Fentanyl Vs. Alfentanil in Acidosis

  1. fentanyl has a pka of 8.4 and is 91% ionized at a ph of 7.4. alfentanil has a pka of 6.5 and is 11% ionized at a ph of 7.4, and also has a lower volume of distribution than fentanyl. i believe alfentanil is more lipophillic but don't quote me on that. if a patient is in metabolic acidosis say ph of 7.0 wouldn't the alfentanil become even more unionized <11% and since it's pka is lower it would have a bigger effect on a patient than giving fentanyl? another argument is that at that ph you might get a delayed effect with the alfentanil and you might tend to push more and eventually overdose the patient. i believe acidosis would have a greater impact on drugs with a lower pka that are highly non-ionized than one like fentanyl. neither nagelhout or katzung touch on the affects of acidosis/alkalosis on opioids.


    thanks,
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  2. 3 Comments

  3. by   jwk
    Quote from soliant12
    fentanyl has a pka of 8.4 and is 91% ionized at a ph of 7.4. alfentanil has a pka of 6.5 and is 11% ionized at a ph of 7.4, and also has a lower volume of distribution than fentanyl. i believe alfentanil is more lipophillic but don't quote me on that. if a patient is in metabolic acidosis say ph of 7.0 wouldn't the alfentanil become even more unionized <11% and since it's pka is lower it would have a bigger effect on a patient than giving fentanyl? another argument is that at that ph you might get a delayed effect with the alfentanil and you might tend to push more and eventually overdose the patient. i believe acidosis would have a greater impact on drugs with a lower pka that are highly non-ionized than one like fentanyl. neither nagelhout or katzung touch on the affects of acidosis/alkalosis on opioids.


    thanks,
    first thought? with a ph of 7.0, selection of narcotics wouldn't be my biggest concern.
  4. by   soliant12
    Quote from jwk
    First thought? With a pH of 7.0, selection of narcotics wouldn't be my biggest concern.
    just a hypothetical number it is more due to the environment acidic/basic rather than strictly a pH number
  5. by   louloubell1
    Quote from jwk
    First thought? With a pH of 7.0, selection of narcotics wouldn't be my biggest concern.
    LOL... my thoughts too. No need for narcs in a dead person......

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