The use of an N2O/O2 mixture is a useful method for rapid lung collapse. The present study determined that the EC50 of N2O in O2 for rapid lung collapse was 27.7%.
The underlying mechanism of an N2O/O2 inspired gas mixture leads to rapid lung collapse may attributed to a “second gas” effect, which is the rapid absorption of N2O facilitating O2 uptake, or to a concentration effect, or to gas solubility . During OLV, the nonventilated lung collapses initially due to elastic recoil, and the remaining gas is then removed by absorption into the pulmonary capillary blood . Thus, in the present study, for complete lung collapse by elastic recoil, both nonventilated and ventilated lumens of the DLT were opened to the atmosphere for 60 s, then the nonventilated lumen was clamped for gas uptake. The average time of plural opening in the present study was approximately 60 s (mean, 59.6 ± 12.2 s), which is consistent with previous studies reporting on plural opening in VATS . Then, a verbal rating scale [7, 8], scored from 0 (no lung deflation) to 10 (maximal lung collapse), was used by the surgeon to score the patient’s lung collapse condition. Other studies [13, 14] have also used a four-point ordinal scale (1, extremely poor to no collapse of the lung; 2, poor partial collapse with interference with surgical exposure; 3, good total collapse, but the lung still contained residual air; and 4, excellent to complete collapse with perfect surgical exposure). To evaluate the condition of the lung, however, defining a “success” and “fail” condition is a necessary step for determining EC50 using the up-and-down method. Compared with a four-point ordinal scale, a verbal rating scale from 0 to 10 appears to be more accurate for scoring lung collapse condition. Moreover, in our pilot study, virtually all surgeons regarded LCS ≥ 8 as a proper condition for lung manipulations; thus, we defined LCS ≥ 8 as “success” and < 8 as “fail”.
In a study investigating the use of a b-blocker as a lung isolation tool, the LCS of 50% N2O in O2 was significantly higher compared with that of 100% O2 at 5 min after opening the pleura; however, < 50% patients’ LCS was ≥8 . In another study, in which DLT was used as the lung isolation tool, when 50% N2O was applied, the average LCS was 9 at 10 min after opening the pleura, although the investigators did not report LCS at 5 min after opening the pleura . When 30% N2O in O2 was used in our pilot study, approximately 50% of patients had an LCS ≥ 8. Differences in LCS 5 min after opening the pleura between our study and the study investigating b-blockers as the lung isolation tool may largely be attributed to the different isolation tools and the surgeon’s personal LCS scoring criteria.
In previous studies [7, 8], the target gas mixtures of N2O and O2 were used at the time of preoxygenation during anesthesia induction, and the gas concentrations before OLV were equal to the target concentrations. In the present study, 100% O2 was used for preoxygenation, and the selected N2O and O2 gas mixtures were then used after intubation. However, before OLV, all selected N2O and O2 gas mixtures were equal to the target mixtures. Therefore, it appears that using O2 for induction, and switching to N2O and O2 after intubation is more applicable because a “more O2 induction period” is safer than one that involves less. Regarding operation type, all patients in the present study underwent VATS for lung surgery, which is the primary surgery type for lung tumors, and the enrolled cases in previous studies mainly underwent open thoracotomies. Compared with open thoracotomies, the lung collapse condition is more important for VATS; thus, data from the present study are more applicable to modern clinical practice(s).
The present study had several limitations. First, for the purposes of this study, we determined the success or failure of lung collapse based on the surgeons’ scoring scale, which was not entirely objective. However, similar to the methods used in previous studies, using more objective criteria, such as the distance of the collapsed lung to the chest wall, appears to be less clinically relevant due to varying sizes of patient chests. Therefore, the most clinically relevant assessment of the lung collapse condition is the surgeon’s opinion. Second, the tidal volumes were 8 mL kg-1 ideal body weight during both 2LV and OLV without PEEP. However, this has been associated with increased postoperative complications and mortality. Furthermore, an adequate amount of PEEP was shown to be effective in reducing stress to the dependent lung and V/Q mismatch . Applying PEEP to the dependent lung should also influence the primary outcome. In fact, LCS was assessed by a surgeon who could have been confounded by a more inflated dependent lung. Third, all patients in the present study demonstrated relatively normal results on pulmonary function testing (including 3 smokers) and body mass indices. As such, the results of our study may not be applicable to patients with poor pulmonary function test results, or to obese patients and/or smokers. Lastly, the duration of administration of the O2/N2O admixture was from the confirmation of DLT with FOB to the time of skin incision, and unfortunately, we did not record the time of this period. These concerns may be addressed in future studies.