Standardized Alveolar Air Sampling Procedure

We collected the alveolar air samples using a standardized procedure [16 (link)]. Because VOC concentrations may be affected by diet, flow rate, and anatomical dead space [17 (link),18 (link)], all subjects were required to stop eating and smoking for 12 h before the air sampling. The air was then taken after intubation with an endotracheal tube and before surgery. To prevent contamination from the upper airway, we sampled alveolar air from the endotracheal tube with a capnometer (Masimo, Irvine, CA, USA). Under the visual control of a carbon dioxide-controlled sampling device, the alveolar air was taken from the breathing circuit during the alveolar phase of expiration [19 (link)] (Figure 1). To maintain a consistent flow rate of 125 mL/s, we set the ventilator to a tidal volume of 500 to 600 mL, a respiratory rate of 8–10/min, and an inspiratory-to-expiratory time ratio (I:E) of 1:2. To decrease the influence of humidity, all breath samples were dehumidified by a heat-moisture exchanger and then collected in a 1-L Tedlar bag (SKC Inc., Eighty Four, PA, USA).