Introduction
General anesthesia may induce respiratory complications in which the protective effect of airway reflexes will be decreased. Respiratory complications are among serious issues during each surgery that should be accurately managed to inhibit associated morbidity such as cardiac arrest during the peri-, intra-, and postoperative management. The ability to protect patent airway and provide effective positive pressure ventilation (PPV) is a principal during any anaesthesiological events (1).
Endotracheal tube (ETT) is commonly applied for managing respiration in a setting of general anesthesia which is mostly used in severely injured patients, or anesthetized patients to inhibit asphyxiation or airway obstruction. EET has been proposed as the main standard method for maintaining airway from aspiration (2). Applying this device needs sufficient operator expertise and has some limitation from different anatomical points of view including (3) local trauma, stress-response reflex, and malpositioning of the ETT.
The laryngeal mask airway (LMA), invented in 1983 by Dr Archie I. J. Brain, is a supraglottic airway device suggested as a beneficial alternative for managing and monitoring respiration during spontaneous or controlled ventilation in adults and children (4). Compared with ETT, the prevalence of using LMA is increasing due to its less invasive characteristics and minimal associated cardiovascular, respiratory, and postsurgical airway complications and disturbances (5). Using LMA has some advantageous over ETT, including no manipulation of patients` neck, jaw, and head, no compression of facial nerves, lower hemodynamic stress response, and low incidence of cerebral hemorrhage, fast and easy placement and stable positioning, no tracheal edema, improved oxygen saturation, and lower incidence of sore throat in adults (6,7). However, there are various possible disadvantages of LMA such as: gastric insufflation and aspiration, inadequate alveolar ventilation, and impossibility of suctioning the airway or using drugs endotracheally (8,9).
Postoperative respiratory tract complications are important to be controlled due to the possibility of threatening surgery outcomes (10). Postoperative coughing can increase the venous pressure that can lead to increased cerebral blood flow, intracranial pressure, and regional brain oxygen saturation (11).
In this systematic review, we study the differences reported in literature between ETT and LMA during general anesthesia in adults regarding the incidence of coughing as an air-related complication.

Methods
We searched PubMed to retrieve all the relevant articles to the clinical question of this study with the following search term: (laryngeal airway mask OR LMA) and (endotracheal tube OR ETT). Coughing was considered as the outcome of the included studies. No date limitation was included in  search strategy of this study. Only English language articles were selected to be extracted. Reference lists of the included articles were also searched manually to prevent missing any relevant article. Irrelevant articles were omitted by studying the title, abstract, and eventually the full text of the obtained articles, at first search. Inclusion criteria were all the randomized controlled clinical trials that studied the efficacy of LMA and ETT in adults undergoing general anesthesia during any surgical operation. Exclusion criteria were all the different types of articles and case reports, studies on pediatrics, experimental studies, non-English language articles, and those that did not compare LMA and ETT. Data on authors, patients’ characteristics, surgery type, percent of coughing in each study, and odds ratio as the effect size of each article are expressed in Table 1.

Results
Overall, 484 articles were found at the initial search that some were excluded after studying the title, abstract, and the full text. Eventually, 15 articles were included in this study for data extraction as the most relevant studies to the question of this review. Flowchart of the main studies is presented in Figure 1.
 included studies showed the significant differences between LMA and ETT regarding the incidence of coughing during the recovery time of patients under general anesthesia in different types of surgeries.
NNT which has been presented in Table 1 for each study shows that applying LMA significantly reduces the incidence of coughing compared with ETT in surgeries with general anesthesia.
Higher incidence of coughing by using ETT during general anesthesia might be due to the insertion process of ETT in trachea, which can irritate the airway and lead to the incidence of postoperative respiratory complications (12).
Evaluating the incidence of coughing and other associated airway complications has a clinical importance for choosing the best treatment approach. It seems that the incidence of coughing during recovery time can increase the possibility of further postoperative complications in some specific surgeries.
According to the study of Akhtar et al., coughing could increase the intraocular pressure to more than 50 mmHg, which was unfavorable following cataract and intra-ocular ophthalmic surgeries and could lead to iris or vitreous prolapse (12); thus it should be inhibited. Similar results would happen regarding increased intracranial pressure by coughing during cranial surgeries (26). Intraocular increased pressure is specifically vital in patients undergoing eye injuries. In this regard, applying LMA could inhibit the possibility of compromising outcomes of some surgeries by reducing the incidence of coughing. Webster et al. mentioned that coughing could increase the possibility of bleeding following intranasal surgeries (25).
Sharma et al. mentioned that pulmonary functions were depressed in lower level compared with other surgeries during peripheral limb surgeries, therefore applying LMA could reduce the possibility of coughing and the consequence postoperative complications (27).

Conclusion
This study showed that using LMA as a suitable alternative for ETT in adults could significantly reduce the incidence of coughing, an airway complication following general anesthesia.

Acknowledgement
We would like to thank Clinical Research Development Unit of Ghaem Hospital for their assistant in this manuscript.

Conflict of Interest
The authors declare no conflict of interest.