Management of the High-Risk Late-Gestation Mare
Each mare should receive a complete physical examination, and foaling histories should be obtained. Signs of impending parturition (sacroiliac ligament and perineal relaxation, mammary development, and mammary secretion electrolyte concentration) should be assessed daily.
The reproductive tract may be evaluated by rectal palpation, and transabdominal ultrasonography may be performed at regular intervals to detect changes in the fetus, fetal fluids, or placenta. Prolonged periods of feed restriction should be avoided to prevent maternal hypoglycemia.13 Estimation of progestin concentrations in the maternal circulation, using a commercial progesterone assay, is recommended.18When treating a pregnant mare for any medical or surgical condition, consideration must also be given to the fetus. Any illness or disease that affects the mare's cardiovascular system has the potential to affect placental perfusion and the integrity of the fetoplacental unit. Hypotension, toxemia, and hypoxemia are examples of conditions that can alter uteroplacental blood flow and jeopardize the pregnancy. Diseases that stimulate prostaglandin production have the potential to initiate labor and delivery. Illnesses that produce prolonged periods of anorexia in the late-term mare can also contribute to premature delivery. The effect of various drugs on the placenta and fetus should be considered when treating the pregnant mare. If delivery is not imminent, then many drugs will pass through the placental and fetal circulation and be cleared by the maternal liver and kidneys. Due to the epitheliochorial nature of the equine placenta, some drugs will not cross the placental barrier at all. If drugs are administered to the mare and the fetus is delivered shortly thereafter, then the neonate must rely on its renal and hepatic function to process, degrade, and excrete those drugs.
In most instances, as long as fetal well-being and placental integrity are closely monitored, the goal of most therapies is to treat the maternal condition and maintain the pregnancy as long as possible to achieve an acceptable degree of fetal maturation.The use of antibiotics in placentitis should be based on knowledge of common pathogens and the likelihood of achieving therapeutic concentrations at the site of infection. Studies using microdialysis probes inserted into the allantoic fluid of normal mares reported therapeutic concentrations of penicillin and gentamicin in the allantoic fluid after 22,000 IU/kg of potassium penicillin G (IV q6h) and 6.6 mg/kg gentamicin (IV q24h), albeit with peak concentrations in allantoic fluid 80% of that achieved in maternal serum.76 Studies in two mares with experimentally induced bacterial placentitis again confirmed passage of both drugs across the placenta. The combination of trimethoprim and sulfadiazine (30 mg/kg PO bid) and the phosphodiesterase inhibitor pentoxifylline (8.5 to 10 mg/kg PO bid) also readily crosses the fetal membranes in both healthy mares and in animals with experimental placentitis.17 In many cases, medical treatment of the placental infection and prolongation of pregnancy is associated with a good outcome. Maternal treatment includes systemic antibiotics, flunixin meglumine, pentoxifylline, and altrenogest.13,77 In an experimental model of placentitis, induced through inoculation of S. equi subspecies zooepidemicus, the combination of trimethoprim sulfamethoxazole, pentoxifylline, and altrenogest was highly effective at prolonging gestation and improving foal viability when contrasted with mares that received no treatment.77 Because of the usual presence of mixed gram-positive and gram-negative placental and fetal infections, a broad-spectrum antibiotic that reaches therapeutic levels in the fetus and fetal fluids should be selected. Therapeutic options include penicillin and gentamicin, trimethoprim-sulfonamide, or ceftiofur.
Trimethoprimsulfonamide combinations or doxycycline have in vitro activity against several nocardioform actinomycetes.59 Low doses of flunixin can be administered to decrease inflammation and prevent prostaglandin-mediated induction of delivery.Altrenogest (Regu-Mate, 0.044 to 0.088 mg/kg, 10 to 20 mL of 0.22% solution for 500-kg mare, PO q24h) is commonly used in late pregnancy to prevent preterm delivery, possibly through interference with prostaglandin regulation and oxytocin receptors.17 The rationale for altrenogest use in late pregnancy has been questioned, as plasma progestagen concentrations are typically raised in mares with placental disease.78 The evidence for altrenogest use in mares with placentitis is not strong, with studies using a model of experimental placentitis failing to document an advantage of the drug over antimicrobials alone.79,80 Altrenogest readily crosses the placenta and reaches relatively high concentrations in the fetal fluids and the newborn foal.81 Concentrations in the healthy foal decline rapidly over the first 6 hours postpartum. The effects of altrenogest on the foal are largely unknown, but a reduced neutrophil-to-lymphocyte ratio in foal blood has been reported.82 It has been recommended to withdraw altrenogest after 320 days' gestation.
In a group of mares with experimentally induced ascending placentitis after inoculation with S. equi subspecies zooepidemicus, the authors reported that estrogen supplementation (estradiol cypionate 10 mg IM q3d for 3 treatments), coupled with trimethoprim sulfamethoxazole (30 mg/kg IV q12h for 10d) and flunixin meglumine (1.1 mg/kg IV q24h for 10 days), resulted in a normal gestation period and foal survival rates similar to control animals.79 This was a superior result than that of mares treated with antibiotics, flunixin, and altrenogest.
If there are large areas of placental thickening, IV DMSO (0.5 to 1 gm/kg) can be administered to decrease placental edema, although direct evidence is lacking.
Pentoxifylline has also been administered in attempts to improve placental perfusion.If premature delivery appears unavoidable, then one to three doses of maternal corticosteroids can be used with the hope of stimulating and accelerating fetal lung maturation through enhanced surfactant production. This is done with the knowledge that corticosteroids are anecdotally associated with a heightened risk of laminitis, particularly in mares with underlying disease. Normal Thoroughbred mares given 100 mg of dexamethasone intramuscularly on gestation days 315, 316, and 317 delivered foals around 2 weeks earlier than saline- treated control mares.83 The interval between the last dose and parturition was only 1 to 8 days. Resultant foals were smaller but appeared mature and had normal neutrophil-to- lymphocyte ratios. There was evidence of lingering adrenal suppression in some foals.
In the high-risk mare, it is important that the delivery be attended by knowledgeable personnel and that all supplies, drugs, and equipment required for diagnosing and correcting a dystocia and stabilizing the mare and foal be organized and close at hand. A spontaneous, vaginal delivery is generally preferred in the high-risk mare, both because of the profound problems associated with the untimely delivery of a premature foal and the complications sometimes associated with induced labor or cesarean section. (See the Maturity section in Chapter 17). There are instances, however, when an induced birth or cesarean section is indicated or preferred.
Induction of parturition should be considered with the following:
• Severe fetal distress noted on prenatal assessment
• Evidence of premature placental separation or a history of premature placental separation associated with dead or asphyxiated foals
• Hydrops allantois and/or amnion
• Unproductive stage I labor
• Uterine inertia
• Impending prepubic tendon rupture
• Life-threatening maternal illness
Indications for cesarean section may include the following:
• Pelvic injury or abnormality resulting in obstruction of the birth canal
• Gastrointestinal crisis requiring surgery
• Severe dystocia
• Insufficient, thickened placenta associated with fescue toxicity in the mare
• Catastrophic and terminal illness or injury in the mare, such as gut rupture or fractured limbs
If induction of parturition or cesarean section is elected, every effort should be made to ensure that the fetus is mature and ready to be born; the usual result of induction at an inappropriate time is a nonviable newborn.
Three essential criteria are a gestation of longer than 330 days, good-quality colostrum in the udder, and softening of the cervix.84 The scheduling needs of the veterinarian or owner should never be the only criterion used for determining the timing of delivery. A slow, continuous oxytocin infusion administered at a rate of 1 unit/ minute usually results in delivery within 20 to 40 minutes.13 Alternatively, multiple intravenous or intramuscular injections of 10 to 20 units of oxytocin every 10 minutes have been recommended.84,85 Other investigators have shown that smaller intravenous doses of oxytocin (2.5 IU administered every 15 to 20 minutes until rupture of the chorioallantois or a total of 20 IU of oxytocin has been administered) is an effective, and perhaps more physiologic, method of induction.86Induction of parturition in the mare has been associated with more violent, painful contractions than spontaneous labor and a higher incidence of premature placental separation and neonatal asphyxiation. Cesarean section also predisposes to neonatal peripartum asphyxia. Maternal hypotension secondary to general anesthesia and the weight of the maternal abdominal contents on the aorta and vena cava both compromise uteroplacental circulation. For further details concerning anesthesia of the late-term mare, the reader is referred to other texts.87 Abdominal support with nonsteroidal antiinflammatory drugs and altrenogest has been used to maintain the pregnancy in a mare with hydrous amnion until day 321, when spontaneous delivery occurred.88 The delivery was complicated by uterine inertia, maternal postpartum hypovolemic shock, and cardiac arrhythmia, but both mare and foal survived.
Foal Resuscitation resuscitation kit, along with a self-inflating resuscitation bag (the bag component of a commercial air mask bag unit [AMBU; Ambu Inc., Linthicum Heights, Md.). Intubation provides the most effective method for ventilation support.
A 9-mm tube will suit most foals, although a large tube may be needed in foals larger than 55 kg. Passage of the nasotracheal tube is facilitated by keeping the head in extension. The tube is passed through the ventral meatus into the nasopharynx to the larynx. Rotation of the tube can aid in passage through the arytenoid cartilages and into the trachea. The tube is inserted fully to reduce dead space, such that the tube end is positioned in the midtracheal region and the cuff is inflated. An adult resuscitation bag (typically 1600-mL capacity with 2600-mL oxygen reservoir bag) is connected to the tube and manually compressed to provide positive-pressure ventilation at an initial rate of 10 breaths/minute. The bag has an adjustable escape valve that prevents barotrauma. The valve is typically set at 35 to 45 cm H2O.90 The thoracic wall should rise appropriately without air leaks around the cuff. Ideally, the bag is connected to an oxygen cylinder using a flow rate of 8 L/min, but ventilation with room air is acceptable. The oxygen flow rate can be adjusted on the basis of the minute volume that is delivered.92 Assisted ventilation should be ceased every 3 minutes to monitor for spontaneous ventilation. The use of chemical respiratory stimulants, such as doxapram hydrochloride, is no longer indicated in the treatment of secondary apnea.If there is asystole or the heart rate is persistently less than 40 beats per second, then cardiac compressions are indicated without delay. It has been recommended that compressions occur at a rate between 100 and 120 per minute, with complete thoracic wall recoil between each compression.89,91 The compressions should be forceful and contribute around 25% to 30% of normal cardiac output. Ideally, compressions are continuous and there should be a compression-to-ventilation ratio of 30 : 2.91
Drugs are indicated if the heart rate fails to increase with supported ventilation and thoracic compressions. The drugs can be administered intravenously via the jugular vein or intraosseously, using a 12-gauge intraosseous infusion needle.89 The most popular medications for cardiovascular resuscitation include epinephrine and vasopressin. Epinephrine can initially be administered using a bolus dose of 0.01 to 0.02 mg/kg (1 mg/mL, 0.5 to 1.0 mL/50-kg foal), repeated every 3 to 5 minutes due to a short half-life. A higher dose is recommended if the foal fails to respond to other therapies (0.1 mg/kg; 1 mg/ mL, 5 mL/50-kg foal). Vasopressin is a peripheral vasoconstrict- ing agent that can be administered at a bolus dose of 0.6 U/ kg IV92 It can be used alone or in combination with epinephrine.91 The effects last approximately 10 to 20 minutes. The use of sodium bicarbonate in CPCR continues to be controversial, but evidence indicates a potentially deleterious effect of myocardial function, and in the absence of any positive survival data it is no longer recommended in human CPCR.91,94
Defibrillation is indicated in ventricular arrhythmias, including ventricular fibrillation and ventricular tachycardia without palpable pulse pressure.90 The technique should not be used in the management of bradyarrhythmias or asystole. Recommendations include an initial defibrillation of 2 J/kg, followed by a discharge of 4 J/kg 30 to 60 seconds after treatment with epinephrine or 1.5 mg/kg, repeated in 5 minutes for lidocaine, and continued cardiac compressions.89 Amiodarone has been used in adult horses with refractory ventricular tachycardia; the recommended dose is 5 mg amiodarone/kg/h for 1 hour, followed by 0.83 mg amiodarone/kg/h during 23 hours, although boluses over 2 minutes have been used safely in normal adult horses.95,96
Resuscitation should be discontinued in the absence of spontaneous cardiac output within 10 minutes of initiating CPCR. Successful resuscitation is associated with spontaneous ventilation at a normal rate, depth, and effort and a regular cardiac rhythm with a rate greater than 60 beats/min. The foal should be monitored closely over the following 60 minutes and then at a lower level for a further 48 hours, as there is a heightened risk of postasphyxia complications.