Anesthetic Considerations in Cesarean Section of the Dog or Cat

Maria Killos, DVM

Resident, Anesthesiology

University of Minnesota, College of Veterinary Medicine

Physiologic Changes Associated with Pregnancy and their Importance in Anesthesia:

• Pregnancy increases plasma volume, with a corresponding decrease in hematocrit.  A pregnant animal with a normal hematocrit is dehydrated.  As much as possible, correct dehydration before anesthesia.  Minimally, these patients will require fluid support during anesthesia in order to remain normotensive.
• 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.  
• Cardiac output (CO) increases approximately 30‑40%
  

• Achieved by an increased heart rate ( CO = HR x SV )
• Increased blood volume and decreased SVR may cause patients with pre‑existing cardiovascular disease to decompensate

• Blood pressure falls slightly

o       Secondary to generalized vasodilation resulting in a fall in systemic vascular resistance (SVR) and decreased afterload

o       Mechanical compression by the gravid uterus (positional) adds to hypotension, especially as the mother is positioned for surgery

• Gastrointestinal motility is typically slowed.  Risk of vomit or regurgitation is higher than with non-pregnant patients.  A secure airway (with a cuffed endotracheal tube) can help to prevent aspiration.
• A gravid uterus, especially in the case of a large litter, will increase abdominal pressure and thus the work of breathing.  A ventilator, or other mechanical ventilation, will allow you to be sure the mother is ventilating adequately.  This is especially important because fetal hypoxemia is a potentially fatal complication.

·        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

• 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 (related to the vascular engorgement)

General Considerations in the peri-operative period:

• Time is of the essence!  Pre-clip the surgical site before induction.  Work quickly!!!!
• Maternal stability under anesthesia is very important.  Monitor vitals including blood pressure, heart rate and rhythm, oxygenation status (pulse oximeter).  An IV catheter will be crucial for providing intravenous fluids and supportive medications.
• Maintain maternal blood pressure to maintain placental blood flow
• Fluid therapy
• Avoid excessive anesthetic depth
• Position the mother during surgery to minimize blood flow impedance
• Maintain maternal oxygenation to avoid fetal hypoxemia
• Supplement with oxygen
• Ventilate as needed
• Correct anemia if needed
• 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

Preparing for neonatal resuscitation

Equipment:

• Oxygen source and either small endotracheal tubes or a mask for assisted ventilation
• Dry towels to gently rub the neonates and stimulate breathing
• Suction (a syringe will do in a pinch, otherwise a mouth suction device works well)
• Syringes with 25 gauge needles: 1 drop from a 25 gauge needle is a standard measurement for resuscitation medications in the neonate
• Suture to tie the umbilical cord AFTER the neonate is resuscitated
• Source of warmth such as a circulating water blanket, but do not place neonates directly on the warming device because they are at high risk for burns!

Drugs:

All of the following drugs can be given sublingually in the neonate.  One drop from a 25 gauge needle is a standard starting dose.

• Doxapram to stimulate respiration if needed
• Atropine in case of neonatal bradycardia (<100 bpm)
• Reversal agents such as Naloxone (opioid antagonist) or Flumazenil (benzodiazepine antagonist), depending on the drugs chosen for the mother.
• Dextrose or another sugar source such as karo syrup or honey
• Warm sterile fluids such as normal saline or lactated Ringers (can be given subcutaneously if needed)

Anesthetizing the Bitch/Queen

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

Benzodiazepines:  Diazepam, Midazolam

(–) May cause profound sedation in neonates

(+) Reversible with flumazenil

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)

Acepromazine

(–) Profound, long lasting neonatal sedation

(–) Not reversible

(–) Maternal vasodilation increases risk of hypotension

Dissociatives: Ketamine, Tiletamine (Telazol)

(–) Not reversible

(–) May cause uterine vasoconstriction leading to fetal hypoxemia

Thiobarbiturates:  Thiopental

(–) Not reversible

(+) Rapidly redistributed and fetal liver can metabolize some, so low doses are not too depressive

Propofol

(+) Ultra short acting

(–) Not reversible

(–) May cause transient apnea in the dam.  Correct by establishing an airway and ventilating as needed.

Etomidate

(+) Minimal cardiovascular effects on mother or fetus

(–) Not reversible

(–) Relatively expensive

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.

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.

Inhalants:  Isoflurane, Sevoflurane, Halothane

(–) All cross the placenta rapidly

(+) Degree of fetal depression is dose‑dependent, so can readily be minimized

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

A few sample protocols:

1.  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

2.

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

3. 

• 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.

References and Further Reading

Moon PF, Erb HN, Ludders JW, Gleed RD, Pascoe PJ. Perioperative management and mortality rates of dogs undergoing cesarean section in the United States and Canada. JAVMA 1998. 213(3):365-9.

Moon PF, Erb HN, Ludders JW, Gleed RD, Pascoe PJ. Perioperative risk factors for puppies delivered by cesarean section in the United States and Canada. JAAHA 2000. 36(4):359-68.

Moon-Massat PF, Erb HN. Perioperative factors associated with puppy vigor after delivery by cesarean section. JAAHA 2002. 38(1):90-6.

Thurmon JC, Tranquilli WJ, Benson GJ. Lumb and Jones’ Veterinary Anesthesia.  1996 Third edition; 818-828

Funkquist PME, Gorel CN, Lofgren AJ and Fahlbrink EM. Use of Propofol‑Isoflurane as an Anesthetic Regimen for Cesarean Section in Dogs. JAVMA 1997. 2113; 313‑317.

Luna SP, Cassu RN, Castro GB, Texiera Neto FJ, Silva Junior JR, Lopes MD. Effects of four anesthetic protocols on the neurological and cardiorespiratory variables of puppies born by caesarean section. Veterinary Record 2004. 154: 387-389.

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