Intrinsic positive end-expiratory pressure during ventilation through small endotracheal tubes during general anesthesia: Incidence, mechanism, and predictive factors

Study Objective To assess the safety of mechanical ventilation and effectiveness of extrinsic positive end-expiratory pressure (PEEP) (PEEP_e) in improving peripheral oxygen saturation (SpO₂) during direct microlaryngeal laser surgery; to assess the incidence, amount, and nature (dynamic hyperinflation or airflow obstruction) of ensuing intrinsic PEEP (PEEP_i); and to find a surrogate PEEP_i indicator. Design Quasiexperimental. Setting S. Raffaele Hospital (Milano), November 2009 to December 2010. Patients Fifty-two adults scheduled for direct microlaryngeal laser surgery. Exclusion criterion is pregnancy. Interventions Twenty-one percent O₂ mechanical ventilation through 4.5- to 5.5-mm internal diameter endotracheal tubes; in 29 patients, after measurement of PEEP_i, an identical amount of PEEP_e was added; and PEEP_i. Measurements SpO₂, peak (Paw_peak) and plateau (Paw_plateau) airway pressure, and end-expiratory carbon dioxide were measured every 5 minutes. Respiratory compliance (C_rs) was computed. PEEP_i was measured (end-expiratory occlusion method). Main Results PEEP_i ≥ 5 cm H₂O occurred in 14 patients (27%) after intubation, in 16 (30%) at the beginning, and in 14 (27.3%) at the end of surgery. Thirty-one patients (59.4%) exhibited PEEP_i ≥ 5 cm H₂O on at least 1 time point. PEEP_i at the beginning of surgery was positively correlated with Paw_plateau, C_rs, tidal volume, and body mass index. Body mass index was the only predictor for the occurrence of PEEP_i ≥ 5 cm H₂O. At the beginning of surgery, the Paw_plateau receiver operating characteristic curve predicting PEEP_i ≥ 5 cm H₂O had area under the receiver operating characteristic curve of 0.85; best cutoff value of 15.5 cm H₂O (sensitivity, 88.9%; specificity, 75%; correctly classified cases, 86.1%). When PEEP_e was applied, in 23 cases (82.1%), total PEEP equaled PEEP_e+ PEEP_i; in 3 (10.7%), it was lower; and in 2 (7.1%), it was higher. Application of PEEP_e increased SpO₂ (Prs (Pi (mostly due to dynamic hyperinflation) is common. Hemodynamic complications, barotrauma, and O₂ desaturation (reversible with PEEP_e) are rare. Paw_plateau provided by ventilators is useful in suspecting and monitoring the occurrence of PEEP_i and allows detection of lung overdistension as PEEP_e is applied.