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) (PEEPe) 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 (PEEPi); and to find a surrogate PEEPiindicator. 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 O2mechanical ventilation through 4.5- to 5.5-mm internal diameter endotracheal tubes; in 29 patients, after measurement of PEEPi, an identical amount of PEEPewas added; and PEEPi. Measurements SpO2, peak (Pawpeak) and plateau (Pawplateau) airway pressure, and end-expiratory carbon dioxide were measured every 5 minutes. Respiratory compliance (Crs) was computed. PEEPiwas measured (end-expiratory occlusion method). Main Results PEEPi≥ 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 PEEPi≥ 5 cm H2O on at least 1 time point. PEEPiat the beginning of surgery was positively correlated with Pawplateau, Crs, tidal volume, and body mass index. Body mass index was the only predictor for the occurrence of PEEPi≥ 5 cm H2O. At the beginning of surgery, the Pawplateaureceiver operating characteristic curve predicting PEEPi≥ 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 PEEPewas applied, in 23 cases (82.1%), total PEEP equaled PEEPe+ PEEPi; in 3 (10.7%), it was lower; and in 2 (7.1%), it was higher. Application of PEEPeincreased SpO2(P<.05) and Crs(P<.05). Conclusions During ventilation through small endotracheal tubes, PEEPi(mostly due to dynamic hyperinflation) is common. Hemodynamic complications, barotrauma, and O2desaturation (reversible with PEEPe) are rare. Pawplateauprovided by ventilators is useful in suspecting and monitoring the occurrence of PEEPiand allows detection of lung overdistension as PEEPeis applied.