Shunt systems

shunt systems

1. Shunt Systems Dr SANJOG CHANDANA (MIND)

2. Introduction Insertion of  cerebrospinal fluid  (CSF) devices for the management of hydrocephalus is one of the most common procedures performed in neurosurgery. Many CSF shunt components are commercially available There is no consensus which devices are the best for a given indication. No single shunt or catheter design is suitable for all patients.

3. History 1890s, J. Miculicz developed a gold, flanged hollow tube that diverted CSF from the ventricle to the subgaleal space, but this valveless device was only rarely effective. 1914, Heile described the first diversion of CSF from the lumbar subarachnoid space to the peritoneum with the use of a valveless rubber tube,- unsuccessful. 1939, Torkilsden described a shunt from the lateral ventricles to the cisterna magna for obstructive hydrocephalus that was modestly successful

4. History 1949, Matson described a shunt from the lumbar subarachnoid space to the ureter. M odern CSF shunt devices  by the publication of Nulsen and Spitz’s paper D escribing a ventriculojugular shunt with a ball-and-spring differential-pressure valve.

5. History The first shunt made with silicone was the Spitz-Holter valve, a slit valve designed by engineer John Holter for his son, who had hydrocephalus. Pudenz and colleagues  a distal-slit valve and a sleeve valve, both differential-pressure silicone valves for use in ventriculo -atrial shunts. Initial preferred site for shunt placement was the vascular system. Due to complications and identification of peritoneum site.

6. Shunt hydrodynamics Pressure Flow Resistance

7. PHYSICS

8. Pressure Pressure is force (F) per unit area (A). For a cylindrical column of  fluid , as in a shunt tube T he pressure at the base of the tube is equal to the weight divided by the tube’s surface area, which is equated with the height of the column (h) multiplied by the density of the  fluid  ( ρ ) and the force of gravity (g) P =  ρ  • g • h

9. Pressure In shunt systems : Pressure is generally measured in relation to atmospheric pressure  0. Pressure expressed in : mmHG / mmH2O. 1 mm HG = 13.65 mm H2O. Cerebrospinal space  One column. Right atrium – Zero , in supine position.

10. Pressure When a person is sitting or standing  Jugular venous pulse. The pressure in head is slightly negative, and in the Lumber CSF is positive. The pressure in the abdominal cavity – varies according to body habitus, abdominal wall tone – can be generally considered to atmospheric pressure. Pleural cavity – negative intrapleural pressure.

11. Pressure Shunt systems depends in the difference between the two ends of thee shunt. Which is also responsible for flow in the shunt.

12. FLOW and RESISTANCE Flow ( Q ) in a tube is defined as the volume of fluid ( V ) passing a point in space during a given time (t). Millimeters / minute . Flow from one end of the shunt system to the other is defined by the equation Q =  Ι P/(R T  + R V ), where  Ι P is the difference in pressure between the ventricle and distal catheter location , R T  is the resistance of the tube, and R V  is the resistance of the valve .

13. FLOW and RESISTANCE Resistance to the flow of  fluid  through a shunt system (R T  + R V ) depends on a number of factors. Because flow of  fluid  through catheters is laminar (smooth), resistance of catheters (R T ) is defined by Poiseuille’s law: RT = 8 L u /Pgr4 R – radius of the tube, L = length of the tube , u = viscosity of the fluid (CSF ), p = density of fluid, g = force of gravity.

14. FLOW and RESISTANCE Lab studies  90 cm long distal catheter – provides an additional resistance to flow that is approximately equivalent that provided by a differential – pressure valve. The increase in CSF viscosity – ( eg. Proteinaceous CSF ) – doesn’t have a great effect. When most proteinaceous – 7 % CSF flow reduced.

15. FLOW and RESISTANCE CSF viscosity decreases with increasing temperature; flow rates at body temperature are approximately 30% higher than at room temperature. important implications in new shunt designs, particularly those in which CSF flow occurs through a very small orifice Shunt catheter resistance rises as a fourth power of the radius. S tandard catheter diameter of 1.0 to 1.6 mm

16. FLOW and RESISTANCE Debris and air bubbles in the shunt valve or catheter significantly increase turbulence and restrict the diameter of the lumen, both of which significantly increase resistance to flow. The pressure gradient driving CSF flow in a ventriculoperitoneal shunt system is determined by the formula.

17. IVP = intraventricular pressure Pgh = ( h – difference in vertical height between the head and distal height) OPV= opening pressure of the valve IAP = intra abdominal pressure.

18. I n the upright position, the predominant influence on the pressure gradient (and therefore CSF flow) is hydrostatic pressure, not opening pressure of the valve

19. SIPHONING On ce the patient moves to the upright position and the valve opens, the hydrostatic forces acting on the shunt system will predominate and result in excessively high flow rates, despite negative intracranial pressure (ICP).

20. In a valveless system, ICP would continue to fall until IVP = − ρ hg to balance the siphon effect. Such a drop in ICP does not occur in a normal brain because there is no posture-related change in the CSF–sagittal sinus pressure gradient

21. “sinking skin-flap syndrome”. Low pressure symptoms – 10 %. Ventricular collapse. Tearing of bridging veins Subdural hematoma formation Premature suture closure Aquired aqueductal stenosis Slit ventricle syndrome

22. Components of Shunt systems 1 – ventricular catheter - proximal 2 – a valve 3 – distal catheter.

23. Ventricular ( proximal ) and distal catheter Silicone elastomer or Polyurethane Mixed with barium / tantalum. Entire Barium coated – may leach barium over time ,, local tissue reaction Calcification loss of elasticity strength of distal catheter tubing FOCAL TETHERING FRACTURE USE OF SINGLE STRIP OF BARIUM

24. Ventricular end – rounded tip, multiple holes . MC – catheter obstruction. Secondary to growth of choroid plexus and glial tissues. Flanged tips – no changes. 500 micro m diameters.

25. Distal catheter Blunt / Open end. Distal slit valves. Medicated with : Rifampicin + Clindamycin. Also carry risk of allowing resistant strains to emerge.

26. Parker and colleagues – 2011 : antibiotic impregnated shunt significant reduction in incidence of in both adult and paediatrics population. Klimo and coworkers : significant benefit 2011.

27. Valves Fixed differential pressure valves Flow regulated valves Programmable valves

28. Fixed differential pressure valves First to be developed Valves close to prevent flow of CSF when the difference in pressure across the valve ( driving pressure ) drops below a fixed threshold ( closing pressure of the valve ). When pressure exceeds OPV – valve opens. Q = (delta)P / R, Q = flow, (delta)P = driving pressure , R = total resistance

29. Differential pressure valves Ball in spring Diaphragm Slit Miter Despite difference goal to achieve normal ICP. Available in Low , Medium, High pressure gradient .

30. P roblems with fixed-pressure valves increased was that of overdrainage , which occurs by and large secondary to “siphoning.” In the recumbent position, the proximal and distal ends of the shunt are at nearly equal height, and the hydrostatic pressure  ( ρ gh )  in the shunt is more or less zero. T he shunt will equilibrate ( Ι P = 0) when IVP becomes OPV + DCP W hen the proximal end lies at a greater height than the distal end (i.e., when the patient sits or stands), siphoning occurs.

31. Under these circumstances, the hydrostatic pressure is no longer zero and contributes to the driving pressure through the shunt by  ρ gh . Ultimately, there is rapid flow of CSF through the shunt to equilibrate this additional pressure  flow persists until IVP becomes (OPV + DCP) −  ρ gh . I f the hydrostatic pressure exceeds OPV + DCP, IVP will become negative Low pressure symptoms – 10 %.

32. Anti siphon devices The problem of shunt overdrainage spurred the development of ASDs over 40 years ago. ASDs are coupled ( positioned distal) to a standard differential pressure valve. In general, these devices lie in direct contact with the overlying scalp, and their flow-pressure characteristics are dependent on the pressure gradient between the internal lumen of the shunt and the surrounding atmosphere.  This pressure differential is transmitted through the skin and ASD membrane.

33. W hen the internal shunt pressure falls below the atmospheric pressure (e.g., negative pressure created by postural change to an upright position), T he ASD membrane is drawn inward, which increases resistance and thus decreases flow through the shunt system.

34. Gravitational devices Gravitational devices  to prevent overdrainage , Their mechanism differs from that of ASDs. These devices, which, like ASDs, are add-ons to a differential pressure valve, use a gravity-dependent mechanism to change the shunt’s opening pressure based on body position, from recumbent to upright. Switcher Counterbalance.

35. Flow regulating valves These valves work by increasing the resistance through the valve as the driving pressure increases. Maintains stable flow rate. 3 stages of operation.

36. At low pressures (stage 1), they function like low-resistance differential pressure valves, and flow increases proportional to the pressure differential until a CSF flow rate of approximately 20 mL/ hr is reached.

37. During stage 2, as the pressure differential continues to increase beyond this point, the valve uses a variable resistance mechanism to maintain flow at a relatively constant rate regardless of pressure. This rate is meant to closely match physiologic CSF production and thereby prevent overdrainage .

38. If the pressure differential exceeds a set threshold (usually 300 mm H 2 O or so), the variable resistance mechanism is overcome and the valve again allows rapid flow of CSF against low resistance (stage 3).

39. Prone to obstruction – due to small outlets.

40. Flow restriction may also be achieved by a device added in series to a differential pressure valve, T he Codman SiphonGuard , which is described as an “anti-siphon and flow-control device.” The SiphonGuard houses two pathways for CSF flow: a primary, low-resistance and a secondary, high-resistance pathway. With normal flow, both pathways function in concert to drain CSF. However, during excessive flow, only the secondary pathway is operational, which is said to decrease the flow rate by 90%.

41. Programmable differential pressure valves Identical in function to standard differential pressure valves. Opening pressure is not fixed. – adjustable. Ball in cone and spring mechanism. Transcutaneous electromagnetic programmer is used to set magnetic rotor to adjust tension on the spring  there by altering opening pressure of the valve.

42. Useful in patient prone to overdrainage . Cost factor. Magnetic Fields affects. Should be verified and readjusted after MRI.

43. Choosing a Valve. Neither ASDs nor flow-regulating valves, for instance, have been shown to prevent overdrainage or lengthen overall shunt survival in well-designed, prospective studies. The Shunt Design Trial randomized 344 hydrocephalic children to undergo treatment with a standard fixed differential pressure valve, with a valve containing an ASD (the Delta valve), or with a flow-regulating valve (the Orbis-Sigma; Cordis, Fremont, CA) . N o significant difference in  the rate of  ventricular  reduction, final  ventricular  size, or overall shunt failure.

44. Choosing a Valve. A multicenter, randomized controlled trial comparing the Codman HAKIM programmable valve to a conventional valve system found no difference between the two systems with regard to shunt survival or rate of complication. In the absence of a clear universally superior valve design, the choice of valve should be adapted to the individual clinical scenario and guided by the surgeon’s sound clinical judgment.

45. Indian shunt systems Chhabra shunt system Upadhyay shunt system Shri Chitra Shunt system

46. Chhabra shunt system Slit and spring

47. Z flow

48. SHUNT SURGERY Ventriculoperitoneal shuts ( MC ) Ventriculoatrial Shunts Ventriculoperitoneal shunts Ventriculosubgaleal shunts. Ventricular resorviours .

49. 0utcomes In the Shunt Design Trial, only 61% of patients were free of shunt failure at 1 year and 47% at 2 years. After the high failure rate in the first and second years following shunt implantation, there appears to be a steady decline in shunt survivability extending for several years. Tuli and colleagues 287  found that patient age at initial shunt placement and time interval since previous revision were important predictors of repeated shunt failure. Most cases of shunt failure are related to mechanical failure (e.g., obstruction), infection, or overdrainage