The fundamental question of why to feed critical patients has been confidently answered by studies in human patients demonstrating increases in infection rates, hospital stays, and antibiotic usage in patients that are fasted or “starved” compared to patients that have been provided nutrition. Whereas the veterinary literature contains relatively few studies directly comparing fasted patients with fed patients, understanding of the physiologic and pathologic abnormalities in simple starvation and stressed starvation is helpful in understanding the need to feed critically ill patients.
“Simple starvation” in animals is an adaptive response to lack of nutrients. During simple starvation, initially gluconeogenesis increases to provide energy in the first 1-2 days. After three days, metabolic rate is decreased to preserve endogenous tissues. Inactive T3 is increased and active T4 is decreased in order to preserve nutrients. If simple starvation continues then ketogenesis and lipolysis commence to continue to provide energy.
“Stressed starvation” occurs in patients undergoing critical illness or injury. Critically ill patients are unable to make the adaptive responses characteristic of simple starvation to preserve endogenous tissues. In critically ill patients, the metabolic rate increases rather than decreases. Derangements in protein, lipid and carbohydrate metabolism result in protein/calorie malnutrition. The derangements in nutrient metabolism in stressed starvation are driven by inflammatory cytokines including tumor necrosis factor. These cytokines promote catecholamine release, and increases in cortisol that result in tissue catabolism.
The goals of nutritional therapy in the critically ill patient are to sustain the host’s responses to injury and infection and to preserve endogenous tissue. Of importance is the point that in critically ill or malnourished patients, the goal of feeding in the acute period is not to increase body mass. Though nutritional support should be started early in the course of disease, patients that are volume depleted, severely dehydrated or with severe electrolyte abnormalities should have these derangements corrected prior to initiation of feeding. Initiating feeding in this sick patient population prior to the correction of volume depletion, dehydration and electrolyte abnormalities increases the risk of re-feeding syndrome. Feeding volume deplete patients can also increase metabolic and oxygen demands.
Enteral nutrition or feeding through the gastrointestinal tract (GI) is the preferred route of nutrient delivery. The benefits of enteral nutrition over parenteral nutrition are many. Enteral nutrition is associated with fewer septic and metabolic complications compared to parenteral nutrition. Enteral nutrition is not only more physiologic, but feeding enterally prevents villous atrophy and promotes the local immune function of the gut. Glutamine is an amino acid required for normal enterocyte health and feeding enterally is the only way to deliver glutamine directly to the enterocytes. Even with placement of feeding tubes enteral nutrition is generally less costly than parenteral nutrition. Because of these notable benefits of enteral nutrition, if the GI tract is working it should be used for nutrition. With a functional or even partially functional GI tract enteral nutrition can be used instead of or in combination with parenteral nutrition. Though assist enteral feedings or more commonly known as syringe feedings can be used to provide some calories, this method of assist feeding is intended for very limited and very short duration (a couple days) supplemental nutrition. Syringe feeding is rarely able to deliver the calorie requirements or adequate nutrients even in the short term. In patients that are nauseous or vomiting, syringe feeding especially in cats can contribute to the development of food aversions. Therefore achieving the goals of nutrient delivery and prevention of the consequences of “stressed starvation” in critically ill patients often means placement of enteral feeding tubes. When deciding on which feeding tube to place, several things should be considered including the anticipated duration of feeding, risk of gastroesophageal reflux or aspiration, risks of anesthesia, the patient’s underlying disease, and the condition of the GI tract.
Feeding Tube Types
Nasogastric (NGT) and nasoesophageal (NET) feeding tubes can be placed with relative ease in most dogs and cats requiring minimal to light sedation for placement. These tubes are small bore feeding tubes intended for relatively short duration feeding (3-7days) of a liquid diet. They are inserted into the ventral meatus via the nares and advanced to terminate in the esophagus (NET) or stomach (NGT) (Figures 1, 2, 3). Feedings can be given as either bolus feedings or as a continuous rate infusion of a liquid diet such as Clinicare™, DogSure™, and CatSure™. In my experience continuous rate infusions of liquid diet are better tolerated by the patient and are easier to implement from a nursing care standpoint based on the frequency and time it takes to deliver enough bolus feedings to meet calorie requirements. For these reasons, these tubes are used primarily in the in-hospital setting and are rarely used in the home setting. NGT have a benefit over NET because they can be used for suctioning of gastric residual volumes which help to decrease nausea and vomiting associated with gastric stasis or distension. Because NG tubes pass through and sit within the lower esophageal sphincter, there is potential to enhance gastro-esophageal reflux. Use of NET requires a functional, non-diseased esophagus. For example, an NET would be an in-appropriate feeding tube for a patient with megaesophagus, esophageal stricture, or esophageal masses. If a patient does not require gastric suctioning and has a functioning esophagus, then NET would be preferred over NGT to reduce risk of gastroesophageal reflux. Both NE and NG tubes pass through the nares and this may present some limitations to using these tubes. Depending on the severity of nasal disease, nasal pathology including fractures, nasal tumors, severe upper respiratory tract infection and congestion, or severe stenotic nares may present relative or absolute contraindications for placing nasal tubes. Most patients will require an e-collar to prevent dislodgement of nasal tubes. The comfort and tolerance of nasal tubes can be improved in many patients with anxiolytic and sedative medications in hospital or nasal instillation of local anesthetic drops such as proparacaine or lidocaine. Some patients are simply intolerant of nasal tubes from a comfort perspective and will persistently sneeze or paw at the tubes, which may limit their usefulness in that subset of patients. Both NG and NE tubes can be removed at any time after placement, are easily and quickly removed, most often not requiring sedation to remove them.
Esophagostomy tubes (e-tubes) are 14 to 19 french tubes that are placed most commonly in cats but can be placed in dogs for moderate to long duration of feeding - weeks to months. There are many commercially available tubes intended for placement as esophagostomy tubes; however, red rubber catheters have been used as esophagostomy tubes and work well. Esophagostomy tubes exit the left lateral cervical region and the tip of the tube lies within the distal 1/3 of the esophagus.
As with NET, esophagostomy tubes require a functional and non-diseased esophagus. The technique for esophagostomy tube placement is relatively easy requiring no special equipment though the procedure does require a short (10 minutes or less) anesthesia. Tube sizes are large enough that gruel or blenderized canned diets can be fed through the tube. The tubes are well tolerated by the patient, not requiring an e-collar to protect them but rather a light neck wrap. This makes them great tubes for longer term feeding at home. Besides mechanical complications with tube clogging (a risk for all enteral feeding tubes), one of the more common complications with esophagostomy tubes are abscesses or infections at the exit site. A local exit site infection can be treated with warm compresses and antibiotics or lancing the site. Only rarely, a severe exit site infection will require the removal of the tube for resolution. Esophageal irritation is an uncommon complication with a chronically placed e-tube simply due to the physical presence of the tube within the esophagus. When no longer needed for feedings, e-tubes can be removed at any time--there is no minimal duration that they need to be in place. They are removed generally without any sedation, and the exit site is allowed to heal by second intention.
Pharyngostomy tubes for feeding are no longer placed or recommended because the placement in the pharynx results in patient intolerance and gagging. Pharyngeal placement of feeding tubes can also contribute to obstruction of airflow.
Gastrostomy tubes (g-tubes) are large bore feeding tubes that can be used for moderate to long term feeding (weeks, to months, to years). Gastrostomy tubes exit directly from the stomach at the left lateral abdominal wall (Figure 6). Gastrostomy tubes bypass the esophagus and therefore an indication for placement of gastrostomy tubes is esophageal disease when the rest of the GI tract remains functional. Placement requires general anesthesia and may require specialized equipment to place. Gastrostomy tubes can be placed percutaneously with endoscopic guidance or ELD device in small dogs (<15 kg), or are placed surgically in larger dogs. These tubes are generally well tolerated by patients long term, with most patients not requiring an e-collar. The large size of the tubes allows for feeding of canned diets through the tubes. In the case of percutaneously placed g-tubes there is a risk for septic peritonitis with premature dislodgement or removal of the tube. Risk of peritonitis is lower in patients that have g-tubes surgically placed because the stomach is usually pexied to the body wall during surgical placement. Due to potential risk of septic peritonitis with early removal, g-tubes must stay in place for a minimum length of time even if you are not using them. This wait period is to allow a mature stoma to form between the stomach and the body wall that will prevent peritoneal leakage of gastric contents after the tube is removed. Some sources say removal can occur as early as 5-7 days; my personal preference, as long as the patient is not bothering the tube, is to wait two weeks to ensure a mature stoma. Tubes are easily removed with no to minimal sedation. The exit site, as with e-tubes are allowed to heal by second intention. Over time, the plastic of the tubes can become stiffened and crack with age, so in patients that require feeding for very long term (years), a new tube may need to be periodically placed through the existing stoma. Another option for patients being fed for many years through a g-tube is placement of a low profile g-tube using the existing stoma. Low profile g-tubes improve the cosmetic appearance of the tube and sit flush with the body wall. Because of this low profile, they are less likely to catch on obstacles or objects.
Jejunostomy tubes (j-tubes) are tubes which terminate or feed into the jejunum. Different methods have been described for placement of jejunostomy tubes including surgical placement directly into the intestine, or advancement through gastrostomy or nasal tubes under endoscopic or fluoroscopic guidance to ensure that the tip resides within the jejunum. Placement requires anesthesia. These tubes are often very small bore and require liquid feedings. Because of the small capacity of the jejunum, the liquid diet must be delivered as a continuous infusion. These tubes are used for short term (5-7days) feeding in hospital only and are not tubes used at home. For j-tubes that are placed surgically directly into the intestines, there is a risk of peritonitis with tube dislodgement. Their small size makes them more likely to experience clogging or kinking. Jejunostomy tubes are indicated in patients in whom the upper GI tract needs to be bypassed. Most commonly, they have been used to enterally feed patients with severe or necrotizing pancreatitis with the rationale that the j-tube is bypassing the gastric and duodenal stimulation of pancreatic enzyme secretion. These tubes are easy to remove with minimal to no sedation.
There are several predictive equations for calculating the amount of calories to feed. The two most common equations for calculating resting energy requirements (RER) are:
RER in kilocalories = 30(Body Weight in Kilograms)0.75
RER in kilocalories = 30(Body Weight in Kilograms) + 70
These two predictive equations give slightly different values. The second equation (non-exponent equation) is easier to input into most calculators and provides a good estimate of RER for patients weighing 2-30 kilograms. The caloric density of the food you are feeding of course depends on the diet you select and whether you need to mix that diet with water to get a consistency that will go down the tube. Some of the more common diets used in feeding tubes and their caloric densities are listed in Table 1.
|Clinicare (liquid diet)||Abbott||1 kcal/ml or 237 kcal/8 oz can|
|DogSure (liquid diet)||PetAg||0.7 kcal/ml or 233 kcal/11 oz can|
|CatSure (liquid diet)||PetAg||0.6 kcal/ml or 201 kcal/11 oz can|
|Recovery (canned diet)||Royal Canin||1.2 kcal/ml or 184 kcal/5.8 oz can|
|Maximum Calorie (canned diet)||Iams||2.1 kcal/ml or 333 kcal/6 oz can|
|A/D (canned diet)||Hills||1 kcal/ml or 150 kcal/5.5 oz can|
When feeding into the upper GI tract, gastric capacity has been reported to be 90 ml/kg in dogs and 45 ml/kg in cats; however, patients who are sick may be unable to tolerate large volumes of feedings. To increase enteral tolerance at initiation of feedings, typically feedings are started at only a fraction of RER (1/2 to 1/3 resting energy requirements) and then are subsequently increased over the course of 2-3 days to finally meet calorie requirements. It is no longer recommended to multiply RER by an illness factor. More frequent feedings to reduce the volume of bolus feedings should be used initially or alternatively a continuous infusion of liquid diet can be used to improve enteral tolerance. None of the feedings tubes prevent the patient from eating voluntarily on their own. If there is not a specific reason to withhold oral feedings in your patient then with all of these tubes, the patients can be offered food to gauge their voluntary food intake.
Complications of enteral nutrition are broadly categorized as mechanical, gastrointestinal, metabolic or infectious complications. Mechanical complications include tube obstruction, premature removal of tube, or tube dislodgement. Gastrointestinal complications include nausea, vomiting, diarrhea, abdominal pain or cramping, intestinal ischemia and mucosal injury. Metabolic complications are often referred to as the “refeeding syndrome” and can include hyperglycemia, lipemia, azotemia, hyperammonemia, hypokalemia, and other electrolyte abnormalities. Infectious complications include tube site infections, peritonitis, cellulitis at the exit sites, and aspiration pneumonia. The mechanical complications and diarrhea are some of the more frequent complications of enteral feeding that are easily fixed with experience with these tubes or easily treated with anti-diarrheals or a change in the diet fed. The severe complications including refeeding syndrome, and septic complications are very rare. The frequency of the severe complications is rare enough that the concern over them is not a contraindication to enteral feeding but rather the patient should simply be monitored for development of complications during initiation of feedings and while tubes are in place.
Procedural videos demonstrating nasogastric tube placement, esophagostomy tube placement and endoscopically guided percutaneous gastrostomy tube placement can be found on this website.