Anesthesia for cesarean section (Proceedings)

Objectives

     • To review pertinent physiological changes that occur during pregnancy

     • To discuss different anesthetic protocols for use during Cesarean section

To summarize the techniques employed by practitioners, and the morbidity and mortality associated with Cesarean section in dogs

Overview

     • An understanding of maternal and fetal physiology is necessary to formulate an effective and safe anesthetic plan for pregnant dogs and cats

     • Any anesthetic drug that is able to cross the maternal blood brain barrier will reach the fetus

     • Maintain maternal blood pressure during anesthesia

     • Prevent hypoxemia by supplementing with oxygen pre and intraoperatively

     • Minimize stress during pre-operative preparation

     • Use drugs that are easily reversed or short-acting when possible

Anesthesia for Cesarean Section

Cesarean section may be indicated for animals with prolonged gestation periods, refractory uterine inertia (primary or secondary), or those with obstructive dystocias. In addition, elective cesarean section may be done in those breeds with consistent fetal oversize (i.e., English bulldogs).

The goals of Cesarean section anesthesia include:

     • Minimal stress on the dam

     • Uncomplicated, rapid recovery

     • Minimal fetal/maternal morbidity/mortality

This presentation will review maernal and fetal factors important to the anesthetist and the advantages and disadvantages of a variety of anesthetic agents and approaches. We will also discuss the protocols that are used by practitioners today.

Pregnancy and the Respiratory System. Prolonged elevation of plasma progesterone concentration during pregnancy results in increased minute ventilation and decreased PaCO2. Increased uterine size results in cranial displacement of the diaphragm and decreased pulmonary functional residual capacity (FRC). FRC is the amount of gas left in the lung after a normal tidal expiration. FRC is normally large (~45 ml/kg) compared with tidal volume (~10 ml/kg), and FRC serves as an oxygen reservoir and helps to optimize the efficiency of gas exchange. Oxygen consumption (and metabolic rate) is also increased in pregnant animals. Taken together, these changes result in accelerated inhalant anesthetic induction and decreased oxygen reserves in pregnant animals. For these reasons, preoxygenation prior to anesthetic induction is indicated and oxygen supplementation during surgery is recommended in pregnant animals. Vigilant monitoring of anesthetic depth is also important to prevent anesthetic overdose.

The Cardiovascular System and Uterine Circulation. Systemic and uterine hemodynamics are altered during pregnancy. Cardiac output increases during gestation, and increases more during parturition. In contrast, systemic vascular resistance and diastolic blood pressure both decrease during pregnancy. Because cardiac work is increased during pregnancy and during labor, animals with cardiac abnormalities (i.e., congestive heart failure) are more likely to show signs of decompensation.

Uterine blood flow is poorly autoregulated in the pregnant female. Thus, uterine blood flow and fetal oxygen delivery vary directly with maternal blood pressure, and studies have shown that maternal hypotension (systolic blood pressure <100 mm Hg for 10-15 min) may be associated with signs of fetal distress (i.e., fetal acidosis and fetal bradycardia). Maintenance of maternal blood pressure should be considered when an anesthetic protocol is chosen and blood pressure should be evaluated frequently during anesthetic maintenance. Intraoperative hypotension should be treated aggressively.

Typically, crystalloid fluid therapy is recommended during anesthesia in pregnant animals in an attempt to maintain systemic blood pressure and uterine blood flow. Crystalloid fluids are usually administered at a rate of 5-10 mL/kg/h in the perioperative period, but may be increased when emergency volume resuscitation is required.

Colloidal fluids are preferred for therapy when plasma oncotic pressure is decreased. Colloid therapy is often initiated with serum albumin concentration is less than 2 g/dL or when plasma protein concentration is less than 4 g/dL. Synthetic colloidal solutions (Dextran or hydroxyethyl starch-containing solutions) are commonly used in both veterinary and human medicine for expansion of plasma volume. Up to 20 ml/kg/24 hr may be administered IV to treat hypotension. Blood and blood components may also be used when appropriate.

Inotropes and vasopressors may also be used to improve maternal blood pressure and fetal blood flow during cesarean section. Since alpha receptor agonists (i.e., methoxamine, phenylephrine) may induce uterine vasoconstriction and decreased uterine blood flow, agents which improve blood pressure as a result of cardiac beta receptor activation are preferred in the treatment of maternal hypotension. Ephedrine is a synthetic catecholamine that acts directly on alpha and beta receptors and indirectly via norepinephrine release. It is frequently used to improve maternal blood pressure and uterine blood flow in human beings and its use has been described in animals. Ephedrine administration is associated with increased systolic and diastolic blood pressure, as well as increased heart rate and cardiac output. A single IV injection will produce effects which last ~5-30 min. It may also be administered intramuscularly. Dobutamine and dopamine are two inotropes frequently used to increase blood pressure. They must be given as continuous infusions, and may be associated with tachycardia and arrhythmias at high doses. In addition, at higher doses dopamine may cause vasoconstriction.

The central nervous system is also affected by pregnancy and parturition. Potency of inhaled and epidural/spinal anesthetics is increased in pregnant animals, partially because of elevated maternal progesterone concentrations. For inhaled anesthetics, a 25-40% increase in anesthetic potency is observed and persists for about five days postpartum. Besides increased potency, the spread of epidural and spinal local anesthetics is also increased in pregnant animals. Thus, induction and maintenance doses of inhalant anesthetics should be decreased. The dose of epidural anesthetics may be reduced by 25-33% in pregnant animals. Increased size of paravertebral venous sinuses may be associated with increased risk of IV injection, systemic absorption, and toxicity of epidurally administered drugs.

Pregnancy also affects digestive function, and is associated with increased gastric emptying time and fluid volume. Gastric contents are also likely to be more acidic in pregnant animals. This implies that regurgitation and aspiration would be both more likely and more dangerous during anesthesia in pregnant animals. Rapid and proper placement of a cuffed endotracheal tube is the most effective method to prevent aspiration of gastric contents.

The time between uterine manipulation/incision and delivery should be short. Kamat et al. recently investigated the effect of uterine manipulation-to-delivery time on Apgar scoring in infants delivered by cesarean section. The Apgar scoring system uses heart rate, respiratory effort, reflex irritability, muscle tone, and color to evaluate human neonate well being at 1 and 5 min after birth. Apgar scores correlate well with fetal acid-base status. The authors showed that a prolonged (> 90 sec) uterine manipulation-to-delivery time resulted in a significantly worse infant Apgar score.

Length of anesthesia time may also affect the neonate. Interestingly, short (< 5 min) or long (> 15 min) anesthetic induction-to-delivery time has been shown to be associated with worse Apgar scores. Presumably, a very rapid anesthetic induction-to-delivery interval does not allow for redistribution of the induction anesthetic agent (i.e., thiopental). In contrast, prolonged anesthetic induction-to-delivery time (>15 min) may be associated with increased fetal distress because of prolonged anesthesia-induced hypotension.

For all practical purposes, the placental barrier may be thought of as very similar to the blood-brain barrier. Anesthetic drugs, which cross the blood-brain barrier due to relatively low molecular weight or high lipid solubility, readily cross the placenta. In addition, because fetal blood pH is lower than maternal blood pH, weakly basic drugs will be trapped in the fetal circulation because of greater ionization (and decreased lipid solubility). Local anesthetics, opioids, and diazepam may accumulate in the fetal circulation to a greater extent than in the maternal circulation. In contrast, neuromuscular blocking agents and glycopyrrolate are found in low concentrations in the fetal circulation.

Local anesthetics may be used alone, but are most commonly used with other anesthetic drugs for cesarean section. Infiltration anesthesia (lidocaine HCl, 5-10 mg/kg maximum dose) is commonly used to supplement neuroleptanalgesia. Lidocaine may also be applied topically to the arytenoids to facilitate endotracheal intubation, epidurally, spinally, or locally (at the incision site). The use of epidural and spinal anesthesia during the anesthesia of human patients undergoing cesarean section is common. In fact, neonatal acid-base status is better following epidural anesthesia than with either spinal or general anesthesia. In veterinary anesthesia, epidural anesthesia may be used in calm animals presented for elective cesarean section. However, logistical considerations (i.e., the time required for effective blockade and the personnel required to attend the animal) preclude the use of epidural anesthesia for emergency cesarean section in most practice situations. When epidural anesthesia is used, preoxygenation, sedation, and the administration of intravenous fluids are still important components of the protocol.

The practitioner should be prepared to care for neonates after delivery. Upon delivery, placental membranes should be removed from the head of the neonate and the oropharynx should be cleared of secretions. Oropharyngeal secretions may be removed by gentle suctioning. Take care to perform this operation quickly, as prolonged suctioning is associated with hypoxemia. The umbilical cord should be clamped approximately 1 inch from the body wall and the placenta removed distally to the hemostat. Drying the puppy with a towel will help to prevent hypothermia and will also stimulate spontaneous ventilation.

In the immediate postoperative period, supplemental O2 administration and maintenance of adequate environmental temperature are important considerations. One way to accomplish both is to place the neonates into an anesthetic induction chamber containing a circulating warm water blanket. Oxygen and warmth may then be easily provided to the newborn.

If narcotics were a part of the anesthetic protocol, naloxone should be administered to the neonate. The narcotic antagonist may be given sublingually (1-5 drops from a 22-gauge needle; approximately 1-5 mg), subcutaneously, or intramuscularly. Naloxone administration may have to be repeated should renarcotization occur. Doxapram (1-5 mg) may be given sublingually, subcutaneously, or via the umbilical vein if respiratory stimulation is necessary. Atropine administration has also been described in the neonate for the treatment of bradycardia (HR < 80 bpm). Remember that hypoxia leads to bradycardia, and atropine should not be substituted for oxygen therapy in the bradycardic, hypoxemic animal.

Summary

     • Maintain maternal blood pressure during anesthesia

     • Prevent hypoxemia by supplementing with oxygen pre- and intraoperatively

     • Minimize stress during pre-operative preparation

     • Use drugs that are easily reversed or short-acting when possible

Reference:

Muir et al. Chapter 24: Anesthesia for Cesarean section. Handbook of Veterinary Anesthesia 4rd ed. Mosby, St. Louis, 2007.