Electrical Impedance Tomography for Real-Time PEEP Monitoring and Atelectasis During Mask Ventilation: A Randomized Controlled Physiological Trial

BACKGROUND: The combination of high inspired oxygen fraction (FiO 2 ) and positive-pressure mask ventilation at induction can lead to atelectasis, increasing the risk of perioperative pulmonary complications. Positive end-expiratory pressure (PEEP) may reduce atelectasis, but prior studies relied on imaging performed before or after induction rather than during ongoing ventilation. We used electrical impedance tomography (EIT) as a bedside, real-time imaging tool. We hypothesized that higher PEEP levels would improve end-expiratory lung aeration and homogenize ventilation distribution without clinically relevant hemodynamic compromise. METHODS: In this prospective, randomized controlled study, 72 adult patients undergoing elective ophthalmic surgery under general anesthesia were enrolled. The effect of three different PEEP levels (0, 5 and 8 mbar) on the distribution of lung ventilation and hemodynamic parameters during mask ventilation at anesthesia induction with 100% (FiO 2 ) were evaluated. Patients were randomized to ascending or descending PEEP sequences to control for order effects. The primary endpoint was the occurrence and the degree of atelectasis represented by the dorsoventral ventilation gradient at each PEEP level as detected by EIT. Secondary endpoints were changes in end-expiratory lung impedance (EELI) (ratio of EELI gain to EELI loss) and changes in lung compliance (ratio of compliance gain to compliance loss) as surrogate markers for atelectasis formation. Hemodynamic parameters were monitored with noninvasive blood pressure, electrocardiography, and electrical cardiometry. RESULTS: EIT was able to show in real-time that higher PEEP levels significantly improved the mean SD dorsoventral ventilation gradient (0.48 0.21 at 0 mbar vs 0.56 0.28 at 5 mbar vs 0.63 0.25 at 8 mbar; p<.001), increased the ratio of gain/loss of the end-expiratory lung impedance of the whole lung (3.1 4.8 at 0 mbar vs 6.1 8.1 at 5 mbar (~5.1 cmH2O) vs 10.5 12.2 at 8 mbar (~8.2 cmH2O); p<.001) and increased the compliance gain/loss ratio of the whole lung (2.7 9.2 at 0 mbar vs 3.8 10.3 at 5 mbar vs 4.3 11.3 at 8 mbar; p=.011), independent of the sequence of PEEP (all data dimensionless). Electrical cardiometry revealed that hemodynamic parameters decreased after induction, but these changes were not associated with specific PEEP levels. CONCLUSIONS: Using real-time bedside EIT our study demonstrates that higher PEEP levels during mask ventilation at anesthesia induction can improve ventilation distribution without adverse hemodynamic effects.