The measure of using O2/N2O mixture is a useful way for fast lung collapse. This study further determined the EC50 of N2O in O2 for fast lung collapse. The results from present study showed that the EC50 of N2O in O2 for fast lung collapse was 27.7%.
The underlying mechanism of N2O/O2 inspired gas mixture leads to fast lung collapse may attributed to a “second gas” effect, which is the rapid absorption of N2O facilitates O2 uptake, or to a concentration effect or to the 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 present study, for the complete lung collapse by elastic recoil, both nonventilated and ventilated lumens of DLT were opened to atmosphere for 60 s, and then the nonventilated lumen of DLT was clamped for gas uptake. The average time of plural opening in present study was around 60 s(59.6 ± 12.2 s), which is consistent with the former studies reported for plural opening of VATS. Then a verbal rating scale[7, 8] from 0 (no lung deflation) up to 10 (maximal lung collapse) was used by surgeon for scoring the patient’s lung collapse condition. There are also other studies[13, 14] used a four-point ordinal scale(1, extremely poor-no collapse of lung; 2, poor-partial collapse with interference with surgical exposure; 3, good-total collapse, but the lung still had residual air; and 4, excellent-complete collapse with perfect surgical exposure.) to evaluate the lung condition, however, defining a “success” and “failed” condition is the necessary step for determining EC50 by up and down method. Compare to the four-point ordinal scale, the verbal rating scale from 0 to 10 seems more accurate for scoring patient lung collapse condition. Moreover, in our plot study, almost all surgeons regarded LCS equal to or more than 8 as a proper condition for lung manipulations, and we thus defined LCS equal to or more than 8 as “success” and less than 8 as “failed”.
In the study of 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 minutes after opening the pleura, however, there were still less than 50% patients’ LCS was more than or equal to 8. In another study which DLT was used as the lung isolation tool, when 50% N2O was applied, the average LCS was 9 at 10 minutes after opening the pleura, although these investigators did not observe LCS at 5 minutes after opening the pleura. In our plot study, when 30% N2O in O2 was used, about 50% patients’ LCS was more than or equal to 8. The differences of LCS at 5 minutes after opening the pleura between our study and the study of b-blocker as the lung isolation tool may largely attributed to the different isolation tools and the surgeon’s personal LCS scoring criteria.
In these former 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 ones. While in present study, 100% O2 was used for preoxygenation, and then the selected gas mixtures of N2O and O2 were used after intubation. However, before OLV, all selected gas mixture of N2O and O2 were equal to the target ones. Therefore, it seems that using O2 for induction, and switch to N2O and O2 after intubation is more applicable, since a “more O2 induction period” is safer that less one. For the operation type, all cases in the present study underwent VATS for lung surgery, which is the main surgery type for lung tumor, and the enrolled cases in these former studies mainly underwent open thoracotomies. Compare to the open thoracotomies, the lung collapse condition is more important for the VATS, and thus the data from present study is more applicable for modern clinical practice.
The present study has several limitations. First, for the purposes of this study, we determined the success or failure of lung collapse based on surgeons’ scoring scale, which was not completely objective. However, like the methods used in previous studies, using a more objective criteria such as the distance of the collapsed lung to the chest wall seems less clinically relevant due to the different sizes of the patients’ chests. Therefore, the most clinically relevant assessment of lung collapse condition is the surgeon’s impression. Secondly, all patients in present study had relative normal pulmonary function tests and body mass indices, the results of our study might not be applicable to the patients with poor pulmonary function tests or obesity. And these concerns are the objectives of our future study.