Neuromuscular blocking agents can be used to secure a good surgical field, but it can also cause delayed extubation or postoperative pulmonary complications. Traditionally, rocuronium which is a commonly used non-depolarizing agent is usually reversed by cholinesterase inhibitors such as neostigmine or pyridostigmine. These drugs act by increasing the concentration of acetylcholine at the neuromuscular junction (a competing antagonist), not by direct antagonists. Consequently, there is a risk of pulmonary complications when cholinesterase inhibitor is not used appropriately. Use of sugammadex can reverse neuromuscular blockade (NMB) quickly, thereby being helpful for spontaneous deep breathing postoperatively. In a previous study, the moderate neuromuscular blockade was not guaranteed during surgery because intraoperative train-of-four (TOF) monitoring was not used and the outcome was focused on the correlation between reversal agent and the overall incidence of postoperative pulmonary complications. However, in the present study, TOF ratio or post-tetanic count (PTC) was repeatedly measured during surgery, thereby the intensity of intraoperative NMB being maintained. Moreover, lung compliance was repeatedly measured during surgery and the correlation between the intensity of intraoperative NMB and postoperative atelectasis which is evaluated by quantitative technique was also investigated.

Particularly in thoracic surgery, one lung ventilation is usually required for the surgical procedure. During one-lung ventilation, the compliance of ventilated lung is decreased and resistance can be increased, thereby the risk of atelectasis being increased. Furthermore, after thoracic surgery, although patients were encouraged to deep breathe, it is difficult to take a deep breath because of various factors. (i.e. pain, chest tube, long retracted time, postoperative interstitial edema, etc.) Therefore, postoperative atelectasis is much more important in patients undergoing thoracic surgery than other types of surgery.

For preventing postoperative atelectasis, the intraoperative intensity of neuromuscular blockade can be a crucial factor. Because deep neuromuscular blockade provides a good lung compliance during mechanical ventilation, peak inspiratory pressure can be decreased, thereby reducing the risk of ventilation-induced lung injury, particularly in one lung ventilation situation.However, there has been still lack of quantitative evidence that deep block is superior to moderate block in the thoracic surgery with one-lung ventilation

For assessment of postoperative atelectasis, plain chest radiography may be used. However, plain chest radiography can provide only a qualitative assessment of atelectasis. Computed tomography can assess the whole lung by its density (HU) and enables a quantitative assessment of postoperative atelectasis. Moreover, it can indicate the location of atelectasis more clearly than plain chest radiography, thus provide detailed information about postoperative lung state. To assess the effect of maintaining deep block and sugammadex reversal on the postoperative atelectasis, using chest CT can provide a much more quantitative and valuable information than conventional chest radiography.