Principles and Techniques of ICP

Measurement

and

Waveform Interpretation

Presented by – Dr. Dhritiman Chakrabarti

Moderated by – Dr. Sudhir Venkataramaiah

Introduction

Alexander Monro – 1783 -

Francois Magendie - 1842 – Idea of Fluid in brain accepted

George Burrows – 1846 – Reciprocity of intracranial CSF and

Blood volumes

Harvey Cushing – 1926 – Reciprocity of CSF, Blood and

Brain in Intact skull

Indications

ICP Monitoring TechniquesInvasive:

Based on Technological differences:

1) External Ventricular Drainage (EVD) – Gold Standard

2) Microtransducer ICP Monitoring Devices

Fiberoptic

Strain Gauge

Pneumatic

Based on location:

1. Intraventricular

2. Intraparenchymal

3. Epidural

4. Subdural

5. Subarachnoid

Non Invasive:

1. Transcranial Doppler Ultrasonography (TCD) – Based on PI

2. Tympanic Membrane Displacement (TMD)

3. OpticNerve Sheath Diameter (ONSD) – via Transocular USG

4. Magnetic Resonance Imaging (MRI) & Computer Tomography (CT)

5. Pupillometry

6. Near-Infrared Spectroscopy (NIRS)

7. EEG

Invasive Techniques

• Type of Monitor?

• Optimal location of sensor?

• Validity of the measured value?

External Ventricular Drain

Gold Standard for ICP monitoring.

Complications:

1) Hemorrhage 5.7% (of clinical importance 0.61%)

2) Bacterial Colonization (0 – 27%) – Prevention by sterility

while insertion, tunneling (10 cm.), avoid routine sampling,

avoid changing catheter.

3) Malposition.

4) Blockage.

EVD inserted for pressure relief, specially in ICSOL, may cause

displacement of intracranial structures due to magnification

of pressure gradients set up due to pathology.

Microtranducers

Strain Gauge

Fiberoptic Device

Pneumatic Sensor

Optimal Location of Sensor

Intercompartmental pressure gradients

TCDBased on Gosling/Pulsatility Index

PI= (FVpeak sys - FVend dia)/(FVmean)

Bellner et al – ICP = (10.972 x PI) - 1.284

2 Depth TCD

A. Ragauskas, DSc, et al

Neurology 78 May 22, 2012

Optic Nerve Sheath Diameter

Cutoff values, varying between 4.8 and 5.9mm for ICP

estimation.

Corr coeff between 0.46 and 0.74

Tympanic Membrane Displacement

Based in intact stapedial reflex and patency of cochlear aqueduct.

High cochlear fluid pressure causes an inward-directed movement of the

tympanic membrane, low cochlear fluid pressure causes an outward

movement.

This movement is measured as the mean volume displacement (Vmean [nL])

Oto-acoustic Emissions

• Change in evoked OAE due to displacement of stapes because

of increased cochlear fluid pressure.

• Distortion product otoacoustic emissions (DPOAEs), evoked

by a pair of primary tones – significant phase shift in 750 –

1500 Hz range with ICP changes.

• Corr Coeff. between invasive ICP and DPOAE phase shift in

degrees – Buki et al – 0.77.

Near Infrared Spectroscopy

• THx (total Hb reactivity index) – Corr. Coeff between THI (from NIRS) and ABP.

• PRx and THx show linear correlation with r = 0.56.

• CPPopt found based on lowest value of CPP in PRx -CPP plot also shows significant correlation with CPPopt from THx-CPP plot. (r=0.74)

• Thus THI may be used as surrogate for ICP when calculating CPPopt.

Pupillometer

• Hand-held infrared system which automatically tracks and

analyzes pupil dynamics over a 3-second time period.

• Neurologic Pupil Index (Npi) – A scalar value derived

from size, percent constriction, latency, constriction velocity,

and dilation velocity.

• Npi 0 – 5.

• Npi < 3 predictor of increased ICP.

• Npi abnormalities started 16 hrs prior to peak

ICP

Chen et al (2011)

Surg Neurol Int. 2011; 2: 82.

EEG for ICP Estimation

Median Frequency

Delta Ratio - ratio of delta

power to the sum of

alpha power and beta

power.

Pressure Index = 1 / (median frequency×delta ratio)

ICP Waveform

• Normal ICP waveform:

P1 = (Percussion wave)

represents arterial pulsation

P2 = (Tidal wave) represents

intracranial compliance

P3 = (Dicrotic wave) represents

aortic valve closure

ICP Waveform Analysis

Time Domain Analysis:

1) Mean ICP and visual inspection of trends.

2) ICP Wave Trends – Lundberg Waves A, B

3) AMP – Pulse pressure amplitude.

4) RAP – Corr. Coeff. b/w AMP and ICP

5) PRx – Pressure Reactivity Index (MAP-ICP corr. coeff.)

6) PRx-CPP Curve – Optimal CPP

7) ICP Variability Analysis: Successive Variation

8) Detrended Fluctuation Analysis

9) Multiscale Entropy Analysis

Frequency Domain Analysis:

1) High Frequency Centroid

2) Slow Wave Power

Fourier Transform: Time domain to frequency domain

Mean ICP

(A) Low and stable intracranial pressure (ICP).

(B) Stable and elevated ICP

(C) ‘‘B’’ waves of ICP.

(D) Plateau waves of ICP

(E) High, spiky waves of ICP caused by sudden increases in ABP.

(F) Increase in ICP caused by temporary decrease in ABP.

(G) Increase in ICP of ‘‘hyperaemic nature’’

(H) Refractory intracranial hypertension

Lundberg Waves A

Lundberg Waves B

Two proposed Mechanisms:

1) Rosner’s Theory – Based on ABP variations (akin to A waves, but

on a smaller scale).

2) Neuropacemaker Theory.

ICP Pulse Pressure Amplitude

AMP – Amplitude of frequency component of ICP

waveform which corresponds to heart rate.

Used for computing RAP.

RAP

• Correlation coefficient between AMP and ICP.

• Represents Compensatory reserve.

PRx• Pressure Reactivity Index: Correlation coefficient between

time averaged slow waves of ABP and ICP.

PRx – CPP Curve

The U shaped plot suggests that at too low CPP, vascular

reactivity is impaired, which could produce ischemia, and at too

high CPP vascular reactivity is also impaired, aggravating the

risk of hyperemia.

ICP Variability Analysis

Time averaged variability (termed Successive Variation) of

mean ICP values correlates positively with increased mortality.

ICP-SV2 showed a positive relationship with ICP, and hematoma

volume , while the relationship with CPP was inversed.

Cut off value for mortality – 2.8 mm Hg.

Breakdown of ICP Waveform

• High frequency components - ?Harmonics

• AMP

• Respiratory Variations

• Slow Waves: IB, B, UB, A

High Frequency Centroid

Used as a measure of intracranial compliance.

Slow Waves

Infra B (IB) below 8 mHz,

B from 8 mHz to 50 mHz,

Ultra B (UB) beyond 50 mHz up to 200 mHz

Is ICP Monitoring Useful?

• There were no RCTs comparing ICP monitoring vs Clinical

examination based management before 2012.

The hidden questions that need to be addressed:-

1) Whether fixed mean ICP thresholds are a good guide?

2) Whether change in treatment protocol due to the monitoring has

deleterious effect on outcome?

3) Whether focus on instantaneous ICP readout rather than trends

influences outcome?

4) Focus on ICP based management rather than CPP?

The authors acknowledge that ICP monitoring (as is any monitoring

modality) is just a useful guide for management.

The outcomes are decided by the differences in management

protocols that the knowledge of the said parameter bring about.

This brings the focus of future research on evaluation of the

parameter being used for management and the management

protocol used for treatment.

ICP Monitoring in Trauma

Brain Trauma Foundation Guidelines – When to use?

Level II:

Intracranial pressure (ICP) should be monitored in all salvageable patients with a severe TBI and an abnormal computed tomography (CT) scan. An abnormal CT scan of the head is one that reveals hematomas, contusions, swelling, herniation, or compressed basal cisterns.

Level III:

ICP monitoring is indicated in patients with severe TBI with a normal CT scan if two or more of the following features are noted at admission: age over 40 years, unilateral or bilateral motor posturing, or SBP < 90 mm Hg.

BTF Guidelines – What to use?

In the current state of technology (as of 2007), the ventricular catheter connected to an external strain gauge is the most accurate, low-cost, and reliable method of monitoring intracranial pressure (ICP).

Association for the Advancement of Medical Instrumentation (AAMI) has developed the American National Standard for Intracranial Pressure Monitoring Devices –

• Pressure range 0–100 mm Hg.

• Accuracy ± 2 mm Hg in range of 0–20 mm Hg.

• Maximum error 10% in range of 20–100 mm Hg.