Measurement of dead space in subjects under general anesthesia using standard anesthesia equipment

BACKGROUND: Pulmonary dead space is the volume of gas that is delivered to the lungs but does not participate in gas exchange. Knowing pulmonary dead space in patients under general anesthesia is clinically useful because it can aid in detecting disease processes such as pulmonary emboli or low cardiac output states. Dead space can be simply calculated by using the Bohr equation; however, it is difficult to measure mixed exhaled carbon dioxide (PECO₂) with a standard anesthesia machine. Previously, a study at our institution demonstrated the carbon dioxide (CO₂) concentration in the bellows of a standard anesthesia machine is an accurate approximation of PECO₂. In this study, we used the bellows PECO₂ measurement and arterial CO₂ (PaCO₂) to calculate pulmonary dead space. We verified the technique by adding known apparatus dead space volumes during anesthesia. METHODS: Subjects were under general endotracheal anesthesia. A sampling line was positioned inside the ventilator bellows and connected to a capnometer. Measurements of PECO₂ and PaCO₂ from an arterial catheter were taken at baseline and after adding 100 mL and 200 mL of dead space to the endotracheal tube. Dead space was calculated using the Bohr equation (alveolar dead space/tidal volume = [PaCO₂ - PECO₂]/PaCO ₂) at baseline and after adding 100 mL and 200 mL of apparatus dead space. RESULTS: The dead space at baseline was 265 ± 47 mL (mean ± SD) in 10 study subjects. After adding 100 mL of dead space to the endotracheal tube, the measured dead space increased by 110 ± 46 mL. The measured dead space increased by 158 ± 39 mL after adding 200 mL. CONCLUSIONS: Our baseline dead space measurements were in the expected range under general anesthesia. When dead space was added, we were able to calculate that an increase in dead space occurred. Our calculation was more accurate after adding a 100-mL volume than after adding 200 mL. We present a simple way to detect trends in dead space in ventilated patients using a Narkomed GS anesthesia machine (Dräger Medical, Lübeck, Germany).