NEONATAL CHARACTERISTICS INFLUENCING FLUID AND DRUG THERAPY¹⁸⁶

It is often observed that neonatal animals are more “sensitive” to the actions of drugs administered at normal adult dosage levels (on a mg/kg bwt basis) and less tolerant of inappropriate fluid administration than adult animals. Differences between neonatal and adult animals in drug effects generally can be attributed to differences in drug distribution, metabolism, or excretion. Some general characteristics of the neonatal period include better absorption of drugs from the GI tract, less drug binding to plasma proteins, increased apparent volume of distribution of drugs that are distributed in the extracellular fluid (ECF) volume, increased permeability of the blood-brain barrier, and slower elimination (i.e., longer half-life) of many drugs. It is important to remember, however, that the foal and the calf are relatively precocious newborns, and much of the data on neonatal differences were generated in considerably less mature species. For instance, glomerular filtration rate (GFR) reaches adult values at 2 days of age in calves versus at least 14 days of age in puppies. Studies of the development of renal function in foals,¹⁸⁷ as well as indirect evidence provided by pharmacokinetic studies of antibiotic agents eliminated primarily by renal excretion,¹⁸⁸ suggest that full-term, 2- to 4-day-old foals also have relatively mature renal function. For a more detailed discussion of neonatal drug disposition, the reader is referred to other texts.^186,189,190

In the neonate, the relative volumes of fluid differ from those in the adult. Total body water in the equine neonate constitutes 70% to 75% of the total body weight, versus approximately 60% in the adult horse. During growth the intracellular fluid compartment remains relatively consistent with regard to size, whereas the ECF compartment decreases as a percentage of body weight, with an increasing body fat percentage. In the 2-day-old foal, ECF volume was 394 ± 29 mL/kg, blood volume was 151 ± 32.8 mL/kg, and plasma volume was 94.5 ± 8.9 mL/kg; at 4 weeks at age, ECF volume was 348 mL ± 45 mL/kg and plasma volume was 61.9 ± 5.9 mL/kg.¹⁹¹ In another study, in a 1-week-old term foal, ECF volume was 44 ± 1.3% of body weight and plasma volume was 6.5 ± 1% of body weight. The ECF volume at 3 weeks of age had decreased to 28 ± 2% of body weight.¹⁹²

Although the neonate has a higher percentage of total body water than the adult, it is more vulnerable to water loss than the adult for several reasons, including increased basal metabolic rate; relatively greater surface area, predisposing to increased heat and water losses; and reduced urine concentrating ability.

BASIC FLUID THERAPY IN THE FOAL

The goal of fluid therapy is to expand vascular volume in an attempt to restore and maintain cardiovascular function, improve organ perfusion pressure, and correct dehydration, acid-base balance, osmolality, and electrolyte disturbances. Fluid therapy is a crucial part of the supportive care of the abnormal neonate. In assessment of the need for fluid therapy, both the state of hydration (sunken eyeballs, decreased skin turgor, dry mucous membranes, generalized weakness, decreased urine output) and the state of circulating volume (heart rate, pulse quality, capillary refill time, temperature of limbs, BP) should be assessed. If the losses are very acute, gross abnormalities in effective circulating volume may not yet be reflected in decreased skin turgor or sunken eyeballs, but heart rate or pulse quality may be abnormal. On the other hand, pulse quality and perfusion may be relatively normal in a neonate with severely sunken eyes and reduced skin turgor. Neonates that appear very thin and malnourished may actually be very dehydrated; with fluid therapy alone, their appearance can dramatically change in a short period. In sick foals fluid therapy should replace existing deficits while supplying maintenance requirements. A foal with moderately to severely sunken eyes is estimated to be 8% to 10% (of body weight) dehydrated. The estimated fluid deficit in a 40-kg animal that is 10% dehydrated would be approximately 4 L.

Laboratory parameters are useful for formulating a fluid therapy plan. Serum electrolyte concentrations may be life-threateningly deranged in foals with a condition such as uroperitoneum or enteritis, and knowledge of specific values can be of great benefit in selection of an appropriate fluid. The affordability of portable blood chemistry analyzers (e.g., IRMA Blood Analysis System, Diametrics Medical, St. Paul, Minn.) makes determination of electrolyte values feasible even in field situations. Poor nursing in a neonate can be detected by measurement of urine specific gravity. The normal nursing foal produces large quantities of dilute urine (specific gravity 1.000 to 1.012). In some cases, laboratory values can be misleading. For example, PCV or total plasma protein is often within the normal range in clinically dehydrated neonates.

The fluid therapy plan is calculated to supply maintenance needs and to replace deficits and current losses. Calculation of fluid deficits is based on the following equation:

Table 19-3 lists fluid replacement volumes for foals based on clinical assessment of dehydration.

Table 19-3 Calculation of Fluid Deficits

Foals experiencing septic or hypovolemic shock may require fluid administration rates of 40 to 80 mL/kg/hr initially until their BP is stable. Normal BP ranges are as follows:

BP can be easily measured indirectly using the coccygeal artery and the noninvasive Doppler or oscillometric method.¹⁹³

The maintenance fluid requirement for a newborn foal is approximately 4 to 6 mL/kg/hr (200 to 300 mL/hr for a 50-kg foal). Maintenance fluid administration in large animal neonates at the rate of approximately 100 mL/kg/day has usually resulted in adequate fluid balance and good urine output, in the absence of fluid deficits or increased fluid losses. If severe diarrhea is present, the daily fluid requirements can reach 15 to 20 L (500 mL/kg/day) or more.

If an animal is mildly to moderately dehydrated and the GI tract is not seriously compromised, fluid requirements can be provided by the enteral route, using milk or commercially available dextrose and electrolyte mixtures. However, if the gut is abnormal or if moderate-to-severe dehydration is present, the intravenous route is the preferred method of fluid administration.

Many types of intravenous catheters are suitable for use in the large animal neonate. Teflon catheters tend to be more thrombogenic than Silastic or polyurethane catheters and therefore should be replaced at more frequent intervals. A 5-inch long, 16-gauge Teflon catheter (Abbocath, Abbott Hospitals, North Chicago, Ill.) placed in the jugular vein and a 2-inch, 16-gauge Teflon catheter (Quik-Cath, Baxter Healthcare Corporation, Deerfield, Ill.) placed in the cephalic vein have both worked well to deliver intravenous fluids to foals. Short catheters placed in peripheral veins can be difficult to maintain and are not suitable for large-volume, rapid fluid replacement. I prefer to use a long-term, 16-gauge, 8-inch, polyurethane catheter (Arrow Catheter, Arrow International, Reading, Penn.) inserted in the jugular vein. This catheter is inserted over a flexible J-wire and can be left in place for 2 to 3 weeks. The use of polyurethane catheters reduces the incidence of thrombophlebitis and eliminates the need for frequent catheter replacement. Smaller-diameter catheters may be more suitable for lambs and kids. Regardless of the type used, it is essential to use aseptic techniques for catheter placement and to secure the catheter firmly to the skin. A combination of superglue and -sutures has been very effective in keeping the catheters in place. Catheter sites are kept as clean as possible, and the site of venipuncture and the vein are watched closely for signs of infection. Specific information on catheter placement and maintenance can be found in other texts.^194,195

The intraosseous infusion technique is an alternative method for rapid delivery of fluids in the critically ill neonate when IV access is not possible. This procedure uses the intramedullary vessels in the bone marrow to gain access to the central circulation. A description of this technique is described in other texts.¹⁹⁶ The optimum type of intravenous fluid administered depends on the electrolyte and acid-base status of the patient. Fluids are available as either crystalloids (e.g., polyionic fluids such as Plasmalyte, Normosol, lactated Ringer’s, saline) or colloids (e.g., plasma, hetastarch). Polyionic fluids are usually used for rapid rehydration and maintenance fluid therapy. These fluids should be isotonic (osmolality 270 to 300 mOsm/L). In most circumstances, in the absence of appropriate laboratory services the use of a balanced electrolyte solution such as lactated Ringer’s or Plasmalyte is satisfactory to replace fluid deficits. Saline solutions may be a more appropriate choice in certain situations: foals with diarrhea are often hyponatremic and hypochloremic; premature foals with immature renal and endocrine function conserve electrolytes poorly and have a tendency to become hyponatremic and hypochloremic; foals receiving diuretics often require additional sodium chloride. Other exceptions to this rule include animals with hyperkalemia, in which potassium-containing fluids are best avoided, and animals with hypernatremia, in which controlled slow reduction of body sodium content is required.

Fresh or frozen plasma is often more effective than crystalloid fluids for volume expansion in seriously ill neonates. Endotoxemia and sepsis produce inflammatory changes in vessel walls. Capillary endothelial permeability is increased, resulting in increased extravasation of fluid and albumin from the capillaries into the interstitium. Rapid infusion of large volumes of crystalloids reduces colloidal oncotic pressure while transiently increasing intravascular hydrostatic pressure. These forces favor movement of fluid out of vessels. Colloidal solutions contain large—molecular—weight molecules that do not freely pass though the capillary membrane. Therefore colloid administration results in increased plasma oncotic pressure, increased plasma volume, and more effective improvement in circulating blood volume. There are synthetic colloids (e.g., dextran, hetastarch) and natural colloids (e.g., plasma, whole blood). Plasma has several advantages over synthetic colloids. It is a good source of protein, opsonins, complement, clotting factors, and immunoglobulins. The disadvantages of plasma include the possibility of an anaphylactic reaction and the need to thaw frozen plasma, which makes it less suitable when rapid fluid resuscitation is necessary. An effective, commercially available synthetic colloid is hetastarch (Hespan, DuPont Pharma, Wilmington, Del.). Hetastarch has been used successfully for rapid fluid resuscitation in equine patients and has caused few adverse reactions.¹⁹⁷

If a neonate remains hypotensive in spite of volume expansion and fluid replacement, pressor agents such as dopamine and dobutamine may be indicated. Dopamine, with its combined α- and β-adrenergic and dopaminergic activity, is preferred. Higher doses will be required for patients in severe septic shock. If the foal fails to respond to high doses (>10 to 15 μg/kg/min), then norepinephrine, a more potent α-adrenergic agent, can be tried. Safe and effective infusion of these agents requires continuous monitoring and some type of infusion pump. Recently, nitric oxide (NO) has been shown to play a role in sepsis-induced hypotension.¹⁹⁸ IV administered new methylene blue, an NO antagonist, has been used to try to reverse severe life-threatening hypotension.

The rate of fluid administration is determined by the degree of dehydration, severity of cardiovascular compromise, and maintenance requirements. Most normal foals can tolerate rapid fluid infusion, but asphyxiated, septic, or premature foals may be oliguric and therefore far less tolerant of overzealous fluid administration. In these cases generalized edema may result. A general rule of thumb is to replace one half the calculated deficit in the first 6 hours of fluid therapy and the rest over 12 to 24 hours. A flow rate of 20 mL/kg/hr or higher (40 to 80 mL/kg/hr) may be needed to treat hypovolemic or septic shock.

In any depressed, weak, or seizuring animal, blood glucose levels should be checked, because hypoglycemia is one of the most frequently observed metabolic derangements accompanying many neonatal diseases. The rapid blood glucose reagent strips and hand-held Glucometers are very useful in making this determination in a field setting. For treatment of hypoglycemia a continuous infusion of 5% to 10% dextrose is sufficient to reach and maintain adequate blood glucose levels in most neonates. Hypertonic glucose boluses (25% to 50%) may aggravate preexisting CNS insults and frequently result in rebound hypoglycemia 30 to 40 minutes after infusion. Therefore they should be avoided. One regimen for treating hypoglycemia is 10% dextrose infusion administered at 5 to 10 mL/kg fairly rapidly, followed by a continuous infusion to supply 4 to 8 mg/kg/min (approximately 5 mL/min of 5% dextrose for a 40-kg neonate). The appropriateness of the therapy should be judged by frequent blood and urine glucose determinations. If hyperglycemia results, the rate of infusion or concentration of solution is decreased (to deliver perhaps 2 mg/kg/min) but not stopped.

Another metabolic derangement commonly observed in the large animal neonate is metabolic acidosis. This disorder may be caused by accumulation of acid, by loss of buffers from the body, or by a combination of the two. Whenever possible, treatment should be directed at correcting the underlying cause of the acidosis. Acidosis caused by low cardiac output or decreased peripheral oxygen delivery should be treated by measures to increase tissue perfusion (e.g., plasma volume expansion, cardiac inotropes, nasal oxygen insufflation). In this type of acidosis there is no actual loss of bicarbonate from the body, and bicarbonate therapy often produces disappointing results and adverse reactions. If respiratory dysfunction is present, considerable caution should be exercised in the use of sodium bicarbonate. Bicarbonate functions as a buffer only in an “open” system in which carbon dioxide can be transported to the lungs and eliminated.¹⁹⁹ Profound fluctuations in BP and cerebral blood flow, intracranial hemorrhage, and decreased oxygen delivery to tissues are possible adverse effects of sodium bicarbonate infusion in human beings.²⁰⁰ In many mildly to moderately acidotic neonates, simple volume expansion with isotonic fluids alone is very effective in correcting the base deficit by improving perfusion. Other more compromised individuals need more aggressive support of the cardiovascular system. Mild acidosis (HCO₃ deficit 5 to 10 mEq/L) associated with dehydration, can be corrected by simple rehydration. Bicarbonate supplementation is recommended when HCO₃ deficits are >10 mEq/L (serum HCO₃ <15 mEq/L) or whenever the pH is <7.2. Bicarbonate deficits can be calculated using the following equation:

An isotonic bicarbonate solution is preferred because excessive bicarbonate administration results in increased CO₂ production, leading to respiratory embarrassment and increased risk of CNS acidosis and hemorrhage. Isotonic bicarbonate can be made by adding 150 mL of 8.4% bicarbonate solution to 850 mL of sterile water, or 200 mL of 5% bicarbonate solution to 800 mL of sterile water. Bicarbonate solutions should be given slowly. Hyperkalemia is often observed with metabolic acidosis because of the transcellular shift of potassium ions into the ECF in exchange for hydrogen ions.²⁰¹ As the metabolic acidosis is corrected, the hyperkalemia resolves. Bicarbonate solution should not be combined with any calcium containing solution or precipitation will occur.

The effect of the bicarbonate replacement therapy should be monitored closely, and the dose adjusted accordingly. Neonates with severe diarrhea because of ongoing losses of bicarbonate through the feces may need considerably more than the calculated deficit to maintain an adequate blood pH until the diarrhea subsides. As in any type of fluid therapy, a plan is devised, the animal’s response to the plan is monitored, and changes are made accordingly.

Hypokalemia can occur in anorexic foals, foals with diarrhea, and those receiving diuretic therapy. Potassium (K) supplementation can be estimated using the following equation:

Potassium can safely be added to fluids at a rate of 20 to 30 mEq/L. The rate of potassium administration should not exceed 1 mEq/kg/hr. If acidosis is present, hydrogen ions are exchanged for intracellular potassium ions, resulting in a relative increase in serum K. As the acidosis is corrected there will be an influx of K ions back into cells, resulting in potential hypokalemia, which must be anticipated during fluid therapy.

NUTRITIONAL SUPPORT OF THE ABNORMAL NEONATAL FOAL

Provision of adequate nutritional support to the compromised neonate is an essential part of critical care but often becomes a major management problem. Reasons for the common failure to provide adequate nutrition to the neonate include underestimating the needs of the ill, stressed animal; a disinterest in nursing on the part of the sick neonate; the need to use alternate methods and routes of delivery for continued oral feeding; and a GI tract that is compromised and intolerant of nutrient intake.

Nutrition of the premature or sick neonate is a science that is in its early stages of development even in human neonatology; much less is known in veterinary medicine. The exact nutritional requirements for optimum growth of the normal-term foal have not even been defined, let alone for the premature, growth-retarded, or debilitated animal whose caloric, protein, mineral, and vitamin requirements might be very different.

Measurements of milk production of mares combined with data concerning the free-choice milk intake of normal orphan foals and premature and sick foals recovering from various illness suggest that a figure of 125 to 150 kcal/kg/day or even higher is close to the normal caloric intake.²⁰² Healthy full-term foals nurse an average of 2 minutes, seven times an hour,²⁰³ consume between 20% and 30% of their body weight in mare’s milk daily, and gain 0.5 to 1.4 kg/day. On this diet a 50-kg foal would consume 10 to 12.5 L of milk a day to receive 120 to 150 kcal/ kg/day. Nutritional requirements may be even higher in disease states such as generalized septicemia, pulmonary disease, or thermal stress or after surgery.

If there is no medical contraindication for oral feeding, and if the GI tract is functional, then enteral nutrition is the preferred and most effective route of nutritional supplementation. Enteral feeding is more physiologic and stimulates normal gut maturation, growth of intestinal villi, production of crypt cells, and hepatic and biliary secretions and brush border disaccharidase enzyme activity. Enterocytes rely on absorption of volatile fatty acids (VFAs) such as glutamine and β-hydroxybutyrate from the gut lumen as their primary energy source. Therefore, even in foals that must be fed parenterally, small volumes of enteral feeds are given to “feed the gut” to prevent gut atrophy.

Foals that are not nursing from the mare can be fed by bottle, bucket, or nasogastric tube. If an effective swallow reflex is present, then bottle-feeding can be used. Udder-bumping and teat-seeking behavior can be stimulated by allowing the foal to approach the bottle from behind and under the handler’s armpit. This technique also reduces the risk of aspiration by preventing overextension of the head and neck. Bucket-feeding allows the foal to drink with its head and neck in a flexed position and is helpful for foals with a weak swallow reflex or foals destined to be hand-raised. Milk should be introduced in a shallow handheld bowl and the foal encouraged to suckle the finger or nipple as its head is lowered into the milk. “On demand” feeding is ideal but often impractical and labor intensive. Foals less than 7 days of age should be fed every 2 hours.

Nasogastric intubation is required if ineffectual swallowing and suckling are present. A small-bore flexible silicone tube (5 to 7 mm internal diameter) with a weighted tungsten end is preferred. Individual choice dictates whether the tube is passed for each feeding or left indwelling. Indwelling tubes can be sutured to the nares or taped to half a tongue depressor, which is then taped to the foal’s muzzle and/or fleece halter. Tubes should be sealed between feedings to prevent aerophagia. Recumbent foals should be maintained in sternal recumbency immediately after tube-feeding to reduce the risk of gastroesophageal reflux and aspiration.

If the foal’s mother is available and milk production is adequate, free-choice nursing is optimal. A nurse mare is probably the next best substitute. Popular enteral formulas include mare’s milk, goat’s milk, and artificial milk replacers. Mare’s milk is preferred. Goat’s milk is acceptable and is higher in fat, total solids, and gross energy than mare’s milk Foals raised on goat’s milk occasionally become constipated. Cow’s milk is not as digestible but can be used if additional sugar is added and some of the fat is removed. This can be accomplished by using 2% skim milk and adding 20 g of dextrose per liter of milk. Various artificial milk replacers are available. The ideal replacer should have 22% crude protein, 15% fat, and less than 0.5% fiber on a dry matter basis. Complications associated with enteral feeding include colic, abdominal distention, diarrhea, constipation, flatulence, misplacement of the nasogastric tube, aerophagia, nasal and pharyngeal irritation from the tube, and aspiration pneumonia.

Delayed gastric emptying and gastroduodenal dysmotility can be improved in some foals with metoclopramide given IV as a slow infusion (0.25 mg/kg/hr) or orally (0.3 to 0.6 mg/kg q4-6h). Overdosage is associated with excitement. Other prokinetic agents are erythromycin (1 mg/kg PO four times per day [qid] or given as a 30-minute infusion qid), which works throughout the GI tract, and cisapride (0.1 to 0.2 mg/kg PO or per rectum q6h), which also affects the entire gut. All prokinetic agents are contraindicated if GI obstruction is suspected. Diarrhea is treated symptomatically with oral bismuth subsalicylate (1 to 2 mL/kg PO q4-6h) and/or loperamide (0.1 to 0.2 mg/kg PO q6h). Diarrhea may also respond to administration of active culture yogurt or an intestinal inoculant containing lactobacillus organisms. Nasopharyngeal irritation from repetitive tubing can be treated with insufflation of a nasopharyngeal spray containing prednisone, Furacin, glycerin, and DMSO.

More details on the feeding of both orphan and sick neonatal foals are contained in review papers^202,204-207 and in Chapter 50.

PARENTERAL NUTRITION

During the past years, PN has been more frequently used to supply at least a portion of the daily nutritional requirements to critically ill foals and calves. PN is indicated whenever feeding via the gut is inadequate or contraindicated. Candidates for partial or total PN include individuals with chronic diarrhea, those with GDUD (foal), a variety of postsurgical patients, foals with botulism, premature and infected animals, and other individuals with GI tracts poorly tolerant of enteral feedings.

PN involves administration of hypertonic solutions containing dextrose, amino acids, lipids, vitamins, electrolytes, and trace minerals. These PN solutions must be administered continuously through a jugular catheter. Complications include metabolic disturbances such as hyperglycemia, hypoglycemia, glucosuria, osmotic diuresis, hyperlipemia, azotemia, and imbalances of trace minerals, vitamins, and electrolytes. Catheter-related problems include thrombosis, phlebitis, and sepsis. Commonly used stock solution for PN include 50% dextrose, 8.5% or 10% amino acids, and 10% or 20% lipid emulsion. Sample calculations are as follows:

Foals receiving PN should have their blood and urine glucose monitored. Serum glucose concentration should remain >80 mg/dL and <180 mg/dL. Serum should be checked for gross lipemia. Heparin can be administered at 10 units/kg as an intravenous bolus to treat lipemia. The amount of glucose, lipid, and amino acids can be varied for each individual foal. Foals must be weaned onto and off of PN slowly. All intravenous lines must be checked routinely for signs of infection. Additional information regarding the use of PN in foals is presented in other articles.²⁰⁷

At the present time, the applications of PN are on a fairly short-term basis compared with human medicine—usually 2 to 3 weeks at the most. Most commonly, parenteral and enteral nutrition has been used in combination; parenteral nutrient delivery is used to supplement, not totally replace, oral intake. Enteral nutrition helps to maintain the intestinal mucosa. Prolonged total PN is associated with reduced intestinal epithelial cell renewal, villous atrophy, and decreased enzymatic activity.²⁰⁸

Although PN can be expensive and difficult to manage, the benefits, such as prevention of a catabolic state or starvation, improved body condition at discharge, and better healing, can far outweigh the drawbacks. Details concerning the use of PN compounds can be found in other references²⁰⁹ and in Chapter 50.

IMMUNE SYSTEM SUPPORT: PLASMA AND COLOSTRUM

Controversy persists as to what serum IgG concentration is protective for newborn foals. There is little argument, however, that healthy foals have postsuckle IgG exceeding 1000 to 2000 mg/dL within 24 hours of birth. There is also agreement that there is a correlation between very low IgG concentrations (IgG <200 mg/dL) and increased foal morbidity and mortality. By definition, IgG <200 mg/dL is complete FPT, and IgG between 200 and 800 mg/dL is partial FPT. Causes of FPT include poor-quality colostrum, failure to ingest adequate colostrum, and inability to absorb adequate amounts of colostral immunoglobulins. Mares produce an average of 1.5 to 2 L of colostrum. Ideally, foals should receive a minimum of 1 L of good-quality colostrum within the first 8 hours of life. In addition to IgG, colostrum contains IgA for local gut immunity, IgM, high caloric density, growth factors, lactoferrin, laxative properties, and leukocytes. If fresh or frozen colostrum is not available, some sources of lyophilized IgG products for oral administration have a longer shelf life and do not require freezing. These products are expensive and may have variable absorption. Regardless of the product, a rule of thumb is to administer a minimum of 40 g of IgG (or 1 g/kg of body weight) to colostrum-deprived foals. Always measure serum IgG concentrations to determine if the supplementation was adequate.

Plasma administration becomes necessary if the foal’s IgG is low and the foal is too old to absorb colostrum or gut function is abnormal. I recommend IgG supplementation for any foal with a serum IgG <200 mg/dL regardless of its health status or environment. If the foal’s IgG is between 200 and 800 mg/dL, I recommend IgG supplementation if one or more of the following conditions exist:

Plasma is administered through an aseptically placed catheter using a blood administration set with an in-line filter. The volume of plasma to give depends on the foal’s IgG, the desired IgG, the foal’s body weight, and the IgG in the plasma and the general health of the foal. The old rule of thumb for plasma administration for FPT is 20 mL/kg or approximately 1 L for a 45-kg foal. In healthy foals, 1 L of plasma with IgG of 1200 mg/dL raises the serum IgG 200 to 250 mg/dL. The same amount of plasma has less effect in foals with sepsis. Ill foals require relatively more plasma because the serum half-life of IgG is less, IgG may be sequestered in intravascular spaces or at sites of inflammation, and IgG may be catabolized more readily. A complete discussion of FPT and its treatment is presented in Chapter 53.

Plasma should be administered at an average rate of 10 mL/kg/hr. Give the first 100 mL slowly, and monitor the foal’s pulse, respiratory rate, and temperature. Possible transfusion reactions and treatment for such reactions are listed in Table 19-4.

Table 19-4 Transfusion Reactions: Signs, Causes, Therapy

IV, Intravenous; RBCs, red blood cells; SC, subcutaneous.

RESPIRATORY SUPPORT

Thoracic radiographs and ABG analysis help determine the severity of lung disease. Lateral radiographs with the foal standing or recumbent help characterize the nature and extent of pulmonary pathology. Diffuse pulmonary infiltrates occur with bacterial and/or viral pneumonia and atelectasis. Cranioventral and caudoventral pulmonary infiltrates are seen with aspiration pneumonia and bacterial bronchopneumonia. Nodular infiltrates suggest discrete abscessation.

ABG analysis determines the degree of pulmonary dysfunction. Portable blood gas machines now make blood -gas analysis easy and affordable. The preferred site for arterial puncture is the great metatarsal artery. A small 25-gauge needle attached to a heparinized 1- or 3-mL Luer slip syringe is used. Hypoxemia (PaO₂ <60 mmHg) with a normal PaCO₂ is caused by ventilation-perfusion mismatching, right-to-left shunting, low inspired O₂, and impaired gas exchange. Hypoxemia accompanied by elevated concentrations of CO₂ is usually the result of hypoventilation caused by respiratory muscle fatigue, central depression of the respiratory center, or neuromuscular weakness as with botulism. Mild hypoxemia can be improved by positioning the laterally recumbent foal into a sternal position. Oxygen supplementation is best administered through a soft nasal cannula inserted into the nasal passage to the level of the medial canthus. The cannula can be sutured or taped to the external nares. Humidified oxygen is administered using a tank or wall oxygen source and a humidifier filled with distilled water. Oxygen flows between 2 and 10 L/min are regulated using a flowmeter. Flow rates are adjusted to keep the PaO₂ between 70 and 100 mm Hg. Long oxygen lines attached to a surcingle allow even ambulatory foals to benefit from continuous oxygen therapy.

Mechanical ventilation is necessary for persistent hypoxemia that is refractory to nasal insufflation or is accompanied by PaCO₂ >70 to 75 mm Hg. A long, cuffed nasotracheal tube is required, as is a ventilator capable of delivering tidal volumes of 10 to 15 mL/kg, respiratory rate of 15 to 25 breaths per minute, proximal airway pressure between 18 and 25 cm H₂O, end expiratory pressure (PEEP) of 0 to 10 cm H₂0, and an inspired oxygen concentration between 0.21 and 1.0. Ventilators that allow the foal to breathe spontaneously between preset ventilator-delivered breaths are tolerated the best by the foal. Guidelines regarding ventilatory support for foals are presented in other review articles.⁷⁰ More recently there have been investigations using noninvasive mechanical ventilation in neonatal foals.⁷²

Foals have a poorly developed cough reflex. Tracheobronchial secretion removal may be enhanced using chest coupage and nebulization with mucolytic agents such as acetylcysteine or dilute bicarbonate solution. Ultrasonic nebulizers using solutions or an Equine Aero Mask using metered dose aerosol inhalers can be used.

Chemical respiratory stimulants can be used to stimulate the central respiratory center. Theophylline, caffeine, and aminophylline are xanthine derivatives commonly used as bronchodilators but can also be used to improve diaphragmatic contractility and to treat periodic apnea associated with hypoxia and prematurity. The safest stimulant is caffeine: loading dose of 10 mg/kg PO once per day (sid) followed by 2.5 mg/kg PO sid as a maintenance dose.

ANTIBIOTIC THERAPY

Foals that become ill or compromised during the first few days of life are at increased risk for infection. Because of the neonate’s immature immune system, localized infections tend to become systemic, leading to septicemia. This explains how foals with diarrhea can develop uveitis and septic joints. Antibiotics are administered to foals for two reasons: prophylactically to prevent infection, and therapeutically to treat existing infection. The most serious infections are those caused by gram-negative bacteria (e.g., E. coli, Klebsiella, Salmonella, Pasteurella, Actinobacillus). The most common gram-positive pathogen is Streptococcus species, which are often encountered as part of a mixed infection involving the respiratory tract and umbilicus. Occasionally, anaerobic infections (e.g., Clostridium, Bacteroides) are encountered as causes of umbilical infections, diarrhea, or aspiration pneumonia.

When antibiotics are used prophylactically, the oral and intramuscular routes of administration can be considered. Penicillin or ampicillin and an aminoglycoside administered IM or IV are good choices if the risk of infection is great. Ceftiofur (IM) or trimethoprim-sulfamethoxazole (PO) are reasonable choices. Prophylactic antibiotics should be given for 3 to 5 days or until the risk factors for sepsis are gone. Once sepsis is confirmed, intravenous administration is the preferred route because gut absorption is too variable. Antibiotic therapy should be continued for a minimum of 7 to 10 days. If localized infections develop, then antibiotics may need to be given for 2 to 3 weeks. In cases of abscess formation and bone infections, therapy is often extended for 1 to 2 months.

TRANSPORT AND REFERRAL

A decision should be made early in the clinical course as to whether the neonate can be taken care of at the farm or whether it should be referred to a neonatal intensive care facility for treatment. If the support staff on a farm are experienced and committed to provision of good nursing care and if appropriate diagnostic facilities are available, many mildly to moderately ill individuals can be successfully treated on the farm and recover within 2 to 4 days. When the neonate is more compromised and in need of considerable supportive care, including continuous intravenous fluid therapy and oxygen supplementation, a more rational decision is to refer, if the animal’s value warrants the expense. The sicker or more immature the neonate, the more complications it is likely to develop during the course of treatment. It is much better to refer a sick neonate early in the course of disease, rather than as a last resort before death.

If a decision is made to refer the animal, the method of transport is extremely important to the outcome of the case. If the foal is recumbent and is showing signs of hypothermia and/or respiratory distress, consider shipping the foal ahead of the mare in a heated vehicle. The mare can be stripped of colostrum and then sedated. Colostrum can be sent with the foal. The mare can be sent later, once the foal’s condition has been evaluated and stabilized at the referral clinic. Save and send the placenta with all compromised foals. Recumbent foals can be restrained in SUVs and cars by wrapping them in a sleeping bag or blanket. Ideally, an attendant should travel with any weak, recumbent, or potentially recumbent foal.

Cold foals can be warmed during transport by increasing the inside temperature of the vehicle and by placing water bottles or heat packs beside the foal. Body temperature, blood glucose, and oxygenation must be maintained during the trip. In the hypoglycemic patient, a continuous glucose infusion during the trip is far better than a glucose bolus given before departure.

If the foal is dyspneic, administer intranasal oxygen at 3 to 6 L/min using an indwelling intranasal cannula. Portable oxygen tanks can be rented from home care pharmacies with a veterinarian’s prescription. A recumbent foal should be kept sternal and turned every 2 hours to minimize dependent lung atelectasis. If the foal is apneic or demonstrating an unusually slow respiratory rate, consider a loading dose of caffeine (10 mg/kg) given PO or per rectum before transport.