Neurosurgery notes/Trauma/Management of trauma/Neuromonitoring/ICP monitoring/Invasive ICP monitoring techniques

Invasive ICP monitoring techniques

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ICP monitoring

Intraventricular catheter (IVC)

  • AKA: external ventricular drainage (EVD) + external pressure transducer via fluid-filled tubing.
    • Other options for IVCs utilize transducers tipped with fiberoptic or strain gauge devices
  • Measurement of ventricular fluid pressure is the current gold standard for measuring ICP as it is the most accurate, low-cost, and reliable method of monitoring ICP

Advantages

  • Most accurate (can be recalibrated to minimize measurement drift)
  • Lower cost
  • Allows therapeutic CSF drainage
    • May help reduce ICP directly, and may drain particulate matter
      • E.g. blood breakdown products after SAH, that could occlude arachnoid granulations

Disadvantages

  • May be difficult to insert into compressed or displaced ventricles
  • Obstruction of the fluid column (e.g. by blood clot, or by coaptation (adaptation) of the ependymal lining on the catheter as the ventricle collapses with drainage) may cause inaccuracy
  • Some effort is required to check and maintain function
  • Transducer must be consistently maintained at a fixed reference point relative to patient’s head
  • Allows accurate ICP measurements only when the ventricular drain is closed
    • If an attempt is made to record the ICP while the catheter is draining CSF, the recorded ICP is always equal to or lower than the drainage level because of hydrostatic laws.

Insertion technique

  • For technique to place catheter in frontal horn, see Kocher’s point
  • The right side is usually used unless specific reasons to use the left are present (e.g. blood clot in right lateral ventricle which might occlude IVC).

Set-up

  • The effect of having an opening on the top of the drip chamber (through an air-filter) is the same as having the drip nozzle open to air, and therefore as long as this filter is not wet or plugged the pressure in the IVC is regulated by the height of the nozzle (as read on the pressure scale; note that the “0” is level with the nozzle).
  • The external auditory canal (EAC) is often used as a convenient external landmark for “0” (approximates the level of the foramen of Monro).

Normal functioning of the IVC system

  • The system should be checked for proper functioning at least every 2–4 hours, and any time there is a change in:
    • ICP (increase or decrease),
    • Neuro exam,
    • CSF output (for systems open to drainage).
  • What to check for
    • Check for presence of good waveform with respiratory variations and transmitted pulse pressures
    • IVCs: to check for patency, open the system to drain and lower the drip chamber below level of head and observe for 2–3 drops of CSF (normally do not allow more than this to drain)
    • For systems open to drainage:
      • Volume of CSF in drip chamber should be indicated every hour with a mark on a piece of tape on the drip chamber, and the volume should increase with time unless ICP is less than the height of the drip chamber (in practice, under these circumstances the system would usually not be left open to drainage).
        • NB: the maximum expected output from a ventriculostomy would be ≈ 450–700ml per day in a situation where none of the produced CSF is absorbed by the patient.
          • This is not commonly encountered.
        • A typical amount ofdrainage would be ≈ 75ml every 8 hrs
      • Drip chamber should be emptied into drainage bag regularly (e.g. q 4 or 8 hours) and any time the chamber begins to get full (record volume)
    • In cases where there is a question whether the monitor is actually reflecting ICP,
      • Lowering the HOB towards 0° should increase ICP
      • Gentle pressure on both jugular veins simultaneously should also cause a gradual rise in ICP over 5–15 seconds that should drop back down to baseline when the pressure is released

IVC problems

  • Air filter on drip chamber gets wet (prevents air from passing through filter)
    • Result: fluid cannot drain freely into drip chamber
      • The pressure is no longer regulated by the height of the drip nozzle)
      • If the outflow from the drip chamber is clamped, then no flow at all is possible
        • If the clamp on the drip chamber outlet is open, then the pressure is actually regulated by the height of the nozzle in the collection bag and not the nozzle in the drip chamber
    • Solution:
      • If a fresh filter is available, then replace the wet one.
      • Otherwise one must improvise (with the risk of exposing the system to contamination):
        • Replace the wet filter with a filter from an IV set,
        • Replace the wet filter with a sterile gauze taped over the opening
  • Air filter on collection bag gets wet: this will make it difficult to empty the drip chamber into the bag
    • This is not usually an urgent problem unless the drip chamber is full and the collection bag is distended tensely with air
    • Filter will dry out with time and will usually start to work again
    • If it is necessary to empty the drip chamber before the filter is dry, then use sterile technique to insert a needle into the bag drainage port and decompress the bag of fluid and air
  • Improper connections:
    • A pressurized irrigation bag with or without heparinized solution should never be connected to an ICP monitor
  • Changing position of head of bed:
    • Must move drip chamber up or down to keep it level with the same external landmarks (e.g. level of auditory canal):
    • When open to drainage, this will assure the correct pressure will be maintained
    • When opened to pressure transducer, will maintain correct zero
  • When open to drain, pressure reading from transducer is not meaningful:
    • The pressure cannot exceed the height of the drip chamber in this situation (because at that point, fluid will drain off), and the opening to the “atmosphere” in the drip chamber will dampen the waveform
  • Drip chamber falls to floor:
    • Overdrainage: leads to
      • Seizures
      • Subdural hematoma formation
    • Solution: securely tape chamber to pole, bed-rail…, check position regularly

Complications

  • Haemorrhagic
    • Over all 7%,
    • 0.8% significant haemorrhage
  • Infection (Ventriculitis and meningitis)
    • 0% - 20% depending on the definition of infection used and the characteristics of the study population.
    • Antibiotic or silver-impregnated catheters are associated with a significant decrease in infection risk

IVC troubleshooting

IVC no longer works

  • Manifestation of problem:
    • Dampening or loss of normal waveform
    • No fluid drains into drip chamber (applies only when catheter has been opened to drain)
  • Possible causes:
    • Occlusion of catheter proximal to transducer ● slide clamp closed or stopcock closed ● catheter occluded by brain particles, blood cells, protein
    • IVC pulled out of ventricle
      • Test:
        • Temporarily lower drip nozzle and watch for 2–3 drops CSF
      • Solution:
        • Verify all clamps are open, and flush no more than 1.5ml of non-bacteriostatic saline (AKA preservative-free saline) with very gentle pressure into ventricular catheter (NB: in elevated ICP the compliance of the brain is abnormally low and small volumes can cause large pressure changes).
        • Note.
          • If no return then brain or clot is probably plugging catheter.
          • If it is known that the ventricles are ≈ completely collapsed then the IVC may be OK and CSF should still drain over time.
            • Otherwise this is a non-functioning catheter, and if a monitor/drain is still indicated then a new catheter may need to be inserted (CT may be considered first if the status of the ventricles is not known).
          • If catheter is clotted by intraventricular hemorrhage, rt-PA may sometimes be used.

ICP waveform dampened

  • Possible causes:
    • Occlusion of catheter proximal to transducer
    • IVC pulled out of ventricle: no fluid will drain
    • Air in system:
      • Solution: allow CSF to drain and expel air
      • Caution: do not allow excessive amount of CSF to drain (may allow obstruction of catheter, subdural formation…). Do not inject fluid to flush air into brain
    • Following decompressive craniectomy: due to the fact that the catheter is no longer in a closed space, this is a normal finding in this setting

Intraparenchymal microtransducers monitor

Types

  • Solid-state devices based on pressure sensitive resistors
    • Eg
      • Codman MicroSensor (MRI compatible)
      • Raumedic Neurovent-P ICP sensor (MRI compatible)
      • Pressio sensor (not MRI compatible)
    • Piezoelectric strain gauge devices
      • Transducer is deformed because of a change in ICP, resistance changes and this is converted into an ICP value.
  • Fibreoptic design.
    • Eg Camino ICP monitor (Not MRI compatible)
    • Transmit light via a fibreoptic cable towards a displaceable mirror at the tip
      • Changes in ICP distort the mirror and the differences in intensity of the reflected light are translated into an ICP value.

Insertion technique

  • Inserted right frontal region at a depth of approximately 2 cm

Downside:

  • It is not possible to recalibrate them after placement
  • There is a degree of zero-drift over time which can result in a measurement error after several days.
  • Cost higher than both conventional and antibiotic impregnated ventricular systems

Location

  • Subarachnoid screw (bolt):
    • Risk of infection 1%, rises after 3 days.
    • At high ICPs (often when needed most) surface of brain may occlude lumen → false readings (usually lower than actual, may still show ≈ normal waveform)
  • Subdural:
    • Less accurate
    • May utilize a fluid coupled catheter (e.g. Cordis Cup catheter), fiberoptic tipped catheter, or strain gauge tipped catheter
  • Epidural:
    • May utilize a fluid coupled catheter, or fiberoptic tipped catheter (e.g. Ladd fiberoptic).
    • Less accurate
  • In infants, one can utilize an open anterior fontanelle (AF):
    • Fontanometry: probably not very accurate
    • Applanation principle:
      • May be used in suitable circumstances
        • If the fontanelle is concave with the infant upright, and convex when flat or head down to estimate the ICP within 1cm H2O.
      • The infant is placed supine, and the AF is visualized and palpated while the head is raised and lowered.
      • When the AF is flat, the ICP equals atmospheric pressure, and ICP can be estimated in cm H2O as the distance from the AF to the point where the venous pressure is 0 (for a recumbent infant, the midpoint of the clavicle usually suffices).
      • If the AF is not concave with the infant erect, then this method cannot be used, because either
        • The ICP exceeds the distance from the AF to the venous zero point OR
        • The scalp may be too thick