Cerebral blood flow is does not depend on? (AIIMS MAY 2008)
|A||Cerebral Metabolic rate|
a.For adequate function of brain, a continuous blood flow is required.
b.Normal cerebral blood flow is 15% of resting cardiac output (750ml /min) or about 50- 55 ml blood /100gm tissue / min.
c.50 - 70 % of CSF is formed in the choroid plexuses and the remainder is formed along the ventricular walls and the blood vessels.
d.VOLUME - 150 ml
e.Rate of CSF production is 550ml / day.
f.The composition of brain CSF is the same as that of brain E.C.F
g.Interruption of CSF for 5-10 seconds causes loss of consciousness and circulatory arrest for only 3-4 min results in irreversible brain damage.
h.Cerebral perfusion pressure is defined as mean systemic arterial pressure minus intra cranial pressure.
i.Option (3) cerebral autoregulation & effect of B.P
j.If systemic B.P drops cerebral perfusion is preserved through - vasodilatation of arterioles of brain; likewise arteriolar vasoconstriction occurs at high systemic pressure to prevent hyper perfusion. But cerebral autoregulation is effective over a range of 80 -180 mm Hg only.
k.Option (2) CBF & effect of pH & CO2
l.CBF increases with hypercapnia & acidosis and decreases with hypocapnia & alkalosis. This forms the basis of use of hyperventilation to lower intra cranial pressure.
Note: Metabolic acid base imbalances produce opposite changes in CSF.
i.Respiratory acid base imbalances (change in arterial CO2 tension) produce parallel changes in CSF
Option (1) CBF & cerebral metabolic rate
ii.Cerebral autoregulation may be disordered focally &unpredictably in disease states such as traumatic brain injury and severe focal cerebral ischemia.
iii.Since about option (3), (2) & (1) it is clearly mentioned in HARRISON'S; the best answer of exclusion should be option (4).