Total parenteral nutrition (TPN) frequently is used to provide adequate nutrition intravenously in small animals and humans, and it is becoming more commonly used in horses and foals when feeding via the gastrointestinal tract is impractical, inadequate, ill-advised, or impossible. The term parenteral means administration by means other than orally. The term "Total" parenteral nutrition is misleading at times because "partial" parenteral nutrition also is used (giving only part of the total daily calories necessary by the intravenous route).
There are three basic nutritional elements that the body uses as building blocks for tissue and the generation of energy: carbohydrates (sugars, of both a simple and complex nature), proteins (and when in their most basic form, amino acids), and lipids (fat). Normally these nutritional components are ingested orally, digested within the gastrointestinal system, and the vital carbohydrates, proteins, and fats are absorbed into the bloodstream from the gastrointestinal system. If the gastrointestinal system is "off-line" for whatever reason, these nutritional factors can be administered directly into the bloodstream.
This treatment can be vital because the daily caloric need exists regardless of the inability to ingest/digest food. In fact, in the case of illness, this becomes an even more important factor, as with many disease states, the daily caloric need can be greatly increased (almost doubled during some conditions).
The normal function of the immune system as well as any healing that might need to take place is highly dependent on adequate nutrition. For example, the caloric needs of a horse recovering from colic surgery might be nearly double that of a normal horse. Without those calories and tissue building blocks, the healing of the surgical site could be delayed. In addition, as the body uses vast amounts of energy and protein stores, a significant amount of weight could be lost if the daily caloric and protein intake is not sufficient.
An important note here is that the body really is not that discriminant when mobilizing carbohydrates, fat, and protein. The substances come from all available sources. (This is one reason why people dieting with a severely protein-deficient diet often develop heart problems--the heart is a muscle and will degenerate and weaken just like all of the other muscles. The scientific term for this is a "catabolic" state, meaning that there is a nutritional deficiency and the body is being self-digested for the sake of supplying the necessary nutritional elements. The converse to this is the so-called "anabolic " state, such as the state many body builders place themselves into with all the high protein/carbohydrate products that load the shelves of health food stores.
A Case Study
A seven-year-old Standardbred mare was referred to the Cornell Large Animal Clinic for acute colic. This mare was accompanied by her one-month-old foal and was in heavy lactation. Physical examination findings included hypothermia (low body temperature), tachycardia (elevated heart rate), cool extremities, reduced peripheral pulses, hypermotile/fluid gastrointestinal sounds, and severe abdominal distension. Rectal exam revealed fluid-filled loops of small intestine and passage of a nasogastric tube yielded approximately 10 liters of green/yellow reflux with a pH of 4.5. Differential diagnosis included proximal enteritis and small intestinal obstruction/strangulation.
Total parenteral nutrition was used to support this mare, which had acute proximal enteritis, an inflammatory disease affecting the upper gastrointestinal system. The cause is unknown. However, a clostridial (bacterial) enterotoxin (perfringens or deficle origin) is suspected. This disease causes massive gastric and small intestinal ileus (absence of or severely decreased motility) for extended periods of time, thus necessitating that the patient be maintained NPO (nothing per os or by mouth) potentially for weeks at a time.
Proximal enteritis causes a distension of the stomach with fluid backed up from the small intestine. There usually is a continual need to drain this fluid build-up via a nasogastric tube and prohibit the ingestion of food or water. The condition required the mare to be totally food restricted for 13 days and severely food restricted for another seven. Total parenteral nutrition was used without complication, and it is thought to have significantly aided in her recovery.
The mare was treated with potassium-penicillin, gentamicin (another antibiotic), and cimetidine (the anti-acid drug Tagamet) for broad spectrum antibiotic coverage and to decrease gastric acid production, respectively. Fluid therapy consisted of a balanced electrolyte solution with daily adjustments in content based on acid base and electrolyte evaluation of the mare's blood. The mare was evaluated every two hours and refluxed via an indwelling nasogastric tube every two to four hours. She was maintained NPO and refluxed an average of 25 liters of yellow/
green, relatively acidic reflux per day from the stomach tube.
Due to the lack of improvement and the continued gastric reflux, a barium GI series was performed on Day 8, and a possible site of stricture in the small intestine was suspected. An exploratory surgery was performed on Day 9; however, no obvious area of stricture formation was observed. At that point, the foal had been taken away to a nurse mare due to the mare's deteriorating body condition (body weight was 830 pounds). TPN was formulated and started on Day 10 and was continued through Day 15. Antibiotics, cimetidine, and balanced fluid therapy were continued. The quantity of reflux decreased to approximately 5 liters per day and the nasogastric tube was removed on Day 12. She then was offered one-third of a bucket of water and several handfuls of grass.
By Day 14, she was getting one-half bucket of water, one-third bucket of grass, and several handfuls of hay every four hours On Day 15, she was eating three-quarters of a flake of hay every four hours and weighed 870 pounds. By Day 18, she was eating free-choice hay and water and small bran mashes twice a day; the fluids were discontinued and antibiotic/cimetidine therapy continued for another two days. Her body weight was 915 pounds. The mare's condition continued to im-prove over the next five days, and she was discharged after 23 days of hospitalization.
This case demonstrates a complex set of energy demands. For 10 days this horse had no energy intake whatsoever, during which time she had the energy drain of normal body/life maintenance and milk production combined with her illness, surgery, and stress. After the foal was removed, there was a reduction in energy required for milk production; however, this is not complete until the gland is completely dried off. On Day 9, there was an added energy requirement created by the elective exploratory surgery. The energy demands placed this mare in a catabolic state, and she was relying on carbohydrate, protein, and oxidation of fat for energy. The affect of this metabolic switch includes protein catabolism, reduced immunocompetence, abnormal lipid metabolism, and delayed healing. She was experiencing protein and energy malnutrition as a circumstance of the disease process. This "starvation" was further accentuated by the energy needs in addition to maintenance, i.e., a factor for the elective surgery, a factor for the disease process and stress, and a factor for lactation.
The National Research Council uses digestible energy to calculate the daily maintenance requirement of energy. Typically, when dogs and cats are placed on TPN, the energy requirement is calculated using resting energy requirements, as they are hospitalized and thus not as active. The maintenance requirement accounts for a "moderate" degree of activity and "the energy required to find food and water." The portion required for activity is appropriate, but in the majority of today's domestic animals (dogs, cats, and horses), there really is no energy required to "find food and water" as it usually is provided for them free of any activity. There is, however, a significant energy requirement to consume it, i.e., activity of muscles of mastication and so on. This should be tied into the maintenance energy requirements regardless of the definitions. A rough calculation using the NRC guidelines shows the following energy status--a somewhat complicated set of formulas to follow accurately.
MAINTENANCE: DE (digestible energy)=1.4 + 0.03 (379 kg BW [body weight])= 12,700 kcal/day
LACTATION: DE=0.04 (379 kg BW) (0.792)=12,000 kcal/day
For small animals undergoing elective surgery, the resting energy requirement should be multiplied by 1.25 to account for increased energy demands. Although there is no specific factor in the large animal TPN literature, it is worth considering.
The combined energy requirements for the horse in this case could potentially be upward of 25,000 kcal (kilocalories)/day with no energy input. This state was leading to significant weight loss and had the potential for severely hindering the mare's recovery and a potentially fatal outcome.
Energy should be supplied to spare the horse from the ongoing catabolism draining the body's nutrients. Approximately four hours after a meal, NPO glucose is depleted. By 16 hours after a meal, stored liver sugars have been depleted and the catabolic state is maximized. These dynamics are for the dog and cat, and they might be slightly different in the horse since up to 50% of the horse's energy requirement can be met via volatile fatty acid production by the fermentation of hay in the large colon. Fermentation (assuming that the GI tract is full prior to going NPO) in a NPO horse might continue for a considerable length of time (days), perhaps supplying a form of energy from the "stored" fuel in the colon. Regardless, these animals soon will be in a catabolic state and have a net negative nitrogen (protein) balance.
Protein has to be provided for the healing of diseased tissue. Protein is needed to feed the rapid protein turnover by the regenerating gastrointestinal mucosal cells, wound healing, and production of plasma proteins.
Water soluble vitamins (especially the B vitamins) should be supplemented since they are intimately involved with energy metabolism, and they have the potential to be flushed from the body by diuresis (increase kidney activity and urine production) caused either by the fluid therapy and/or TPN-induced glucose in the urine. There also is reduced production of B vitamins as the large colon empties and bacterial fermentation decreases. Micronutrients such as zinc and copper are important for energy metabolism and tissue repair, and their supplementation should be considered in the long-term NPO case.
The first step was to remove the energy drain of the foal. Even though the mare rapidly was becoming emaciated, she continued to lactate at a significant rate. The mare was in very poor body condition, but had normal blood protein values. Serum protein, however, is not a good indicator of the state of malnutrition in the short term. The mare's energy need was calculated using 30 kcal/kg BW @ 379 kg=11,370 kcal/day. This value was slightly below the maintenance value of 35 kcal/kg for a mature, non-lactating, healthy horse and was thought to be a good starting point as the foal was removed. It is better to be under rather that over the actual requirements to reduce the risk of complications.
A TPN supplement was formulated. The TPN mix contained 6.5 liters of 8.5% protein, 2.7 liters of 50% dextrose, and 2.5 liters of 20% emulsified fat. This was divided into a constant infusion rate of 479 ml/hour over 24 hours. On the first day, the rate was 375 ml/hour in order to reduce the chances of sudden hyperglycemia and/or lipemia (too high blood sugar or lipid, respectively).
A catheter was aseptically placed in the right jugular vein, covered with sterile bandages, and dedicated to the TPN. Another catheter had been placed in the left jugular vein to serve IV fluids, electrolytes, and medications. It is important to leave the TPN catheter dedicated only to TPN as the risk of sepsis and thrombosis (vein clotting) is great if adequate sterility is not maintained. The TPN solution is a perfect culture media for bacteria, so it is necessary to take extra precautions not to contaminate the set-up. The administration sets were changed every 24 hours and the pre-mixed TPN solution was kept refrigerated until three hours before its use. The mare was cross-tied during TPN administration to assist in preservation of the catheter.
A complete blood count, serum chemistry panel, and serum triglycerides (fats) were performed prior to starting TPN, 48 hours into treatment, and again on the last day. Over the five-day course of therapy, hydration, blood electrolytes, and acid base status were monitored daily and adjustments in the fluid therapy were made accordingly. All blood parameters stayed within normal limits. Blood glucose was only slightly elevated on the sample days, and no glucose in the urine (a signal that the blood sugar is too high) was observed.
Careful monitoring is critical. Potential complications of TPN include hyperglycemia (too high blood sugar), hyperlipemia (too high blood fats), glucosuria (sugar in the urine), azotemia (signs of kidney trouble), electrolyte aberrations, dehydration, phlebitis (inflammation of the vein around the catheter), and septicemia (infection within the bloodstream). In order to minimize risk to the patient, it is necessary to be aware of these complications early in order to appropriately adjust the TPN therapy.
It was noted that the mare's attitude was much improved 24 hours after the start of TPN. She was less depressed and became much more active in the stall as therapy continued. It also was noticed that she had an increased interest in food. Prior to TPN, she showed little interest in the horses being fed around her; after a day of TPN, she became more aware and agitated when surrounding horses were eating. When offered food on Day 13, a strong appetite was demonstrated. There was a 40 pound increase in weight during the course of TPN, but this was most likely attributable to the feed consumption on Days 13, 14, and 15 in conjunction with the TPN. The TPN calories were slightly below maintenance for her body weight, and the added bulk of the recently consumed food could account for most of this weight gain.
During the course of therapy, there were no complications with tolerating the TPN formula or the catheter. This horse was shown to tolerate 30 kcal/kg of BW per day of TPN in the aforementioned protein: carbohydrate: lipid ratio for five days. Other reports have demonstrated horses which tolerated 35.8 kcal/kg of BW per day for 10 days without complications.
Total parenteral nutrition can be a significant adjunct to supportive therapy in a horse which has limited intake via the enteral route and should be considered in such cases.
When To Use TPN
There are numerous other types of illness states that might benefit from the use of TPN. The therapy could prove extremely useful in the case of foals or neonates with illnesses which prevent the normal ingestion of milk. Foals have high energy demands that, as with adults, are increased in the face of disease or healing wounds/surgical incisions. The limitations of TPN include expense. The materials necessary to blend the formula to be administered are relatively expensive. In addition, the skill of a pharmacist, or other experienced medical professional, is necessary to calculate the appropriate ratio of carbohydrates, lipids, and proteins. The formulas must be made in an especially clean environment and used soon after blending. The components are a prime breeding ground for bacteria, so great care must be taken in the formulation and use of TPN. This care also must be carried over into the hospital environment with respect to care of the specific catheter used for the TPN administration. Due to the special care necessary in both the formulation and administration of TPN, it usually is limited to the hospital environment.
In addition, it usually is routine to monitor several aspects of blood chemistry on a daily basis to ensure that the body is tolerating the specific formulation of TPN and its rate of administration, both of which might need to be adjusted if indicated by the blood profile.
In conclusion, TPN is somewhat limited to hospital application and might be cost prohibitive in adult horses, but can be of significant benefit in the supportive care for a number of disease states in both adult horses and foals.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: Winter Horse Care Concerns