2018A Question 06

Describe the washout (not offset) of sevoflurane from a patient following two hours of general anaesthesia. You may wish to use a graph to illustrate the description.

Examiner Report

54% of candidates achieved a pass in this question.

To achieve a pass in this question the following were required :

• A clear explanation of the elimination kinetics of Sevoflurane with due consideration to a time course appropriate to the termination of anaesthesia after two hours
• A washout graph including appropriate axis labelling, a multi exponential decay curve, time course and half times consistent with volatile washout
  It was very difficult to pass the question without a washout graph. Wash-in graphs did not score points.
• A description of the movement of Sevoflurane out of the alveolus, from plasma to alveolus, from the various compartments to plasma and their respective time courses and a description of factors which underpin those kinetics; including depth and duration of anaesthesia, alveolar ventilation and fresh gas flows, compartmental blood flows and solubilities

Common errors included:

• Confused graphs with no or inappropriate time courses indicated, nonsensical axis labelling or graphing of wash-in and wash-out on the same graph
• Detailed descriptions of Fick’s Law which wasted time and did not score points
• An overemphasis on the role of cardiac output in washout and incorrect assertions that increased cardiac output speeds washout
• Conflation of washout and awakening from anaesthetic as the same thing
• The incorrect assertion that following two hours the fat is completely saturated with Sevoflurane
• The confusion of partial pressures, concentrations and amounts

Model Answer

Structure:

• Emergence modeling
• Final end-tidal partial pressure
• Elimination from V₁

Emergence Modeling

Washout time is a function of:

• Final partial pressure in effect site prior to washout
• Rate of elimination from V₁

Compartment saturation at two hours:

• Equilibration time constants:
  • τ= 1/k
  • τ= (V x λ) / Q
  • Where: (V = volume of tissue, λ = tissue: Blood partition coefficient, Q = blood flow)

Final Effect-Site Partial Pressure

Effect site partial pressure:

• Correlates with that in V₁
• May not be included in all kinetic models
• Rate of equilibration between effect site and V₁ is:
  • $\propto$ CBF (per unit mass, not percentage CO to brain)
  • $\propto {1 \over Brain-blood \ partition \ coefficient}$

Elimination from V₁