Pharmacokinetics Pharmacokinetics of Inhalation of Inhalation Anesthetics Anesthetics Dr.Doaa Kamal Dr.Doaa Kamal Department of Anesthesia, Ain Department of Anesthesia, Ain Shams University Shams University Email adress: Email adress: [email protected][email protected]
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Pharmacokinetics of Inhalation Anesthetics Dr.Doaa Kamal Department of Anesthesia, Ain Shams University Email adress: [email protected].
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Pharmacokinetics of Pharmacokinetics of Inhalation AnestheticsInhalation Anesthetics
Dr.Doaa KamalDr.Doaa KamalDepartment of Anesthesia, Ain Department of Anesthesia, Ain Shams UniversityShams UniversityEmail adress: Email adress: [email protected]@hotmail.com
PharmacokineticsPharmacokinetics::The study of The study of AAbsorption.bsorption. DDistribution.istribution. MMetabolism.etabolism. EExcretion.xcretion.AimAim::
For a better understanding of the factors For a better understanding of the factors that governthat govern inductioninduction andand recoveryrecovery from from anesthesia.anesthesia.
KEY POINTsKEY POINTs::
►► Alveolar partial pressureAlveolar partial pressure (FA)(FA) governs the governs the partial pressure of volatile anesthetic in all body partial pressure of volatile anesthetic in all body tissues: all must approach and ultimately equal tissues: all must approach and ultimately equal alveolar partial pressure.alveolar partial pressure.
► ► EquilibrationEquilibration occurs occurs rapidlyrapidly between between alveolaralveolar & & brainbrain anesthetic partial pressure. anesthetic partial pressure.
► ► Therefore Therefore Alveolar concentrationAlveolar concentration is ultimately is ultimately the the principal factorprincipal factor in determining the in determining the onset of onset of actionaction..
►►DiffusionDiffusion occurs from a region of occurs from a region of higher higher concentrationconcentration to a region of to a region of lower lower concentrationconcentration (down a (down a concentration gradientconcentration gradient).).
(A) Factors Affecting (A) Factors Affecting InspiredInspired gas gas concentration concentration (Fi)(Fi)1)1) Fresh Gas Flow rateFresh Gas Flow rate:: ↑↑ FGF → ↑ speed of induction & recovery. FGF → ↑ speed of induction & recovery.
(If pt awakens in the middle of surgery on (If pt awakens in the middle of surgery on low flow → low flow → ↑↑ FGF). FGF).
2)2) VolumeVolume of breathing circuit of breathing circuit ((apparatus dead space): ): ↑ volume → ↑ volume → slower induction dt.dilution of anesthetic slower induction dt.dilution of anesthetic gases. (Eg. MRI breathing circuits = 3m, gases. (Eg. MRI breathing circuits = 3m, circuit priming for pediatrics, circuit priming circuit priming for pediatrics, circuit priming for single breath technique for sevoflurane).for single breath technique for sevoflurane).
3)3) AbsorptionAbsorption by the breathing circuit: by the breathing circuit: rubber tubing absorbs rubber tubing absorbs ˃˃ plastic & silicon plastic & silicon → → delays induction & recovery. Not used for pt delays induction & recovery. Not used for pt with suspected malignant hyperthermia with suspected malignant hyperthermia (release of trace anesthetic gases).(release of trace anesthetic gases).
3- Contraindication of N3- Contraindication of N22O in presence of O in presence of closed air spaces.closed air spaces.
(1) (1) Concentration Effect/ 2Concentration Effect/ 2ndnd Gas Effect Gas Effect
NN22OO diffuses rapidly from alveoli to diffuses rapidly from alveoli to blood much faster than blood much faster than NN22 diffuses out. diffuses out. This decreases the total volume of gas This decreases the total volume of gas in the alveolus → ↑ fractional in the alveolus → ↑ fractional concentration of other anesthetic concentration of other anesthetic agent.agent.
(2) (2) Diffusion HypoxiaDiffusion Hypoxia ((opposite the concentration effect)opposite the concentration effect)
►►At the end of anesthesia after discontinuation At the end of anesthesia after discontinuation of of NN22OO, , NN22OO diffuses from blood into the diffuses from blood into the alveoli much faster than alveoli much faster than NN22 diffuses from diffuses from alveoli into the blood.alveoli into the blood.
►►↑↑ Total volume of gas in the alveolus Total volume of gas in the alveolus →→ fractional concentration of gases in the alveoli fractional concentration of gases in the alveoli is diluted by is diluted by NN22OO →→ ↓ PaO↓ PaO22 & & PaCOPaCO22 → → hypoxia. This occurs in the first 5-10 mins of hypoxia. This occurs in the first 5-10 mins of recovery. Therefore it is advised to use 100% recovery. Therefore it is advised to use 100% OO22 after discontinuation of after discontinuation of NN22OO..
(3) (3) N2O is Contraindicated in Presence of Closed N2O is Contraindicated in Presence of Closed Air SpacesAir Spaces
The higher solubility of The higher solubility of NN22OO than than NN22 in the in the blood makes it contraindicated in the presence blood makes it contraindicated in the presence of closed air spaces:of closed air spaces: Intestinal obstructionIntestinal obstruction.. PneumothoraxPneumothorax.. PneumocephalusPneumocephalus.. Air embolismAir embolism.. TympanoplastyTympanoplasty..
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