SPEED OF INDUCTION & RECOVERY

SPEED OF INDUCTION & RECOVERY


Introduction:

  • Induction & recovery determined by alveolar concentration.
  • Alveolar concentration related to B: G coefficient.

Blood: gas partition coefficient (B: G coefficient):

  • Measure of agent solubility in blood.
  • Low blood solubility (low B:G coefficient) – High concentration in alveolar air.
  • Diffuse less via alveolar capillary membrane due to low blood solubility.
Relationship between alveolar conc., b: G coefficient & induction-recovery:
  • As induction & recovery determined by alveolar concentration.
  • Faster induction & recovery in agents with less B: G partition coefficient.
  • Slower induction & recovery in agents with high B: G partition coefficient.

Examples:

  • Desflurane – Minimum B: G partition coefficient (least blood solubility) – Fastest onset & recovery.
  • Methoxyflurane – Maximum B: G partition coefficient (maximum blood solubility) – Slowest onset & recovery.
  • Speed of onset & recovery in decreasing order(means increasing order of B: G partition coefficient & blood solubility):
  • Desflurane (0.42) > Cyclopropane (0.44) > Nitrous oxide (0.47)Sevoflurane (0.69) > Isoflurane (1.38) > Enflurane (1.8) > Halothane (2.4) > Cholorform (8) > Trilene (9) > Ether (12)Methoxyflurane (15)

Diffusion coefficient:

  • Diffusivity: Ability to be mobile within substance of a membrane.
  • Expressed as a “diffusion coefficient”.
  • Low diffusion coefficient (low diffusion capacity) = Fast induction & recovery.
  • High diffusion coefficient = Slow induction & recovery.
  • Desflurane – Minimum diffusion coefficient.
  • Methoxyflurane = Maximum diffusion coefficient.
Diffusion coefficient in recovery:
  • During recovery, reverse occurs.
  • Eg: N20 with low blood solubility –> rapidly diffuses into alveoli.
  • Dilutes alveolar air -> partial pressure of alveolar oxygen reduced –> Hypoxia.
  • Referred “Diffusion hypoxia” (Fink effect).
Diffusion hypoxia:
  • Occurs in 1st 5-10 mins of recovery.
  • Seen with N2O & xenon.
  • Prevented by continuing 100% O2 inhalation for few mins after discontinuing N2O.
  • Not to be directly switched over to air.

Exam Important

  • Induction & recovery determined by alveolar concentration.
  • Blood: gas partition coefficient (B: G coefficient) is measure of agent solubility in blood.
  • Faster induction & recovery in agents with less B: G partition coefficient.
  • Slower induction & recovery in agents with high B: G partition coefficient.
  • Desflurane with minimum B: G partition coefficient & least blood solubility has fastest onset & recovery.
  • Methoxyflurane with maximum B: G partition coefficient & maximum blood solubility has slowest onset & recovery.
  • Speed of onset & recovery in decreasing order means increasing order of B: G partition coefficient & blood solubility.
  • Desflurane (0.42) >Sevoflurane (0.69) >  Methoxyflurane (15).
  • Desflurane has minimum diffusion coefficient.
  • Methoxyflurane has maximum diffusion coefficient.
  • Diffusion hypoxia is in accordance with “Fink effect”.
  • Diffusion hypoxia occurs in 1st 5-10 mins of recovery.
  • Diffusion hypoxia is seen with N2O & xenon.
  • By continuing 100% O2 inhalation for few mins after discontinuing N2O prevents diffusion hypoxia.
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