ANESTHESIA

Physiologic Changes

• Pregnancy increases plasma volume, with a corresponding decrease in hematocrit
• A pregnant animal with a normal hematocrit is dehydrated
• These patients will require fluid support during anesthesia in order to remain normotensive.

Physiologic Changes

• As the mother's cardiovascular system adapts to the demands of fetuses, oxygen consumption increases as does carbon dioxide production.
• Mild maternal hyperventilation leading to mild respiratory alkalosis is common.

PHYSIOLOGIC CHANGES

• Cardiac output increases approximately 30-40%
• Blood pressure falls slightly

PHYSIOLOGIC CHANGES

• Gastrointestinal motility is typically slowed
• Risk of vomit or regurgitation is higher than with non-pregnant patients

PHYSIOLOGIC CHANGES

• A gravid uterus, especially in the case of a large litter, will increase abdominal pressure and thus the work of breathing.

PHYSIOLOGIC CHANGES

• Increased pain threshold related to increased progesterone and endogenous endorphin levels lead to: o Reductions in inhalant anesthetic requirements (decrease MAC up to 40%) o Reduced opioid requirements (to achieve analgesia)
• o Decreased enzymatic breakdown of opiates also reduces opioid requirements

PHYSIOLOGIC CHANGES

• Doses of local anesthetics required for epidural anesthesia are decreased due to: o Venous engorgement of the epidural space o Increased spread secondary to increased epidural pressure

General Considerations

• Time is of the essence! Pre-clip the surgical site before induction. Work quickly!!!!
• Avoid excessive anesthetic depth
• Position the mother during surgery to minimize blood flow impedance o Maintain maternal oxygenation to avoid fetal hypoxemia

General Considerations

• Nearly all drugs that you give the mother will cross the placenta to the fetuses
• Choose drugs that cause minimal cardiovascular depression or that are reversible
• Short-acting drugs will aide neonatal resuscitation
• You can always add longer-acting drugs for the mother after the neonates are delivered

ANESTHETIZING

• Opioids: Buprenorphine, Butorphanol, Hydromorphone, Morphine, Oxymorphone (+) Reversible with naloxone (+) Shorter acting opioids may not necessitate providing neonates with multiple doses of reversal agent (-) Among this group, buprenorphine is most difficult to reverse so is not recommended for c-section analgesia (-) May cause bradycardia and respiratory depression in the mother and fetus

ANESTHETIZING

• Benzodiazepines: Diazepam, Midazolam (-) May cause profound sedation in neonates (+) Reversible with flumazenil

ANESTHETIZING

• Alpha-2 adrenergic agonists: Medetomidine, Detomidine, Xylazine (-) May cause profound sedation in neonates (-) Maternal bradycardia and cardiac arrhythmias are possible (-) May cause decreased blood flow to the uterus, compromising fetal oxygen delivery (this is proven in cattle, not other species)

ANESTHETIZING

• Acepromazine (-) Profound, long lasting neonatal sedation (-) Not reversible (-) Maternal vasodilation increases risk of hypotension

ANESTHETIZING

• Dissociatives: Ketamine, Tiletamine (Telazol) (-) Not reversible (-) May cause uterine vasoconstriction leading to fetal hypoxemia

ANESTHETIZING

• Thiobarbiturates: Thiopental (-) Not reversible (+) Rapidly redistributed and fetal liver can metabolize some, so low doses are not too depressive

ANESTHETIZING

• Propofol (+) Ultra short acting (-) Not reversible (-) May cause transient apnea in the dam. Correct by establishing an airway and ventilating as needed.

ANESTHETIZING

• Etomidate (+) Minimal cardiovascular effects on mother or fetus (-) Not reversible (-) Relatively expensive

ANESTHETIZING

• Anticholinergics: Atropine, Glycopyrrolate (+/-) Atropine rapidly crosses the placenta and will increase fetal heart rate. This can lead to fetal tachycardia, or may correct fetal bradycardia induced by other drugs given to the mother. Glycopyrrolate is less likely to cross the placenta due to its larger molecular size, so will have minimal fetal effects.

ANESTHETIZING

• Local anesthetics: An epidural or regional analgesia such as a line block with a local anesthetic (bupivicaine, lidocaine) will provide pain control and can allow reduced doses of other systemic drugs. A calm patient may be awake or slightly sedated in the *front end* while puppies are being surgically delivered under epidural analgesia. However, remember that local anesthetics have some side effects including vasodilation due to sympathetic blockade. They should be used with caution in a cardiovascularly compromised patient as they may push the patient into critical hypotension.

ANESTHETIZING

• Inhalants: Isoflurane, Sevoflurane, Halothane (-) All cross the placenta rapidly (+) Degree of fetal depression is dose dependent, so can readily be minimized

ANESTHETIZING

• Nitrous oxide (-) May lead to fetal hypoxia secondary to maternal hypoxia. Monitor maternal oxygenation (pulse oximeter) and decrease nitrous oxide as needed to maintain saturation >95% (+) Allows reduction of dose of more potent inhalants

PROTOCOLS

• If the mother is calm and quiet, and can be catheterized without sedation * Induce anesthesia with propofol to effect * Intubate and maintain a light plane of anesthesia with sevoflurane or isoflurane in oxygen * Line block or epidural for pain management * After neonates are delivered, add hydromorphone or oxymorphone IV for additional maternal pain control

PROTOCOLS

• * Sedate with fentanyl and diazepam * Epidural containing bupivacaine and morphine * Provide oxygen via facemask

PROTOCOLS

• * Mask induction with sevoflurane or isoflurane in oxygen * Intubate and maintain a light plane of anesthesia with sevoflurane or isoflurane in oxygen * After neonates are delivered, add hydromorphone or oxymorphone IV for maternal pain control.

WORK CITED

• www.tc.umn.edu/~rootk001/killos_anesthesia_c-sx.htm
• Maria Killos, DVM, MS, DACVA Virginia-Maryland Regional College of Veterinary Medicine