Insertion length of NG tube determined by the distance from the tip of the nose to the ear lobe and from the ear lobe to the xiphoid process of the sternum or the distance from the hairline of the forehead to the xiphoid process of the sternum [15]. Some scholars proposed that insertion length of NG tube should be extended 10 - 15 cm in adults [16] [17] [18]. A Meta analysis shows that extended insertion length of NG tube 10 cm can reduce the incidence of reflux, chocking coughs, aspiration, pneumonia [18] [19]. Other scholars improved the measurement methods for premature infants, neonates and catheterization [20] [21] [22]. However, these studies did not compare with X-ray and report the location of NG tube. Furthermore, this measuring method is affected by age, height, anatomical structure, body position and other factors [23] [24] [25]. Therefore, this method is limited to evaluate the placement of NG tube.

The PH of gastric juice was 3.9, the PH of pulmonary bronchial secretion was 7.73, and the PH of intestinal juice was 7.35, which is used to distinguish the variety of liquid [26]. PH ≤ 5.5 is contributed to ensure placement of nasogastric tube [27] [28]. The sensitivity of a pH ≤ 5.5 to correctly identify gastric samples was 68% and the specificity was 79% [29]. The accuracy of this method was 94% in children [26]. The alert recommended testing with PH indicator paper as the first line check [30]. However, Borsci [31] found that in 45.7% of cases aspiration could not be achieved only using this method, and the likelihood of misreading the strips that may lead to errors of decision making with an adverse impact on a patient is 11.15%. The determination of PH value, considering a value lower than 5 as the cut-off for the correct placement, is not indicated in patients who take some medications, such as H₂ blockers, and doesn’t distinguish between intestinal and tracheal placement [32]. Due to its uncertainty of this method, a cross-sectional survey in 383 intensive care units from 20 European countries reported that only 3.5% of ICUs used this method to confirm the location of nasogastric tube [4].

Hearing bubbling sound over epigastric region while air injected via tube perceived as evidence of NG tube placement. This method is widely used, and 51.93% - 84.7% of nurses used this method to confirm the location of NG tube [4] [33]. The uncertainty of this method was documented by American group Metheny in 1990 following a study. In the study, the average percentage of correct classifications was 34.4%, three subjects with feeding tubes inadvertently positioned in the respiratory tract, air insufflations were clearly audible in 2 of the 3 cases [34]. The accuracy of this method was 67% in children [26]. The effect of auscultation will be lower when nurses wear disposable caps, protective clothing and other personal protective equipment. Therefore, auscultation of injected air to predict NG tube location is unreliable and it should not be used alone [12] [30].

X-ray is the gold standard for NG tube placement [24] [32] [35] [36] [37] [38] [39]. This method can timely detect gastric tube heterotopia and prevent complications [35] [40]. This method was usually used to confirm the position of NG tube [37] [41]. However, due to the uncertainty of ray frequency and radiation exposure, it is not a first-line solution for the tip location of NG tube [4] [30] [42].

Sonography was firstly used to confirm the tip of a naso-enteric tube feeding during the passing of pylorus in 1996 [43]. Bedside sonography is a sensitive method for confirming the position of NG tubes and performs in a shorter time than X-ray. Compared with X-ray, the sensitivity of sonography could reach 92.2% - 100%, and average time-consuming of this method was 42 - 140 s. [7] [37] [41] [44] [45] [46] [47]. The sensitivity of subxiphoid sonography and neck sonography and air-water mixture combined could be greatly increased [48]. The main difficulties were found in the visualization of the esophagogastric junction and the antrum in the transabdominal longitudinal scan in obese patients because of the interposition of gas. Left lateral decubitus and the injection of 50 ml of normal saline could facilitate the sonographic exam [7]. However, this location method is not wildly used in intensive care unit [4]. Further research should be conducted on the feasibility of nurses using sonography to verify the location of NG tube.

Laryngoscope and gastroscope were used to NG tube placement [49] [50] [51] [52]. Laryngoscope can direct the NG tube into the esophagus. However, laryngoscope and gastroscopy are invasive procedures, and complications such as hypopharyngeal and esophageal perforation may occur [53]. Video-guided laryngoscope reduces nasogastric intubation time compared to manual and direct laryngoscope intubation [54]. However, endoscopic procedures may produce droplets which could increase the risk of respiratory infections among medical staffs. Due to the risk of respiratory and invasive injury, endoscopy was used for these mechanical ventilated patients who have difficult nasopharyngeal anatomy. These is no study to compare the effect of this method with X-ray. More researches are needed to explore the effect of endoscopy on location of NG tube.

Electromagnetic navigator was used to confirm the tip of a NG tube by Williamsin in 1996 [55]. Electromagnetic navigator is composed to electromagnetic transmitter, receiving device and display device. Electromagnetic transmitter is at the tip of the guide wire inside the nasogastric tube to monitor the position of the tip. Receiving device receives signal from electromagnetic transmitter and the placement of nasogastric tube could be visible through display device. The placement of an enteral feeding tube is also visible in critically ill patients with slow gastric emptying [56]. The sensivity of electromagnetic navigator was 98%, compared with chest X-ray [39]. This method is mainly applied to locate the nasojejunal tube [55] [56] [57] [58] [59]. It is needed to explore the effect of electromagnetic navigator on location of the tip of the NG tube.