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/Q and respiratory mechanics were measured at positive end-expiratory pressure 0 cm H ₂O during bilateral lung ventilation and one-lung ventilation and, subsequently, during one-lung ventilation at 5 or 10 cm H ₂O of positive end-expiratory pressure. Shunt fraction and high V/Q 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 H ₂O to 5 cm H ₂O and 10 cm H ₂O resulted in a shunt fraction decrease of 5% (0 to 11) and 11% (5 to 16), respectively (P < 0.001). The Pao ₂/Fio ₂ ratio increased significantly only at a positive end-expiratory pressure of 10 cm H ₂O (P < 0.001). Driving pressure decreased from 16 ± 3 cm H ₂O at a positive end-expiratory pressure of 0 cm H ₂O to 12 ± 3 cm H ₂O at a positive end-expiratory pressure of 10 cm H ₂O (P < 0.001). The high V/Q 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/Q and reduce driving pressure.