Verification using airbag movement at the base of the gastric tube is the latest method for identifying tube position. When the tube is in the respiratory tract, airbag movement occurs at the base of the SNGT tube. The airbag expands during expiration and collapses during inspiration. These movements will occur as long as the end of the tube is inside the nose at the top of the epiglottis. Once the end of the tube is advanced deeper, two possibilities occur. One is that the airbag will stop moving when the end of the tube enters the esophagus properly. The other is that the airbag will continue to move and deflate when the end of the tube enters the trachea. If this happens, the tube must be withdrawn immediately before going deeper and damaging the bronchi, alveoli, and lung pleura. This method of verifying the tube position is important because gastric fluid is not easily obtained during NGT insertion. According to Boeykens et al. (2014), aspiration can only be obtained directly in 48.6% of patients. In 33.5% of patients, aspiration was obtained after additional measures such as air insufflation into the NGT, lateral positioning, and retrying after 1 hour, which delays feeding. As for 18.4% of patients, this method is feasible because of the small amount of gastric fluid .
The results of this study are supported by several other studies that took advantage of the differences in physiological aspects between the respiratory and digestive tracts. According to Fan et al. (2017), the method of verifying the NGT position can be performed with capnography/colorimetric capnometry. Capnography is the continuous analysis and recording of carbon dioxide (CO2) levels using infrared technology . According to Chau et al. (2011), colorimetric devices as accurate as capnography can detect CO2 levels during NGT placement. This seems to be a promising method for confirming NGT placement, especially when NGT aspiration is not obtained.20 However, the drawback of this method lies in the false-positive results associated with reflux contamination of gastric contents. Carbonated drinks or drugs such as sodium bicarbonate have the potential to induce CO2 production in the stomach .
Our results show once incidence of air movements (expansion and deflation) during SNGT 100% TV insertion in the stomach. These results are in accordance with the findings of Hani et al. (2015) that showed that air in the stomach can move out through the NGT. Tests of a modified gastric tube in pig test animals revealed that the presence of air in the stomach prevented gastric flushing . According to Mari , the presence of excessive air in the stomach is caused by various factors such as visceral hypersensitivity, behavior-induced abdominal wall phrenic reflex, effects of poorly fermented carbohydrates, and changes in the microbiome. According to Sherwood , the abdominal muscles contract during expiration. They press the diaphragm upward to the narrow chest cavity. Organs in the abdomen, including the stomach, can be compressed when the abdominal muscles contract. If excessive gas is present in the stomach and a gastric tube is attached, the air in the gastric tube can move outward through the tube during expiration. This causes the movement of air in and out of the gastric tube, similar to inspiration and expiration.
Observations during the study showed that the SNGT airbag remained static when the tip of the tube was in the esophageal tract before reaching the gastric marker boundary (xiphoid process). This occurred in both the SNGT 50% TV and SNGT 100% TV treatment groups. On the basis of these findings, the procedure for inserting the SNG tube must be adjusted. The esophagus can be a substitute for marking the position of the tube in the ideal digestive tract. The organ has a lower sphincter that prevents the entry of air and fluids from the stomach. When the end of the tube is in this channel, the air in the stomach will not interfere with the movement of the SNGT airbag. Thus, the middle part of the manubrium sterni can be used as a boundary marker for the second SNG tube instead of the xiphoid process, whereas the trachea and esophagus are just behind the manubrium sterni .
The results of this study did not show a significant difference in insertion accuracy between the SNGT 50% TV, SNGT 100% TV, and conventional NGT (p > 0.05). This is because all gastric tube insertions led to proper tube placement in the digestive tract on the first insertion attempt. All gastric tube insertions were performed by senior trained staff. Insertions performed by trained senior staff in accordance with the standard procedures can prevent improper gastric tube insertion. Chauhan et al. (2021) suggested the importance of safety checks and correct interpretation of chest radiographs by trained senior medical staff. Misplacement in the pulmonary tract can occur when the system designed by the hospital does not ensure that staff have received competency-based training and does not provide a documentation format that includes all patient safety checks . According to Cao et al. (2020), the incidence rate of pulmonary complications caused by malpositioning of the NGT in the tracheobronchial tree branches ranges from 1.2% to 2.4%. These complications include perforation, pneumothorax, pneumonia, lung abscess, and even severe acute respiratory distress syndrome . For this reason, methods to prevent incorrect tube insertion in the respiratory tract must be developed. The verification method used so far has limitations related to convenience and cost. For example, radiographic verification carries a risk of cancer due to repeated exposure. The use of endoscopes and sensors is expensive. The use of a laryngoscope is also not easy for all health workers to perform. Meanwhile, the SNGT can more simply and inexpensively detect tube insertion errors early before entry into the bronchi and lungs.
The spirometric measurements showed that the TVs of the Macaca fascicularis test animals were 52.8 and 50 ml. The two test animals were 4 years old and weighed 4–5 kg. The results of this study are slightly different from those of Iizuka et al.  that showed a TV range of 36–40 ml for 11 male Macaca fascicularis.
The results showed a significant difference between the two types of airbags with sizes of 50% and 100% of the TV (≤0.05). The movement of the airbag whose size was 50% of the TV appeared greater than that of the airbag whose size was 100% of the TV. However, the changes were small, namely an average of 3.5 mm for the SNGT 50% TV airbags and 2 mm for the SNGT 100% TV airbags. This is due to an open system in the upper respiratory tract. Air from the trachea will exit through the oral and nasal cavities. The monkey's oral cavity was not closed during the insertion process of the NGT and SNGT tubes. As a result, more air was released through the oral cavity because of its larger hole diameter than that of the nostrils.
The resistance of the line in the tube affects the flow of air into the airbag. The tube has a diameter of 0.2 cm and a length of 120 cm. The tube diameter is small enough to reduce the amount of air that can pass through it, following Poiseuille's law, which states that the smaller the radius of a channel, the greater its resistance . In addition, the loss of space in the airbag that is deflated during insertion will also affect its movement. Air that fills the tube will enter and fill the previously unfilled airbag. According to Ivanov (2015), the loss of air in the equipment can interfere with the amount of gas exchange that is expected to occur. Even a loss of space that is too large can match the TV . Thus, the SNGT requires an airbag with a volume smaller than the TV to allow for easier observation of airbag movement.
The results of two pH paper tests showed a pH value of 7, which contradicts the general view that the pH of the stomach is <7. According to Hatton et al. , the gastric pH of primates ranges from 2 to 6. The results of this study are consistent with those of a study by Fan et al. (2017) that evaluated the effectiveness of pH 6, pepsin 100 g/ml, and trypsin 30 g/ml in fluid aspirated from a gastric tube as criteria for proper tube placement. These three indicators correctly classified all cases of misplacement in the respiratory tract and 93.4% of cases of misplacement in the digestive tract. Although this method seems promising, laboratory testing is needed to evaluate these biochemical markers. According to Metheny et al. (2017), the gastric proteolytic enzyme pepsin is a potential marker for examining tube position in the digestive tract. In a study of gastric aspiration in 32 critically ill infants, investigators used Western blot immunoassay with a sensitivity of 1 g/ml for a mean (SD) pepsin concentration of 111.9 (36.8) g/ml .