1. MEASURING ICP AND CBF Thinking inside the box…
Dr Hannah Rose
SpR Eastbourne April 2007

2. Why do we measure ICP and CBF?
Raised ICP and Low CBF = POOR outcome following brain injury
Need continuous monitoring – albeit no evidence to suggest better outcome with ICP monitoring
28 Cremer OL, van Dijk GW, van Wensen E, et al. Effect of intracranial pressure monitoring and targeted intensive care on functional outcome after severe head injury. Crit Care Med 2005; 33:2207–13
(CPP=MAP–ICP)

3. Uses Severe HI SAH – hyperaemia vs vasospasm CVA
Intracerebral haematoma
Meningitis
Encephalitis
Perioperative Neurosurgical patients
Carotid Endarterectomy

4. Methods Intracranial Pressure Monitors Transcranial US doppler
Intraventricular
Intraparenchymal
Subdural
Transcranial US doppler
Jugular Bulb Oximetry
Near Infra-red Spectroscopy
Invasive Brain Tissue Oximetry
Microdialysis

5. ICP Monitoring N = 7-15 mm Hg (Supine) Infants 1.5-6/ Child 3-7
ICP= pulsatile, changes with respiratory and cardiac cycles
>15 mmHg = Pathological
>15mmHg in Benign path = Rx
>20-25 in Severe HI = Rx
No consensus…..
Not necessarily uniform distribution of ICP
Also no consensus on ideal CPP in HI

6. Intraventricular ICP monitoring
GOLD standard
Measure and Rx Raised ICP
Infection risk 6-11%
Difficult to place in swollen brains!
Reference pt = External auditory meatus (Foramen of Munro)

7. ICP Monitoring Alternatives
Intraparenchymal
Low infection risk
Zero drift and calibration problems
Strain gauge or fibre optic catheter tips
Resistance change or change in reflection of light beam sensed with change in ICP
Measures local pressures
Subdural, epidural – Inaccurate

8. ICP waveforms - Lundberg
A waves – plateau waves – steep ^ICP for 5-20mins. Always pathological. Intact autoregulation. Decreased intracranial compliance
B waves – related to change in vascular tone /min
C waves – little significance. Oscillations 4-8/min
Refractory intracranial hypertension- when ICP increases over a few hours to very high values and leads to the death of the patient unless aggressive measures such as craniectomy are instituted

9. CBF monitoring Transcranial Doppler Monitoring
Easy to use, noninvasive, repeatable
Measures basal cerebral bld flow velocity flow via doppler equation
Used to differentiate vasospasm from hyperaemia (Lindegaard Index)

10. Jugular Bulb Oximetry Base of skull-level C1/C2
Aspiration port +/- sats probe
N sats 55 – 71%
Dep on paO2, CBF and O2 extraction
 = hyperaemia / shunt d/t raised ICP
 = Hypoxia, low CBF, Increased ICP or seizures/pyrexia
Limitations: accuracy, frequent recalibrations, local complications, no focal info, rapid aspirations

11. Near-infrared Spectroscopy
Infrared light with a shallow and deep detector
Difference in absorption d/t brain
Beer-Lambert Law related algorithm displays oxy/deoxy Hb conc’s
Research only at present
Significant errors

12. Invasive Brain Tissue Oximetry
Measure brain oxygen levels directly
Licox (Clark Electrode)
Neurotrend ( ‘Fluorescent Quenching -> T, pO2,pCO2, pH)
Measure local changes in oxygenation missed by SjO2
? Role---multimodal Rx

13. Microdialysis Intracerebral fine coaxial catheter
Low flow dialysis fluid through catheter
Fluid removed regularly and analysed
Energy related substances (glu,lac,pyr,aden,xan)
Neurotransmitters (glut,asp,GABA)
Tissue damage markers (glyc,K,cytokines)
Exogenous substances (drugs)
 lactate/pyruvate = tissue ischaemia
Research only at present

14. Further Reading
L. A. Steiner and P. J. D. Andrews Monitoring the injured brain: ICP and CBF Br. J. Anaesth : 26-38; doi: /bja/ael110
K Pattinson, G Wynne-Jones and C Imray Monitoring intracranial pressure, perfusion and metabolism
Continuing Education in Anaesthesia, Critical Care and Pain, Vol 5 No