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

Authors: Savino Spadaro, M.D., Ph.D. et al

Anesthesiology 3 2018, Vol.128, 531-538.

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 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/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 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/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.

What We Already Know about This Topic

• Low tidal volume is necessary to prevent lung injury during one-lung anesthesia but can potentiate development of atelectasis. Although positive end-expiratory pressure can reduce atelectasis, it may cause hyperinflation and increased alveolar dead space.

What This Article Tells Us That Is New

• Shunt fraction and ventilation/perfusion ratio ( ) distribution was measured in 41 patients undergoing one-lung anesthesia. Positive end-expiratory pressure at 5 or 10 cm H₂O, applied in random order, resulted in lower shunt fraction and driving pressure, without increasing dead space.