Barbiturates Flashcards
What are the chemical origins of barbiturates?
Barbiturates are derived from barbituric acid. (Barbituric acid is
formed from the condensation of urea and malonic acid.)
Stoelting RK, Hillier SC. Pharmacology and Physiology in
Anesthetic Practice. Philadelphia, PA: Lippincott Williams and
Wilkins; 2006: 127.
To what receptor do barbiturates bind with to exert
their clinical action and where is this receptor
located?
Barbiturates bind with the GABA-A receptor found in neurons in
the central nervous system.
Katzung BG, Masters, SB, & Trevor AJ. Basic and Clinical
Pharmacology. 12th ed. New York: McGraw-Hill; 2012: 378.
What chemically defines the oxybarbiturates? What
are some examples of oxybarbiturates?
Oxybarbiturates are known as true barbiturates and have an
oxygen at the second carbon atom. Examples of
oxybarbiturates include pentobarbital, methohexital,
secobarbital, and phenobarbital.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 176.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed.
Philadelphia: Elsevier Saunders; 2011: 103.
What chemically defines the thiobarbiturates? What is an example of this class of drug?
The thiobarbiturates replace the oxygen at the second carbon
atom with a sulfur atom. Thiopental is an example of a
thiobarbiturate.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed.
Philadelphia: Elsevier Saunders; 2011: 176.
Are barbiturates weak acids or weak bases?
They are weak acids and are prepared as sodium salts.
Stoelting RK, Hillier SC. Pharmacology and Physiology in
Anesthetic Practice. Philadelphia, PA: Lippincott Williams and
Wilkins; 2006: 127-130
What chemically defines the methylated oxybarbiturates? What is an example of this class of drug?
The methylated oxybarbiturates retain the oxygen at the second
carbon atom, but have a methyl group on the first carbon atom.
Methohexital is a methylated oxybarbiturate.
Stoelting RK, Hillier SC. Pharmacology and Physiology in
Anesthetic Practice. Philadelphia, PA: Lippincott Williams and
Wilkins; 2006: 127.
How do barbiturates affect cerebral blood flow and
metabolism?
Barbiturates reduce cerebral metabolic rate, and reduce
cerebral blood flow (by INCREASING cerebrovascular
resistance).
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 583.
How do barbiturates produce an inverse steal
phenomenon in the brain?
Barbiturates produce vasoconstriction only in normal cerebral
tissue resulting in preferential blood flow to ischemic areas
while simultaneously reducing total cerebral blood flow. This is
referred to as the Robin Hood effect or inverse steal
phenomenon.
Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s
Clinical Anesthesiology. 5th ed. New York, NY: McGraw-Hill;
2013: 583
What is the pKa of thiopental?
The pKa of thiopental is 7.6.
Stoelting RK, Hillier SC. Pharmacology and Physiology in
Anesthetic Practice. Philadelphia, PA: Lippincott Williams and
Wilkins; 2006: 129-130.
Does thiopental provide analgesia?
No. In fact, in low doses, thiopental has proven to produce
hyperalgesia.
Ouellette RG, Joyce JA.Pharmacology for Nurse
Anesthesiology. Sudbury, MA: Jones and Bartlett Learning;
2011: 43.
Can thiopental be reconstituted with Lactated
Ringer’s solution? Why or why not?
Barbiturates must be reconstituted with sterile saline or sterile
water. Even though barbiturates are weak acids, they are
prepared with anhydrous sodium carbonate which makes the
powder alkaline. If Lactated Ringer’s solution or any other
acidic solution is used, the thiopental will precipitate out of the
solution.
Miller RD, Pardo MC. Basics of Anesthesia. 6th ed.
Philadelphia: Elsevier Saunders; 2011: 103.
Can thiopental cross the placental barrier?
Yes. Thiopental is extremely lipid-soluble and can easily cross
the blood-brain barrier or the placental barrier.
Stoelting RK, Hillier SC. Pharmacology and Physiology in
Anesthetic Practice. Philadelphia, PA: Lippincott Williams and
Wilkins; 2006: 130, 136.
What accounts for the short duration of action of
thiopental? What part does metabolism play?
The short duration of action of thiopental is attributed to the
rapid redistribution away from the brain and back into the
systemic circulation. Metabolism does not play a significant role
in the duration of action of a single dose of thiopental.
Ouellette RG, Joyce JA.Pharmacology for Nurse
Anesthesiology. Sudbury, MA: Jones and Bartlett Learning;
2011: 43.
What are the cardiac effects of an induction dose of
thiopental and what is responsible for these effects?
Thiopental produces dose-dependent decreases in stroke
volume, cardiac output, and arterial blood pressure. This is
mainly due to increased venous capacitance and myocardial
depression.
Longnecker DE, Newman MF, Brown DL, Zapol WM.
Anesthesiology. 2nd ed. New York: McGraw-Hill; 2012: 694-
695.
Thiopental does not undergo significant elimination
via the kidneys. Why is this?
Only about 1% of thiopental is eliminated unchanged by the
kidneys. The primary reason for this is because thiopental is
80% protein-bound and the protein-drug complex is not easily
filtered at the glomerulus.
Ouellette RG, Joyce JA.Pharmacology for Nurse
Anesthesiology. Sudbury, MA: Jones and Bartlett Learning;
2011: 43.
