Abstract: Objective To study the effect of different mean arterial pressures (MAP) on arterial to end-tidal CO2 partial pressure
difference [(Pa-et)CO2] and the intrapulmonary shunt (Qs/Qt) in patients undergoing thoracic surgery during one-lung
ventilation (OLV). Methods Forty-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). Results PetCO2 and PaCO2 were well correlated during two-lung ventilation and
OLV in group A (P<0.05). In group B, [(Pa-et)CO2] at T2 was significantly higher than that in group A, but PetCO2 was 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. Conclusion PetCO2 reflects the dynamic changes of
PaCO2 under normal blood pressure during OLV. In the hypotension period, when [(Pa-et)CO2] increases and the correlation
coefficient between PetCO2 and PaCO2 lowers, PetCO2 may not accurately reflect the changes of PaCO2 and blood gas analysis
is warranted.