Usefulness of the Capnograph during One-Lung Ventilation

The Influence of Sampling Site of Airway Gas on a-_ETDCO₂

Abstract

The influences of the sampling site and shunt ratio of airway gas on arterial to end -tidal PCO₂ difference (PaCO₂-P_ETCO₂ difference: a-_ETDCO₂) were examined during one-lung ventilation in six patients scheduled for elective surgical procedures of esophageal cancer. During one-lung ventilation, anesthesia was maintained with enflurane(0.75-1.5%)in 100% oxygen supplemented with epidural lidocaine (1%) and small amount of fentanyl. Using a capnograph(Ohmeda 5200 CO₂ monitor, Oxicap), measurements of P_ETCO₂were performed, the airway gas being sampled from two different sites: one from the catheter placed into the endobronchial tube of the double lumen tube (the catheter tip near the carina) (A) and the other from Y-piece portion of the anesthesia circle(B). During one-hour period of one-lung ventilation, P_ETCO₂ was significantly smaller(29.4±2.3 vs 32.8±1.8mmHg) and a-_ETDCO₂ was greater (10.2±1.5 vs 6.8±0.9mmHg)in sampling site B when compared to those in sampling site A. The increase in a-_ETDCO₂ appeared to be greater during one-lung ventilation than that during bilateral-lung ventilation. There were significant correlations between P_ETCO₂ and PaCO₂ in both sampling sites. Whereas, no significant correlation was found between a-_ETDCO₂ and shunt ratio. In summary, during one-lung ventilation, especially when the sampling site for capnography is distant from carina, a-_ETDCO₂ becomes greater than as expected during bilateral-lung ventilation.