D.E. Mason, DVM, PhD, DACVA
Anesthesia for cesarean section can be a straightforward process, administered to a healthy animal with full-term neonates. Sometimes anesthesia for cesarean section may involve a dam that is metabolically compromised, to salvage the viability of neonates from a uterus that contains a dead and decomposing fetus. Or a cesarean section may be required in an animal in which no live offspring are expected. Each of these scenarios presents a different challenge to the anesthetist and may require a different approach in drug choices and intra-operative management. Prior to choosing the anesthetic approach it is worthwhile to understand the reason for the cesarean section. The most important factors to consider when choosing anesthesia for cesarean section are: 1) the health status of the mother; 2) the viability of the offspring; 3) is the surgery an emergency? 4) how to provide pain control for the mother; and 5) the ability to perform the surgery with the anesthetic technique that is chosen.
The incidence of maternal complications of cesarean section can be low if one provides adequate patient support, including oxygenation and fluid therapy and anesthetic monitoring. There are two categories of anesthesia for cesarean section--regional anesthesia and general anesthesia.
Regional techniques include line blocks and epidural anesthesia. Epidural anesthesia involves placing local anesthetic into the epidural space and results in anesthesia of the caudal half of the body so that the surgery can be completed with no muscle tone at the operative site and virtually no pain for the mother. However, in contrast to human labor and delivery, epidural anesthesia in small animals does not include the advantage of avoiding systemic drugs that may affect her offspring. Without some form of significant sedation the mother is awake and may not remain cooperative in dorsal recumbency throughout the surgical procedure. Both dogs and cats generally require sedation in addition to their epidural block to achieve adequate restraint for surgery and many of the commonly used sedatives and tranquilizers in small animal practice have the potential to significantly depress the fetus at the time of delivery (see below). Epidural anesthesia requires greater skill compared to a line block, however the degree of block is often more complete. A benefit of using an epidural is that the epidural can provide pain relief after the operation for a period of hours. The addition of an opioid such as morphine (0.05 mg/kg) to the epidural can provide many hours of analgesia in the mother at a dose much lower than that used for systemic administration of the drug.
Epidural technique: An epidural can be performed with the patient in either sternal or lateral recumbency, but the technique is easier with the animal in sternal recumbency. One should palpate the space lying just caudal to the palpable dorsal spinous process of the seventh lumbar vertebra and in front of the sacrum. A 22-gauge spinal needle is advanced through the skin into the lumbosacral space. Staying at the midline will decrease the likelihood of puncturing the venous plexus and seeing blood upon removal of the stylet from the spinal needle. As one advances the spinal needle a distinct "pop" may be felt as the needle passes through the interarcuate ligament into the epidural space. The best test to confirm proper placement of the needle into the epidural space is a "loss of resistance" test. After removal of the stylet, making sure that no CSF or blood is present in the needle, a test injection can be made using a syringe with several milliliters of saline and a large air bubble. If one is in the epidural space, there should be little resistance to injection of saline and the air bubble in the syringe should not be deformed while making the injection. If there is resistance to injection, the air bubble will deform and this indicates that you need to try again to place the needle within the epidural space. Administration of 1 ml/6 kg body weight of 2% lidocaine (approximately 3 mg/kg) is sufficient for performing cesarean section
Infiltration technique (line block): Simpler to perform, but requiring a greater volume of local anesthetic, the line block is only effective at the site of the surgical incision and does not immobilize the rear limbs of the mother, so adequate sedation is necessary to insure cooperation of the patient throughout the procedure. This block is performed by multiple intradermal and subcutaneous injections of local anesthetic while advancing the needle along the line of the proposed surgical incision. The amount of local anesthetic needed is a function of the length of the incision, however, one should not exceed 8 mg/kg total dose of lidocaine in the dog or 5 mg/kg total dose of lidocaine in the cat. If the volume of drug at that dose range is not sufficient to extend the length of the incision, one can dilute the agent with 0.9% saline. Bupivacaine (2 mg/kg maximum dose) or ropivacaine (3 mg/kg maximum dose) can also be used for infiltration anesthesia.
Other recommendations when choosing to use regional anesthesia include the use of an endotracheal tube, if the mother is sedated adequately. If intubation is not possible when using a regional anesthetic technique, deliver oxygen by mask. Any period of hypoxemia in the mother is also a period of hypoxemia in the fetus and this can markedly decrease fetal viability at birth.
General anesthesia has some advantages over regional anesthesia for cesarean section. General anesthesia can be induced quickly. Oxygenation and ventilation are more easily controlled and the airway is protected with endotracheal tube placement. Patient restraint is optimal with general anesthesia. However, fetal depression can be substantial due to the systemic administration of potent anesthetics. Hypotension is more likely with inhalant anesthetics, which could significantly compromise a metabolically unstable mother and impair uterine blood flow decreasing the viability of the fetus.
There are some general recommendations regarding procedure when using general anesthesia in cesarean sections. Prior to starting anesthesia, prepare as many things as possible; assemble equipment (anesthetic and surgical) clip the site for surgery, and apply an initial surgical scrub to the surgical site. It is important to minimize anesthesia time up to the point of delivery of the offspring. Always plan to use an endotracheal tube in the mother to decrease the possibility of regurgitation and aspiration, which can occur more easily during pregnancy as a result of increased intra-abdominal pressure and dorsal recumbency.
Virtually all sedatives, tranquilizers, analgesics and anesthetics used commonly in veterinary anesthesia exert their effect after crossing the blood-brain-barrier. If a drug crosses the blood-brain-barrier, it crosses the placenta as well, so there are few choices that will not have some effect on the fetus during cesarean section. Phenothiazines, like acepromazine are not associated with significant fetal respiratory depression, but they may cause hypotension in the mother and impair uterine blood flow. Acepromazine should only be considered in a healthy mother prior to elective C-section with a maximum dose of 0.02 mg/kg IV. Alpha-2 agonists can cause significant fetal depression, often proportional to the level of depression obtained in the mother. They are not recommended for cesarean section. Opioids cross the placenta and may concentrate in the fetal blood due to lower pH. However they can be useful due to the analgesia they provide, their safe cardiovascular profile, and their contribution towards sedation. The fetal depression can be reversed by administration of naloxone to the neonate at the time of delivery. Diazepam tends to concentrate in the fetal blood in a 2:1 ratio compared to the maternal circulation. Significant fetal depression is associated with the use of diazepam.
There have not been extensive studies of outcome regarding various general anesthetic agents or techniques for cesarean section, however there is some evidence in the literature that would suggest that puppy vigor at the time of delivery is most impaired by techniques that include ketamine. Ketamine may increase uterine tone and as a result diminish uterine blood flow nearing the time of delivery. Puppy vigor appears to be superior when rapid intravenous induction is achieved with propofol compared to either ketamine or thiopental. Inhalation agents produce a rapid fetal effect which is proportional to the depth and duration of maternal anesthesia. The use of isoflurane or sevoflurane, with their low solubility and rapid elimination can provide rapid recovery of vigor in the neonate, provided the puppy or kitten will breathe.
Based on this information my current approach to cesarean section in any mother for whom viability of the puppies or kittens is expected, is to induce anesthesia with intravenous propofol (3-8 mg/kg) to effect for intubation. Then the mother is maintained under anesthesia with sevoflurane, although isoflurane is acceptable as well. Once the puppies or kittens are delivered, the mother is administered an analgesic agent to improve quality of recovery and to provide some post-operative pain relief. Typical choices for opioid in our practice are morphine 0.3 mg/kg, hydromorphone 0.1 mg/kg (dogs only), or buprenorphine (0.01 mg/kg).
Neonatal survival is also influenced by adequate support of the puppy or kitten upon delivery. The airway should be immediately cleared by wiping away the fluid and membrane around the muzzle using a clean, dry towel while holding the animal in a head downward position. Some will advocate suctioning the airway with a bulb syringe, although the fluid present in the mouth and nasal passages will generally run out with gravity and the fluid in the lungs will be absorbed once the animal begins expanding its lungs with respiratory efforts. The umbilical cord should be clamped and tied within 2 to 5 cm of the abdominal wall. Vigorous but gentle physical stimulation will promote neonatal activity and help to initiate respiratory effort. Continue to rub the puppy or kitten with a clean, dry towel, occasionally stopping to observe for respiratory effort and to palpate a cardiac impulse through the chest wall. If an opioid was administered to the mother prior to delivery of the puppy, one can place a drop of naloxone under to tongue of the neonate to reverse the opioid's effects on the puppy. Don't be too hasty to give up on the puppy or kitten. Stimulation should continue for at least 10 minutes in any puppy or kitten that has a palpable cardiac beat. One to two drops of doxapram can be placed on the tongue if the neonate is reluctant to breath, but the efficacy of this technique is not established. A small endotracheal tube can be made from an 18-gauge or 20-gauge over-the-needle catheter by removal of the stylet. This can be used to intubate the neonate and deliver initial positive pressure breaths in order to promote elimination of any residual inhalation agent causing prolonged respiratory depression. Once the neonate begins to breathe, vocalize and move about it should be kept in a warm environment until such time that the mother is recovered from anesthesia and one can introduce the puppies or kittens to their mother.
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