Pulmonary oxygen exchange during endobronchial anesthesia: effect of tidal volume and PEEP

To determine the effects of tidal volume (VT) and positive end-expiratory pressure (PEEP) on pulmonary oxygen exchange during endobronchial (one-lung) anesthesia, the authors studied the effects of VT at 8 and 16 per cent total lung capacity (TLC), at zero end-expiratory pressure (ZEEP), and at 10 cmH2O PEEP in 16 patients in the lateral position. Anesthesia was maintained with halothane and oxygen. During two-lung ventilation (FIO2 0.99), mean PaO2 and physiologic shunt (Qs/Qt) were 421 +/- 12 mmHg and 0.22 +/- 0.02, respectively. During one-lung ventilation, PaO2 decreased and venous admixture (or Qs/Qt) increased in every patient. The magnitude of this decrease correlated directly with preoperative forced expiratory volume in one second (FEV1) (r = 0.66, P less than 0.005). A VT of 16 per cent of TLC at ZEEP resulted in the highest mean PaO2 (210 +/- 30 mmHg) and lowest Qs/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 per cent TLC ventilation, mean PaO2 decreased from 210 +/- 35 to 162 +/- 25 mmHg (P less than 0.05). PEEP did not significantly affect PaO2 during 8 per cent TLC ventilation. At both levels of VT, PEEP reduced mean Qt by approximatley 10 per cent (P less than 0.01) and increased compliance (P less than 0.01). However, PEEP did not significantly affect mean Qs/Qt or mean arterial or pulmonary arterial pressures at either level of VT. There was considerable variation in PaO2 and Qs/Qt among patients.