Using a diagram, explain the effect of PaO2, PaCO2 and MAP (mean arterial pressure) on cerebral blood flow (60% marks). Outline the effects of propofol and ketamine on cerebral blood flow (CBF), cerebral metabolic requirement for oxygen (CMRO2), and cerebral venous oxygen saturation (40% marks).
Graphical depictions of the effect of Mean Arterial Pressure, oxygen tension and carbon dioxide tension on cerebral blood flow were common and in general accurate. Mention of factors that affected, and regulation of, the MAP vs CBF graph was expected in order to pass this question well.
The effect of propofol and ketamine on the CBF was well answered. Propofol and ketamine have an opposite effect on cerebral haemodynamic and metabolic rate. Propofol produces a dose dependent reduction in CBF with proportionate reduction in CMRO2, and thus a minimal change in cerebral venous O2 Sat. Propofol doesn’t affect the autoregulatory curve of CBF and the PaCO2 response. Ketamine produce a dose dependent increase in CBF and a mild increase in CMRO2.
Recommended sources: Guyton Textbook of Medical Physiology Chp 61; Goodman and Gilman
The pharmacological basis of therapeutics 11th edition pgs 350-352.
With any luck, this graph is "in general accurate". It attempts to bring all of the factors onto one graphical field, mindful of the fact that they are only related by the fact that they are measured in mmHg
It would be uncontroversial to say that propofol decreases, and ketamine increases, cerebral blood flow. The question on the effects of propofol and ketamine on cerebral metabolism is more delicate, mainly because most textbooks will still tell you that ketamine increases CMRO2. For example, this is what you will find in the 23rd edition of Katzung (p. 456). However, modern human data (eg. Långsjö et al, 2003) suggests that this is not a real effect, and that ketamine either has no effect on cerebral metabolic rate, or only a mild and regional effect (mainly increasing CMRO2 in the frontal lobe and limbic system). Unfortunately, question writers base these questions on textbooks (often, textbooks from their own distant youth), which means you really have to give the wrong answer here to score any marks. As always, the basic principle remains: don't start an argument with the examiner.
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