Shunt Valves — How They Work

Educational reference only. Device behaviour depends on the exact model, configuration, catheters and patient. Confirm against the implant record, the current IFU and your attending before changing any setting.

The 60-second version

• A shunt valve is a one-way resistor: it opens when the pressure difference across it (inlet minus outlet) exceeds its opening pressure, and the faster CSF flows, the higher that pressure climbs.
• Lying down, that difference is roughly ventricular pressure minus distal-cavity pressure.
• Sitting or standing, the column of CSF in the distal catheter adds suction — the siphon effect — so a valve that is right for lying can overdrain upright.
• Programmable valves let you change the opening pressure through the skin. Siphon-control and gravitational devices add resistance only when it is needed upright.
• Valves are labelled in mmH₂O (Codman, Medtronic, Sophysa) or cmH₂O (Miethke); ICP monitors read in mmHg. 1 mmHg ≈ 13.6 mmH₂O.

1. Pressure units

Manufacturer shorthand: Miethke uses 1 mmHg ≈ 1.36 cmH₂O (and 1 cmH₂O ≈ 0.74 mmHg); Sophysa states 1 mmH₂O = 9.807 Pa = 0.074 mmHg.

Use the quick unit converter for any other value.

2. What “opening pressure” really means

It is a pressure-flow curve, not a single number. Each valve passes more CSF as the pressure difference rises, so manufacturers quote the pressure at a reference flow — and they do not all use the same one:

Catheters add resistance. Integra’s L-P IFU shows the gap: a HAKIM valve set at 30 mmH₂O measured 61 mmH₂O with a 120 cm peritoneal catheter at 20 mL/h, and setting 200 measured 229. Sophysa labels the valve alone and notes the catheters add their own resistance; Medtronic’s small-lumen peritoneal catheter limits flow about six-fold versus a standard one at 20 mL/h.

So “the same number” on two brands is only approximately the same pressure — which is why the converter reports the closest setting, the difference, and the next setting above and below.

3. Posture, hydrostatics and the siphon effect

Miethke’s IFUs give the cleanest model. With P_L the valve opening pressure lying, P_S the opening pressure upright, P_H the hydrostatic column from the ventricles (foramen of Monro) down to the distal cavity, and P_B the abdominal pressure:

• Lying: IVP = P_L + P_B
• Upright: IVP = P_H − P_S − P_B — i.e., the hydrostatic column pulls CSF out unless the valve’s upright opening pressure nearly matches it.

In an upright adult the column is tens of centimetres (42 cm in Miethke’s worked example), far above any lying setting — so a differential-pressure valve alone lets IVP fall well below zero when the patient stands: overdrainage, with slit ventricles, low-pressure headache, hygroma or subdural haematoma. Raising the setting to stop it simply under-drains the patient when lying. The fix is a device that adds resistance only upright, which is what the next two sections are about.

Medtronic simulates the upright patient by testing at −50 cmH₂O hydrostatic pressure; Miethke aims for a slightly negative standing IVP (the DualSwitch valve targets −5 to +5 cmH₂O).

4. Valve mechanisms

How programmable valves change setting: HAKIM — a codified magnetic signal drives a stepper motor that turns a cam under the spring. CERTAS Plus — a magnetically rotated construct. Strata — a magnet-driven mechanism read with hand tools or StrataVarius. Polaris — a rotor held by a magnetic lock (two opposed micro-magnets a one-directional field cannot release). Sophy — a magnetic rotor with no lock. Miethke — a rotor held by a brake that releases only when the adjustment tool presses on the valve (M.blue adds an “Active-Lock”).

5. Siphon-control (“anti-siphon”) devices

These respond to the suction or flow surge of upright posture rather than to posture itself.

• Medtronic Delta chamber — a normally closed chamber that opens to positive ventricular pressure but stays closed to negative distal pressure. Built into the Delta valve, Strata / Strata II and (as a siphon-control diaphragm) StrataMR II. Example: Strata II PL 1.5 is 70–90 mmH₂O lying but 85–105 mmH₂O at −50 cmH₂O hydrostatic.
• Codman SiphonGuard — flow-activated: a sudden surge closes the central ball-and-cone and diverts all CSF through a long spiral secondary path (resistance 1 cmH₂O per mL/h); when flow falls it reopens, and flow never fully stops. It does not activate at low flow, works in any orientation distal to the valve, and should be primed slowly (~0.5 mL/min). Integrated in some HAKIM, HAKIM Precision and CERTAS Plus models.
• Strata NSC (“no siphon control”) is the same adjustable mechanism without the Delta chamber — its upper levels are much higher (PL 2.5 ≈ 200–220 vs 135–155 mmH₂O on Strata II).

6. Gravitational (“anti-gravity”) units — built-in and add-on

Principle: a heavy tantalum ball rests on a ball-and-cone seat. Lying flat, the ball rolls off the seat and adds no resistance; upright, its weight must be lifted, adding a fixed opening pressure. Intermediate angles add a graded amount. They must be implanted parallel to the body axis.

Is the M.blue an attachment? No — the M.blue is a complete valve whose gravitational unit is the adjustable part. The true attachments are the SHUNTASSISTANT family, proSA, SiphonX and the Integra GCA, which you add in series to a separate differential-pressure valve.

Total upright opening pressure of a gravitational system = DP unit + gravitational unit. Example (Miethke): DP set to 10 cmH₂O with a 25 cmH₂O gravitational unit → 10 cmH₂O lying, 35 cmH₂O upright.

7. MRI and programmable valves

Fixed valves have nothing to reprogram, but every valve makes some artifact; the non-metallic Medtronic Delta and CSF-Flow Control valves make essentially none. Outside the scanner, strong everyday magnets can also move a setting — Integra warns that magnets over 80 gauss (household or loudspeaker magnets) near a HAKIM valve can change it, while Miethke’s braked rotors and Sophysa’s Polaris lock are designed to resist them.

8. Adjusting a setting safely — manufacturer rules

• Identify the valve first. If a patient arrives from elsewhere, get a skull film and identify the model before anyone tries to adjust it (X-ray guide).
• Codman HAKIM: while becoming familiar, change by no more than ±40 mmH₂O per 24 h and monitor for 24 h; X-ray to confirm. Keep ≤1 cm of tissue over the mechanism. An inverted valve (white marker on the left) is programmed by first setting 200, then 210 − the desired pressure (190 and 200 cannot be reached).
• Miethke proGAV 2.0 / proSA: change by ≤8 cmH₂O per adjustment step — the rotor takes the short way round, so 3 → 18 must go 3 → 11 → 18. Verify with the compass after every step; postoperative swelling can make the first days difficult. Vibration (e.g., jogging) can transiently lower the opening pressure by 25–35% (laboratory data).
• Codman CERTAS Plus: tissue over the valve <10 mm; the electronic tool kit must not enter the MR suite; a safety stop sits between settings 1 and 8.
• Sophysa Polaris / Sophy: the locator’s pressure range must match the valve’s (range dots on X-ray); clockwise raises, counter-clockwise lowers.
• Medtronic Strata: the StrataVarius gives a numeric readout (e.g., PL 2.0 = 115 mmH₂O); the Strata NSC LP valve can only be adjusted with StrataVarius.
• Reservoirs: frequent pumping can itself overdrain (Miethke).
• Document manufacturer, model, setting, siphon/gravitational components and distal site on every note.

9. Manufacturer starting-pressure recommendations (non-binding)

These are the manufacturers’ own suggestions — not clinical guidelines — and Miethke stresses they must be individualised (lower the gravitational unit for low mobility or high BMI; no gravitational unit for bedridden patients).

M.blue (DP unit 5 cmH₂O for all; upright total in brackets):

proGAV: DP 5 cmH₂O standard for NPH; gravitational unit 20 cmH₂O (children ≤5 y, adults >60 y) or 25 (children >5 y, adults ≤60 y); −5 if shorter than 1.60 m, +5 if taller than 1.80 m.

paediGAV: <6 months 4/24; 6 months–5 years 9/24; >5 years 9/29 cmH₂O (lying/upright).

Sizing an add-on gravitational unit (Miethke method): choose the unit so that DP valve + gravitational unit ≈ (diaphragm-to-foramen-of-Monro distance) − 5 cm. Worked example from the IFU: 42 cm distance, DP valve 13 cmH₂O → 42 − 13 − 5 = 24 → select the 25 cmH₂O unit → standing IVP ≈ 25 + 13 − 42 = −4 cmH₂O. The DualSwitch valve uses the same distance-minus-5-cm rule for its upright chamber.

10. Recalls and safety notices

• Medtronic Strata II & Strata NSC — FDA Class 2 recall Z-1121-2017 (Jan–Jun 2017): reports of the set performance level disagreeing with the X-ray reading; the IFU warning was updated. FDA record
• Medtronic StrataMR (1st generation) — FDA Class 1 recall Z-1702-2017 (Feb–Sep 2017): locator-tool misalignment could set the rotor wrong, giving higher opening pressure and underdrainage. FDA record

11. Using the converter well

1. Choose the source valve and setting (or type a pressure) and the posture you care about.
2. Read the result in all four units, plus the lying and upright opening pressures for that setting.
3. Look at the closest equivalent on the target valve and its neighbours above and below — then pick using clinical context, not just the smallest difference.
4. Remember the caveats: different reference flows, ±10–35 mmH₂O tolerances, catheter resistance, and fundamentally different siphon and gravity behaviour.

Open the converter →

Related on Pterion Prep

• Reference: CSF shunt systems, types and valves — shunt configurations, malfunction workup, complications
• VP shunt · Shunt revision · Lumboperitoneal shunt

Key literature

• Drake JM, et al. Randomized trial of cerebrospinal fluid shunt valve design in pediatric hydrocephalus. Neurosurgery 1998. PubMed
• Pollack IF, et al. (Hakim-Medos Investigator Group). A randomized, controlled study of a programmable shunt valve versus a conventional valve. Neurosurgery 1999. PubMed
• Lemcke J, et al. Safety and efficacy of gravitational shunt valves in patients with idiopathic normal pressure hydrocephalus (SVASONA). J Neurol Neurosurg Psychiatry 2013. PubMed
• Huang AP, et al. Antisiphon device: a review of existing mechanisms and clinical applications to prevent overdrainage in shunted hydrocephalic patients. Biomed J 2022. PubMed
• Baird LC, et al. Pediatric hydrocephalus guidelines, Part 5: effect of valve type on cerebrospinal fluid shunt efficacy. J Neurosurg Pediatr 2014. PubMed
• Valve identification on X-ray (open access, the source of this site’s radiographs): Rhomberg T, et al. Acta Neurochir 2024 (doi); Vach M, et al. Clin Neuroradiol 2025 (doi); Giancardo L, et al. Sci Rep 2018 (doi).

Manufacturer sources (instructions for use, brochures, FDA 510(k) and recall records) are linked on every card in the valve library.