Equine Neonatal Critical Care--Keeping it Simple
ACVIM 2008
John Madigan, DVM, MS, DACVIM
Davis, CA, USA

Specialized units for critical care of ill newborn foals are part of private practice referral centers and University Teaching Hospitals. Neonatal foals require special considerations because of their unique transitional state post birth from the in-utero environment with depressed awareness, fetal circulation using the placenta for oxygen and carbon dioxide gas exchange, and reduced organ activity such as reduced gut motility (See Figure 1) etc. A large number of systems must be turned on by the foal and when illness occurs during the neonatal period this normal smooth transition is interrupted and must be dealt with by the attending clinician and technicians.

Before going into specific details I would like to point out that the single most important factor in the success of equine neonatal critical care has been the delivery of nursing care and monitoring by animal health technicians. Without the nursing staff and their skills of observation and dedication to deliver of precise 24hr per day veterinary medical care, the field of neonatal medicine would have never developed and grown as it has since the creation of the first foal intensive care unit in the early 1980's.

Figure 1.

 

In utero

After birth

CNS/ Mentation

Depressed

Responsive

Respiratory

upwards arrowpulmonary vascular
resistance, atelectasis

Rapid lung expansion

Cardiovascular

Fetal circulation

Mature circulation

Gastro-intestinal

Minimal absorption

Full digestion

Renal

Minimal urine output

Complete control of fluid balance

Musculo-skeletal

Non-ambulatory

Fully ambulatory

Thermoregulation

Environment controlled

Self controlled

Pain sensation

Minimal

Full

I have further categorized these unique differences compared to adult equine critical care patients which will need to be monitored in all neonatal foal critical care cases.

Body temperature fluctuations in the neonate can cause problems and the foal is very prone to hypothermia. Wet foals undergo rapid heat loss due to evaporation. Foals placed on cold surfaces lose heat by conduction. Foals lose heat by radiation when adjacent (without) direct contact to walls or windows with negative temperature gradients across them. Foals exposed to low environmental temperatures and drafts lose heat by conduction. Additionally foals lack fine tuning of their body temperature set points and can easily become overheated and hyperthermic. Conditions such as excessive struggling and seizures can produce significant hyperthermia requiring a complete cessation body warming efforts. Hyperthermia causes increased caloric demands, affects ventilation and pulmonary function and lowers the threshold for seizures.

Carbohydrate metabolism is altered with neonatal disease and hypoglycemia is a clinical problem. The foal is born with very low stores of glycogen in liver, muscle or brain. Blood glucose rapidly drops in the first 2 hours post foaling and a source of milk or energy must be provided to the foal if it fails to nurse by 3 hours. Glucose is required for cellular energy metabolism and brain function. Dextrose containing fluids need to be considered in ill neonatal foals. A STAT blood glucose is part of all entry data bases for ill neonatal foals. Normal blood sugar values range from 80 mg/dl to 160 mg/dl post feeding. Recent findings in human critical care have correlated somewhat lower survival in patients where patients experience hyperglycemia. However, recent studies reveal patients monitored by intensive insulin administration have more complications. The ability to control blood glucose by intravenous fluids requires frequent monitoring and will be affected by oral feedings. Partial parenteral nutrition is commonly used in weak foals that cannot receive adequate energy by the oral route. Rates of infusion and use of insulin with glucose infusions need to be individually determined for each foal.

The use of 5% dextrose alone is not a preferred starting fluid because the glucose is rapidly metabolized and free water is all that is left leading to lowered levels of blood sodium levels. In general adding 20 ml of a 50% dextrose solution to each liter bag of lactated ringers solution or plasmalyte 40, making a (1% glucose solution) provides a reasonable starting approach. Other clinicians prefer starting with 0.45% NaCl + 2.5% dextrose solution. Because the need from energy from glucose may exceed allowable fluid volumes to the newborn, once stabilized concentrated glucose solutions such as 50% glucose are used at separate infusion rates to that of maintenance fluids often starting in the range of 20-25ml/hr to a 50 kg foal.

Immune function is less than in adults. Neutrophil function is less in the first 24 hours of life and for the first few months the foal has reduced mononuclear cell function with regard to certain pathogens. Complement (works together with antibodies to fight bacterial infection) levels are lower in young foals than adult horses. Specific antibody cannot be generated until a secondary response develops which can take several weeks. Foals have limited responses to cytokines (factors produced in adult horses and humans when an infection is present) and rarely have fevers even when infections are present. This means that the minimum data base includes a blood culture and/or culture of a joint, umbilicus, or urine at admission. Additionally, a sepsis score is calculated which takes into account risk factors, white blood cell count, presence of bands, IgG level, and other factors in making an early decision about the likelihood of infection.

Infections in the neonate often involved mixed infections with gram-positive and a gram-negative bacteria and which can rapidly progress to systemic inflammatory response and signs of septic shock. Gram-negative bacteria release endotoxin which triggers a cascade of events leading to SIRS (systemic inflammatory response syndrome) and full blown septic shock. Mortality is very high for gram-negative sepsis in humans and in foals. Early identification and prompt treatment are essential as well as provision of critical care if advance stages of sepsis are present. This mandates frequent assessment of cardiovascular stability and monitoring including heart rate, respiratory rate, mucous membrane color, jugular distensibility, pulse quality, blood gases, pulse oximetry, and systemic mean blood pressure via tail Doppler.

Septicemia (bacterial infections reaching the blood stream and circulating bacteria throughout the body) can occur with other conditions affecting the foal. Foals with neonatal maladjustment syndrome, birth hypoxia, ruptured bladders, angular limb problems which limit nursing, and other conditions may be accompanied by sepsis.. Foals which have not nursed have an 'open gut' waiting for ingestion of intact immunoglobulins from colostrum. Bacterial translocation (movement of bacteria across the open gut, is considered a likely source of bacteria which then enter the blood stream shortly after birth. Therefore, antibiotic coverage is essential when a risk of sepsis is present. Antibiotics will not reverse the SIRS and must be given prior to the development of the sepsis cascade to be most beneficial. Late administration of antibiotics renders the foal at risk for joint infections where the bacterial settle in the highly vascular growth plates of the young foal.

Normal values for blood clinical chemistry tests and hematology are different in neonatal foals compared to adult horses. It is important to understand the normal parameters of the foal for interpretation of laboratory findings. The normal 24 hr old foal should have a 2:1 neutrophil lymphocyte ratio shortly following birth. Premature foals have low neutrophils and high lymphocyte counts. Foals have lower serum protein than adults. On a chemistry panel certain values are very different then adult horses. For example the serum alkaline phosphatase levels are much higher than in adults and likewise GGT (gamma glutamyltransferase), bilirubin and AST levels are elevated. This is normal for the foal. Always consult a table of normals for the neonatal foal not the adult horse.

Antibiotics and other drugs are absorbed, metabolized, and excreted at different rates in term neonates than adults. It is important to use specific doses of medications that are appropriate for the neonatal foal. A larger volume of distribution and reduced liver metabolism of some drugs will alter the dose and frequency of administration of some drugs. With regard to infection fighting drugs like gentomycin, an aminoglycoside drug, foals metabolize this class of drug similar to adults and do not have a unique susceptibility to toxicity. Foals need to be kept hydrated when using aminoglycoside compounds. Always consult a current updated therapeutic dosage table for neonatal foals.

Body position affects blood oxygen levels. Foals in the sternal position have better lung perfusion that those in recumbency. Blood oxygen levels can drop by over 40 mmHG by being in the lateral recumbency. Always put the foal in sternal position when performing post birth resuscitation and attempt to maintain the foal in sternal position when resting in the ICU. Bean bags have worked well and there are other V shaped devices to keep the foal sternal.

Prolonged hypoxia can produce irreversible damage and cardiopulmonary failure due to reversion to fetal circulation. Shortly after birth when the first breath takes place the pulmonary arteries relax and increased blood flow to the lungs in facilitated compared to the in utero state of limiting blood flow to the lungs and using blood flow to the placenta for gas exchange. This system will switch back to high pulmonary pressure, and right to left shunting of blood and reopening of the ductus arteriosis when the foal is compromised. For example prolonged external cold doubles pulmonary vascular resistance, as does prolonged hypoxia. The pulmonary vessels constrict and the body attempt to shunt blood to the placenta, which is now gone, and less blood flow goes to the lungs and leading to limited oxygen exchange leading to more hypoxia despite providing supplemental nasal oxygen insufflation.

A neonates condition can change rapidly compared to adult horses. High risk or sick neonatal foals need frequent observation for detection of new problems which may occur rapidly. Referral of valuable ill neonatal foals to critical care units where around the clock care and observation is provided may be required for optimal care. Within the neonatal care unit a detailed record keeping system and frequent monitoring is required. An example of the kinds of considerations that need to be made for the ill foal are can be found in Manual of Equine Neonatal Medicine, 4rd edition, JE Madigan, editor.

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John Madigan, DVM, MS DACVIM
University of California - Davis
Davis, CA


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