OBJECTIVES: One-lung ventilation (OLV) during thoracoscopic surgery is associated with a significant decline in arterial PO(2) in patients with severe pulmonary emphysema and patients with preserved lung function. The authors hypothesized that patterns of arterial PO(2) changes are different in these 2 patient groups. DESIGN: Prospective nonrandomized study. SETTING: University hospital. PARTICIPANTS: Twenty-five patients undergoing thoracoscopic interventions: 16 with severe pulmonary emphysema and 9 patients without emphysema. INTERVENTIONS: Continuous arterial blood gas measurement (PaO(2), PaCO(2), pHa) during OLV of the left lung in left lateral position using the Paratrend 7 blood gas monitoring system (PT7; Pfizer Hospital Products Group, High Wycombe, UK). MAIN RESULTS: The decrease of PaO(2) was delayed in patients with severe emphysema. Steady state (defined as DeltaPaO(2) <7.5 mmHg/min) was reached after 18 +/- 4 minutes compared with 11 +/- 3 minutes (mean +/- standard deviation) in patients with normal lung function (p = 0.0002). PaO(2) values at steady state were comparable (p = 0.49); the pattern of changes in PaO(2) for the first 15 minutes of left-sided OLV was significantly different between the groups (p = 0.0004). The difference of predicted versus measured PaO(2) at steady state was -48 +/- 160 mmHg for patients with emphysema and -51 +/- 60 mmHg for patients with normal lung function (p = 0.019). CONCLUSION: During OLV, oxygenation is better preserved for a longer period of time in patients with severe pulmonary emphysema as compared with patients with normal lung function. In contrast to patients without emphysema, prediction of oxygenation during OLV for the individual patient with emphysema is unreliable because of large interindividual differences.