Table 1 Basic functioning of different neuromonitoring devices

EEG

EEG gives information about cortical

S: Global information of cerebral activity

Ketamine increases the activity of excitatory

activity

L: EEG trace needs experienced operators for

neurons and high frequency oscillations

Global information

Scalp electrodes

interpretation

Nitrous oxide increases the amplitude of high frequency activity

Dexmedetomidine causes slow oscillations with deeper sedation but easily awakened patient

Propofol increases theta, alpha, spindle beta power, slow waves and delta activity

pEEG

Translation between analogic signal of

S: Bedside application

Ketamine increases the activity of excitatory

EEG to digital (numeric). Awake

S: Easy interpretation

neurons and high frequency oscillations

status = 100; general anesthesia = 40–60;

S: Availability

Nitrous oxide increases the amplitude of high

suppressed EEG = 0

L: Muscle artifact

frequency activity

Regional information

L: Translation between analogic signal of

Dexmedetomidine causes slow oscillations

Adhesive pads

EEG to digital

with deeper sedation but easily awakened

DSA is a colored trace obtained from

L: Delay between event and measure

patient

EEG frequencies and transformed into

L: Cerebral activity in limited area (frontal)

Propofol induces slow waves, until suppressed

decibels of bi-hemispheric activity that

L: Several noisy elements can interfere with

pEEG with increases in dosage

can change from red (more frequent) to blue (rare)

the signal

L: Effects of medications

Sevoflurane effects on BIS are unclear

L: Validation in non-older adults

Type

Methodology

Strengths (S) and limitations (L)

Action of medications

SSEPs

SSEPs measured by stimulating a peripheral sensory nerve and recording the signal transmitted to the sensory cortex

Bedside application

S: Bedside application

S: Availability

L: Interference with electric devices

L: Interpretation by expert

Ketamine increases cortical SSEP amplitude Dexmedetomidine affects amplitude minimally

Propofol has minimal effects on SSEPs. Sevoflurane affects SSEPs in a dose-dependent way

Barbiturates increase latency and decrease amplitude of SSEPs

Benzodiazepines reduce amplitude and increase latency

Opioids do not significantly affect SSEPs, but remifentanil prolongs SSEP latency

MEPs

MEPs measured by transcranial stimulation of the cortex and recording the signal at the spinal cord level, peripheral motor nerves, or the muscles

S: Bedside application

S: Availability

L: Interference with electric devices

L: Interpretation by expert equipment

Ketamine increases amplitude at increased frequency of MEPs

Dexmedetomidine causes a decrease in MEP amplitude

Propofol has excitatory effects on MEPs Sevoflurane has a depressant effect on MEPs Benzodiazepines attenuate MEPs

TCD

Investigation of local blood flow and

velocities in the circle of Willis

2 mHz probe placed in acoustic windows

(i.e., transtemporal)

Measure of nICP and eCPP

Cerebral autoregulation

Critical closing pressure

Diastolic closing margin

Midline shift

Emboli, obstruction, stenosis

S: Bedside application

S: Availability

L: Need for experienced operators

L: Availability of windows

Ketamine may affect cerebral hemodynamics Propofol decreases the tone of the venous capacitance vessels and decreases cerebral metabolism

Remifentanil reduces cerebral blood flow velocity despite constant perfusion pressure Thiopental decreases CBF velocities Benzodiazepines decrease CBF velocity

ONSD

Measure of nICP and eCPP

S: Bedside application

S: Easy interpretation

S: Availability

ONSD is larger with propofol in comparison to sevoflurane

Pupillometry

Pupillometry measures the diameter of

S: Bedside application

Remifentanil determines miosis and reduces

the pupils and the pupillary light reflex

S: Easy interpretation

PLR

NPi is an algorithm using parameters to

S: Availability

Propofol determines miosis and reduces CV

determine pupillary light response, with a

L: Agitated or confused patients can be

Barbiturate titrated to burst suppression

scale 0–5, < 3 is abnormal

difficult to evaluate

reduces CV

Maximum and minimum pupil diameter

L: Patients with scleral edema, periorbital

Droperidol causes miosis and reduces PDR

(mm) refers to diameter at rest and peak

edema, intraocular lens replacement prior

Metoclopramide causes miosis and reduces

constriction

Latency is the time (seconds) delay between light stimulus and pupillary constriction

ocular surgical procedures can limit assessment with pupillometry

L: Ambient light can influence the measure L: Expensive

PDR

CV is the distance (mm) of constriction divided by duration (seconds) of constriction

Dilation velocity is the distance (mm) of re-dilation divided by duration (seconds) of re-dilation

ype

Methodology

Strengths (S) and limitations (L)

Action of medications

NIRS

NIRS measures cerebral oxygen

S: Bedside application

Propofol and dexmedetomidine equally

saturation by using a near-infrared light

S: Easy interpretation

preserve cerebral oxygenation and do not

passing through adhesive pads and

S: Availability

affect neurological outcome

tissues. The light is therefore adsorbed by

L: Regional evaluation of cerebral oximetry

Cerebral oxygenation may be better preserved

oxyhemoglobin and deoxyhemoglobin, thus obtaining a value reflecting the local

that may not reflect global changes in hemodynamics

with sevoflurane than propofol

Midazolam and morphine may alter cerebral

amount of oxygen within the frontal region

A decrease of 20% from baseline can be associated to the reduction of CBF, hypoperfusion and neurologic symptoms Regional measure

L: Bias with skin color and gender

L: Variations due to systemic extracranial perfusion

L: Elimination of oxygen degradation products in patients with liver diseases that can alter the absorption of light

oxygenation and hemodynamics

Adhesive pads

Various devices are available with different algorithms and components, including Masimo (Masimo Corp., Irvine, CA), INVOS (Medtronic, Minneapolis, USA)