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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.
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.
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.
Formulating an anesthetic plan in the pregnant patient
The pregnancy status of a patient is crucial when formulating a plan for a pregnant animal. If the anesthesia is to be performed on a patient that is not currently due to deliver, an anesthetic plan that minimizes a decrease in cardiac output and renal blood flow should be selected. A plan that avoids alpha-2 agonist drugs, maintain blood and fluid volumes, and blood pressure should be selected. Patients presenting for an elective Cesarean section can be given an epidural to help minimize the need for systemic drugs. Emergency Cesarean sections or dystocia should use techniques that allow for a quick induction and maintenance so that the puppies/kittens can be delivered as quickly as possible. A protocol based around propofol and inhalant anesthetics have been shown to achieve the highest puppy survival numbers. The use of thiopental or ketamine based protocols have shown to result in higher puppy mortality rates.
Anesthesia for the neonatal/pediatric patient
The neonate can be one of the most difficult patients to anesthetize. Not only does there small size limit what the practitioner can do technically, their unique physiology requires and understanding of how drugs impact physiologic systems so that appropriate protocols can be selected.
Puppies and kittens are considered neonates until 4 weeks of age and pediatric until 12 weeks of age. During these periods many of the physiologic systems (cardiac, respiratory, renal, hepatic) are undergoing maturation and disruptions in their development can result in long standing organ dysfunction.
In the cardiovascular system, the heart has a large number of non-contractile fibers that necessitate the heart contract at near maximal force for an appropriate ejection fraction. Therefore, cardiac output is particularly reliant upon rate to provide adequate blood flow. Anesthetic/analgesic drugs that can cause bradycardia can subsequently result in a tremendous drop off in cardiac output. Drugs such as alpha-2 agonists should be avoided in any patient less than 12 weeks of age. Additionally, these patients tend to be a lower pressure system and therefore need mean blood pressures of less than adult animals. The use of pressure agents to achieve blood pressure numbers similar to adults is not necessary and can result in vascular damage.
The respiratory system has differences from adults not only physiologically but anatomically as well. The tongue tends to be larger and the upper airway smaller compared to body size and put these patients at a higher risk for airway obstruction. Intubation of these patients is therefore recommended. These patients also have a high chest wall compliance low functional residual capacity and a higher metabolic demand for oxygen. Hypoventilation, hypoxemia and hypoxia are significant risks and when using agents that depress ventilation, positive pressure ventilation may be required.
Kidney maturation tends to happen later in the pediatric portion of life and anesthetic techniques that protect the kidneys are necessary. Appropriate fluid and electrolyte balance in recommended and non-steroidal anti-inflammatory medication should be avoided. Fluid overload is not tolerated by neonatal and pediatric patients and therefore mechanisms such as syringe pumps to deliver fluids should be considered to prevent accidental administration of excessive fluids.
The hepatic microsomal systems are also immature in the neonate and drug metabolism is minimal compared to adults which can result in a prolonged duration of action of administered medications. Glycogen storage is nearly non-existent and these young animals need exogenous sources of glucose to prevent hypoglycemia. Not only should dextrose be added to administered IV fluids, but fasting should be minimized and should not be done in nursing animals.
Thermoregulation is also a problem for the neonatal/pediatric patient. With a high surface area to body weight ratio, these patients loose heat at an astounding rate. The use of blankets and warming devices should be done to prevent shivering as this is an incredibly inefficient mechanism for heat generation and consumes a large amount of energy.
Anesthetic protocol built around benzodiazepines, opioids, and inhalants are the norm with neonatal/pediatric patients. With proper monitoring and care, the best possible outcomes can be achieved.