Propofol
Propofol is the standard induction agent for general anesthesia in obstetrics, favored for its fast onset and short duration of action. It replaced thiopental, the historical standard for cesarean delivery, which has not been marketed in the United States since 2011. Like thiopental, propofol is highly lipid-soluble and largely unionized at physiologic pH, so it diffuses across the placenta within a single circulation time. Propofol is about 98% bound to plasma proteins (thiopental about 75 to 85%), yet it still crosses rapidly: bound drug dissociates as free drug leaves the maternal plasma, and the UV/MV ratio at delivery after a 2 to 2.5 mg/kg induction dose is about 0.7. Fetal levels then fall quickly because the drug redistributes out of the maternal plasma, which lowers the gradient driving transfer.
Fetal Effect: Due to its high lipid solubility, propofol rapidly enters the fetal circulation, and its effects on the neonate can be observed almost immediately after birth. At a standard induction dose of 2 to 2.5 mg/kg, Apgar scores and umbilical blood gases are comparable with those after thiopental; transient depression of Apgar scores and of neurobehavioral scores (mild sedation, reduced tone and altered motor responses on the NACS) appears when larger doses (about 2.8 mg/kg or more) are used or when induction is followed by a propofol infusion before delivery. These effects are short-lived, and the neonate usually recovers without significant intervention as the drug redistributes and is metabolized. Timing matters less than dose for the induction agents: because maternal levels fall by redistribution, a longer induction-to-delivery interval under general anesthesia does not by itself depress the neonate, whereas a uterine incision-to-delivery interval longer than about 3 minutes does predict fetal acidosis and low Apgar scores, through impaired uteroplacental perfusion rather than drug. Keeping the induction dose in the standard range is therefore the main lever for protecting the neonate.
Etomidate
Etomidate is a non-barbiturate hypnotic agent used for induction in obstetric anesthesia (0.2 to 0.3 mg/kg) when hemodynamic stability is critical, for example in hemorrhage or severe cardiac disease, because of its cardiovascular stability and rapid onset of action. It is particularly favored in patients where maintaining cardiovascular function is critical, as it causes minimal cardiovascular and respiratory depression compared to other anesthetics. Etomidate is highly lipid-soluble and has a short duration of action, making it an ideal choice for quick induction, especially in emergency situations. Etomidate exerts its effects by enhancing GABAergic activity, increasing the duration of chloride channel opening, and thus producing sedation and anesthesia. It is highly lipid-soluble, allowing it to cross lipid membranes easily, including the placental barrier. At physiological pH, a significant portion of etomidate remains unionized, which facilitates its passage through cell membranes. Etomidate has a high protein-binding capacity, with around 76% of the drug bound to plasma proteins, mainly albumin, which limits the free fraction available for placental transfer. However, its lipid solubility still permits moderate placental transfer, with a UV/MV ratio of about 0.5 in women given etomidate for cesarean delivery.
Fetal Effect: Etomidate can pass through the placental barrier and reach the fetus shortly after maternal IV administration. Studies in pregnant ewes have shown that about 45% of the drug present in the maternal circulation becomes available to the fetus, demonstrating a significant placental transfer. However, during continuous etomidate infusion, the placenta appears to act as a partial barrier, as fetal plasma etomidate concentrations plateaued at lower levels than maternal plasma levels, indicating reduced drug accumulation in the fetus. While fetal exposure to etomidate is almost 40% of maternal plasma concentrations during the infusion period, the placenta may still limit the total amount reaching the fetus. Despite placental transfer, the fetus clears etomidate quickly, with mean residence times (MRT) for the drug being similar in both mother and fetus (around 22 minutes). This rapid elimination reduces the risk of drug accumulation in fetal tissues and contributes to the absence of long-term neonatal side effects. Fetal plasma concentrations after continuous etomidate infusion were comparable to those required for effective hypnotic action in humans, suggesting the possibility of fetal sedation during prolonged maternal etomidate use. However, concentrations following a single IV bolus may not be sufficient to induce fetal hypnosis. Nonetheless, neonatal sedation and respiratory depression are typically mild and resolve quickly post-delivery, as the fetus metabolizes the drug efficiently. Two other effects are worth knowing: etomidate transiently suppresses adrenal 11-beta-hydroxylase, and neonates exposed at induction have measurably lower cortisol for about the first 6 hours of life (without described clinical consequence after a single dose), and maternal myoclonus and pain on injection are common.
Ketamine
Ketamine, originally introduced as an anesthetic and analgesic, has gained attention for its misuse as a recreational drug and, more recently, as a rapid-acting antidepressant. In obstetrics it is the induction agent of choice for the hypotensive or hemorrhaging patient and for the severe asthmatic (0.5 to 1 mg/kg), because it raises blood pressure and heart rate through sympathetic stimulation; for the same reason it is avoided in preeclampsia and other hypertensive states. Ketamine is small, lipid-soluble and only moderately protein bound, so it crosses the placenta essentially completely: UV/MV ratios of about 1.0 to 1.3 are reached within minutes of an intravenous dose, meaning fetal levels can briefly exceed maternal levels. Induction doses above about 1.5 to 2 mg/kg are associated with increased uterine tone, lower Apgar scores and neonatal depression, so the dose is kept at or below 1 mg/kg. Separately, concerns have emerged regarding the neurotoxic effects of ketamine on developing brains when used during pregnancy, because of its distribution into fetal tissues, especially the brain.
Fetal Effects: Ketamine readily crosses the placenta, exposing the developing fetus to significant levels of the drug shortly after maternal administration. Studies have demonstrated that fetal ketamine exposure can disrupt critical processes in brain development, such as synaptogenesis and neurogenesis, leading to potential long-term cognitive and behavioral impairments. Animal research has shown that prenatal exposure to ketamine is associated with neurotoxicity, including increased neuronal apoptosis, oxidative stress, and impaired synaptic plasticity, which may manifest as learning, memory, and behavioral deficits in offspring later in life. These findings underscore the need for caution when using ketamine during pregnancy. The human evidence should be kept in proportion: the FDA's 2016 warning about anesthetic neurotoxicity (which names ketamine along with propofol, the volatile agents and benzodiazepines) concerns repeated or prolonged exposures of more than 3 hours in the third trimester, not a single induction dose, and the American College of Obstetricians and Gynecologists (2019) states that no human data show that a single, brief anesthetic exposure in pregnancy harms the developing brain. A single induction dose of ketamine for a cesarean delivery is not contraindicated on these grounds.
