FRCA Notes

Speed of Onset

  • The speed of onset of an inhaled anaesthetic is dependent on factors which affects its alveolar partial pressure (and consequently the arterial and cerebral partial pressures)
  • Factors increased alveolar partial pressure will increased speed of onset

Inspired concentration

  • Higher inspired concentration (Fi) = more rapid rise in PAlveolar = faster onset
  • Factors affecting Fi include:
    • Concentration set on vaporiser
    • Fresh gas flow
    • Volume of breathing circuit
    • Amount absorbed by anaesthetic machine and breathing circuit

Alveolar minute ventilation

  • Higher alveolar ventilation results in a faster rise in PA and therefore in Parterial and PBrain
  • Alveolar minute ventilation is itself dependent on several factors, such as:
    • Ventilatory settings in the ventilated patient
    • Respiratory depressant effects of the volatile agent (and other drugs) in the spontaneously ventilating patient

FRC

  • A larger FRC will effectively dilute inspired concentration
  • Therefore there'll be a slower onset of anaesthesia with a larger FRC
  • Those patients with a small FRC have reduced volume to dilute the inspired agent, therefore PA rises rapidly and there is a fast onset of anaesthesia

Irritability

  • Desflurane and isoflurane are poor agents for gaseous induction due to their irritability

  • Cardiac output
    • A higher cardiac output maintains a concentration gradient between the alveolus and pulmonary capillaries
    • Therefore PA rises slowly, as agent is constantly being transported away in blood, and equilibration with Pa is also slower
    • Therefore a lower cardiac output favours a more rapid onset of anaesthesia and more rapid equilibrium
  • The myocardial depressant effect of the volatile agents therefore has a positive feedback effect on their speed of onset