POCUS to confirm positioning of feeding tubes

Introduction

Nutritional support is now an important part of the management of critically ill patients. In fact, such patients are at risk of malnutrition due to anorexia and the catabolic effects of illness 1, and a rapid and appropriate refeeding regime can limit the onset of malnutrition 2. Enteral feeding is preferred to parenteral nutrition to maintain intestinal mucosa integrity, as well as the intestinal microbiome and function 1,2. In both human and veterinary medicine, rapid initiation of enteral feeding is associated with reduced mortality and fewer complications during hospitalization; current guidelines recommend initiation of early onset feeding for animals with complete anorexia or dysorexia for several days (3 days in cats and 5 days in dogs) using a feeding tube 3. The refeeding volume is adjusted according to the period of anorexia, and any observed complications, such as vomiting or regurgitation 1,3. In veterinary medicine, different types of feeding tube (FT) can be used, depending on the duration and type of illness. Nasogastric (NGT) and nasoesophageal tubes are the most commonly used, especially for feeding periods of less than 10 days 4. This type of tube is inexpensive, easy to set up and generally does not require anesthesia for placement 3,4.

Tube placement

Importantly, the position of the NGT must be confirmed before any refeeding commences, in order to ensure that the tube is in the digestive tract, and more specifically, in the stomach. Inadvertent airway placement is estimated to occur in 5% of all human NGT insertions, and the complications resulting from misplacement are numerous and may even be fatal: aspiration pneumonia, pneumothorax, mediastinal emphysema, subcutaneous emphysema, pulmonary hemorrhage, empyema, hemothorax, bronchopleural fistula or esophageal perforation, malabsorption, diarrhea and even peritonitis 1,5. Therefore, correct tube position, and more specifically the position of the distal end of the tube, must be verified to minimize the risk of complications and to increase patient safety 5. In human medicine, several methods have been described to achieve this: thoracic radiography is the gold standard, but induces stress and exposes the patient to radiation. This has led to research for other methods, such as auscultation of the stomach projection area after insufflation of 10 mL of air through the tube, measurement of the length of tube from nose to the umbilicus, gastroscopy, pH measurement of aspirated fluid, or capnography. None of these methods are particularly sensitive nor specific enough and require additional thoracic radiography to confirm correct tube placement.

POCUS for assessment of tube placement 

In this context, the use of POCUS (Point-of-Care Ultrasound) has been investigated in human medicine to assess the position of NGTs. Ultrasound enables rapid, non-invasive, repeatable and radiation-free bedside assessment, and some studies have been carried out in human medicine to assess the accuracy of this technique in evaluating feeding tube positioning 6,7,8,9,10. These showed a sensitivity ranging from 92.8-100% for POCUS ultrasound detection of the NGT, with a total procedure time of 10 minutes 9. Inflation via the feeding tube (using anything between 5 mL of saline mixed with 5 mL of air, up to 50 mL of saline) has been shown to create a hyperechoic fogging image in the stomach and improved sensitivity to confirm good positioning of the NGT 9,10. However, specificity could not be assessed, as only a few tubes were (mis)placed in the respiratory tract, so it remains unclear whether this method is optimal for detecting NGT misplacement.

In veterinary medicine, right lateral thoracic radiography remains the gold standard method for confirming the position of a feeding tube. However, it requires transport of the animal, often unstable, to the imaging unit, radiation exposure for the veterinary staff, and significant cost. An initial study has assessed that gastric ultrasound will reliably evaluate the position of oro-gastric tubes in newborn puppies and kittens (less than 24 hours old) 11, but to date only three studies have been performed to assess the feasibility and reliability of POCUS to confirm the correct position of a NGT 12,13,14. One study showed that scanning the left neck using a linear probe had an excellent sensitivity (95.6%) and high specificity (83.3%) to detect the esophageal position of the tube in 51 non critically ill dogs 12. A second study had 100% sensitivity to detect the esophageal position of a feeding tube using neck ultrasound in a population of 25 non-critically ill cats 13. The authors of this study also performed gastric ultrasound using a convex probe to identify the NGT end in the stomach and obtained 100% sensitivity, but a specificity of only 75% 13, and an injection of 5 mL saline-air mixture followed by repeat ultrasound did not improve the results obtained. However, the moderate specificity shown in this study should lead to cautious use in the clinical setting. Finally, recent preliminary results of a study were published reporting on 37 dogs and 21 cats hospitalized in an intensive care unit 14, and again a scan of the left neck showed excellent sensitivity to detect esophageal positioning of the tube. To confirm the gastric location of the NGT tip, a combination of gastric ultrasound assessment associated with air inflation gave the best sensitivity and specificity (68.2% and 73.3% respectively) 14. POCUS was faster for obtaining images when compared to radiography (6 minutes [range 5-8] vs. 11 minutes [range 5-15] (p=0.0008)). The remainder of this article will detail how to use POCUS to detect esophageal and gastric positioning of a feeding tube.

Technique

The animal can be placed either in right lateral or sternal recumbency, with the neck held extended by a helper if needed, at the operator’s discretion. Clipping of the hair is not usually necessary, but may improve visualization.

Esophageal ultrasound

Esophageal ultrasound can be undertaken with a linear probe (6-12MHz). A left transverse neck view is first performed to detect the trachea, with the thyroid gland caudally (Figure 1). The trachea is identified as a curved structure with a hyperechoic tissue/gas interface associated with reverberation artefacts. The jugular and thyroid veins, muscles and thyroid gland, carotid artery, internal jugular vein and vagosympathetic trunk are identified if possible (Figure 2). The cervical esophagus appears as an oval structure to the left of the trachea on this view, and it should be possible to identify the five layers of the esophageal wall; hyperechoic adventitia, hypoechoic muscularis, hyperechoic submucosa, hypoechoic mucosa and hyperechoic mucosal surface 15.

In both transverse and longitudinal ultrasound images, the NGT will appear as a double parallel hyperechoic line, with an acoustic shadowing artifact distal to the tube (Figure 3). If the operator cannot visualize the tube initially, additional maneuvers can be performed, such as moving the tube backwards and forwards, applying pressure on the trachea or cricoid cartilage, or changing the patient’s position.

As soon as the tube is visualized in the transverse view, or if the operator cannot see it, the probe is rotated 90° to obtain a longitudinal view of the esophagus. The cervical esophagus will appear as a long tubular structure on this view (Figures 4 and 5).

Stomach ultrasound

Gastric ultrasound can be performed using a convex or micro-convex probe (2-5MHz). The probe is placed in the subxiphoid area, slightly parasagittal, and oriented towards the left cranial abdominal quadrant to identify the distal esophageal sphincter (Figure 6). The gastric cardia is visible between the liver and abdominal aorta. The tip of the tube appears as a double parallel hyperechoic line in the stomach (Figure 7), although the presence of gas can lead to difficulties in visualizing gastric contents.

Air-inflation

To confirm the gastric location of the tube end, an additional maneuver can be performed by making a quick injection of air or saline-air mixture through the tube while the ultrasound probe is still in the subxiphoid position. Note that instillation of liquid into the respiratory tract if the tube is misplaced can induce discomfort, coughing or respiratory distress in a critically ill animal. Moreover, because our patients vary in size, the amount of water required can be hard to determine, but the literature reports use of 4 mL saline with 1 mL of air in cats 11 and 10-15 mL of air in dogs 10,12. If the NGT is within the stomach the operator should be able to perceive a hyperechoic “fog” after inflation.

Conclusion

The use of POCUS ultrasound seems promising in veterinary medicine, but to date few studies have been carried out. Left cervical POCUS is a reliable tool to confirm an esophageal position of a feeding tube, but further studies are needed to determine if subxiphoid POCUS can be used to confirm the gastric end of a feeding tube, and a number of questions remain unanswered, particularly with regard to the specificity of the ultrasound technique.