Blood Gas Partition Coefficient

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 It is the ratio of the amount of anaesthetic agents present in the blood compared to gas when two phases are of equal volume and pressure and in equilibrium at 37 degrees Celsius. 

⇒ Also called as Ostwald coefficient of blood gas. 

⇒ It expresses the solubility of the agent in blood. 

⇒ The higher the blood/gas partition coefficient, the more the agent is soluble. It will take longer to raise the agent's partial pressure in the blood. 

Anaesthesia is related to the partial pressure of the anaesthetic agent at the effect site. 


↑ B/G partition coefficient: 

  • ↑ Duration Onset time  of anaesthesia 
  • ↑ Duration of Offset time of anaesthesia 


↓ B/G partition coefficient: 

  • ↓ Duration of Onset time
  • ↓ Duration of Offset time 


The blood gas partition coefficient of the volatile agents:

  • Halothane: 2.4 
  • Isoflurane: 1.4
  • Sevoflurane: 0.6 
  • Desflurane: 0.4 
Halothane is present in the blood 2.4 times more than that present in the alveoli.  

The concentration of anaesthetic agents in the blood comprises the portion that is not dissolved in the plasma and the portion that is dissolved in the plasma and bound to the plasma protein. The agents with higher partition coefficients bind more to the plasma protein and produce little partial pressure in the blood. 

High partition coefficient → higher lipophilicity → higher potency → higher solubility → more anaesthetic agents are in dissolved form → slower onset 


Source: 

Pharmacology for anaesthesia and intensive care, 4th edition 

Smith and Aitkenhead's Textbook of Anaesthesia, 7th edition 

Morgan and Mikhail's Clinical anesthesiology, 6th edition 

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