Pulmonary Oxygen Exchange during Endobronchial Anesthesia

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Abstract
To determine the effects of tidal volume (VT) and positive end-expiratory pressure (PEEP) on pulmonary O2 exchange during endobronchial (1-lung) anesthesia, the effects of VT at 8 and 16% total lung capacity (TLC), at zero end-expiratory pressure (ZEEP) and at 10 cmH2O PEEP were studied in 16 patients in the lateral position. Anesthesia was maintained with halothane and O2. During 2-lung ventilation (FIO2 [fractional concentration of inspired O2] 0.99), mean PaO2 and physiologic shunt (.ovrhdot.Qs/.ovrhdot.Qt) were 421 .+-. 12 mmHg and 0.22 .+-. 0.02, respectively. During 1-lung ventilation, PaO2 [arterial partial pressure of O2] decreased and venous admixture (or .ovrhdot.Qs/.ovrhdot.Qt) increased in every patient. The magnitude of this decrease correlated directly with preoperative forced expiratory vol. in 1 s (FEV1) (r = 0.66, P <0.005). A VT of 16% of TLC at ZEEP resulted in the highest mean PaO2 (210 .+-. 30 mmHg) and lowest .ovrhdot.Qs/.ovrhdot.Qt (0.35 .+-. 0.02), probably as a result of end-inspiratory alveolar recruitment with the least pulmonary blood flow redistribution. When 10 cmH2O PEEP was applied during 16% TLC ventilation, mean PaO2 decreased from 210 .+-. 35 to 162 .+-. 25 mmHg(P < 0.05). PEEP did not significantly affect PaO2 during 8% TLC ventilation. At both levels of VT, PEEP reduced mean .ovrhdot.Qs by approximately 10% (P < 0.01) and increased compliance (P < 0.01). PEEP did not significantly affect mean .ovrhdot.Qs/.ovrhdot.Qt or mean arterial or pulmonary arterial pressures at either level of VT. There was considerble variation in PaO2 and .ovrhdot.Qs/.ovrhdot.Qt among patients.