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5.4 Monitoring Depth of Anesthesia (Including
Bispectral Analysis Index)
Bispectral analysis index could help us deliver an appropriate level of anesthesia
and/or sedation. During postoperative period, during the time interval from patient
transfer to ICU until tracheal extubation, the overall patient status (including hemo-
dynamic, pulmonary, hematologic, and consciousness) is not yet prepared for extu-
bation. Meanwhile, the residual effects of anesthetics are usually vanished in this
time, and the patient needs some degree of sedation.
Besides, frequently it happens in a number of patients that some invasive proce-
dures are needed (like intubation, orotracheal suctioning, central line insertion, and
chest tube change), or less frequently, it happens that more time is needed for full
recovery: some need prolonged intubation and mechanical ventilatory support or
hemodynamic support, mandating additional sedation/analgesia. Level of sedation/
analgesia should be monitored to deliver adequate analgesic agents while prevent-
ing over-administration. Bispectral analysis index monitor could help us in such
cases to improve sedation/analgesia level.
Depth of anesthesia scoring for bispectral analysis index is defi ned as
following:
• >80: awake
• 60–80: sedation state
• 40–60: surgical anesthesia
• <40: deep anesthesia
In some studies, bispectral analysis index is considered as the “most widely used
method at the present time” for monitoring sedation as part of an integrated moni-
toring approach for assessment of sedation/analgesia in critical patients. Also,
bispectral analysis index is demonstrated to be a “valid measure of wakefulness
after cardiac surgery.”
Another important issue is the legal aspects of delivering appropriate level of
sedation/analgesia to create both patient satisfaction and amnesia and so prevent
patient recall (Simmons et al. 1999 ; Drummond 2000 ; Riker et al. 2001 ; Brocas
et al. 2002 ; Frenzel et al. 2002 ; Courtman et al. 2003 ; Deogaonkar et al. 2004 ;
Watson and Kane-Gill 2004 ; Fraser and Riker 2005 ; Freye 2005b ; Hernandez-
Gancedo et al. 2006 ; Payen et al. 2007 ; Palanca et al. 2009 ; Lamas and Lopez-Herce
2010 ).
5.5 Evoked Potentials
Evoked potentials are used as monitoring devices to check the functional integrity
of the central and peripheral nervous system, especially the different neural circuits
and pathways. Although both EEG and evoked potentials assess the electrical activ-
ity of the nervous system, there are some differences between these two neural
monitoring, discriminating evoked potentials from EEG:
• Evoked potentials have lower-voltage amplitude compared to EEG.
• Evoked potentials assess the neurologic response to a stimulus (sensory or
motor).
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• Evoked potentials are not limited to the cortical areas of the nervous system; in
other words, evoked potentials monitor cortical areas, deeper neural structures of
the CNS, spine, and also peripheral nervous system.
• In periods when EEG is fl at (like therapeutic hypothermia), evoked potentials
could still work and monitor the functional integrity of the nervous system.
Evoked potentials are divided into three main categories:
1 . Somatosensory-Evoked Potential (SSEP) : Somatosensory-Evoked Potential
(SSEP) which monitors the functional integrity of the ascending pathways, from
the peripheral receptors (median or ulnar nerve for upper extremity and posterior
tibial nerve or peroneal nerve for lower extremity) up to the multiple spinal seg-
ments and then to the contralateral thalamus, reaching fi nally to cortex; a sub-
type of this monitoring modality is called visual-evoked potential (VEP) which
incorporates visual stimuli as the sensory input.
2 . Motor-Evoked Potential (MEP) : Motor-Evoked Potential (MEP) which monitors
the motor pathway activated in response to electrical stimulus, from the cortical
areas down to the related nuclei and corticospinal tracts and fi nally to motor units.
3 . Auditory-Evoked Potential (AEP) : Auditory-Evoked Potential (AEP) which
objectively monitors the neural pathway involved in hearing from cochlea in ear
to the auditory (8th cranial) nerve to brainstem then related brain ganglia and
fi nally to related cortical areas; the fi rst milliseconds of this monitoring controls
the brainstem function involved in auditory pathway and is called brainstem
auditory-evoked response (BAEP).
The above modes have a relatively wide application in perioperative care, includ-
ing cardiac surgical procedures. Their application in the postoperative period of
cardiac surgery patients could be very useful and help us gather important objective
data; however, this is not a common practice; one could stress on the following as
the main indications of indications of evoked potentials in postoperative period:
• During periods of postoperative-controlled (therapeutic) hypothermia, EEG
becomes fl at; however, evoked potentials could monitor functional integrity of
the nervous system even at such states.
• Monitoring the physiologic integrity of the neural system while the patient is
hemodynamically unstable, deeply sedated, has altered consciousness state, or is
comatose; possibly other nervous system monitoring could not assess the func-
tional status of the patients in these periods of time.
In summary, evoked potentials could help the clinicians in assessment of neural
system integrity with objective and reproducible data which are a new window besides
the routine CNS monitoring; possibly, their postoperative application in cardiac sur-
gery patients would be more common in future years (Hill and Chiappa 1994 ; Nuwer
et al. 1995 ; Sloan 1995 , 2004 ; Rodriguez 2004 ; Freye 2005a , b ; Toleikis 2005 ; Sloan
and Jameson 2007 ; Amantini et al. 2008 ; Martin and Stecker 2008 ; Sloan et al. 2012 ).
5.6 Cerebral Oximetry
Cerebral oximetry is a relatively new technology for cerebral monitoring though it
has passed more than 35 years from the fi rst publication regarding the in vivo appli-
cation of the technique; however, it has been used in a number of procedures
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including cardiac surgery. Successful previous studies have shown the use of this
monitor in both animal models and in human studies involving some high-risk pro-
cedures like cardiac and vascular surgery and in patients with underlying acute cere-
bral events though there are still some controversies.
The technology of “near-infrared spectroscopy” (NIRS) was described for this
purpose in 1977 by Professor “Frans Jöbsis.” NIR light has a specifi c characteristic
which is the power to penetrate a wide range of body tissues (including bone) and,
unlike pulse oximetry, utilizes the refl ection phenomenon of light rather than the
process used in pulse oximetry; transmission of light from a small part of the body
(e.g., a fi nger) is the technology used in pulse oximetry; in other words, this technol-
ogy utilizes passage of near-infrared (NIR) light in the range of 700–1,000 nm,
through skull and underlying tissues using self-adhesive optodes used on the scalp
above the eyebrows; part of the NIR light is absorbed by biologic chromophores,
especially two main chromophores oxyhemoglobin (OHb) and deoxyhemoglobin
(HHb) and cytochrome oxidase, and the rest of the light is returned back; the
returned portion of NIR light is used for data collection, processing, calculation, and
demonstration of the absorbed fraction of light in each of the above chromophores;
in this process, the “modifi ed Beer-Lambert Law” is used for calculations; fi nally
these data are processed by the machine software to demonstrate the fi gures of rSO
2
on the monitor screen. NIR light is produced by “light-emitting diodes” (LEDs) and
absorbed by silicon photodiodes. In summary, its mechanism is by differential
absorption of near-infrared spectroscopy (NIRS) in the range of 700–1,000 nm,
through skin and bone.
One of the key features of cerebral oximetry by NIRS is that rSco
2
fi gures are in
appropriate range only if the brain tissue receives adequate oxygenated blood,
which mandates not only adequate cerebral oxygenation but also appropriate state
of cerebral hemodynamic , patency of cerebral arteries , and effi cient cerebral
venous drainage . Possibly this is why in some high-risk patients like cardiac surger-
ies (including CABG patients), regional cerebral oxygen saturation monitoring
(rSco
2
) prevents episodes of severe cortical hypoxia; so, application of this monitor
has the potential to signifi cantly reduce the incidence of major organ dysfunction
and “improve outcome,” though some controversies exist yet and there are studies
questioning the effect of cerebral oximetry application on patient outcome (espe-
cially the systematic review recently published by Zheng et al. ( 2013 ).
A number of practical notes for utilization of this monitoring are:
• Monitors the in vivo state of the CNS.
• Monitoring is noninvasive, real time, and portable.
• Measures both oxygenation and perfusion of the CNS (i.e., mandates appropriate
hemodynamic of the CNS).
• Monitors not only the CNS status but also the hemodynamic status since it moni-
tors both oxygenation and perfusion of the CNS.
• A drop more than 20 % from baseline or an absolute decrease below 50 % in
measured rSco
2
values is considered as cerebral hypoxia.
• Could be used for preoperative, intraoperative, or postoperative monitoring (all
over the perioperative period).
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• Does not need pulsatile fl ow like pulse oximetry for monitoring during CPB.
• Is one important member of the “multimodal CNS monitoring” in cardiac
surgery.
• Fine attachment and correct positioning of the probes is very important for its
application, and this is a very important application point which could bias the
measurements.
• For patients receiving hemodynamic support by ECMO, cerebral oximetry is a
very useful and promising CNS monitoring; also, if the patient would be under
non-pulsatile fl ow with ECMO, this device could be more practical than many
other CNS monitors.
• Could be used as a predictor for successfulness of extubation in cardiac surgery
patients.
• Its application in high-risk patients (like the elderly and the patients undergoing
surgery with underlying cerebral vascular disorders) is recommended more than
the others.
• Underlying diseases like decreased cardiac output, pulmonary problems, ane-
mia, and underlying disorders of the cerebral vessels are considered as con-
founders which could decrease the rSco
2
readings and create bias.
If a decrease in rSco
2
is observed, the following steps are suggested by Murkin
et al. and Denault et al. as a useful approach for relieving the etiologic disorder
causing cerebral ischemia:
1. Control head position; if head is rotated extensively, turn it to normal position.
2. Check the possibility of arterial or venous occlusion due to arterial or venous
cannula physical effects.
3. Check for mean arterial pressure (MAP) and treat it if it is low.
4. Control arterial saturation (by pulse oximeter or blood gas analysis); if it is low,
rule out possible causes of systemic desaturation.
5. Treat possible hyperventilation which could decrease arterial PaCO
2
, especially
if it is below 35 mmHg.
6. Treat possible anemia to increase hematocrit above 30 %.
7. Assess the cardiac function including the situation of the heart using methods
like echocardiography and also SjvO
2
; relieve potential underlying etiologies
including treatment of failing heart.
8. Check cerebral oxygen consumption; if it is increased, rule out and treat “con-
vulsions” and/or “hyperthermia.”
9. But if cerebral oxygen consumption is normal, rule out increased “intracerebral
hypertension” and/or “cerebral edema”; for this purpose, use the help of imaging
modalities.
(Rolfe 2000 ; Ferrari et al. 2004 ; Casati et al. 2005 , 2006 ; Polito et al. 2006 ;
Denault et al. 2007 ; Hoshi 2007 ; Murkin et al. 2007 ; Wolf et al. 2007 ; Wright 2007 ;
Fischer et al. 2009 ; Hasegawa and Okita 2009 ; Huppert et al. 2009 ; Murkin and
Arango 2009 ; Slater et al. 2009 ; Vohra et al. 2009 ; Brady et al. 2010 ; Erickson and
Cole 2010 ; Fedorow and Grocott 2010 ; Green and Paklet 2010 ; La Monaca et al.
2010 ; Palombo et al. 2010 ; Sellmann et al. 2010 ; Svyatets et al. 2010 ; Veel et al.
2010 ; Andritsos et al. 2011 ; Bronicki and Chang 2011 ; Hoshi 2011 ; Lampe and
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Becker 2011 ; Lima et al. 2011 ; Radovanovic and Radovanovic 2011 ; Rao and
Durga 2011 ; Roggenbach and Rauch 2011 ; Smith 2011 ; Cyrous et al. 2012 ; Ghosh
et al. 2012 ; Hankey 2012 ; Kertai et al. 2012 ; Li 2012 ; Scheeren et al. 2012 ; Tsygan
2012 ; Foster et al. 2013 ; Hampton and Schreiber 2013 ; Moerman et al. 2013 ;
Murkin 2013 ; Seule et al. 2013 ; Zheng et al. 2013 ; Zulueta et al. 2013 )
5.7 Jugular Venous Oxygen Saturation (SjvO
2
)
Monitoring the CNS oxygenation status has been used for more than 60 years:
Gibbs et al fi rst in 1942 and then Datsur et al in 1963. Assessment of the jugular
venous oxygen saturation (SjvO
2
) is an “indirect surrogate indicator for global oxy-
genation of the cortex” and also is an indicator of the balance between cerebral
blood fl ow (CBF) and cerebral metabolism rate of oxygen (CMRO
2
). SjvO
2
moni-
toring could be used to monitor the trend of change in the following items:
• Oxygen uptake by the brain tissue (i.e., the difference between brain arterial and
venous difference which equals CNS oxygen uptake).
• Arterial and venous difference of CNS blood gases and their related parameters.
• Arterial and venous difference of CNS blood glucose levels.
• Arterial and venous difference of CNS blood lactate levels; impaired CNS perfusion
leads to decreased cerebral oxygenation which would activate anaerobic metabo-
lism leading to increased lactate level in the jugular venous bulb blood samples.
The normal values for SjvO
2
are lower than the normal values for global mixed
venous oxygen saturation (which is usually measured by pulmonary artery cathe-
ter). This is due to the fact that oxygen uptake and consumption in the cerebral tis-
sue is much higher than the global body oxygen uptake and consumption.
5.7.1 Contraindications for SjvO
2
Catheter Insertion
According to Shaaban et al. absolute contraindications for SjvO
2
catheter insertion are:
• Injuries in the cervical spine
• Bleeding diathesis
• Local neck trauma
• Local infection
Also, relative contraindications could be:
• Compromised drainage of the cerebral venous system
• Patients having tracheostomy
SjvO
2
measurements are done either using the conventional technique with serial
measurements of SjvO
2
and other factors or using a fi beroptic catheter with near-
infrared light in the catheter tip; however, this conventional technique has the limi-
tation of being “point assessment” needing serial measurements but could not
provide real-time data. On the other side, malpositioning of the catheter tip could
cause reading errors or the near-infrared light of the fi beroptic catheter may be out
of its defi ned range, and hence, misreading would occur. Also, rotation of the head
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to either side could affect the venous return and distort the measurements. The tech-
nology used in fi beroptic SjVO
2
monitor is similar to the oximetry technology used
for some types of pulmonary artery catheters.
5.7.2 Technique of Catheter Insertion
SjvO
2
is assessed through a catheter introduced to the internal jugular vein using the
following technique:
• The technique is through the internal jugular vein, usually the right side.
• Usually, the right IJV is the dominant vein for cerebral venous drainage; however,
the mixing pattern between right and left hemispheres is not always the same.
• SjVO
2
catheter insertion is exactly the same as central venous catheter (CVC)
insertion; the anterior triangle approach is the preferred one; however, the needle
and guidewire direction should be a cephalic one (compared to CVC direction).
• Seldinger technique is used for guidewire insertion.
• After guidewire insertion, the catheter should be inserted and conducted upward
until resistance is sensed, or in the awake patient, a sense of pressure in skull
base is noted by the patient; also, Doppler sonography or sizing the inserted
length of the catheter by an external sizer.
• The catheter is sent from the right internal jugular vein over the guidewire to the
cephalic direction.
• The catheter tip is sent cephalad to the “common facial vein outlet”; from there,
it is sent to the jugular bulb; the jugular bulb is the dilated portion of the internal
jugular vein; it is located distal to the jugular foramen (i.e., the anatomic opening
in the bony skull from which the jugular vein exits).
• Inside the jugular bulb, the catheter tip should be positioned in the roof of the
bulb; this is the site for SjVO
2
measurements.
• The location of the catheter tip should be documented by lateral or anteroposte-
rior neck X-ray; it should be located at the level of the mastoid process; in lateral
neck X-ray, this location is equal to the lower border of the fi rst cervical spine or
the inferior margin of the orbital rim; also, if we draw a line between the two
mastoid processes, the tip of the catheter should be just cephalad to this line.
• Another way is the surface landmarks: the surface landmark for jugular vein bulb
is 1 cm anterior and 1 cm below the mastoid process.
• The measurements made by the catheter should be calibrated, in vivo or in vitro,
per information provided by the manufacturer.
• If the catheter tip is misplaced and attached to the vessel wall or if the catheter
tip has move in either direction (cephalic or caudal) more than 2 cm from the
bulb of jugular vein, the readings would be biased due to venous sample “con-
tamination” and would fall in the biased range of measurement.
• Also, the speed of sampling should be less than 2 mL/min, to prevent mixing of
cerebral venous blood with the venous blood from extra cranial veins; otherwise,
mixing of these two samples would result in erroneous over-readings, falsely
above the real normal values.
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5.7.3 Complications of SjvO
2
Catheter
Complications of SjvO
2
catheter insertion are similar to the complications of central
venous catheter insertion and could classifi ed as those related to catheter insertion
(like tissue or arterial injuries) and those related to the presence of catheter (like
possible increased risk of catheter infection or thrombosis).
5.7.4 Data Collection by SjvO
2
Catheter
Normal values for SjvO
2
are between 55 and 85 %; however, most studies have
declared the normal value for SjvO
2
in the range of 55–75 %. Anyway, values lower
than 50 % are considered abnormal and accompanied with poor outcome in the
majority of studies.
As mentioned above, SjvO
2
is an indirect surrogate index of global cerebral
perfusion; so, it has a high specifi city but its sensitivity is low; in other words,
SjvO
2
could detect global cerebral ischemia but cannot detect exactly the loca-
tion of the ischemic region in cerebral hemispheres. Also, as mentioned by
White and Baker, the following factors could affect the accuracy of SjvO
2
readings:
• Simultaneous hemoglobin concentration
• Saturation of the systemic arterial blood
• Core body temperature
• Level of CO
2
in the arterial blood
5.7.5 Arterial-Jugular Vein Oxygen Gradient (AjvDO
2
)
Another important index measured by calculating SjvO
2
is the gradient between
arterial and jugular vein oxygen content, abbreviated as AjvDO
2
. Schell and Cole
have done this calculation using the sequence of following formulas:
DO CBF CaO
22
=´
CMRO CBF CaO CjvO
222
=´ -
()
AjvDO CaO CjvO
22 2
=-
in which:
D O
2
: cerebral O
2
delivery
CBF: cerebral blood fl ow
CaO
2
: arterial O
2
content
CMRO
2
: cerebral O
2
consumption
CjvO
2
: jugular vein O
2
content
AjvDO
2
: arterial-jugular vein oxygen gradient
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Considering the above equations, AjvDO
2
could be calculated by solving the
above formulas:
AjvDO
CMRO
CBF
2
2
=
Schell and Cole have declared that normal AjvDO
2
values are between 4 and
8 mLO
2
/100 mL of blood. Values less than 4 denote that O
2
delivery is more than O
2
consumption (i.e., luxury O
2
delivery), while values above 8 demonstrate O
2
short-
age in brain tissue which could be due to increased CMRO
2
or decreased CBF
(desaturation state). According to Schell and Cole, each of the above ranges for
AjvDO
2
is associated with a number of differential diagnoses discussed here.
SjvO
2
> 75 or AjvDO
2
< 4 (Luxury O
2
Delivery): This is due to decreased CMRO
2
(like hypothermia, administration of sedatives), increased CBF (like hypothermia),
increased arterial oxygen content, or other causes like bran death.
SjvO
2
< 50 or AjvDO
2
> 8 (Desaturation State): This is due to the following etiolo-
gies which should be considered and ruled out in the postoperative period for
patients undergoing cardiac surgery:
• Increased CMRO
2
(like fever, hyperthermia, or seizure)
• Decreased CBF (like systemic hypotension, cerebral arterial vasospasm,
hyperventilation- induced hypocapnia)
• Increased intracranial pressure (like brain edema or impaired cerebral venous
return)
• Arterial hypoxia (impaired ventilation due to lung pathology or ventilator prob-
lems, impaired hemoglobin oxygenation, or impaired transfer and delivery of
oxygen to tissues including brain)
According to White and Baker, in cardiac surgery patients, low systemic perfu-
sion pressure during cardiopulmonary bypass, low hematocrit values, and rapid
rewarming are the three main causes of intraoperative low SjvO
2
(Hatiboglu and
Anil 1992 ; Sheinberg et al. 1992 ; Dearden and Midgley 1993 ; Croughwell et al.
1995 ; von Knobelsdorff et al. 1997 ; Gupta et al. 1999 ; Ip-Yam et al. 2000 ; Nandate
et al. 2000 ; Schell and Cole 2000 ; Shaaban Ali et al. 2001 ; Kadoi et al. 2002 ; Smythe
and Samra 2002 ; White and Baker 2002 ; Chieregato et al. 2003 ; Kadoi and Fujita
2003 ; Kawahara et al. 2003 ; Perez et al. 2003 ; Sarrafzadeh et al. 2003 ; Oh et al.
2004 ; Stocchetti et al. 2004 ; Alten and Mariscalco 2005 ; Shimizu et al. 2005 ; Wright
2007 ; Rohlwink and Figaji 2010 ).
5.8 Transcranial Doppler (TCD)
TCD was presented to the clinical practice for the fi rst time by Rune Aaslid and
colleagues in 1982 in order to assess blood fl ow velocities in cerebral arteries. TCD
is a noninvasive monitor applicable to either operating room or intensive care unit
patients, both in adult or pediatric patients. TCD is used to assess CNS vascular
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status through measurement of cerebral blood fl ow velocity ( of course not the blood
fl o w ); usually the mean fl ow velocity ( FV mean ) is used for assessments. This
device is based on using ultrasonic Doppler signal, in the low frequency (1–2 MHz)
using pulse-waved Doppler; the Doppler shift in signals gained during passage of
red blood cells through the cerebral arteries is calculated by the device to demon-
strate the blood fl ow velocity in the arteries under assessment.
Although electroencephalography (EEG) could assess the integrity of physio-
logic function in the cortical regions of brain, the subcortical areas are usually
neglected by EEG; this is while the cognitive functions are mainly affected by the
latter nuclei, while the blood fl ow to such regions is assessed more effectively by
TCD; however, these two technologies assess different aspects of CNS integrity and
function, and this is why their complementary use is more justifi ed in higher-risk
patients than their competitive comparison. Such high-risk patients may benefi t,
regarding CNS outcome, in the perioperative period by using both devices.
The following characteristics are among the main features of for assessment of
the cerebral blood fl ow:
• Noninvasiveness
• Fast
• Real-time assessment
• Relatively low price
• Accurate
• Continuous monitoring
• The possibility for repeated assessments (reproducible)
• Indirect information from CNS arteries
• Application both in the operating room and outside of it (including the intensive
care unit)
• Application to all age ranges
TCD has been used for cardiac surgery patients mainly during the intraoperative
period to detect and prevent sandblasting events, microemboli and macroemboli
caused by aortic clamping and manipulations; if there is severe aortic atherosclero-
sis, preventive surgical methods should be used to prevent stroke in high risk of
stroke; even, it may lead to an important change in the surgical strategy to prevent
unwanted postoperative CNS insults. However, in postoperative period, more wide-
spread use of TCD could be anticipated. Some of the potential applications of TCD
in the perioperative period of cardiac surgery patients could be as:
• Adequacy of CBF and arterial blood fl ow through the cerebral arteries
• Assessment of the CNS arterial patency (e.g., to rule out cerebral arterial
vasospasm)
• Noninvasive monitoring of intracranial pressure (ICP)
• Assessment of brain death
• Assessment of CNS autoregulatory response (i.e., CNS cardiovascular response
to a spectrum of stimuli like hypercapnia or blood pressure changes)
• Assessment of postoperative CNS perfusion in patients undergoing cardiac sur-
gery (with or without cardiopulmonary bypass) with underlying borderline
carotid stenosis
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• Assessing the effi cacy of antithrombotic therapies in reducing the platelet
embolic load sent to the CNS
• Measurement of effective downstream pressure
The arterial fl ow is mainly determined by the following factors:
• Cerebral blood fl ow (CBF)
• The inherent vascular anatomy
• The anatomy of the collateral circulation and their perfusion ratio
• The insonation window and hence insonation angle
• The anatomical diameter of the artery
These confounding factors could affect the reliability of the assessments:
• Underlying hemodynamic status especially blood pressure (including systolic,
diastolic, and mean arterial blood pressure)
• Arterial pressure of CO
2
• Gender; female have a bit higher arterial velocity, possibly to compensate for
lower hematocrit values than men
• Age (cerebral blood fl ow has the lowest values after birth, i.e., 25 cm/s; reaching
to its peak at the age of 4–6 years, i.e., 100 cm/s and then decreasing gradually
with a steady velocity to reach 40 cm/s at the seventh decade of life)
• Diurnal time pattern of blood pressure and CNS perfusion (blood velocity
remains the lowest at 11 A.M)
• Patient posture (i.e., the arterial fl ow is different while the patient is seated com-
pared to supine position)
• Intracranial pressure (ICP)
• Level of consciousness, including administration or discontinuation of analgesic
regimens
For gaining the Doppler data, one needs appropriate acoustic (or insonation) win-
dow; this window is usually an anatomic area in which the bone plate of the skull is
thin or there is a normal anatomic window in the skull; the following locations are the
main appropriate acoustic (or insonation) windows in the skull for performing TCD:
1 . Transtemporal window (the supra-zygomatic portion of the temporal bone) usually
used for assessment of anterior, middle, and posterior cerebral arteries (ACA, MCA,
and PCA); this is the most commonly used approach for TCD; for performing TCD
through this window, the patient should be supine, and an imaginary line is drawn
between tragus of ear and the lateral canthus of the eye; the area from this lineup to
about 2 cm should be searched while the probe is perpendicular to the bone and gel
is applied; MCA blood fl ow has a good correlation with total cerebral blood fl ow;
this is among the reasons why using this window has gained great popularity.
2 . Transorbital window (through the eye globe) usually used for assessment of the
ophthalmic arteries and also some cavernous portions of the internal carotid
artery (i.e., the carotid siphon).
3 . Suboccipital window used for assessment of the posterior cerebral circulation,
mainly the basilar and vertebral arteries used for anterior cerebral artery “ACA”
and posterior cerebral artery “PCA.”
4 . Retromandibular window mainly for assessment of cervical segments of the
carotid artery.
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