

7) Alveolar Cycling
Alveolar Cycling is present, if during a reduction in driving pressure (∆P) of about 50% for a few breaths, − Crs at lower ∆P decreases by more than 3% in any ROI (in comparison to global Crs at normal ∆P)

8) Overdistension
Overdistension is present, if during a reduction in ∆P of about 50% for a few breaths − Crs at lower ∆P increases by more than 3% in any ROI (in comparison to global Crs at normal ∆P) For diagnosing alveolar cycling and overdistension, a reduction in ∆P by 50% for at least 5 breaths must always be performed.

9) PEEP Changes
Driving pressure (∆P) must always be kept constant during any changes in PEEP by changing inspiratory pressure by the same value.

"Last PEEP-increase" refers to
− the last successful recruitment maneuver (that is always accompanied by PEEP increase) OR − the last PEEP increase because of "alveolar cycling". Unsuccessful recruitment maneuvers are not referred to as "Last PEEP increase".

Treatment courses of individual patients during EIT-based adjustment of ventilator settings
Abbreviations: RM = recruitment maneuver; ∆P/2 = reduction in driving pressure (∆P) of about 50% for a few breaths; CW = compliance win; CL = compliance loss; OD = overdistension; AC = alveolar cycling.   4.5 7.8 -5.9 -0.8 6.6 6.5  Table S5. Standard deviation of regional ventilation delay (SDRVD), transpulmonary plateau pressure (PTP,plat), end-expiratory transpulmoary pressure (PTP,exp) and tidal power in individual patients after mechanical ventilation according to the ARDS Network protocol low positive end-expiratory pressure (PEEP) table (ARDSNet) and after four hours of mechanical ventilation according to the electrical impedance tomography based protocol (EIT).