8 Dynamics of Cerebrospinal Fluid: From Theoretical Models to Clinical Applications
185
P ss is considered to be a constant parameter, determined by central venous
pressure. However, it is not certain that interaction between changes in CSF pressure
and P ss does not exist in all circumstances: in patients with idiopathic intracranial
hypertension, P ss is frequently elevated due to stenosis of transverse sinuses. Similar
situation can be seen in venous sinus thrombosis.
The coefficient R (symbol R CSF is also used) is termed the resistance to CSF
reabsorption or outflow (units: [mmHg/(mL/min)]).
Storage of CSF is proportional to the cerebrospinal compliance C (units:
[mL/mmHg]):
Storage = C ·
dp
dt
(8.3)
The compliance of the cerebrospinal space is inversely proportional to the
gradient of CSF pressure p and the reference pressure P o (8.4) [2]:
C =
1
E · (p − p o )
(8.4)
Some authors suggest that the relationship (8.4) is valid only above a certain
pressure level called the ‘optimal pressure’ [33]; however, this is still a point
of some dispute. The coefficient E is termed the cerebral elasticity (or elastance
coefficient) (units: [mL −1 ]). Elevated elasticity (>0.18 mL −1 ) signifies a poor
pressure-volume compensatory reserve [41]. This coefficient has been confirmed
to be useful in predicting response to third ventriculostomy [40]. Coefficient E is
inversely associated with the resistance to CSF outflow [41].
The reference pressure P o is a parameter of uncertain significance. Some authors
suggest that it is the pressure in the venous compartment and may be equal to P ss .
Others assume that this variable can be neglected [25].
The relationship (8.4) expresses a fundamental law of the cerebrospinal dynamic
compensation: When the CSF pressure increases, the compliance of the brain
decreases.
Combination of (8.1) with (8.2) and (8.4) gives a final Eq. (8.5):
1
E · (p − p o )
·
dp
dt
+
p − p b
R
= I (t)
(8.5)
where I(t) is the rate of external volume addition and P b is a baseline pressure.
The model described by this equation may be presented in the form of its electric
equivalent (Fig. 8.2) [31].
Equation (8.5) can be solved for various types of external volume additions I(t).
The most common in clinical practice is:
(a) A constant infusion of CSF (I(t) = 0 for t < 0 and I(t) = I inf for t > 0) (see
Fig. 8.3):
185
P ss is considered to be a constant parameter, determined by central venous
pressure. However, it is not certain that interaction between changes in CSF pressure
and P ss does not exist in all circumstances: in patients with idiopathic intracranial
hypertension, P ss is frequently elevated due to stenosis of transverse sinuses. Similar
situation can be seen in venous sinus thrombosis.
The coefficient R (symbol R CSF is also used) is termed the resistance to CSF
reabsorption or outflow (units: [mmHg/(mL/min)]).
Storage of CSF is proportional to the cerebrospinal compliance C (units:
[mL/mmHg]):
Storage = C ·
dp
dt
(8.3)
The compliance of the cerebrospinal space is inversely proportional to the
gradient of CSF pressure p and the reference pressure P o (8.4) [2]:
C =
1
E · (p − p o )
(8.4)
Some authors suggest that the relationship (8.4) is valid only above a certain
pressure level called the ‘optimal pressure’ [33]; however, this is still a point
of some dispute. The coefficient E is termed the cerebral elasticity (or elastance
coefficient) (units: [mL −1 ]). Elevated elasticity (>0.18 mL −1 ) signifies a poor
pressure-volume compensatory reserve [41]. This coefficient has been confirmed
to be useful in predicting response to third ventriculostomy [40]. Coefficient E is
inversely associated with the resistance to CSF outflow [41].
The reference pressure P o is a parameter of uncertain significance. Some authors
suggest that it is the pressure in the venous compartment and may be equal to P ss .
Others assume that this variable can be neglected [25].
The relationship (8.4) expresses a fundamental law of the cerebrospinal dynamic
compensation: When the CSF pressure increases, the compliance of the brain
decreases.
Combination of (8.1) with (8.2) and (8.4) gives a final Eq. (8.5):
1
E · (p − p o )
·
dp
dt
+
p − p b
R
= I (t)
(8.5)
where I(t) is the rate of external volume addition and P b is a baseline pressure.
The model described by this equation may be presented in the form of its electric
equivalent (Fig. 8.2) [31].
Equation (8.5) can be solved for various types of external volume additions I(t).
The most common in clinical practice is:
(a) A constant infusion of CSF (I(t) = 0 for t < 0 and I(t) = I inf for t > 0) (see
Fig. 8.3):