Half Life of Phenobarbital

The detailed description of half life of phenobarbital is discussed here. Thiopentone produces hypnosis within 30 to 40 seconds of intravenous injection. For the half life of phenobarbital , the time required for equilibration of the brain with the thiopental concentration in the blood (effect-site equilibration time) is rapid.

Recovery after a small dose is rapid, with some somnolence and retrograde amnesia.

Also for the half life of phenobarbital the, prompt recovery after a single IV dose of thiopental, reflects redistribution of this drug from the brain to inactive tissues. Skeletal muscles are the most prominent sites for initial redistribution of highly lipid soluble barbifurates such as thiopental.

Therefore, when skeletal muscle perfusion is reduced as in hypovolaemia or when skeletal muscle mass is decreased such as in elderly thiopental plasma concentrations are increased because of less dilution, resulting in the potential for exaggerated cerebral and cardiac depression.

Repeated intravenous doses lead to prolonged anesthesia because fatty tissues act as a reservoir; they accumulate thiopentone in concentrations 6 to 12 times greater than the plasma concentration, and then release the drug slowly to cause prolonged anesthesia.

Maximal deposition of thiopental in fat is present after about 2.5 hours, and this tissue becomes a potential reservoir for maintaining plasma concentrations of the drug, which leads to prolonged context-sensitive half life of phenobarbital .

Elimination of the drug from the body depends almost entirely on metabolism. Only < 1% of these drugs are recovered unchanged in urine. There is no pharmacokinetic difference of clinical significance exists between the R(+)- and S(-)thiopentone enantiomers.

The apparent volume of distribution of thiopental is 233 L. The elimination half-life is 11.5 h and the plasma clearance value is 150 mL/min.

The elimination half life of phenobarbital is shorter (6.1 +1- 3.3 hours) in infants and children, due solely to greater hepatic clearance. Thus, recovery time after large or repeated doses may be more rapid for infants and children than for adults because of the higher clearance.

The apparent volume of distribution in pregnant is 564 L +7- 343 and systemic plasma clearance is 0.286 L/min +7- 0.156, and elimination half life of phenobarbital (t112) is 26.1 h +7- 12.6.

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