Optimal PEEP during one lung ventilation with capnothorax. An experimental study

Background: One-lung ventilation (OLV) with capnothorax is used to facilitate thoracoscopic surgery and carries the risk of hypotension, hypoxemia, carbon dioxide retention and ventilator induced lung injury. The optimal positive end-expiratory pressure (PEEP) during these circumstances is unknown and the PEEP level chosen in clinical praxis is often a compromise between hemodynamic and respiratory considerations. We therefore investigated the physiological effects of a recruitment maneuver followed by a decremental PEEP trial in order to find the optimal PEEP level.Methods: Eight anesthetized, muscle relaxed piglets were subjected to right sided OLV with a CO2-insufflation pressure of 16 cm H2O into the contralateral pleural cavity. Following a recruitment maneuver, a decremental PEEP trial from PEEP 20 cm H2O to zero end-expiratory pressure (ZEEP), was performed. Regional pulmonary ventilation and perfusion were recorded with electrical impedance tomography. End expiratory lung volume (EELV) and hemodynamics were also recorded. Computerized tomography (CT) was used to confirm EIT- and EELV measurements and visualize the effects of the decremental PEEP trial.Results: The best arterial oxygenation was reached at a PEEP level of 12 cm H2O (49 ± 14 kPa) and then gradually deteriorated to ZEEP (11 ± 5 kPa) (p <0.001). The lowest driving pressures were recorded at PEEP 14 cm H2O (19.6 ± 5.8 cm H2O) and then increased to (38.3 ± 6.1 cm H2O at ZEEP (p <0.001). EIT maps showed that ventilation shifted from the dorsal parts of the ventilated lung and perfusion shifted from the ventilated to the non-ventilated lung with down titration of PEEP from 12-14 cm H2O to lower PEEP-levels (p = 0.003, p = 0.02, respectively). Arterial CO2-levels increased and pH decreased at initiation of OLV with capnothorax but were then stable during down titration of PEEP to 12 cm H2O. With further decrement of PEEP, the hypercapnic acidosis worsened from pH 7.19 ± 0.06 to 7.04 ± 0.09 (p = 0.002). Optimal hemodynamic condition was also found at 12 -16 cm H2O, with mean arterial pressure significantly higher at PEEP 12 cm H2O (84 ± 9 mm Hg) compared to ZEEP (63 ± 15 mm Hg) (p = 0.005).Conclusion: Optimal PEEP was closely related to the level of the capnothorax insufflation pressure. With insufficient PEEP, ventilation/perfusion mismatch in the ventilated lung and redistribution of blood flow to the non-ventilated lung occurred.