Stephen J. Birchard DVM, MS, Diplomate ACVS

Tuesday, June 14, 2016

Anesthesia for Cesarian Section in Dogs and Cats: Dr. Lisa Ebner, DACVAA

Cesarean section (c-section) is a commonly performed surgical procedure in small animal practice. In the first segment of this 2-part blog on c-section, anesthetic management of the pregnant bitch or queen is presented by board certified anesthesiologist Dr. Lisa Ebner. In the 2nd segment we will focus on surgical management. The readers are referred to other publications for material on diagnosis and medical treatment of dystocia in small animals.            

As with any anesthesia and surgery the primary goal is to induce and maintain a surgical plane of anesthesia to eliminate pain and allow a safe recovery of the animal upon completion of the procedure. In Cesarean section the neonates complicate the anesthetic management. Anesthetic protocols must consider the health and well being of the unborn puppies and kittens as well as the mother.

With a planned c-section, the gestation length is known, the patient has been properly fasted, and the procedure takes place during normal business hours when plenty of staff members are available to assist.  This contrasts with the emergency c-section where often the patient has been straining for hours, may be in a compromised metabolic state and the viability of the puppies may be compromised. In clinical studies both maternal and puppy mortality was significantly increased with emergency compared to planned c-sections (1,2).

Pre-anesthetic Considerations
Anesthesia planning begins with assessment of patient risk. Obtain a thorough history that includes any concurrent medical conditions and medications.  Determine how long the bitch has been in labor and if any puppies, dead or alive, have been delivered.  A thorough physical exam is important and diagnostic imaging, such as abdominal ultrasound or radiographs, is very helpful. (Fig. 1) 
Fig. 1: Lateral radiograph of late term pregnancy in a dog.

Abdominal ultrasound is more sensitive in detecting fetal viability; while abdominal radiographs help determine the number, size, and position of the fetuses.  If possible, assessment of fetal heart rate can be a good indicator of a healthy (150-220 bpm) vs. a stressed fetus (100-150 bpm).  Laboratory evaluation of the bitch or queen is also indicated.  Selection of lab tests will depend on patient assessment, but a minimum database of PCV, total solids, BUN, glucose, ionized calcium, and other electrolytes prior to anesthesia is recommended. 

Physiologic Changes With Pregnancy
Pregnancy causes significant alterations in the mother’s physiology that need to be considered. Due to higher plasma volume in relation to the number of RBCs present, the pregnant bitch or queen has a “relative” anemia. Therefore if the PCV is within the normal range then she may actually be dehydrated. Pain or catecholamine release can cause tachycardia and increased cardiac output.  Compression of the great vessels in the abdomen during pregnancy does not appear to have the same potential in animals as it does in humans.  Based on clinical studies, small dogs under 25 kg do not appear to exhibit postural influences on systemic blood pressure but large breed, full term bitches could have the potential for a decrease in blood flow to the uterus (3,4). This effect may be exaggerated in animals that are dehydrated or have other illness affecting their cardiopulmonary system. The gravid uterus also displaces the diaphragm cranially, leading to a decreased tidal volume in the patient.  Hyperventilation may also be present due to pain and distress in a patient presenting for a c-section.  Overall there is an increased need for oxygen coupled with lower functional residual capacity that ultimately can result in hypoxemia. 

Anesthesia Protocol
Before beginning anesthesia consider administering 3 to 5 minutes of pre-oxygenation with 100% O2 provided by a tight-fitting oxygen mask (as long as this does not cause undo stress to the animal).  Hypocalcemia may be present in small breed dogs, dogs with large litters, or with uterine inertia. This abnormality should be corrected along with any fluid deficits prior to anesthesia and surgery.

To reduce complications and depression of the neonates, anesthesia and surgery time should be as short as possible.  Clip and perform the initial scrub of the patient prior to induction of anesthesia.  The surgeon should already being scrubbed, gowned, gloved and ready to begin draping as soon as the final surgical prep has been done.  The actual time from induction of anesthesia to removal of the neonate(s) should ideally be less than 5 minutes. 

Anesthetic Drug Selection
Avoid premedication in the pregnant animal because opioids are associated with respiratory depression in the neonates and other tranquilizing or sedating drugs, such as acepromazine or alpha2 agonists, are associated with decreased vigor in the newborns.  Even though some of the premedication drugs are reversible by administration of an antagonist, it is still best to avoid then because there is an increased risk of morbidity and mortality for the neonates.

Intravenous Induction
An intravenous catheter should always be placed for administration of IV fluids and emergency drugs if needed during anesthesia.   After adequate time for pre-oxygenation, the bitch or queen can be induced with propofol IV to effect (typically 4 mg/kg) and intubated with an appropriately sized endotracheal tube that is inflated to prevent a leak at 20 cm H2O.

Inhalation Anesthetics
For anesthesia maintenance isoflurane or sevoflurane, in oxygen, is acceptable.  Ideally, the concentration of the inhalant should be kept as low as possible to prevent the animal from responding to surgical stimulation but also prevent the dose-dependent vasodilation and accompanying hypotension associated with use of inhalants. 

Local or Regional Anesthetics
In order to keep the inhalant concentration to a minimum, local-regional anesthesia is often incorporated.  A line block can be performed preoperatively and will continue to provide analgesia post-operatively.  Bupivacaine (0.5%) is often selected for the line block due to its prolonged duration of action compared to lidocaine (4 to 6 hours vs. 60-90 minutes).  However, the time to onset is slightly longer than lidocaine. 

Another technique that can be included in the anesthetic protocol is a lumbosacral epidural with lidocaine (2%).  Lidocaine is selected because the shorter duration of action is actually preferred since most patients will go home with the owner soon after recovery from anesthesia.  The volume of drug used in the epidural should be decreased about 20-25% compared to a non-pregnant patient because the epidural space is actually decreased in a pregnant patient due to increased size of epidural veins.  It is an option to add morphine to the lumbosacral epidural.  Be careful performing epidurals in animals that are hemodynamically unstable because the local anesthetic drug can have a vasodilatory effect in the epidural space, leading to hypotension.  Other contraindications for epidural include infection of the skin at the site of needle puncture, coagulopathy, and an obese patient that makes palpation of the landmarks unreliable.  Although acceptable to perform c-section with only an epidural technique, such as is done in humans and many large animals, it would depend on the comfort and experience level of the anesthetist since an incomplete block may occur and the patient could respond to surgical stimulation.  Also, the patient would not have a protected airway and is at risk for aspiration pneumonia if regurgitation occurs. 

Neonatal Care
Once the neonates are delivered have several experienced staff on hand to help resuscitate them.  Organize the personnel as well as necessary drugs and supplies prior to induction.  After delivery gently clear fluid from the oral cavity of neonates of fluid with a bulb syringe.  Do not “swing” the neonate to clear its airway, as this has been associated with intracranial trauma (5).  If not already done by the surgeon, clamp the umbilical cord with a small hemostat and place a ligature with absorbable suture around the cord. Typically the cord is dipped in povidone-iodine solution after it is transected. 

Rub the neonates with a soft, absorbent towel immediately to stimulate spontaneous ventilation. (Fig. 2) 
Fig. 2: Newborn puppy being gently rubbed with a dry towel to stimulate respirations.
If there is no sign of spontaneous breathing, administer supplemental oxygen and preferably manual breaths after endotracheal intubation (should be attempted if it can be carefully done).  Most endotracheal tubes will be too large for neonates, but removing the stylet from a 16 gauge IV catheter and using only the polyurethane flexible catheter portion is an option.  If no heart-beat can be palpated, begin gentle cardiac compressions (ideally > 180 bpm).  If the puppy or kitten is bradycardic, this is most likely due to hypoxemia and not due to increased vagal tone.  Drugs that can be used for neonate resuscitation include epinephrine, doxapram, and naloxone (only if opioids were given to the mother).  The dose for each drug is typically 1-2 drops administered under the tongue.  The neonate may also benefit from an oral dose of 2.5% dextrose if it is able to swallow a few drops.  Finally, an important aspect of neonatal care is to keep them warm by placing them in an incubator or under a heat lamp until they can be with their mother when she is fully recovered. 

Postoperative Analgesia
The postoperative pain management of the bitch is always a cause for concern due to the possibility of transfer of drugs in to the milk and therefore the neonate. Based on current research many analgesic drug classes may be safely given during the lactation period without adverse effects on the neonates (6).  Drugs that have high lipid solubility, low molecular weight, and are non-ionized are secreted more easily in to the milk.  However, it appears that only 1-2% of the maternal dose goes in to the neonate.  So opioids such as morphine or hydromorphone post-operatively would be a suitable choice for the mother. Animal studies indicate that buprenorphine has the potential to inhibit milk production. Tramadol use in lactating dogs and cats has not been specifically studied, but in humans it appears that short-term use during establishment of lactation is compatible with breastfeeding (7).  Studies have not established the safety of NSAIDs, such as carprofen, in pregnant or lactating dogs.  So it is generally recommended to avoid NSAIDs in pregnant patients and only use a single dose with caution in nursing dogs.  Clinical judgement of the attending veterinarian should prevail when it comes to selecting an analgesic drug for a patient and it should be based on assessing the patient for pain, considering the species, health status of the patient, and potential for effects on the nursing neonates.  Signs of pain in the mother could include vocalizing, not allowing to neonates to nurse, trouble moving around, and a lack of interest in food. (8)

In summary, anesthesia and cesarean section can be safely performed on dogs and cats when the anesthetist carefully selects a drug protocol, properly prepares the animal for the anesthetic episode, minimizes the anesthetic time, and provides supportive care to the neonates and mother postoperatively.

Table: Summary of Anesthesia Protocol for C-section in Dogs and Cats

Pre-medication: no drugs; (3-5 minutes of pre-oxygenation, clip and initial scrub of surgical site)
Induction: Intravenous propofol
Maintenance: isoflurane or sevoflurane
Local or regional anesthesia: lidocaine line block on proposed incision site or lidocaine epidural
Postoperative analgesia for mother: Tramadol for bitch, buprenorphene for queen

Reference List:
1. Moon PF, Erb HN, Ludders JW, et al 2000. Perioperative risk factors for puppies delivered by cesarean section in the United States and Canada. J Am Anim Hosp Assoc 36: 359-368.
2. Moon PF, Erb HN, Ludders JW, et al 1998. Perioperative management and mortality rates of dogs undergoing cesarean section in the United States and Canada. J Am Vet Med Assoc 213: 365-369.
3. Probst CW, Webb AI. 1983. Postural influence on systemic blood pressure, gas exchange, and acid/base status in the term pregnant bitch during general anesthesia. Am J Vet Res 44: 1963-1965.
4. Probst CW, Broadstone RV, Evans AT. 1987. Postural influence on systemic blood pressure in large full-term pregnant bitches during general anesthesia. Vet Surg 16: 471-473.
5. Grundy S, Liu S, Davidson A. 2009. Intracranial trauma in a dog due to being “swung” at birth. Top Companion Anim Med 23: 100-103.
6. Raffe, Marc. Veterinary Anesthesia and Analgesia: the 5th edition of Lumb & Jones. Wiley Blackwell: 2015. 34.
7. Ilett KF, Paech MJ, Page-Sharp M, et al. Use of a sparse sampling study design to assess transfer of tramadol and its O-desmethyl metabolite into transitional breast milk. Br J Clin Pharmacol. 2008 May;65(5):661-6. doi: 10.1111/j.1365-2125.2008.03117.x. Epub 2008 Feb 20.
8. Aarnes, Turi and Bednarski, Richard. Canine and Feline Anesthesia and Co-Existing Disease. Wiley Blackwell: 2015. 16.

No comments:

Post a Comment