A Modified Method to Assess Tidal Recruitment by Electrical Impedance Tomography

Avoiding tidal recruitment and collapse during mechanical ventilation should reduce the risk of lung injury. Electrical impedance tomography (EIT) enables detection of tidal recruitment by measuring regional ventilation delay inhomogeneity (RVDI) during a slow inflation breath with a tidal volume (V_T) of 12 mL/kg body weight (BW). Clinical applicability might be limited by such high V_Ts resulting in high end-inspiratory pressures (P_EI) during positive end-expiratory pressure (PEEP) titration. We hypothesized that RVDI can be obtained with acceptable accuracy from reduced slow inflation V_Ts. In seven ventilated pigs with experimental lung injury, tidal recruitment was quantified by computed tomography at PEEP levels changed stepwise between 0 and 25 cmH₂O. RVDI was measured by EIT during slow inflation V_Ts of 12, 9, 7.5, and 6 mL/kg BW. Linear correlation of tidal recruitment and RVDI was excellent for V_Ts of 12 (R² = 0.83, p < 0.001) and 9 mL/kg BW (R² = 0.83, p < 0.001) but decreased for V_Ts of 7.5 (R² = 0.76, p < 0.001) and 6 mL/kg BW (R² = 0.71, p < 0.001). With any reduction in slow inflation V_T, P_EI decreased at all PEEP levels. Receiver-Operator-Characteristic curve analyses revealed that RVDI-thresholds to predict distinct amounts of tidal recruitment differ when obtained from different slow inflation V_Ts. In conclusion, tidal recruitment can sufficiently be monitored by EIT-based RVDI-calculation with a slow inflation of 9 mL/kg BW.