Anesthesia for cesarean section (Proceedings)
• To review pertinent physiological changes that occur during pregnancy
• To discuss different anesthetic protocols for use during Cesarean sectionTo summarize the techniques employed by practitioners, and the morbidity and mortality associated with Cesarean section in dogs
• 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.