Anesthesia for pregnancy or cesarean section and for neonates (Proceedings)
Anesthesia of the pregnant dog or cat falls into two categories, anesthesia of a pregnant animal for a procedure unrelated to the pregnancy and anesthesia of a pregnant animal specifically for a problem related to the pregnancy/cesarean section. Anesthesia of a pregnant animal for procedures unrelated to the pregnancy is often not problematic unless the animal is in a compromised state. Compromised patients should be stabilized and treatment geared toward the specific problems. Anesthesia of the pregnant animal for problems related to the pregnancy/dystocia/Cesarean section will be covered here. With proper knowledge of the physiology of the pregnant animal and the use of anesthetic/analgesic drugs that minimally impact the delivered neonate, the best possible outcomes can be achieved.
Physiologic changes in the pregnant patient
The physiology of a pregnant animal begins to change shortly after becoming pregnant and continues to change up to parturition. Significant changes occur in the cardiovascular, respiratory, gastrointestinal and neuro-endocrine systems.Cardiovascular system
As the fetus increases more energy requirements are necessary and blood flow to the uterus is paramount. This is done through a gradual increase in the cardiac output and blood volume in the pregnant animal. As cardiac output increases, cardiac contractility approaches maximum, thus in the face of blood loss, hypotension, or hypovolemia, compensatory mechanisms are stressed and easily fail.
An increase in oxygen requirement is also present during pregnancy. Thus there is a higher requirement for alveolar ventilation. However, as the fetus and uterus expands, pressure on the diaphragm decreases the thoracic cavity volume and therefore there is a decrease in total lung volume and functional residual capacity. As a result, an increase in respiratory rate is necessary and these patients are susceptible to hypoxemia during even short bouts of apnea or hypoventilation.
As the uterus expands during pregnancy and certainly just prior to parturition, the abdominal cavity becomes limited for space. This results in a delay in gastric emptying and ingesta transit time through the gastrointestinal system. Additionally, there is a decrease in esophageal sphincter tone. Combined, these pregnancy associated conditions increase the likely hood of regurgitation and aspiration during induction and recovery setting up a possibility for pneumonia. Additionally, there is an increase in gastrin levels which decrease the pH of gastric fluid which, if aspirated, can cause a chemical pneumonitis.
Progesterone, a steroid hormone, is elevated during pregnancy. Progesterone has sedative/anesthetic effects on its own; therefore doses of exogenously administered anesthetic/analgesic drugs should be reduced to prevent a relative overdose. Anesthetic drug clearance is also decreases. Patients should be monitored closely to prevent excessive anesthetic depth.