Stanley L. Marks, BVSc, PhD, DACVIM (Internal Medicine, Oncology), DACVN
Professor of Small Animal Medicine, University of California, Davis, School of Veterinary Medicine, Davis
Davis, CA, USA
Rationale for Enteral Nutritional Support
Enteral feeding is indicated in patients who cannot ingest adequate amounts of calories, but have sufficient gastrointestinal function to allow digestion and absorption of feeding solutions delivered into the gastrointestinal tract via an enteral feeding device. The most important stimulus for mucosal cell proliferation is the direct presence of nutrients in the intestinal lumen.
Patient Selection for Nutritional Support
Objective methods of assessing nutritional status such as body composition measurement (anthropometry, impedance measurements, dual energy X-ray absorptiometry) are still in their infancy in veterinary medicine, with the result that a subjective global assessment of the patient's nutritional status needs to be performed. This technique is based on easily collected historical information (changes in oral intake, degree of weight loss, presence of vomiting or diarrhea) and changes found on physical examination (muscle wasting, body condition, and presence of edema or ascites). Although body weight is routinely determined in sick animals, it is important to appreciate its limitations. One cannot equate the appearance of the animal with its state of nourishment because body weight does not differentiate between fat, lean tissue, and extracellular water. Determination of the animal's serum albumin concentration and total lymphocyte count are insensitive determinants of nutritional status because of the large number of disease processes that influence these parameters unrelated to the effects of malnutrition. Nutritional support should be considered for animal's demonstrating recent weight loss exceeding 10% of optimal body weight or for those whose oral intake has been or will be interrupted for more than 5 days. Animals with increased nutrient losses from chronic diarrhea or vomiting, wounds, renal disease, or burns should also be considered for nutritional support.
ENTERAL FEEDING ACCESS DEVICES
Most feeding tubes today are made of polyurethane or silicone. These materials have tended to replace the older polyvinylchloride feeding tubes that tends to stiffen when exposed to digestive juices and are more irritating to patients, necessitating frequent tube replacement. Silicone is softer and more flexible than other tube materials with a greater tendency to stretch and collapse. Polyurethane is stronger than silicone, allowing for a tube of this material to have thinner walls and thus a larger internal diameter, despite the same French size. The flexibility and decreased internal diameter of silicone tubes may lead to clogging or kinking of the tube. Both polyurethane and silicone do not rapidly disintegrate or embrittle in situ, providing a longer "wear". The French (F) unit measures the outer lumen diameter of a tube (one French unit is equal to 0.33 mm). Tubes that are too flexible may be chilled before placement to increase stiffness.
Nasoesophageal tubes are a simple and efficient choice for the short-term (less than 10 days) nutritional support of most anorectic hospitalized animals that have a normal nasal cavity, pharynx, esophagus, and stomach. Nasoesophageal tube feeding is contraindicated in animals that are vomiting, comatose, or lack a gag reflex; it is also contraindicated with maxillary fractures. Polyvinylchloride or red rubber tubes are the least expensive tubes for dogs and cats, although the polyvinylchloride tubes may harden within 2 weeks of insertion and cause irritation or ulceration of the pharynx or esophagus. Tubes made of polyurethane or silicone are more expensive; however, they are less irritating and more resistant to gastric acid, allowing prolonged usage. An 8 to 10 F by 109 cm (43 inch) tube (preferably with a guidewire) is suitable for dogs weighing more than 15 kg. A 5 to 8 F by 56 to 109 cm (22 to 43 inch) tube is recommended for dogs weighing less than 15 kg and for cats.
The length of tube to be inserted into the distal esophagus is determined by measuring the distance from the tip of the nose to the eighth or ninth rib. This will help verify the correct placement of the tube in the distal esophagus rather than the stomach, and decrease the likelihood of reflux esophagitis. Desensitization of the nasal cavity with four or five drops of 0.5% proparacaine hydrochloride is recommended. The tube tip should be lubricated with a water soluble lubricant or 5% lidocaine ointment to facilitate passage. The tube is passed by maintaining the animal's head in the normal angle of articulation and gently directing the tip of the tube in a ventromedial direction. The tube should move with minimal resistance through the ventral meatus and nasopharynx and into the esophagus. In dogs, the presence of a small ventral ridge at the proximal end of the nasal passage necessitates directing the tip of the tube dorsally initially to allow passage over the ventral ridge and into the nasal vestibule. Nasoesophageal intubation is more difficult to perform in dogs because of their long, narrow nasal passages and extensive turbinate structures. In the dog, the tube is directed in a ventromedial direction while pushing the external nares dorsally. This maneuver opens the ventral meatus and guides the tube into the oropharynx.
If the tube is unable to be passed with minimal resistance into the oropharynx, it should be withdrawn and redirected because it could be positioned in the middle meatus with its tip encountering the ethmoid turbinate. Once the tube has been passed to the level of the attached "butterfly" tape, it should be secured as close to the nostril as possible, with either suture material or glue. A second tape tab should be secured to the skin on the dorsal midline between the eyes. An Elizabethan collar is usually required for dogs to prevent inadvertent tube removal; however, most cats do not require such a device. Removal of the tube is facilitated by clipping the hair that is attached to the glue.
After placement, the tube position is checked by injecting 5 to 10 ml of air while auscultating the cranial abdomen for borborygmi, or by infusing 3 to 5 ml of sterile saline or water through the tube and observing for a cough response. Confirmation of correct tube placement can also be obtained by obtaining a lateral survey thoracic radiograph and observing the position of the radiopaque tube in the esophagus. The most common complications associated with the use of nasoesophageal tubes include epistaxis, dacrocystitis, rhinitis, tracheal intubation and secondary pneumonia, and vomiting.
A major disadvantage of nasoesophageal feeding tubes is their small diameter, necessitating the use of liquid enteral formulas. Commercially available canned pet foods that are diluted with water will invariably clog the feeding tube. The caloric density of most human and veterinary liquid enteral formulas varies from 1.0 to 1.5 kcal/ml. Diets are fed full strength on continuous (pump infusion) or bolus feeding schedules.
Esophagostomy feeding tubes are easily inserted, and insertion only requires light general anesthesia with isoflurane or heavy sedation, and intubation with a cuffed endotracheal tube. The technique is minimally invasive and no specialized endoscopic equipment is needed. The patient should be placed in right lateral recumbency, and the left lateral cervical region clipped and aseptically prepared for tube placement. A 14 to 20-F red rubber catheter, silicone catheter, or polyurethane catheter should be premeasured from the mid-cervical esophagus to the eighth rib, and marked with a permanent marker to ensure the distal end of the catheter terminates in the distal esophagus. Three basic techniques for placement of a midcervical esophagostomy tube have been described (technique using curved Rochester-Carmalt, Mixter, or Schnidt forceps, percutaneous feeding tube applicator technique, and percutaneous needle catheter technique).
Despite the potential for esophageal scarring and stricture formation, esophageal stricture or a persistent esophagocutaneous fistula has not developed. The most common minor complication is peristomal inflammation, with peristomal abscessation occurring infrequently. Most of the inflammatory reactions are mild and respond to thorough cleansing with topical antibiotics. Other less common complications include regurgitation of the tube into the oral cavity and tube obstruction.
Gastrostomy tube feeding is indicated for long-term (weeks to months) nutritional support of anorectic or dysphagic animals. Gastrostomy feeding tubes are of comparatively large diameter (20 to 24 F), allowing the economic use of blended pet foods and the direct administration of medications. Gastrostomy tube feeding is contraindicated in animals with persistent vomiting, decreased consciousness, or gastrointestinal obstruction. Caution should be exercised in conditions under which the stomach cannot be apposed to the body wall (severe ascites, adhesions, space-occupying lesions).
Gastrostomy tubes can be placed percutaneously or during laparotomy. Placement is usually accompanied via a percutaneous endoscopic gastrostomy (PEG) technique, or a blind percutaneous gastrostomy (BPG) technique. There are a variety of feeding tubes that can be utilized for gastrostomy feeding including latex, polyurethane, and silicon tubes with French-Pezzer mushroom, balloon, bumper, or silicone dome tips. One can modify the catheters by cutting off and discarding the flared open end of the catheter and cutting off two 20-mm pieces of tubing (to be used as internal and external flanges) from the same end of the catheter. The end of the catheter opposite the mushroom tip is trimmed to facilitate its introduction into the larger opening of a disposable plastic micropipette. Make a small stab incision through the center of each flange and fit one flange over the cut end of the catheter, sliding it down until it rests against the mushroom tip. The other 20-mm piece of tubing will be used as an external flange that lies against the abdominal wall. It is not recommended to cut the small nipple on the mushroom tip to enhance the flow of food through the tube. Removing the tip of the mushroom compromises the integrity of the mushroom and hinders percutaneous removal of the tube.
Complications related to PEG tubes include those associated with placement of the tube (splenic laceration, gastric hemorrhage, and pneumoperitoneum), and delayed complications such as vomiting, aspiration pneumonia, tube extraction, tube migration, and stoma infection. Splenic laceration can be minimized by insufflating and trans-illuminating the stomach prior to placement of the needle or catheter into the abdominal wall. The author has recognized a discordant number of large-breed dogs that have had major complications secondary to the stomach falling off the silicone dome at the end of the gastrostomy tube. The stoma appeared normal in all dogs, with the unfortunate consequence that food was introduced intraperitoneally in several dogs. This complication occurred despite the placement of an internal flange between the dome and the gastric mucosa. For this reason, the author recommends that all dogs heavier than 30 kg do not have a PEG procedure, and instead have a gastrostomy tube placed surgically. Minor complications include pressure necrosis at the stoma site and cellulitis.
Surgical placement of gastrostomy tubes has been superceded by the percutaneous techniques because of the ease and speed of placement, lower cost, and decreased patient morbidity associated with the non-surgical techniques. A surgical approach is indicated in obese patients, patients with esophageal obstruction, or in situations where the patient requires a laparotomy for reasons other than tube placement.
Enterostomy tubes are indicated in patients unable to tolerate intragastric or intraduodenal feeding, despite having normal distal small intestine and colon function. Specific indications for feeding via jejunostomy tube include gastric outlet obstruction, gastroparesis, recurrent/potential aspiration, proximal small bowel obstruction, and partial gastrectomy. Jejunal tube feeding minimizes the stimulation of pancreatic secretion and is a viable route for patients with severe pancreatitis.
POSTOPERATIVE CARE AND ASSESSMENT
Calculation of Nutritional Requirements
An estimate of an animal's nutrient requirements is needed to determine the minimum amount of food necessary to sustain critical physiologic processes. The resting energy requirement (RER) is the animal's energy requirement at rest in a thermoneutral environment and in a postabsorptive state. A linear formula can be applied to determine the RER of dogs and cats weighing at least 2 kg but less than 45 kg. Alternatively, one can utilize an allometric formula that can be applied to dogs and cats of all body weights (BW).
Linear formula: RER (kcal/day) = (30 x BWkg)+70
Allometric formula: RER (kcal/day) = 70 (BWkg 0.75)
Hospitalized patients should be fed at their calculated RER initially, realizing that their actual energy requirement is likely to change over the course of the disease process through recovery. Use of "fudge factors" extrapolated from the human literature to calculate the energy requirements of critically ill animals is discouraged, particularly in the early phase of nutritional support. Close observation of changes in body weight, physical examination findings (decreased subcutaneous fat stores, muscle wasting, and presence of edema or ascites), and ongoing losses (diarrhea, vomiting, exudative wounds), will help determine whether to increase or decrease the patients caloric intake towards the illness energy requirement (IER) or RER, respectively.
Feeding should be delayed for 24 hours after placing a gastrostomy tube, to allow return of gastric motility and allow formation of a fibrin seal. In contrast, feeding can be instituted immediately following esophagostomy tube placement once the animal has fully recovered from anesthesia. Diet can be administered as bolus feedings or continuous infusion when feeding via esophagostomy and gastrostomy tube. If continuous feeding is employed, it should be interrupted every 8 hours to determine the residual volume by applying suction to the feeding tube. If the residual volume is more than twice the volume infused in one hour, feeding should be discontinued for 2 hours, and the rate of infusion decreased by 25% to prevent vomiting. Treatment with metoclopramide (1 to 2 mg/kg/24 hour as a continuous infusion) may be used to enhance gastric emptying and prevent vomiting.
With bolus feeding, the required daily volume of food should be divided into four to six feeds. Patients are usually fed approximately 25% of their caloric requirement on the first day of feeding, with a gradual increase of 25% of the caloric requirement per day. Most patients are able to reach their energy requirement by the fourth or fifth day of feeding. The food should be warmed to room temperature and fed slowly through the tube to prevent vomiting. Flushing of the tube with 15 to 20 ml of lukewarm-warm water helps prevent clogging. Before each feeding, aspirate the tube with an empty syringe to check for residual food left in the stomach from the previous feeding. If more than half of the last feeding is removed from the stomach, skip the feeding and recheck residual volume at the next feeding.
Esophagostomy Tube Removal
Unlike gastrostomy tubes, an esophagostomy tube can be removed the same day it is placed if necessary without concern for leakage and development of secondary complications. The dressing and sutures are removed while the tube is held in place. The tube is then occluded by kinking and pulled out using gentle traction. The ostomy site should be cleaned, antibiotic ointment applied, and a light dressing placed around the neck. The dressing should be removed in 24 hours, and the ostomy site inspected. The ostomy site should close within 24-36 hours. Skin sutures are not needed for closure of the ostomy site.
Gastrostomy Tube Removal
For percutaneously placed tubes, it is recommended that the tube be left in place for a minimum of 14 days. Animals receiving immune-suppressive therapy or patients that are severely debilitated may require longer for a peritoneal seal to form. The tube should only be removed when oral food intake is sufficient to meet the patient's caloric requirement. One of two methods of Pezzer tube removal can be applied. The tube can be cut at the body wall and the mushroom tip pushed into the stomach to be passed in the feces. This method is safe in medium to large size dogs, because the mushroom and internal flange should be easily passed in the stool. Alternatively, a stylet can be inserted into the tube to flatten the mushroom tip, while exerting firm traction on the tube. This method is recommended for cats and small dogs, because the mushroom can cause intestinal obstruction. Removal of the MILA catheter is accomplished by deflating the bumper which occurs once the Y-port adapter is removed. Catheters with a dome are removed by gentle, but firm traction on the tube. The gastrocutaneous tract should seal with minimal or no leakage within 24 hours.
Gastrostomy and Esophagostomy Tube Replacement
The PEG tube may malfunction or be prematurely removed by the patient, requiring replacement. If the gastrostomy tube is removed within 14 days of placement (before establishment of the gastrocutaneous tract), a PEG procedure should be performed to evaluate the gastric mucosa and verify correct positioning of the replacement gastrostomy tube. If the tube is inadvertently removed once the gastrocutaneous tract is well healed, one can replace the original catheter with a balloon-type catheter (Flexiflo) or a low-profile gastrostomy device (LPGD). Both catheter types do not require an endoscopic procedure or anesthesia for placement. The gastrostomy "button" is a small, flexible silicone device that has a mushroom-like dome at one end and two small wings at the other end that lies flush with the outer abdominal wall. A one-way antireflux valve prevents reflux of gastric contents through the top of the tube. There are two types of LPGD's: obturated and non-obturated. The obturated device has an enlarged mushroom tip that must be stretched for placement in the stomach by using a special introducer. The non-obturated tube works like a Foley catheter and does not require forceful entry into the gastrostomy stoma. The length of the gastrocutaneous fistula must be precisely determined to guide correct selection of the appropriate "button" shaft length. This is accomplished with a special stoma measuring device provided with the kit. The main advantages of the LPGD include their durability due to their silicon material, decreased likelihood of inadvertent removal by the patient, and their aesthetically pleasing appearance to the clients.
Feeding Tube Displacement
This is a relatively common problem, particularly with nasoesophageal tubes. Displacement of the tube can lead to aspiration, diarrhea, or in the case of gastrostomy tubes, peritonitis. Gastrostomy tubes should be marked with tape or a marking pen at the level of the skin to help verify the position of the tube. Detachment of the stomach from the abdominal wall with consequent intraperitoneal leakage of gastric contents can occur in large-breed dogs, and an internal flange should be placed in these animals to minimize dislodgement of the tube.
Obstruction of the feeding tube is one of the most common complications of enteral feeding. Most obstructions are secondary to coagulation of formula, although obstruction by tablet fragments, tube kinking, and precipitation of incompatible medications can also result in tube obstruction. Nasoesophageal tubes are prone to obstruction because of their small diameters, and obstruction also occurs up to three times more frequently in patients fed by continuous vs. bolus feedings. Sucralfate and antacids have been reported to precipitate with enteral formulas and cause tube obstruction. Several "remedies" have been advocated to relieve tube obstruction. Warm water injected with gentle pressure and suction will relieve most obstructions. For more unyielding obstructions, carbonated water is instilled into the tube and allowed to sit for one hour before applying gentle pressure and suction. Pancreatic enzyme infusions and meat tenderizer have also been advocated to dissolve tube obstructions. On rare occasions, the passage of an angiographic wire down the lumen is needed to unclog the tube. Tube obstructions can be minimized by flushing the feeding tube with warm water before and after administering medications or enteral feedings. Tablets should be crushed and dissolved in water prior to administration through the feeding tube, if no alternative form of medication is available.
Inflammation of the Stoma Site
Signs of inflammation with or without discharge or fever may indicate infection of the stoma site. This must be differentiated from fasciitis as a simple wound infection can usually be treated locally with dilute povidone-iodine solution, topical povidone-iodine antibacterial ointments, and more frequent dressing changes. To minimize this problem from occurring, ensure that the PEG tube can be rotated following its placement and leave a 5-mm space between the external flange and the skin.
References are available upon request.