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GB
PHySICS BaCKGROund
with the body in a horizontal position, the
intraventricular pressure in healthy individuals
is positive. In order to regulate this pressure
through a differential-pressure valve, only the
pressure in the abdominal cavity needs to be
taken into account.
In standing position, the ventricular pressure
should be slightly negative. To adjust this pres-
sure through valve drainage, the paediSHUNT-
ASSISTANT/SHUNTASSISTANT is implanted
in addition to a differential-pressure valve. Only
in this way the hydrostatic pressure minus the
pressure in the abdominal cavity can be com-
pensated for.
IVP
Intraventricular pressure
PVli
Opening pressure in horizontal position
(adjustable unit only)
PVst
Opening pressure in vertical position
(adjustable + gravitational unit)
PB
Pressure in the abdominal cavity
PHyd
Hydrostatic pressure
horizontal:
IVP = PVli + PB
standing:
IVP = PHyd - PVst - PB
IVP
Fig. 2: Calculating the intraventricular pressure
14
funCTIOn Of THE valvE
with
the
paediSHUNTASSISTANT/SHUN-
TASSISTANT in horizontal position, the valve
sealing ball (3) and the weight-ball (2) are not in
the valve seat. hence, in this configuration the
paediSHUNTASSISTANT/SHUNTASSISTANT
is open, and the force brought to bear by the
weight of the ball does not counteract the fluid
pressure. The ventricular pressure is now con-
trolled exclusively by the differential-pressure
valve installed.
Fig. 3: horizontal position of the
paediSHUNTASSISTANT/SHUNTASSISTANT
IVP
P Hyd
P Vst
P Vli
P B
P B
In the vertical position, the paediSHUNTASSIS-
TANT/SHUNTASSISTANT and the differential
pressure valve work together. If the pressure at
the ball seat exceeds the force exerted by the
weight of the tantalum ball, the valve sealing ball
and weight ball are raised, thereby allowing the
lateral flow towards distal of fluid through the
outlet port. This prevents any further increase
in pressure. In order to achieve optimal pres-
sure regulation for each individual patient, the
SHUNTASSISTANT is available with five diffe-
rent pressure ratings, in addition to the paedi-
SHUNTASSISTANT, which was developed es-
pecially for pediatric hydrocephalus.
a)
b)
Fig. 4: vertical position of the paediSHUNTASSIS-
TANT/ SHUNTASSISTANT a) closed b) open
SElECTInG THE aPPROPRIaTE valvE
The choice of an appropriate paediSHUNTAS-
SISTANT/SHUNTASSISTANT is determined by
the ventricular pressure that needs to be achie-
ved for the standing position. The sum total of
the opening pressures of the differential pres-
sure valve and the paediSHUNTASSISTANT/
SHUNTASSISTANT
used
determines
ventricular pressure for the patient in standing
position.
InstructIons for use |
It is recommended that the paediSHUNT-
ASSISTANT/SHUNTASSISTANT is selected for
which the sum total of the opening pressure
of
the
paediSHUNTASSISTANT/SHUNTAS-
SISTANT and of the differential pressure valve
– measured in terms of water column height
– equals aproximately 5 cm less than the di-
stance from the peritoneum (diaphragm) to the
third ventricle (foramen Monroi).
Suggested procedure (example):
1) Measure the distance between the peritone-
um (diaphragm) and the third ventricle (fora-
men Monroi) (e.g. 42 cm).
2) Select the differential pressure valve to be
implanted as a supplement to the paediSHUN-
TASSISTANT/SHUNTASSISTANT,
a reading of the valve pressure rating (e.g. 13
cmh
O). If that value is available in mmhg only,
2
the pressure has to be converted into cmh
1 mmhg = 1.36 cmh
O(e.g. 9.6 mmhg =13
2
cmh
O)
2
3) Subtract the pressure of the differential pres-
sure valve from the distance measured between
the peritoneum and the third ventricle (e.g. 42
cmh
O - 13 cmh
O = 29 cmh
O)
2
2
2
4) Subtract 5 cmh
O from the computed figure
2
(e.g. 29 cmh
O - 5 cmh
O = 24 cmh
2
2
5)
choose
the
paediSHUNTASSISTANT/
SHUNTASSISTAN specified for the pressure ra-
ting closest to this value. for the example given
here, the 25 cmh
O pressure rating would be
2
selected. The ventricular pressure in standing
position would then be:
25 cmh
O +13 cmh
O - 42 cmh
O = -4 cmh
2
2
2
thus inside the physiological range. conflicting
statements are found in the literature as to what
constitutes an ideal ventricular pressure in hy-
drocephalus patients in a standing position. In
the example above, the assumption was made
that a slightly negative pressure comes clo-
sest to fulfilling the patient's needs. A definite
standard value cannot be given here, however.
Applicable values will vary according to patient
and anamnesis.
the
GB
and
take
O.
2
O).
2
O
2
15
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