Physiologic Evaluation of Ventilation Perfusion Mismatch and Respiratory Mechanics at Different Positive End-expiratory Pressure in Patients Undergoing Protective One-lung Ventilation

Background: Arterial oxygenation is often impaired during one-lung ventilation, due to both pulmonary shunt and atelectasis. The use of low tidal volume (V-T) (5 ml/kg predicted body weight) in the context of a lung-protective approach exacerbates atelectasis. This study sought to determine the combined physiologic effects of positive end-expiratory pressure and low V-T during one-lung ventilation. Methods: Data from 41 patients studied during general anesthesia for thoracic surgery were collected and analyzed. Shunt fraction, high (V)over dot/(Q)over dot and respiratory mechanics were measured at positive end-expiratory pressure 0 cm H2O during bilateral lung ventilation and one-lung ventilation and, subsequently, during one-lung ventilation at 5 or 10 cm H2O of positive end-expiratory pressure. Shunt fraction and high (V)over dot/(Q)over dot were measured using variation of inspired oxygen fraction and measurement of respiratory gas concentration and arterial blood gas. The level of positive end-expiratory pressure was applied in random order and maintained for 15 min before measurements. Results: During one-lung ventilation, increasing positive end-expiratory pressure from 0 cm H2O to 5 cm H2O and 10 cm H2O resulted in a shunt fraction decrease of 5% (0 to 11) and 11% (5 to 16), respectively (P < 0.001). The PaO2/FIO2 ratio increased significantly only at a positive end-expiratory pressure of 10 cm H2O (P < 0.001). Driving pressure decreased from 16 +/- 3 cm H2O at a positive end-expiratory pressure of 0 cm H2O to 12 +/- 3 cm H2O at a positive end-expiratory pressure of 10 cm H2O (P < 0.001). The high (V)over dot/(Q)over dot ratio did not change. Conclusions: During low V-T one-lung ventilation, high positive end-expiratory pressure levels improve pulmonary function without increasing high (V)over dot/(Q)over dot and reduce driving pressure.