During mechanical ventilation, inadvertent positive end-expiratory pressure (auto-PEEP) results when there is insufficient time for the lung to return to functional residual capacity prior to initiation of the next breath. Auto-PEEP has generally been seen in patients with obstructive lung disease. We investigated whether auto-PEEP might also result when high minute ventilation is provided via a small endotracheal tube. Method: We ventilated a test lung 40 times per minute with a tidal volume of 1.1 liters and assessed auto-PEEP and air trapping under 12 different conditions: lung compliance was set at 40, 80, and 120 ml/cm H2O, and tubes of 6-, 7-, 8-, and 9-mm diameter were tested at each compliance. Results: Auto-PEEP was present under all 12 conditions but increased markedly with the smaller tubes, reaching more than 30 cm H2O with the 6-mm tubes. Air trapping correlated directly with tube resistance and compliance. Conclusion: At high minute ventilation, the use of small endotracheal tubes may result in auto-PEEP and unintended lung distention. (Respir Care 1986;31:1080-1082.)
