Ch 19-21

Important Features of Intravenous Anesthetic Protocols

Most important features of intravenous protocols are a smooth, excitement-free induction phase with a slow lowering of the body into sternal and lateral recumbency, minimal cardiopulmonary depression, no reactions to surgical stimuli, and a calm recovery with a single attempt to stand and minimal ataxia Other desirable traits include good muscle relaxation and analgesia, as well as the ability to assess depth of anesthesia and to modify it in a quick and predictable manner

What are injectable anesthetic combinations currently used for?

Injectable anesthetic combinations are currently used for anesthesia induction and for short, minor surgical procedures (up to 45 minutes)

Intravenous Induction in Adult Horses

For anesthesia induction, the best suited drug and one that is most commonly used is ketamine, but circumstances may warrant use of tiletamine-zolazepam, alfaxalone, or propofol instead

For compromised horses, a mixture of equal volumes of diazepam (5 mg/mL) and ketamine (100 mg/mL) is a safe method for anesthesia induction and usually requires about 1 mL/25 kg of the mixture (0.1 mg/g diazepam plus 2 mg/kg ketamine) Reduces the cardiovascular-compromising side effects of a2-agonsists that are usually used in horses for sedation before anesthesia induction

Intravenous Induction in Foals

In very young foals, where a2-agonst sedation is generally not recommended, the administration of propofol or alfaxalone to effect may be used for anesthesia induction, often in combination with a benzodiazepine

Ketamine for Intravenous Anesthesia

Provides good somatic analgesia without inducing hypnosis

Not suitable as a sole agent because it may cause excitement, seizure-like activity, and muscle rigidity

Recovery at the end of anesthesia with ketamine is usually quick (complete within 30 minutes) and coordinated

Respiratory depression is minimal

Ketamine's sympathomimetic action is ideal to counteract the bradycardia caused by the drugs used as sedatives

Because the eyes remain open and the reflexes are only minimally depressed, the assessment of anesthetic depth can be difficult and requires some familiarization over time Movements as a result of awakening may occur very suddenly and may be of a strong nature, therefore it may be preferable to administer ketamine in healthy horses according to a fixed time scheme rather than to effect

Ketamine and Guaifenesin

Guaifenesin is a centrally acting muscle relaxant and should be formulated at a concentration of no greater than 5% (50 mg/mL) solution because higher concentrations are associated with significant irritation of the veins and can cause intravenous hemolysis Guaifenesin (5%) is administered to the sedated horse to effect (preferably under pressure, because effective dosages are large: 50 mg/kg or 500 mL for a 500 kg horse). When the horse begins to relax (buckle at the carpus), the induction drug, most commonly ketamine (2 mg/kg) should be administered

Ketamine and Benzodiazepines for Anesthesia

Depending on the dosage of benzodiazepine used, they may increase the respiratory depression caused by ketamine To reduce the risk of apnea and ataxia during recovery, low dosages of benzodiazepines (0.02-0.04 mg/kg diazepam or midazolam IV) are necessary For anesthesia induction followed by inhalation anesthesia where mechanical ventilation is available, higher dosages (up to 0.1 mg/kg IV) have been used to improve relaxation and simplify intubation Because a2-adrenoeptor agonists also provide muscle relaxation, if a low dose of benzodiazepine is used under field conditions, a high end of the dose of sedatives may be chosen

Ketamine and Lidocaine For Anesthesia

An IV bolus of 3 mg/kg lidocaine in ponies anesthetized with xylazine/ketamine for castration did not reduce the need for additional dosing with xylazine/ketamine for maintenance of unconsciousness and recumbency during castration but the time to standing was prolonged by lidocaine

S(+)-Ketamine for Intravenous Anesthesia

Currently licensed form of ketamine is a racemic mixture of the S(+) and the R(-) enantiomers S(+) ketamine is the active compound

At a dosage rate of 50% to 66% of the racemic ketamine, the effects of S(+) ketamine were comparable to the racemic form

Recoveries were smoother and of better quality when S(+)-ketamine was used in combination with xylazine administered as boluses for up to 40 minutes to perform castrations

S(+)-ketamine given as a constant rate infusion in combination with inhalation anesthesia also resulted in better recoveries than the racemic, ketamine at double the dosage rate

Tiletamine for Intravenous Anesthesia

Similar to ketamine, dissociative agent

Commercially available as a powder in a fixed (1:1) combination with zolazepam, a benzodiazepine, and it is reconstituted in sterile water and can be stored refrigerated for up to 2 weeks

Used in horses and other equidae at dosage ranges of 1.1 to 1.65 mg/kg IV after sedation with a2-adrenoreceptor agonists

Depending on the dosage, the recumbency time was considerably longer than with ketamine combinations and respiratory depression was more pronounced

Ataxia during recovery was more accentuated than with ketamine with horses making several attempts to stand

Propofol for Intravenous Anesthesia

Short acting anesthetic that provides very good hypnosis and muscle relaxation but poor analgesia

Not suitable as a sole agent in adult horses because inductions are unpredictable and large volumes are required which make it impractical and prohibitively expensive

Low-dose propofol (0.4 mg/kg) used in in combination with ketamine following xylazine sedation provides both a smooth induction of anesthesia and recovery from it

In the neonatal foal, the use of propofol alone administered to effect for anesthesia induction (approximate dosage: 2 mg/kg IV) provides a good alternative to ketamine as induction is smooth, and because the duration of action is short, recovery is timely Relaxation is better and therefore endotracheal intubation is easier than with ketamine Not recommended in hypotensive foals, as it can cause hypotension and further compromise these patients

Alfaxalone for Injectable Anesthesia

For anesthetic induction or short field procedures, it can be used like ketamine (sedation with a2-adrenoreceptor agonist followed by anesthesia induction with alfaxalone 1 mg/kg IV in combination with benzodiazepine or guaifenesin

Incidence of muscular tremors is higher with alfaxalone

Recovery is also consistently less predictable than with a2-adrenoreceptor agonist and ketamine combinations

Currently is cost prohibitive for routine use as an induction agent in horses

Triple Drip

Consists of a dissociative drug, muscle relaxant, and a2-agonist (e.g. ketamine-guaifenesin-a2 agonist) Benzodiazepines have been substituted for guaifenesin in this combination

Ketamine Combinations for TIVA

Ketamine combinations can be safely used for up to 1.5 hours to perform minor surgeries under clinical practice conditions A recumbent horse breathing air will, however, become hypoxemic To prevent this complication during procedures of 20 minutes or longer, inspired air should be supplemented with oxygen (10-15 L/min, via a nasal or endotracheal tube) A higher inspired fraction of oxygen (FiO2) and ventilatory support may be provided using an oxygen demand valve, if needed If the duration of anesthesia exceeds 2 hours, ketamine should not be used because it produces active metabolites that are eliminated very slowly and will influence the recovery period negatively With ketamine infusions, monitoring of anesthesia is different because when compared to inhalation anesthesia, horses maintain ocular and pharyngeal reflexes and occasional nystagmus, and consequently appear lightly anesthetized even in the absence of response to surgery Because of this reflex activity, injectable-based combinations are not suitable for certain ocular or pharyngeal procedures

Ketamine-Guaifenesin-a2-Adrenoceptor Agonist (Triple Drip) for TIVA

Cardiovascular and respiratory depression is relatively minor

Should be restricted to procedures of up to 1.5 hours in duration because of the cumulative effects of ketamine metabolites and because large doses of guaifenesin may result in severe ataxia during recovery

Anesthetic induction performed prior to triple drip anesthesia should not include guaifenesin so that the total amount of guaifenesin given to the individual remains below the toxic dose where muscle rigidity and respiratory depression become evident

While a2-adrenoreceptor agonists provide good analgesia, adjunctive medications to target other aspects of the pain pathway (e.g. NSAIDs, local anesthetics, opioids) can be useful during TIVA as with inhalation anesthesia maintenance

Ketamine-Midazolam-a2-Adrenoceptor Agonist for TIVA

A protocol using medetomidine for sedation and ketamine (2.5 mg/kg IV) and midazolam (0.4 mg/kg IV) for anesthesia induction, followed by a mixture containing 0.8 mg/mL midazolam, 40 mg/mL ketamine, and 0.1 mg/kg medetomidine at a rate of about 0.09 mL/kg/h for maintenance of anesthesia was used satisfactorily for castrations lasting 38 +/- 8 minutes

Hypoxemia was noted and oxygen supplementation for procedures over 60 minutes was recommended

Propofol for TIVA

In contrast to ketamine, propofol is an ideal anesthetic for prolonged TIVA Short, context-sensitive half-life, permitting rapid recovery

Used in combination with different a2-adrenoceptor agonists, guaifenesin, ketamine, and opioids

Problems mainly of respiratory nature (apnea and severe respiratory acidosis) and the relatively high cost of propofol have prevented widespread use of propofol in clinical practice

While recovery quality after propofol-based combinations is routinely judged to be excellent, extreme respiratory depression and cost limit its use

Alfaxalone for TIVA

Rapidly cleared from plasma and is therefore theoretically suitable for longer anesthesia

Following an uneventful 3 hours of TIVA with alfaxalone, there was some evidence of excitement and hyperesthesia evident in six of the eith horses during recovery

Because of the poor recovery characteristics and the currently high cost, alfaxalone's use for this purpose is not recommended

What ages do anesthetists consider neonates?

Up to 1 month of age

What ages do anesthetists consider pediatric?

1-3 months

What ages do anesthetists consider juvenile?

3-4 months old

From an anesthetic standpoint when can foals be treated like young adults?

Once they have acquired mature cardiopulmonary function and metabolic pathways and can be safely weaned (i.e. at 3-5 months of age)

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Neonate Cardiovascular

Transition from fetal to neonatal circulation

Risk of return to fetal circulation

HR, not SV-dependent cardiac output

Low systemic vascular resistance

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Pediatric/Juvenile Cardiovascular

More SV-, less HR-dependent cardiac output

Increasing systemic vascular resistance

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Neonate Respiratory

Maturation of pulmonary microanatomy, neuromuscular control, compliance, surfactant production

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Pediatric/Juvenile Respiratory

Higher Vmin and RR with normal Vt

Close to normall PaO2

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Neonate Nervous

Immature central, autonomic, and peripheral nervous system function

Higher BBB permeability

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Pediatric/Juvenile Nervous

Matured central, autonomic, and peripheral nervous system function

Close to adult BBB permeability

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Neonate Metabolism and Tissue Composition

High ECF compartment, CBV, CPV

Low glycogen reserves

No fiber intake

High body surface area (heat loss)

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Pediatric/Juvenile Metabolism and Tissue Composition

Higher ECF but close to adult CBV and CPV

Larger glycogen reserves

Increasingly more fiber intake

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management = Neonate Liver

Maturing liver function in first 3-4 weeks

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Pediatric/Juvenile Liver

Overall close to mature

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Neonate Renal

Immature

Reduced concentrating ability

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Pediatric/Juvenile Renal

Overall mature

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Neonate Hematology and Biochemistry

Decrease in PCV often due to neonatal isoerythrolysis

Gradual increase in WBC

Elevated serum enzyme activities

Most Relevant Aspects of Foal Physiology the Affect Anesthetic Management - Pediatric/Juvenile Hematology and Biochemistry

Normalizing PCV

Adult WBC

Elevated serum enzyme activities

Fetal Circulation

The primary function of the circulatory system of both the fetus and newborn is to deliver O2 and nutrients to metabolizing organs and return deoxygenated blood to the gas exchange organ to replenish the O2 and eliminate waste products including CO2 In the fetus the gas exchange organ is the placenta and its vascular connections are in a parallel arrangement with the other systemic organs, which are remove from the pulmonary circulation To supply deoxygenated blood to the placenta and return oxygenated blood to systemic organs, a series of extracardiac shunts (ductus venosus, patent ductus arteriosus) and intracardiac communication (foramen ovale) are necessary Oxygenated blood arriving in the fetus via the umbilical vein flows through the ductus venosus and the caudal vena cava toward the right side of the heart. Having entered the fright atrium, most blood is diverted away from the noninflated lung to the left atrium via the foramen ovale, an opening in the atrial septum that connects both atria Only a small portion of blood that entered the right atrium continues its path through the right ventricle into the pulmonary artery, from where it bypasses the lung by flowing through the patent ductus arteriosus into the aorta, a process that depends on the high pulmonary arterial resistance prevailing within the fetus's not yet aerated lung

Transition from Fetal to Neonatal Circulation

At birth, the function of gas exchange is abruptly transferred from the placenta to the lungs and therefore from the systemic to the pulmonary circulation The trigger for this to occur is the onset of breathing, leading to aeration of the lungs and subsequently a dramatic decrease in pulmonary arterial resistance, thereby promoting blood flow through the pulmonary vasculature As a result, the blood pressure in the left atrium increase relative to the pressure in the right atrium, pushing a valve that lies over the foramen ovale on the left side of the atrial septum against it and thus largely preventing any further right-to-left blood flow through the foramen ovale From this moment onward, venous and arterial circulations are basically separated, and not only are the fetal shunts unnecessary, but their persistence may compromise circulatory functions The decrease in circulating prostaglandins that accompanies perinatal adaptation also promotes the closure of the ductus arteriosus over time Closing of the ductus arteriosus does not occur immediately at birth and therefore right-to-left shunting may continue; murmurs consistent with a patent ductus arteriosus may be auscultated and blood flow within the ductus arteriosus may be echocardiographically detected in normal foals during the first 3-5 days of life and occasionally for several weeks after birth Even partial reopening is possible up to the moment of complete fibrosis of this pathway which usually occurs within 2-3 weeks As part of the transition from fetal to neonatal circulation, the left ventricular wall increases in thickness in parallel with a rise in systemic vascular resistance, reflecting the shift from physiological right ventricular hypertrophy during fetal life to physiological left ventricular hypertrophy in postnatal life

Hemodynamic Function in Foals

Cardiac output (CO) is defined as the amount of blood ejected by the heart per minute and is calculated as the product of heart rate (min-1) and stroke volume (mL) Most appropriate index of overall cardiovascular function When normalized to body weight, referred to as the cardiac index (CI) (mL/min/kg)

The CI in resting foals up to 2-3 months of age is markedly higher when compared to adults and is primarily rate dependent

If the CO is adjusted for metabolic size (kg0.75) the average CI in foals is approximately twice that of adults, but the average stroke volume index 30% less

The normal heart rate of a resting equine neonate is significantly higher than in adults to maintain a higher CO

In this early period of life, any drug with heart rate decreasing properties, such as a2-adrenoreceptor agonists, may compromise the hemodynamic function to an extent that the neonate cannot tolerate

From 4 months of age onward, heart rates reach close to adult values and remain relatively stable throughout the remainder of the first year

Systemic arterial blood pressure is substantially lower in the early days of life, but pulse pressure amplitude is higher in the neonate compared to the adult owing to a lower vasomotor tone and hence systemic vascular resistance

By 1 month of age, foals tend to have a lower CI and heart rate, but a larger stroke volume, and their mean arterial pressure increases during this period because of a marked increase in vascular resistance indicative of the maturing sympathetic branch of the autonomic nervous system

Respiratory System of Foals

At birth, neither neuromuscular control, of ventilation nor the lung itself is fully developed in foals

Pony lungs are microanatomically more mature at birth than horse lungs but still sufficient surfactant production is lacking and gas exchange occurs across terminal air spaces and more primitive alveoli

Compliance of the chest wall is large in the neonate but lung elasticity is decreased During inspiration, the still weak diaphragm and intercostal muscles of the neonate can easily expand the chest and the intrathoracic volume with relatively low cost of muscle strength and without producing much of a negative intrathoracic pressure Low parenchymal compliance of the neonatal lung requires much more of a negative intrapleural and intrathoracic pressure to optimally inflate the lung Functional residual capacity (FRC; i.e. the gas volume left in the lung after a normal expiration) and tidal volumes are markedly smaller than in the adult In the immediate postnatal period foals are hypoxemic with PaO2 values significantly lower than during adult life, while the arterial carbon dioxide tension (PaCO2) values are similar Since the needs of the rapidly developing organism are higher, O2 consumption (6-8 mL/kg/min) exceeds that of the adult horse by two- to threefold, requiring increased respiratory minute ventilation To compensate for the smaller FRC and tidal volume, newborn foals typically breathe up to 60-80 times per minute, which in the fourth to sixth week declines to 30-40 breaths per minute for the remainder of the first 3 months of life before gradually approaching adult values

Neonates close the upper airway at the end of expiration and therefore do not allow the lung to collapse easily (often referred to as "auto-positive end-expiratory pressure" or auto-PEEP"), this is lost in the intubated foal during anesthesia This in conjunction with a lower sensitivity of the respiratory center to changes in arterial oxygen tension (PaO2) and PaCO2 and most prominent after sedation with a2-adrenergic drugs, particularly predisposes neonatal foals to hypoxemia and hypercarbia

Nervous System Development in Foals

Neurogenesis of the cerebellar cortex is fairly complete in the newborn foal, indicating that the horse's brain is maturing quite rapidly

Most of the large neurons differentiate early during fetal development, small neurons and neuroglia differentiate later and myelination of nerve fibers is incomplete at many levels of the nervous system at the time of birth Transmission of nerve impulses from the periphery to the central nervous system is slower than in the adult and the ability to localize stimuli may be relatively poor When a newborn foal is traumatized, it may or may not respond quickly enough with target-oriented nocifensive reflex responses, but this does not preclude functioning nociceptive pathways or pain sensation and thus calls for appropriate analgesic treatment locoregional anesthesia, or even general anesthesia whenever a foal is exposed to noxious stimulation

The blood-brain barrier to proteins and other macromolecules, principally a property of tight junctions between the cells, is well formed early in brain development, while postnatal modifications in tight junction structure is in part responsible for the decline in blood-brain and blood-cerebrospinal fluid (CSF) permeability to small molecular compounds such as many endogenous substances, nutrients, and drugs

During the early postnatal period, still open tubulocisternal endoplasmic reticulum components of cerebral endothelial and choroid plexus epithelial cells close, thereby increasingly restricting the diffusion of endogenous substances and drugs from the blood into the central nervous system

At birth parasympathetic nervous activity dominates while sympathetic innervation of the heart and vasculature is still immature May explain the low systemic vascular resistance and systemic blood pressure as well as higher rate of bradyarrhythmias observed in the newborn foal subjected to hypoxemia and/or hypothermia than adults

Total Body Water Content in Neonates

Total body water content is around 72% to 74.4% +/- 2.4% of total body mass and does not change much over the first 5 months of life

ECF Compartment in Foals

ECF compartment is about 1/3 larger in foals on a per kilogram body weight basis than in adults as are the blood and plasma compartments Higher ECF volume provides a larger volume of distribution for many drugs which must be taken into account for appropriate drug dosing and prediction of drug uptake and distribution in the body

Water Redistribution in Foals

Because of the higher capillary permeability in the neonate yet greater systemic arterial blood pressures postpartum, intravascular water rapidly redistributes into the interstitial space, where it accumulates As a result, no sustained increase in intravascular volume occurs, which in the animal triggers diuresis by modulating release of vasopressin, renin, and ANP Consequently, neonates, especially ill neonates, retain administered fluid over a much longer time and thus do not handle large fluid loads well The expanded interstitial space in the neonate serves as a reservoir for fluid and can be rapidly mobilized in situations of acute hemorrhage or hypovolemia, restoring total blood volume much faster than an adult The neonate can tolerate a greater blood loss before any significant decrease in blood pressure and tissue hypoperfusion is noted

How much do foals aged 11-18 days drink?

246 g/kg

How much do foals age 30-44 days drink?

202 g/kg

Maintenance Fluid Rates in Neonatal Foals

80-120 mL/kg/day (3.5-5.0 mL/kg/h) in foals up to 1 month of age

Holliday-Segar Formula

Used to calculate the daily fluid requirements for each foal

100 mL/kg/day for the first 10 kg of body weight + 50 mL/kg/day for the second 10 kg of BW + 20-25 mL/kg/day for the remaining BW

Hypoglycemia in Foals

Glycogen reserves in the liver and muscle are smaller in the newborn foal than in neonates of other species and last only for a few hours, making the foal more susceptible to hypoglycemia and energy deficits if it doesn't nurse Hypoglycemia (glucose <75 mg/dL) at hospital admission was associated with poorer survival to hospital discharge and was commonly associated with sepsis, SIRS, and bacteremia

Thermoregulation in Foals

Much higher body surface area to weight ratio, thin skin, and scare subcutaneous fat tissue (poor insulation) increase environmental heat loss in the neonate

Conduction, convection, radiation, and evaporation all paly a role and can expose the newborn foal especially to rapid heat loss

Term equine neonates have the ability to generate heat through shivering and can also respond with nonshivering (cellular) thermogenesis and behavioral actions

Anesthetic drugs and commonly used sedatives will interfere with thermoregulation and therefore promote extended periods of hypothermia

Hepatic Function and Development in Foals

Liver is the principal site of drug metabolism

Microsomal cytochrome P450 enzyme system is primarily responsible for transforming lipophilic compounds to polar and pharmacologically less effective or inactive substances (phase I reactions), whereas glucuronidation and other conjugation processes (phase II reactions) render the metabolites more hydrophilic, facilitating renal elimination

Functional maturity of the liver is incomplete at birth, and therefore the capacity to metabolize endogenous substances such as bilirubin or drugs is markedly lower in newborn foals than in the adult horse The metabolism and half-lives of organic waste products (e.g. bilirubin) are expected to be prolonged, causing higher plasma concentrations to persist in the newborn foal Drugs that now have longer plasma half-lives may accumulate on repeated dosing, thereby extending their effects and slowing elimination from the body As blood flow to the liver increases after birth, enzyme induction begins with exposure to various endogenous and exogenous substances Microsomal enzyme activity increases rapidly during the first 3-4 weeks of life, while conjugation processes approach activity levels similar to those measured in the adult more gradually By 6-12 weeks postpartum most hepatic metabolic pathways are completely functioning

Renal Function and Development in Foals

Renal development in terms of glomerular number is complete by 30-40 weeks of gestation although the kidney volume continues to grow until 50-90 weeks of postnatal life

On a per kilogram body weight basis, the glomerular filtration rate and effective renal plasma flow of the full-term newborn foal is already comparable to that of the adult

Foals have a relatively greater renal tubular internal surface area available for reabsorption, but reduced renal concentrating ability in the postpartum period compared with adult animals

Normal urine output in neonatal foals is reported to be approximately 6 mL/kg/h but then decreases gradually over the subsequent 12 weeks of life

Normal USG in newborn foals after the first 24 hours post partum is usually hyposthenuric (<1.008) and is reported to range from 1.001 to 1.027

Excretion, clearance, and fractional electrolyte excretions in 4 day old foals are similar to adults for Na+ but somewhat higher for K+, P+, and Ca++

BUN values of less than 2 mmol/L (<6 mg/dL) are normal up to 3 months of age, whereas the mean adult value is less than 3.5 mmol/L (<9.8 mg/dL)

Blood Volume in Neonates

Blood volume in neonates is higher than in adults (approximately 13-15% of total body weight) and then decreases to near adult values (8-10%) by 12 weeks of age

PCV and Hemoglobin in Foals

PCV and hemoglobin values typically increase to maximum values at birth as a result of placental blood transfusion and then gradually decline Decline is accelerated in patients suffering from neonatal isoerythrolysis, an alloimmune hemolytic anemia caused by antibodies in the mare's colostrum against the newborn's erythrocytes that is accompanied by neonatal icterus Thereafter, values increase again, reaching levels of adult animals by about 1-3 months of age

There is no fetal hemoglobin in the equine species, but levels of 2,3 diphosphoglycerate (2,3-DPG) in fetal and therefore neonatal erythrocytes is slightly lower, thereby increasing the affinity of hemoglobin toward O2 and thus facilitating O2 loading of hemoglobin in the lung while impeding O2 offloading at the tissue sites

WBC Count in Foals

Total WBC count increases gradually after birth, attributable to an increase in neutrophils

Lymphocyte numbers decrease immediately after birth to resume adult levels by about 3 months of age Numbers may be normal or reduced in foals with immunodeficiency

Total Plasma Protein Content in Foals

At birth total plasma protein content varies widely and then increases following colostrum intake, although the albumin concentration remains relatively constant

Bilirubin in Foals

Marked hyperbilirubinemia in the first week of life is a common finding and can be attributed to an accelerated breakdown of neonatal erythrocytes and immature hepatic function

Serum Enzyme Activities in Foals

Serum enzyme activities (including CK, SDH, GGT, lactate dehydrogenase, and AST) have been reported to be transiently elevated in the first few weeks after birth as a result of hepatocellular maturation

Serum Lactate Concentrations in Foals

Serum lactate concentrations are high immediately after birth (3-5 mmol/L) likely because of temporary tissue hypoperfusion and hypoxia, but then soon decrease to normal values (<2 mmol/L)

Fasting Prior to Anesthesia in Foals Up to 2 Months of Age

Nursing foals up to 2 months of age with little fiber intake should not be muzzled prior to anesthesia and have free access to their mother Suckling helps maintaining adequate blood glucose levels, liver glycogen reserves, and hydration status

Fasting Prior to Anesthesia in Older Foals

Older foals with more solid food intake may be muzzled and held off feed for 3-6 hours prior to anesthesia dependent on their diet

Sedation of the Neonate (1 month or younger)

Foals up to 14-21 days of age usually do not require any chemical restraint or tranquilization to be handled prior to induction for brief and less invasive diagnostic or surgical procedures

If sedation is required or the animal is older than 2-3 weeks, a benzodiazepine derivative is the preferred choice for lack of serious adverse cardiopulmonary effects If infusion or repeated drug dosing is anticipated to maintain sedation, midazolam may be a better choice because the propylene glycol vehicle in other benzodiazepine preparations can cause metabolic acidosis, nephrotoxicity, hyperosmolarity, and subsequent tissue irritation and hemolysis

In the more mature neonate (>2-3 weeks postpartum) benzodiazepines may be supplemented with an opioid and/or a low dose of xylazine or (dex-)medetomidine to enhance sedation and provide some analgesia

If desired, benzodiazepine's effects can be reversed at the end of the procedure using flumazenil (Romazicon, 0.025-0.1 mg/kg IV), the opioid component with naloxone (Naloxone, 10-15 ug/kg IV) or levallorphan (Lorfan; 22 ug/kg IV), and xylazine can be reversed with yohimbine (Yocon; 0.1-0.2 mg/kg IM) or atipamezole (Antisedan; 0.05-0.2 mg/kg IV/IM)

Induction and Maintenance of Anesthesia in the Neonate (1 month and younger)

Induction and maintenance of general anesthesia can be achieved with one of the currently approved volatile anesthetics (isoflurane [Isoflo], sevoflurane [Sevoflo], or desflurane [Suprane] in O2) or an injectable agent such as ketamine, propofol, or alfaxalone

Use of a volatile anesthetic alone offers several advantages in neonates Both rapid uptake and elimination of the anesthetic via the lungs because of high minute ventilation and CO Easy and rapid adjustment of anesthetic depth if untoward cardiovascular or respiratory depression or arrhythmias occur Elimination of the anesthetic independently of hepatic and renal function

Considering the high O2 consumption and predisposition of neonates to develop hypoxemia when being deeply sedated or anesthetized, it is recommended to have them breathe O2 or at least an O2-enriched gas mixture (FiO2 > or = 0.5) regardless of the technique used

The volatile agent can be delivered via the rebreathing circuit and the previously placed nasotracheal tube or face mask

Following preoxygenation the vaporizer output is incrementally increased up to the maximum output setting At a fresh gas flow rate of 2.5-5.0 L/min, the volatile anesthetic concentration rises rapidly in the breathing circuit with onset of anesthesia typically occurring within 3-8 minutes Sevoflurane and desflurane are characterized by a 50 and 64% lower blood solubility than isoflurane, respectively, and therefore induction is somewhat faster with these agents than with isoflurane

If mask induced, the foal should be orotracheally intubated as soon as it loses consciousness and the swallow reflex 8-14 mm endotracheal tube used

Ketamine is currently the most commonly used agent for induction of anesthesia in the equine neonate, typically following sedation with a benzodiazepine derivative alone or in combination with an opioid and/or low-dose xylazine Effects last 10-20 minutes

Propofol or alfaxalone may be administered slowly (over 45-60 seconds) either with or without benzodiazepine sedation to effect to avoid respiratory depression and apnea

Induction of anesthesia with thiopental or other barbiturates should be avoided in neonates because of the prolonged recovery period

In most neonates, anesthesia is maintained with one of the volatile agents because of the immature hepatic and renal functions present in the newborn that give rise to drug accumulation and prolonged awakening from anesthesia if injectable agents are being infused or repeatedly administered For isoflurane an average anesthetic vaporizer setting of 2.8 +/- 0.1% has been reported and an Etiso concentration of 1.5 +/- 0.4%

In foals undergoing major trauma surgery, a balanced anesthesia regimen involving intermittent (q1-2h) subcutaneous administration of low-dose medetomidine (1-2 ug/kg) or (dex-)medetomidine (0.5-1 ug/kg) may offer advantages over administering only an inhalant anesthetic

Recovery to a standing position occurs quickly, usually within 15 +/-1 minutes after 86 +/- 4 minutes of isoflurane anesthesia and within 27 +/- 18 minutes after 133 +/- 66 minutes of isoflurane anesthesia

Propofol in the neonate allows maintenance of anesthesia without risk of untoward drug accumulation and prolonged recovery

A study of xylazine sedation in healthy 10- and 28- day old foals indicated a decrease in heart rate by 20-30% without causing second-degree AV blocks that are typically seen in adult horses In addition, a biphasic (initial increase followed by a decrease) change in blood pressure, similar to that in adult horses, occurred but MAP did not fall below 60 mm Hg

Recovery time after CRI of propofol for 60-122 minutes ranged from 15-32 minutes and foals sucked within 10 minutes of standing

The quality of both anesthetic induction and recovery was good after alfaxalone administration and the majority of monitored physiological parameters were clinically acceptable in all neonates but after alfaxalone injection some foals became transiently hypoxemic, underscoring the need for proxygenation prior to and oxygen supplementation throughout the procedure Average duration of anesthesia from induction to first movement and from induction to standing were 19 and 37 minutes, respectively Alfaxalone has an elimination half-life of approximately 23 minutes in equine neonates

MAC of Isoflurane in a Neonate

0.84%

What anesthetic equipment should be used for a neonatal foal?

Circle rebreathing systems and anesthesia equipment designed for use in humans or small animals are well suited for equine neonates

Depending on the size of the animal 3-5 L rebreathing bags or bellows are sufficient

Preoxygenation of Neonates

Intubation using a 6-9 mm cuffed silicone nasotracheal tube with length of 30-55 cm that is passed through the nostril and ventral nasal meatus into the trachea with inflation of the cuff or a tight fitting mask is used Attached to anesthetic circuit delivering O2 or an O2-enriched gas mixture at 40-60 mL/kg/min for 3-5 minutes before anesthesia is induced

What is the fresh gas flow rate that is used for neonates?

4-10 mL/kg/min

Sedation for the Pediatric/Juvenile Foal (1-4 months old)

Systemically healthy foals 4-8 weeks of age (>120-150 kg) or older are more difficult to physically restrain and frequently require sedation for minor procedures In the younger pediatric foals, sedation with one of the benzodiazepine derivatives offers the advantage of few adverse cardiovascular and respiratory effects, yet profound calming and immoblization In fractious individuals or foals older than 2 months, benzodiazepine administration often causes inadequate sedation and muscle relaxation or even excitement similar to what is described in the adult horse In these foals, a2-adrenoceptor agonists such as xyalzine as well as detomidine or romifidine provide more reliable sedation and muscle relaxation and additional profound analgesia Hemodynamic and respiratory side effects observed after a2-agonist administration in up to 2- to 3- month old foals are similar to those noted in adults with the exception of atrioventricular blocks, which occur rarely in younger foals Xylazine has been shown to cause hypothermia in foals Unlike in adult horses, a2-agonists do not seem to produce hypoinsulinemia and hyperglycemia in 4 week old foals, denoting differences in pancreatic responses to these drugs in early life Foals at this stage have a high metabolic rate but lesser glycogen stores in skeletal muscle than adults do so they may run out of glucose fuel during periods of long surgeries Lower doses of xylazine provide usually adequate sedation for 15-30 minutes and are associated with minimal cardiovascular and respiratory changes, making this drug the agent of choice for use in foals of this age group as compared to detomidine and romifidine, the effects of which not only last longer but also carry a higher risk for untoward effects including arrhythmias Acepromazine in clinically common doses produces overall mild but long-lasting sedation and may be used to enhance and prolong sedation with xylazine Clinically relevant hypotension secondary to vasodilation is a rare observation in normovolemic foals and therefore not a concern

Induction and Maintenance of Anesthesia in the Pediatric/Juvenile Foal (1-4 months old)

An IV technique is often considered the preferred method of induction of anesthesia

Ketamine is currently the most commonly used agent for induction of anesthesia in pediatric and juvenile horses and to obtain good muscle relaxation Usually combined with a benzodiazepine unless this type of drug has already been administered for sedation Alternatively and preferably only in foals older than 3-4 months of age, ketamine may be coadministered with the centrally acting muscle relaxant guaifenesin which is administered IV to effect (dropping of head, general muscle relaxation and calmness, fetlock knuckling) at a rate of 2-3 mL/kg/min To avoid inadvertent guaifenesin toxicity from an overdose, the infusion container should only contain up to the calculated maximum dose for the individual foal (about 50 mg/kg)

Following xylazine administration, ketamine in combination with diazepam produces anesthesia in 4 to 6 week old foals, typically of 10 minute duration Ketamine may be replaced by propofol or alfaxalone for induction of anesthesia but respiratory depression and/or hypoxemia are complications in the more mature foal Ketamine and propofol may be combined for induction Thiopental in conjunction with a benzodiazepine or guaifenesin is suitable for induction in the more mature foal and in foals with seizures or brain trauma it is the preferred technique

Anesthesia is most commonly maintained with isoflurane in O2 or an O2-enriched gas mixture, although sevoflurane and desflurane are being used as well

An initially high gas flow rate of 3-5 L/min and vaporizer dial setting of 3% to 5% for isoflurane, 4% to 6% for sevoflurane, and 9 to 10% for desflurane ensures rapid rise of the volatile anesthetic concentration in the breathing circuit and airway of the foal

Infusions of lidocaine or ketamine plus propofol have been used in foals in an attempt to reduce the required dose of the volatile anesthetic and provide better analgesia In the older foal, combination with an a2-agonist such as dexmedetomidine may be considered for painful orthopedic or trauma surgery

Total intravenous anesthesia techniques have been applied in foals 5% guaifenesin containing xylazine and ketamine has been used frequently This can also be used after sedation of the foal at a rate sufficient to rapidly induce anesthesia For maintenance of anesthesia the drip can be continued at a rate of 2-3 mL/kg/h Anesthesia may also be maintained with propofol or alfaxalone administration may be combined with techniques of locoregional anesthesia or infusions of lidocaine, ketamine, or (dex-)medetomidine

Anesthetic Equipment Used in Pediatric/Juvenile Foals

Circle rebreathing systems and anesthesia equipment designed for use ini humans or small animals are not suited for foals when bodyweight exceeds 125-150 kg and hence a large animal anesthesia machine equipped with at least a 3L to 10L rebreathing bag, bellows, or breathing cylinder (accommodating the three- to fourfold of an average tidal volume of 10-15 mL/kg) is required

Orotracheal Tube Sizes for Pediatric/Juvenile Foals

BW 70-100 kg - 14-16 mm (57-70 cm long)

BW 150-200 kg - 18-22 mm (75-90 cm long)

BW 250-400 kg - 22-24 mm (90-100 cm long)

Fresh Gas Flow Rate for Pediatric/Juvenile Foals

4-6 mL/kg/min

What is the appropriate bladder width-to-tail girth ratio for oscillometric blood pressure cuffs?

1:2 to 1:3

Monitoring of the Cardiovascular System of the Foal During Anesthesia

In the critically ill foal, additional hemodynamic monitoring including central venous pressure, urine output, and cardiac output recordings may become necessary

Two minimally invasive techniques have bee developed that are appropriate for assessing cardiac output in foals under clinical conditions Lithium dilution (LidCO) technique Noninvasive cardiac output technique (NICO) based on the Fick principle and partiat rebreathing of CO2 Ultrasound velocity dilution technique (UDCO)

Monitoring of the Respiratory System of the Foal During Anesthesia

Device dependent technology and software algorithms, transducer type, placement site, and tissue perfusion at the site of recording markedly influence the accuracy of a pulse oximeter readout

Under physiologic conditions, ETCO2 changes with alveolar partial pressure of CO (PACO2) and therefore with PaCO2

Changes in ETCO2 may reflect changes in circulatory function (CO) as blood transports CO2 from the periphery to the lungs

Arterial blood gas analysis is the most accurate technique for evaluation of pulmonary gas exchange

What is the ETCO2 value that indicates hypercapnia in a foal? What does hypercapnia indicate?

Excess of 45 mmHg

Hypoventilatiion

What is the ETCO2 value that indicates hypocapnia in a foal? What does hypocapnia indicate?

Below 35 mmHg

Hyperventilation

Monitoring of Blood Glucose of the Foal During Anesthesia

Blood glucose levels below 40 mg/dL may produce deleterious central nervous effects such as seizure activity, cerebral depression, and even permanent neuronal damage, all of which are difficult to detect under general anesthesia

Fluid Management of the Foal in the Perianesthetic Period

Use of balanced electrolyte solutions with a strong ion difference around 28-50 (LRS, Plasmalyte, Normosol-R) has been recommended to avoid the acidifying effect associated with fluids such as physiologic saline solution 0% or 5% dextrose in water

In adults, 20-50% of an isotonic fluid load is retained in the intravascular space 30-60 minutes after infusion, but this is much lower in the neonate, where fluid rapidly accumulates in the interstitial space and escapes regulatory mechanisms of fluid homeostasis As a result, neonates retain infused fluids for a long time and do not handle large fluid loads well which is further complicated by a decrease in urine output under anesthesia

Infusion Rate of IVF in Healthy, Normovolemic Foals During Anesthesia

In the systemically healthy, normovolemic foal undergoing anesthesia, an infusion rate of 7.5 to 10 mL/kg/h has been reported as adequate to maintain an appropriate circulatory volume

Relative hypovolemia caused by anesthetic drug-induced vasodillation, use of high gas flows resulting in a greater than normal respiratory loss, evaporative losses, and intraoperative hemorrhage may temporarily justify an increase up to five times the maintenance rate of 3 to 5 mL/kg/h reported for foals

Volume of IVF Administered to Foals When Volume Deficiencies are Caused by Insensible and Isotonic Fluid Losses

Balanced electrolyte solutions are adequate fluids whenever volume deficiencies are caused by insensible and isotonic fluid losses and may be administered in volumes of up to 50-80 mL/kg, typically given 1/3 at a time followed by reassessment of the foal's volume status They provide rapid extracellular (intravascular and interstitial) rehydration

Hetastarch Dose for Foals if Total Plasma Protein or Albumin Concentrations are Low

Hetastarch at doses of 3 mL/kg at a rate of 10 mL/kg/h supplements crystalloid fluid therapy if TPP or albumin concentrations are low Hetastarch may be administered slowly (0.5-1.0 mL/kg/h) up to a dose of 10 mL/kg/day for treatment in hypooncotic animals

Oxygen Supplementation in Foals During Anesthesia

Supplemental oxygen therapy is principally indicated whenever the SaO2 or SpO2 value in a foal decreases below 90% and the PaO2 value below 60 mmHg

In the foal, multiple factors may contribute to severe respiratory depression and impairment of pulmonary gas exchange leading to poor arterial oxygenation and CO2 retention: persistent pulmonary hypertension drug-induced central respiratory center depression Reduced FRC Exhaustion of respiratory muscle from increased work of breathing Immature lung, lung disease, and airway obstruction Once respiratory minute ventilation decreases below 150-200 mL/kg/min and significant hypercarbia starts to develop and arterial oxygen decreases ventilatory support is needed In spontaneously breathing foals, the PaCO2 commonly rises to values in excess of 80 mm Hg

Supplemental Oxygen Rate that Can Be Delivered to Foals During Anesthesia

Oxygen may be delivered via face mask, nasal cannulae, or a nasotracheal tube at a rate of 5-10 L/min

Mechanical Ventilation in Foals During Anesthesia

Controlled mandatory ventilation (CMV) is the most commonly used mode of mechanical ventilation in foals Ventilator delivers breaths at a preset interval, despite ventilatory efforts made by the animal

Mechanical ventilation may be employed in two different modalities

- Pressure limited

- Volume limited

Statistically significant reduction in ventilation-induced lung injury (VILI) in babies treated with volume limited ventilations

Pressure-Limited Mechanical Ventilation

Peak inspiratory pressure in limited, while volume is variable and is dependent on lung mechanics

Volume-Limited Mechanical Ventilation

Volume is limited, while peak inspiratory pressure is variable (i.e. as the lung and chest wall compliance improves, pressure is automatically weaned to provide the targeted volume)

Typical Settings to Begin Mechanical Ventilation in Neonatal Foals

Typical settings to begin mechanical ventilation in the neonatal foal are a tidal volume of 6-8 mL/kg, a rate of 20-30 min-1, a peak flow of 60-90 mL/min, I:E ratio of 1:2, and a peak inspiratory pressure of 8-12 cm H2O The initial inspired FiO2 value should be set based on the preventilation PaO2 and may be as low as 0.3 to 0.5

It is not necessary to ventilate foals with 100% O2

Foal Recovery from Anesthesia

Foal should remain intubated until it is able to maintain adequate arterial oxygenation (SpO2 >90%)

Continuation of endotracheal or nasal O2 is recommended until the foal has resumed a normal breathing pattern and can maintain adequate arterial oxygenation when breathing room air

Perioperative Pain Mangement in Foals

Given the immaturity of hepatic and renal mechanisms of drug metabolism and elimination during the first 1-2 months of life, local/regional techniques of analgesia and anesthesia (LRAA) should be used over systemic pain therapy whenever possible

Systemic analgesia in foals relies predominantly on administration of NSAIDs, opioids (primarily butorphanol), and lidocaine

Flunixin meglumine, phenylbutazone, ketoprofen, and ibuprofen have significantly slower clearance and greater volume of distribution in neonates than in older foals and adults resulting in prolonged half-lives NSAID doses in neonatal foals may be increased by as much as 1.5 times to induce comparable therapeutic concentrations but dosing intervals should be increased to avoid drug toxicity which can cause gingival and gastrointestinal ulceration, hypoproteinemia, colitis, nephrotoxicity, and platelet dysfunction

In animals up to 3 weeks of age, the elimination half life of butorphanol was 2.1 hours after IV injection and bioavailability was 66 +/-12% (i.e. about twice as long and twice as high, respectively, as in adults In neonates, butorphanol has minimal effects on vital signs, but makes the animals more sedate and even mildly ataxic compared with older foals and adults Also increases nursing behavior Butorphanol exhibits antinociceptive properties only after doses of 0.1 mg/kg and if plasma concentrations of the drug reach or exceed a threshold or 10 ng/mL Intravenous administration of 0.1 mg/kg butorphanol significantly increased thermal nociceptive thresholds for up to 150 minutes in neonatal and 4 to 6 week old foals without apparent adverse behavioral effects Needs to be administered every 3-4 hours or as a CRI to maintain clinically effective plasma concentrations

Foals undergoing soft tissue surgery and especially those with abdominal pain respond well to lidocaine infusion and seem to tolerate such an infusion as well as adult animals

Perianesthetic Complications Requiring Intervention in Foals - Impaired Cardiovascular Function

Perianesthetic complications are not common in this patient population

An arrhythmia was detected in only 4% of foals during the anesthetic period including second or third degree AV blocks, occasional ventricular extrasytoles, hyperkalemia-associated changes in ECG trace, and hypoglycemia, hypothermia, and/or hypoxia-associated bradycardia

Treatment of Bradyarrythmias in Foals During Anesthesia

If atropine (5-20 ug/kg IV) does not restore atrioventricular conduction in bradyarrhythmias, the indirect sympathomimetic drug ephedrine (25-50 ug/kg) or epinephrine (5-10 ug/kg) may do so

Treatment of Ventricular Tachyarrhythmias in Foals During Anesthesia

Ventricular tachyarrhythmias are treated with lidocaine as an initial bolus of 1 mg/kg IV followed by subsequent IV doses of 0.5 to 0.75 mg/kg as required or by CRI (20-50 ug/kg/min) If unsuccessful, quinidine gluconate (0.5-2.2 mg/kg IV every 10 min) or propranolol (0.03-0.1 mg/kg IV) may be administered to control the dysrhythmia

Treatment of Systemic Arterial Hypotension in Foals During Anesthesia

The most frequent hemodynamic complication during anesthesia is systemic arterial hypotension Occurs as a result of anesthetic drug-induced decrease in systemic vascular resistance and/or bradycardia and/or (less frequently) as a result of decreased myocardial contractility Volume replacement therapy is the first line of treatment If this fails to correct hypotension, infusion of dobutamine (primarily used as a positive chronotropic agent in the neonate; 1-5 ug/kg/min), phenylephrine (0.1-3.0 ug/kg/min), norepinephrine (0.05-1.5 ug/kg/min) titrated to effect of IV ephedrine injections at increments of 0.05-0.1 mg/kg are used to increase and maintain mean arterial blood pressures above 60-70 mm Hg

In critically ill foals with severe gastrointestinal disease, blood vessels may become unresponsive to catecholamines and therefore infusion of vasopressin (0.0005-0.001 IU/kg/min) acting through V1 receptors on vascular smooth muscle is required to adequately increase vasomotor tone and thereby diastolic arterial pressure

Perianesthetic Complications Requiring Intervention in Foals - Impaired Respiratory Function

Hypoventilation is frequent with 20% of the foals developing marked arterial hypercarbia (PaCO2 >65 mmHg)

Intraoperative hypoxemia (SaO2 <90%) is rare with only 1% of foals affected

Perianesthetic Complications Requiring Intervention in Foals - Return to Fetal Circulatioin

If newborn foals that present without cyanosis unexpectedly desaturate severely during anesthesia (SaO2 <80%) with PaO2 values decreasing 20-40 mmHg despite behind mechanically ventilated and inhaling 100% O2, a return to fetal circulation with persistent pulmonary arterial hypertension and massive right-to-left shunting of blood flow through the foramen ovale or ductus arteriosus should be suspected May occur more frequently in compromised animals with respiratory disease, prematurity, or septicemia Increasing the anesthetic depth in an attempt to reduce pulmonary vascular resistance may successfully reverse this life-threatening situation Treatment with sildenafil (slowly 0.5-2.5 mg/kg IV), a type 5 phosphodiesterase inhibitor that produces selective pulmonary arterial vasodilatation and thus ameliorates clinical signs of pulmonary hypertension, appears to be more efficacious and reliable An echocardiographic examination should be performed to exclude any primary cardiac cause of cyanosis

Perianesthetic Complications Requiring Intervention in Foals - Cardiac Arrest and Cardiopulmonary Resuscitation

Pulseless electrical activity (PEA) and asystole are the cardiac arrhythmias most commonly associated with cardiac arrest in foals

Cardiopulmonary failure in foals usually occurs secondary to systemic disease or anesthetic overdose that initially cause respiratory arrest This is followed by development of a nonperfusing bradycardia, PEA, and finally asystole

Causes of secondary cardiopulmonary arrest encountered in foals include severe hypovolemia, low cardiac output, severe metabolic acidosis, hyperkalemia (e.g. ruptured bladder), vasovagal reflex, severe hypoglycemia, severe hypothermia, septic shock/endotoxemia, and pulmonary arterial hypertension with return to fetal circulation and right-to-left shunting of blood causing systemic tissue hypoxia Also cardiac tamponade, tension pneumothorax, or trauma have been reported

Atropine is most likely to be of use in animals with asystole or PEA in which a preceding bradyarrhythmia had already been noted or a sudden rise in vagal tone seemed likely 0.02-0.04 mg/kg IV in these cases

Lidocaine may be an appropriate antiarrhythmic agent for treatment of ventricular extrasystoles and prevention of V-Tach or V-Fib after epinephrine administration, or for treatment of forms of V-Fib and V-Tach refractory to electrical defibrillation Initial dose is 1 mg/kg IV and additional IV doses of 0.5 to 0.75 mg/kg up to a maximum dose of 3 mg/kg may be administered at intervals of 5-10 minutes if the arrhythmia persists A CRI of 20-75 ug/kg/min may follow the initial loading dose

During the period of cardiac arrest, large volume fluid administration is contraindicated, especially in a normovolemic foal If fluids are given rapidly, the venous pressure rises, impeding coronary perfusion and return of a normal cardiac rhythm, despite effective chest compressions and doses of epinephrine

Reassessment Campaign on Veterinary Resuscitation (RECOVER) CPR Guidelines

Administration of chest compressions

Ventilation support

Initiation of ECG and ETCO2 monitoring

Obtaining vascular access for drug administration

Administration of reversal agents if any anesthetic/sedative agents have been administered

CPR in Foals

Chest compressions preferentially at a rate of 100/min as soon as nonperfusing cardiac rhythm is diagnosed

Cardiac compressions are most effective when the foal is placed on a firm surface in right lateral recumbency The resucitator places one hand with the fist closed over the foal's heart, while the other hand is placed on top of the first to reinforce the compressor The elbows should remain straight, and the motion for compression should originate from the waist (with the upper body weight powering the compression, resulting in increased endurance) Chest compression results in no more than 25-30% of normal CO Since the anesthetized foal is usually already intubated and attached to an anesthetic circuit, a second person may begin manually ventilating the foal with the rebreathing bag as soon as chest compression commences Rate of 6-10 breaths per minute Essential to discontinue any anesthetic drug administration and flush the anesthetic circuit with O2 prior to initiating ventilatory support If the animal is not intubated, orotracheal intubation with a 7 to 14 mm tube is performed while chest compressions are being administered Delaying cardiac compression significantly decreases the chances of successful resuscitation With effective chest compressions, cardiac output and lung perfusion resume and ETCO2 values will climb to values of 12-20 mmHg Among all drugs tested in CPR only two exhibit significant efficacy in all cardiac arrest situations

- Epinephrine

- Vasopressin

Epinephrine Use in CPR in Foals

Possesses strong vasoconstrictive properties (via activation of a-adrenoreceptors) and has been shown to improve coronary perfusion pressure during cardiac arrest

This increases myocardial blood flow during cardiac compressions and helps resolve myocardial hypoxia and contractile failure

Initially administer a dose of 0.01 to 0.2 mg/kg of epinephrine intravenously every 3 minutes

If jugular venous access is not available, epinephrine may be given by the endotracheal route at a dose of 0.05 to 0.1 mg/kg diluted in 1-2 mL of saline

Intraosseous approach for epinephrine administration using a 12 gauge, 2-3 cm intraosseous infusion needle and doses comparable to those with IV has been described

Complications include ventricular fibrillation, pulseless ventricular tachycardia, and an increase in systemic vascular resistance, all increasing myocardial O2 demand and cardiac workload