27 I Pregnancy, especially the immediate preparturient period, causes significant alterations in maternal physiology A Altered drug pharmacokinetics and pharmacodynamics 4. The weight of the gravid uterus can cause aortocaval compression when the animal is placed on its back; cardiac reserve is decreased because of increased cardiac work 5. Central venous pressure and systemic blood pressure remain relatively unchanged but may increase during labor I Central nervous system (CNS) A Increased progesterone concentration decreases inhalant anesthetic requirement B Vascular engorgement decreases the size of the epidural space; a decreased volume of epidurally administered drug is required A Alveolar ventilation is increased because of progesterone-induced increased respiratory center sensitivity to carbon dioxide 1. Increased respiratory rate and alveolar ventilation and decreased FRC result in a rapid alveolar rate of rise in inhalant anesthetics B Small airways constrict at higher lung volumes A Maternal blood volume is increased by up to 30% B Packed cell volume and plasma-protein concentration are decreased 1. Relative anemia refers to the pregnant animal not tolerating blood loss as well as the nonpregnant animal C Cardiac output is increased 30% to 50% because of increases in stroke volume and heart rate A Placental gastrin secretion increases gastric acidity B The stomach is displaced cranially, and the tone of the lower esophageal sphincter is altered V Other changes: decreased plasma cholinesterase (pseudocholinesterase) I Factors influencing drug transfer across the placenta A Surface area and diffusion characteristics of the placenta B Diffusion properties of drugs 1. High lipid solubility increases diffusion (e.g., barbiturates, inhalants) 2. Lower molecular weight increases diffusion 3. Decreased degree of ionization and protein binding increases diffusion C Relative maternal and fetal drug concentrations 1. Bolus doses of drugs result in rapid transfer of drug to the fetus followed by rapidly declining maternal concentrations 2. Continuous infusion, repeated bolus administration, and the administration of inhalant anesthetics result in elevated maternal drug concentrations and continual drug transfer to the fetus I Anticholinergics—drug effects A Atropine crosses the placental barrier rapidly 1. Fetal tachycardia noted within 10 to 15 minutes of drug administration 2. Fetal disorientation or excitement may be caused by CNS effects of atropine B Glycopyrrolate does not cross the placental barrier in significant quantities because of its large molecular size and charge A Local anesthetic drugs administered by any route cross the placental barrier 1. Drug effects depend on the total dose, the interval between final dose and delivery, and whether epinephrine is used 2. Clinically relevant doses of local anesthetics usually do not produce significant depression in the fetus 1. Lidocaine is the preferred local anesthetic for cesarean section because of its relatively low toxicity 2. Detected in the umbilical venous blood of the fetus within 2 to 3 minutes 3. No correlation has been found between the degree of neonatal depression and the umbilical venous concentration of lidocaine III Preanesthetic drug effects A Drug effects: preanesthetic drugs reduce the amount of potentially more dangerous anesthetic drugs 1. Phenothiazines (acepromazine) a. Rapidly transferred into the fetal blood b. Clinical doses produce little to no apparent effect on the newborn c. α-Adrenergic blockade may produce hypotension resulting in decreased uterine blood flow and fetal hypoxia 2. Benzodiazepines (diazepam, midazolam) a. Concentrations higher in fetal blood than in maternal blood b. Produce minimal respiratory and cardiovascular depression in the fetus and mother c. Duration of effect depends on redistribution away from the CNS 3. α2-Agonists (xylazine, detomidine, dexmedetomidine, romifidine) a. Marked respiratory depression in both the mother and fetus b. Use in low doses and be prepared to administer an antagonist to the newborn (yohimbine, tolazoline, atipamezole) c. α2-Agonists increase uterine tone and pressure in cattle; may be abortifacients; effects in other species are less defined a. Frequently used as preanesthetic medication for sedation and analgesia; some (e.g., morphine) are administered epidurally c. Opioid concentrations may be higher in the fetus than in the mother d. Moderate maternal doses do not produce serious CNS depression; the effect can be reversed by opioid antagonists (naloxone) e. Naloxone has a shorter duration of action than most opioids; neonates redosed as needed (a) Immediately transferred to the fetal circulation (b) Minimal fetal depression if birth is within the first hour of drug administration (a) Causes observable clinical CNS depression in the newborn (b) Has a direct vasoconstrictor effect on placental vessels
Anesthesia for Cesarean Section
Overview
General Considerations
Changes in Maternal Physiology in Advanced Pregnancy
Drug Transfer across the Placenta
Effects from Anesthetic Drugs Administered for Cesarean Section
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
