Neurophysiology Flashcards
In newborns and
premature infants, values are lower than adults at 40–42 ml 100 g21 min2
From 6 months to 3 yr, CBF is thought to be 90 ml 100 g21 min21 and
from 3
to 12 yr at 100 ml 100 g21 min21
In children,
CMRO2 is higher at 5.2 ml 100 g21 min21
compared with 3.5 ml 100 g21 min21 in
adults.3
Neonates have a lower CMRO2 (2.3
ml 100 g21 min21) and a lower CBF, with a
relative tolerance of hypoxaemia.
Arterial PaCO2 has a potent vasodilatory effect
on cerebral blood vessels, leading to an
increase in CBF, which is linear between a
PaCO2 of 3.5 and 8 kPa. At birth, the cerebrovas-
cular response to changes in PaCO2 is incomple-
tely developed. Moderate hypocapnia has less
effect on the newborn brain than in adults and
CBF changes relatively little until severehypocapnia ensues
In adults, the cerebral vas-
culature is less sensitive to changes in PaO2;
CBF does not increase until PaO2 decreases
below 50 mm Hg, and then it increases expo-
nentially. In neonates, the CBF increases in
response to smaller decreases in PaO2
CBF is autoregulated in response to changes in
mean arterial pressure (MAP). Accurate values
for autoregulatory ranges in infants and chil-
dren are currently unavailable, but are probably
related to their normal MAP. Data from animal
and high-risk human neonate studies postulate
the lower limit for autoregulation to be an
MAP of 20– 40 mm Hg.4 Studies of extremely
low birth weight infants show that autoregula-
tion is functional in normotensive but not hypo-
tensive infants.5 Respiratory distress in infants
also leads to impaired autoregulation;2
increases in CBF outside of autoregulation may
contribute to their susceptibility to intraventri-
cular haemorrhage (IVH
In the
infant, before cranial suture fusion, decompres-
sion can occur through an increase in skull
size. The posterior fontanelle closes at about 6
months of age, the anterior fontanelle at around
1 yr–18 months, and final cranial suture
closure may be as late as 10 yr old. Increases
in intracranial volume can only be accommo-
dated if the change is gradual. Acute increases,
such as after traumatic brain injury, will still
result in raised ICP as in adults
