OBJECTIVETo study the effect of different mean arterial pressures (MAP) on arterial to end-tidal CO₂partial pressure difference [(Pa-et)CO₂] and the intrapulmonary shunt (Qs/Qt) in patients undergoing thoracic surgery during one-lung ventilation (OLV).

METHODSForty-two patients undergoing right-sided pulmonary lobectomy were allocated into group A with fluctuation of MAP (at 20 min after OLV) within ± 10% of the baseline (n=22) and group B with lowered MAP by 30% of the baseline (n=20). Arterial and venous blood gas analyses were recorded to calculate [(Pa-et)CO₂] and Qs/Qt at 20 min after induction with two-lung ventilation (T₁), 20 min after OLV (T₂), 30 min after recovery of normal blood pressure (T₃), and 20 min after recovery of two-lung ventilation (T₄).

RESULTSPetCO₂and PaCO₂were well correlated during two-lung ventilation and OLV in group A (P<0.05). In group B, [(Pa-et)CO₂] at T₂was significantly higher than that in group A, but PetCO₂was still correlated with PaCO₂(P<0.05). Qs/Qt increased more obviously in group B than in group A in T₂(P<0.05). Bo obvious correlation was found between the [(Pa-et)CO₂] and Qs/Qt during OLV.

CONCLUSIONPetCO₂reflects the dynamic changes of PaCO₂under normal blood pressure during OLV. In the hypotension period, when [(Pa-et)CO₂] increases and the correlation coefficient between PetCO₂and PaCO₂lowers, PetCO₂may not accurately reflect the changes of PaCO₂and blood gas analysis is warranted.