Page 169 - Edited - Webster HEAD AND NECK - part 1

HN 139

INTRACRANIAL ARTERIAL AND VENOUS PRESSURES

VENOUS PRESSURE ABOUT -45cms.blood/-35mmHg
MEAN ARTERIAL PRESSURE ABOUT 80cms.blood/60mm.Hg
These values are obtained by subtracting the pressure of a column of
blood 50cms. high from the venous & arterial pressures at the bottom
of the column i.e. at the sternal plane. (50cms blood = 35mm Hg).
(SEE Year 1 Notes: “Blood Vascular & Lymphatic Systems” pp 4&5,
ABOUT 50 cms and Fig.7.)
If we treat the arterial and venous systems above the heart as simple
manometers, then at the highest point VERTICALLY above the heart
the pressure will be REDUCED by the pressure of a column of blood
of equivalent VERTICAL height. At the sternal plane (S12,SI133),
venous & mean arterial pressures are known, & the vertical height
from this reference plane can be measured. Remember that the
pressure of a 13.6CMS column of blood is the same as that of a
column of mercury only 1MM high.
Notice that intracranial venous pressure is about 35mm Hg/45cms
blood BELOW atmospheric. These effects are, of course, reduced by
inclining the body from the vertical, and are completely abolished by
lying horizontal when the VERTICAL height is zero.

STERNAL VENOUS PRESSURE ABOUT 5cms.blood/ 3.5mmHg
PLANE MEAN ARTERIAL PRESSURE ABOUT 130cms.blood/95mm.Hg

NOTES:

These phenomena have important corollaries

1. The pressure of cerebrospinal fluid is positive: normally ~10 cms fluid/7.5 mm Hg. The pressure
differential between the subarachnoid space and intracranial venous pressure is essential for re-
absorption of CSF arachnoid villi (HN122; Neuro notes, Vol. I, pp. 12-13).

2. Thin-walled cerebral veins are partially collapsed, because they cannot significantly resist this
pressure differential. Restricting venous outflow by pressing on even one internal jugular vein raises
intracranial venous pressure sufficiently to negate this effect: the intracranial veins expand, and this
results in an increase in CSF pressure. This is the basis of the Queckenstedt’s test for investigating the
continuity between the intracranial and spinal parts of the subarachnoid space (Neuro notes, Vol.
I, .11).

3. The walls of the intracranial venous sinuses are supported by dura mater (HN122) which resists
collapse of the sinuses. If a venous sinus is pierced and opened to the atmosphere (whether by
criminal or accidental violence, or by an unfortunate neurosurgeon), the defect remains patent: if the
subject is upright, air is forced into the venous system (“Blood Vascular & Lymphatic Systems”, Fig.
6A). Such an embolism is rapidly fatal.

4. There is a critical mean arterial pressure in the brain’s arterial tree (~40 mm Hg) necessary to
maintain blood flow compatible with brain function. Since in the upright position (standing or
sitting) intracranial mean arterial pressure is ~35 mm Hg below that in the aortic arch, the critical
intracranial arterial mean pressure for maintaining adequate cerebral perfusion is therefore reached
when the mean systemic blood pressure in the aortic arch is ~75 mm Hg (corresponding to a reading
of e.g. 105 /60) i.e. ~35 mm Hg above this critical level. Fainting ensues, and eliminates the disparity
because the subject becomes horizontal.

5. The effects on the arterial intracranial pressure are exacerbated by arterial narrowing, including
permanent pathology and temporary changes occasioned in the vertebral arteries by turning the head
(HNN 121), or over-treatment of hypertension (see Neuro notes, Vol. I, pp 179-180 for further
discussion).

K.E.W.

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