As the arterial PaCO₂ rises, the V_min increases in an attempt to bring it back to the normal range. The lower end of the curve flattens out because automatic firing from the respiratory centre maintains a minimum V_min (Fig 1a).

Question: What is the effect of opioids or general anaesthetic agents on this curve?

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As the arterial PaCO₂ rises, the V_min increases in an attempt to bring it back to the normal range. The lower end of the curve flattens out because automatic firing from the respiratory centre maintains a minimum V_min (Fig 1a).

Question: What is the effect of opioids or general anaesthetic agents on this curve?

Answer: Drugs that depress the CNS make the chemoreceptors less sensitive to PaCO₂, moving the curve to the right and making it less steep (dashed line) (Fig 1b).