Abstract
Almost every drug used in obstetrical analgesia and anesthesia has been found to cross the placenta, in most instances by simple diffusion. This process is influenced by several factors:
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a)
Degree of ionization: Organic drugs are transferred mainly in the undissociated or non-ionized form charged or ionized particles penetrate with difficulty. Changes in pH which increase the concentration of the undissociated form therefore favor the passage of drugs into the fetus.
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b)
The degree of fat solubility of the non-ionized molecule probably plays the most important role in governing the rate of transfer across the placenta. Drugs with high fat solubility are transferred rapidly, whereas lipid insoluble drugs penetrate poorly.
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c)
The concentration gradient which is related primarily to the quantity of drug administered to the mother, the route of administration and the state of uterine and fetal circulations.
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d)
Substances of low molecular weight diffuse freely across the placenta, whereas those of molecular weight exceeding 1000 do not cross at all.
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References
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Finster, M. (1973). Effects of Obstetric Anesthesia on the Fetus and Newborn. In: Ahnefeld, F.W., Halmágyi, M. (eds) Anaesthesie und Wiederbelebung bei Säuglingen und Kleinkindern. Anaesthesiology and Resuscitation/Anaesthesiologie und Wiederbelebung/Anesthésiologie et Réanimation, vol 71. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-65539-5_3
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DOI: https://doi.org/10.1007/978-3-642-65539-5_3
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