Equine anesthesia carries a higher anesthetic risk for mortality than for other domestic animals, with cesarean sections further increasing risk. A late-term mare with dystocia presents specific anesthetic challenges due to the physiologic changes of pregnancy, the associated procedures, and considerations for the neonatal foal. Dystocic mares will often present with pain, agitation, weakness, tachycardia, and hemoconcentration with potential acid-base and electrolyte abnormalities.
Initial management of these mares includes the rapid administration of appropriate fluids to optimize hemodynamic performance, and sedative and analgesic medications to facilitate examination and a smooth anesthetic induction. Xylazine and detomidine remain the agents of choice for premedication because of their profound and reliable sedation and analgesia. While both drugs are reversible, xylazine has a shorter duration of action and is frequently used for premedication at 0.4-1.0 mg/kg IV. The addition of an opioid provides additional analgesia and can reduce the required dose of the alpha-2 adrenergic agonist; however, it can be transferred to the fetal circulation and may require reversal in the neonatal foal. The administration of a caudal epidural with local anesthetic will provide perineal analgesia and relaxation of the mare, facilitating fetal manipulation.
Currently anesthetic induction is typically performed with a combination of ketamine (2.2 mg/kg IV) and a benzodiazepine such as diazepam or midazolam (0.02 – 0.1 mg/kg IV). Diazepam has been shown to result in measurable circulating concentrations in neonatal foals following administration to the mare; however, respiratory depression is not consistently noted, and any residual effects can be reversed with flumazenil.
Once anesthesia is induced, the mare should be intubated and anesthesia maintained with isoflurane or sevoflurane delivered in oxygen. Maintenance with guaifenesin-ketamine-xylazine has also been reported for anesthetizing dystocic mares in field conditions when inhalational anesthesia is not available.
Attempts are often made at controlled vaginal delivery after induction with the hindquarters of the mare hoisted upward into the Trendelenburg position. Dystocic mares are prone to hypoventilation not only from positioning in dorsal recumbency and the Trendelenburg position, but also from diaphragmatic pressure from the gravid uterus and subsequent reduction in functional residual capacity, making controlled ventilation essential to optimize oxygenation and delivery of inhalational agents.
Hypotension is another common complication due to caval compression by the gravid uterus and abdominal viscera, dehydration, potential blood loss, as well as the vasodilatory effects of oxytocin and anesthetic drugs. This is of particular concern as hypotension increases the risk of equine perianesthetic myopathy. If hypotension is noted, supportive measures including the reduction of inhalant anesthetic, crystalloid and hypertonic fluid administration, and dobutamine infusion should be implemented to improve blood pressure and thus outcome. Once the foal is delivered, additional analgesics can be administered to the mare.
Post-anesthesia rope-assisted recovery is recommended for all post-foaling mares because of the potential for musculoskeletal injury. Muscle weakness from exhaustion, potential electrolyte abnormalities (particularly hypocalcemia) and hypotension, coupled with the increase in ligament laxity and reduction in bone density associated with pregnancy may predispose mares to fractures or joint luxation with uncoordinated attempts to stand. Additionally, obstetrical lubricant can make surfaces extremely slippery and can make solid footing challenging for the mare during recovery.
Administration of sedatives for anesthetic recovery should be done judiciously depending on the condition of the mare to keep the horse sufficiently calm without excessive recumbency to allow more time for elimination of inhalant anesthetics resulting in more coordinated attempts to stand. Continued monitoring of electrolyte, acid-base, and oxygen status can be performed and corrective therapies and administration of supplemental oxygen can be continued for as long as possible throughout the recovery period.
—Stephanie Keating, DVM, DVSc, DACVAA, and Stuart Clark-Price, DACVIM, DACVAA