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56
F. A. Rasulo and N. Zugni
Table 6.1 (contiuned)
Minimal skill
Identication
of other
anatomical
structures
Optic nerve sheath
diameter
Third ventricle (blue line)
Brainstem (red line)
Measurement of the
midline shift
Contralateral temporal
skull bone
L
FVmMCA
FVmICA
6 The Minimal, Intermediate, andAdvanced Skills: How toBoost Your Competencies
Table 6.1 (contiuned)
Minimal skill
Diagnosis of
brain
pathologies
Table 6.2 Intermediate skill
Intermediate skill
Identication and
insonation of arteries
Diagnosis of vasospasm
Lindegaard Index (L.I.)
Identication and
insonation of the
internal ophthalmic
artery
I
..=
57
Identication of
other anatomical
structures
Lateral ventricles
(continued)
58
L
FVmMCA
Table 6.2 (continued)
Intermediate skill
Diagnosis of brain
pathologies
Diagnosis of cerebral
hyperemia and
Lindegaard Index (L.I.)
Intracerebral
hemorrhages (sub,
extradural hemorrhage,
intracranial
hemorrhage) (red line)
Diagnosis of cerebral
circulatory arrest for the
conrmation of brain
death
F. A. Rasulo and N. Zugni
I
..=
FVmICA
Assessment of
cerebrovascular
autoregulation: CO2
reactivity
Diagnosis of
hydrocephalus
Third ventricle (red
line)
ΔCBFV change in CBFV in cm/s per mmHg, ΔPaCO2 change in PaCO2 tension
Absolute CO2
reactivity=ΔCBFV/ΔPaCO
2
C
()
21
C
()
21
6 The Minimal, Intermediate, andAdvanced Skills: How toBoost Your Competencies
Table 6.3 Advanced skill
Advanced skill
Diagnosis of
brain
pathologies
Assessment of
critical closing
pressure
Assessment of
cerebrovascular
time constant (τ)
Diagnosis of venous
pathology (straight
sinus)
rCPABP
=⋅−
τ=C
∙CVR
a
1
1
CVRCaHR
⋅⋅ ⋅
2
+
π
59
Critical closing pressure is
the arterial blood pressure
threshold, below which
small arterial vessels
collapse and cerebral blow
ow ceases
The cerebrovascular time
constant represents the
product of compliance of
cerebral arterial bed and
the vascular resistance
distal to the place of
insonation
τ cerebral arterial time constant (seconds), Ca cerebral arterial compliance, CVR cerebrovascular resistance, CrCP criti-
cal closing pressure, ABP arterial blood pressure, HR heart rate
6.4 Learning Through
Technological Aid
rCPABP
=⋅−
1
1
CVRCaHR
⋅⋅ ⋅
2
+
π
We are at present living in an era of technologi-
In the advanced level the expert sonographer is
capable of performing the most difcult tests and
monitoring techniques which use BUS, such as
evaluation of cerebral compliance, cerebrovascular
time constant, and venous insonation. At this point
the sonographer is experienced enough to act as a
tutor and teach BUS to other students (Table6.3).
cal simulation, not only in medicine but also in
sports, military, commercial pilot training, and
so on. In medicine, simulators are being used
for surgical procedures, anesthesia (difcult
intubation for example), CPR, and obstetrics.
Ultrasound teaching may also be guided through
the use of simulators [14] for example:
60
F. A. Rasulo and N. Zugni
• Simulators
• Automatic signal retriever
• Smartphone probes and software
• Integration with other monitoring systems or
imaging devices (EEG, intraoperative MRI,
MMM)
• Improvements in signal quality (3D imaging)
6.5 Learning Through Guidelines
andPractice Standards
Practice parameters and technical standards are
not inexible rules or requirements of practice
and are not intended to be used in order to establish a legal standard of care.
It should be recognized that adherence to
practice parameters will not assure an accurate diagnosis or a successful outcome. All that
should be expected is that the practitioner will
follow a reasonable course of action based on
current knowledge, available resources, and consensus. The sole purpose of practice parameters
is to assist practitioners in achieving an improvement in outcome.
Factors such as qualication, responsibilities of the physician, and written request for
the examination may differ between countries,
cities, and even hospitals within the same city
itself. A fair example of variation in guidelines
is present for the use of BUS for the diagnosis
of cerebral circulatory arrest. In a recent paper
[15] the authors highlight the great deal of variability which exists among centers and countries
regarding brain death (BD) determination and
state. Although consensus guidance is available
to standardize national processes for the diagnosis of BD, the current variation and inconsistency
in European practice make it imperative that an
international consensus is developed.
Regarding BUS, many societies have created
task forces in an attempt to unify guidelines and
practice standards. The American Academy of
Neurology (AAN), for example, published the
2004 Guidelines for the use of TCD and provide
the indications, sensitivity, specicity, and reference standards for the most common pathologies
evaluated by TCD [16]. However, this document
has been retired by the Guideline Development,
Dissemination, and Implementation
Subcommittee on February 23, 2018, due to no
updates or reafrmation in 5years or less after
the previous publication. Hence, the recommendations and conclusions in all retired guidelines
are considered no longer valid and no longer supported by the AAN.
More recently, the American Society of
Neurophysiologic Monitoring (ASNM) and
American Society of Neuroimaging (ASN)
Guidelines Committees formed a joint task force
and developed guidelines to assist in the use of
transcranial Doppler (TCD) monitoring in the
surgical and intensive care settings [17].
Specically, these guidelines delineate the
objectives of TCD monitoring, characterize the
responsibilities and behaviors of the sonographer
during monitoring, and describe methodological
and ethical issues uniquely relevant to monitoring. They stress that in order to perform a quality
examination, the acquisition and interpretation of
intraoperative TCD ultrasonograms be performed
by qualied individuals, and that the service providers dene their diagnostic criteria and develop
ongoing self-validation programs of these performance criteria in their practice.
In 2010, a multidisciplinary panel of experts
reviewed the published literature on TCD from
1982 through December 2009. Given the emphasis on accreditation of vascular laboratories
[18] they emphasize a need for standardization
of scanning and interpretation processes and
initiated the development of a series of standards and guidelines by experts in transcranial
Doppler and members of the American Society
of Neuroimaging Practice Guidelines Committee
as well as international neuro-sonological
organizations.
Despite the increasing use of brain ultrasound, this technique still remains underused.
An example is presented for one of TCD’s most
common indications in the intensive care unit,
aneurysmal subarachnoid hemorrhage (aSAH)
for surveillance of cerebral vasospasm (CV)
[19]. The authors in this study performed an
6 The Minimal, Intermediate, andAdvanced Skills: How toBoost Your Competencies
61
analysis of nationwide trends in TCD prevalence
by using Nationwide Inpatient Sample (NIS)
data from 2002 to 2011. Teaching hospitals were
examined separately for TCD utilization rates.
The objective was to estimate the proportion of
patients with aSAH receiving TCD monitoring
using the NIS.In teaching hospitals, 2% of the
aSAH patients (95% CI 1.0–4.0) underwent TCD
examination. TCD utilization increased from
<1% during the 2002–2005 period to ≥1.5% during the 2006–2011 period (odds ratio 2.3, 95%
CI 1.0–5.7), an increase also seen in nonteaching
hospitals. They concluded that TCD is underused
nationally in the care of aSAH and that the prevalence of TCD is nearly nonexistent in nonteaching hospitals.
The Intersocietal Commission on
Accreditation of Vascular Laboratories (ICAVL)
[18] has established guidelines for the certication of laboratories making or interpreting
diagnostic ultrasonic measurements of cerebral
blood ow velocity (CBFV) with transcranial
Doppler (TCD) ultrasonography. However, of
the more than 950 ICAVL-approved facilities,
less than 2% are certied for intracranial or TCD
measurements.
Numerous training programs pertaining to
various scientic societies are available and
technological advances in both hardware and
software are making it much easier to obtain the
necessary competence. These include simulators,
certication courses, and user-friendly ultrasound machines.
6.6 Competence
Competence for transcranial color-coded duplex
sonography is rapidly acquired [2].
A broad spectrum of usage scenarios has been
proposed for transcranial Doppler sonography
(TCD) in the intensive care setting (ICU) as a
method to assess intracerebral hemodynamics,
including detection of vasospasm in subarachnoid hemorrhage, arterial steno-occlusive dis-
ease, estimation of intracranial pressure, and
determination of brain death.
As a bedside, easy-to-access, and noninvasive
method, TCD is an attractive tool.
In one study, untrained as compared to trained
TCD operators estimated blood ow velocity
with wide variation, impairing the clinical usefulness of TCD when performed by untrained
operators. In contrast, no similar studies have
been performed in TCCD [20].
It has been shown that TCCD applied to measure the mean ow velocity (MFA) in the MCA
is an easy-to-learn tool yielding accurate and
reliable measurements in volunteers even in the
hands of untrained operators. Competence for
transcranial color-coded duplex sonography is
rapidly acquired [2]. Overall, there was a good
agreement between measurements of untrained
and trained operators. A short-term learning program including an introduction session followed
by either ve supervised and ve non-supervised
examinations (supervised group) or ten nonsupervised examinations (non-supervised group)
was assessed. The supervised program yielded
a more rapid and accurate learning curve due
to active supervision in addition to the effect of
repetitive measurements.
Neulen et al. [21] in a recently published
study examined the aspect of “image guidance,”
where acquired image data were combined with
a TCD system allowing anatomic orientation.
Experienced operators were asked to identify
cerebral vessels by conventional TCD while
inexperienced operators were challenged with
the same task supported by “image guidance.”
While TCD performed by experienced operators
was determined with a mislabeling rate of 37%,
image guidance reduced the mislabeling rate to
10%. Anatomic orientation and visual guidance
facilitate the correct identication of the vessel
of interest. Especially in the setting of inexperienced operators, this advantage may facilitate the
performance of reliable measurements and may
very well explain the favorable learning curve of
TCCD we found in the present study.
62
F. A. Rasulo and N. Zugni
References
1. https://www.asnweb.or/i4a/pages/index.cfm?pageID
=4028&acriveFuII=true
2. Klinzing S, Steiger P, Schüpbach RA, et al.
Competence for transcranial color-coded duplex
sonography is rapidly acquired. Minerva Anestesiol.
2015;81(3):298–304.
3. Echocardiography. Accreditation in adult critical
care echocardiography. https://www.bsecho.org/
media/161652/cc_accreditation_pack_2015.pdf.
4. Bartels E.Transcranial color-coded duplex ultrasound
possibilities and limits of this method in comparison
with conventional transcranial Doppler ultrasound.
Ultraschall Med. 1993;14:272–8.
5. Schoning M, Buchholz R, Walter J.Comparative study
of transcranial color duplex sonography and transcranial Doppler sonography in adults. J Neurosurg.
1993;78:776–84.
6. Rasulo F, et al. Visualizing impending cerebral circulatory arrest caused by intracranial hypertension
following aneurysmal subarachnoid hemorrhage. J
Neurosurg Anesthesiol. 2017;29(1):64–6.
7. Becker G, Bogdahn U, Strassburg HM, et al.
Identication of ventricular enlargement and estimation of intracranial pressure by transcranial
color-coded real-time sonography. J Neuroimaging.
1994;4:17–22.
8. Seidel G, Gerriets T, Kaps M, Missler U.Dislocation
of the third ventricle due to space-occupying stroke
evaluated by transcranial duplex sonography. J
Neuroimaging. 1996;6:227–30.
9. Cardim D, Robba C, Bohdanowicz M, et al. Noninvasive monitoring of intracranial pressure using
transcranial Doppler ultrasonography: is it possible?
Neurocrit Care. 2016;25:473–91.
10. Robba C, Santori G, Czosnyka M, etal. Optic nerve
sheath diameter measured sonographically as noninvasive estimator of intracranial pressure: a systematic review and meta-analysis. Intensive Care Med.
2018;44:1284–94.
11. Robba C, Cardim D, Tajsic T, et al. Ultrasound
non-invasive measurement of intracranial pressure
in neurointensive care: a prospective observational
study. PLoS Med. 2017;14:e1002356. https://doi.
org/10.1371/journal.pmed.1002356.
12. Rasulo FA, Bertuetti R, Robba C, etal. The accuracy of
transcranial Doppler in excluding intracranial hypertension following acute brain injury: a multicenter
prospective pilot study. Crit Care. 2017;21(1):44.
13. Robba C, Poole D, Citerio G, etal. Brain ultrasonography consensus on skill recommendation and competence levels within the critical care setting. Neurocrit
Care. 2019;32:502.
14. Parks AR, Atkinson, Verheul G.Can medical learners
achieve point-of-care ultrasound competency using a
high-delity ultrasound simulator? A pilot study. Crit
Ultrasound J. 2013;5:9.
15. Robba C, Iaquaniello C, Cierio G.Death by neurologic
criteria: pathophysiology, denition, diagnostic criteria and tests. Minerva Anestesiol. 2019;85(7):774–8.
16. Sloan MA, Alexandrov AV, Teheler CH, et al.
Assessment: transcranial Doppler ultrasonography: report of the Therapeutics and Technology
Assessment Subcommittee of the American
Academy of Neurology. Neurology. 2004;62(9):
1468–81.
17. Edmonds HL, Isley MR, Sloan TB, etal. American
Society of Neurophysiologic Monitoring and
American Society of Neuroimaging joint guidelines
for transcranial Doppler ultrasonic monitoring. J
Neuroimaging. 2011;21(2):177–83.
18. http://www.icavl.org
19. Kumar G, Albright KC, Donnelly, et al. Trends in
transcranial Doppler monitoring in aneurysmal subarachnoid hemorrhage: a 10-year analysis of the
nationwide inpatient sample. J Stroke Cerebrovasc
Dis. 2017;26(4):851–7.
20. McMahon CJ, McDermott P, Horsfall D, Selvarajah
JR, King AT, Vail A.The reproducibility of transcranial Doppler middle cerebral artery velocity measurements: implications for clinical practice. Br J
Neurosurg. 2007;21:21–7.
21. Neulen A, Greke C, Prokesch E, et al. Image guidance to improve reliability and data integrity of transcranial Doppler sonography. Clin Neurol Neurosurg.
2013;115(8):1382–8.
Part II
Basic and Advanced Parameters
Flow Velocity, Pulsatility Index,
Autoregulation, andCritical
Closing Pressure
MartaFedriga andMarekCzosnyka
Contents
7.1 Introduction 65
7.2 Flow Velocity 66
7.3 Pulsatility Index 68
7.4 Critical Closing Pressure 68
7.5 Autoregulation 70
7.5.1 Static Autoregulation 70
7.5.2 Dynamic Autoregulation 70
7.5.3 Continuous Monitoring ofAutoregulation 71
References 72
7
7.1 Introduction
Transcranial Doppler (TCD)has been named the
‘stethoscope for the brain’. Apart from cerebral
blood ow velocity and its waveform, secondary indices and derived formulae can be detected.
M. Fedriga
Brain Physics Laboratory, Division of Neurosurgery,
Department of Clinical Neurosciences, University of
Cambridge, Cambridge, UK
Department of Anesthesia, Critical care and
Emergency, Spedali Civili University Hospital,
Brescia, Italy
M. Czosnyka (*)
Brain Physics Laboratory, Division of Neurosurgery,
Department of Clinical Neurosciences, University of
Cambridge, Cambridge, UK
e-mail: mc141@medschl.cam.ac.uk
© Springer Nature Switzerland AG 2021
C. Robba, G. Citerio (eds.), Echography and Doppler of the Brain,
https://doi.org/10.1007/978-3-030-48202-2_7
These measurements might have several useful
clinical applications which have been studied
during the last decades demonstrating that brain
ultrasonography has a strong potential as a safe,
non-invasive repeatable device in acute braininjured patients. However, the strength of this noninvasive technique might become even stronger
when other more invasive assessments of brain haemodynamic are contraindicated, such as in hepatic
failure or during infectious disease. Moreover, it
can be a useful in the intraoperative and perioperative setting. Even though brain ultrasound over the
years has been considered an essential part of the
clinical assessment and management of patients
we cannot deny that it has limitations such as the
necessity of patent transcranial acoustic windows
and operator dependency. TCD and transcranial
colour-coded Doppler (TCCD) have been considered relatively simple methods; however specic
65
66
()
M. Fedriga and M. Czosnyka
skills are required not only to recognise different
landmarks and brain’s anatomical features but
rst and foremost to interpret properly the number
given by the machine. In addiction with transcranial Doppler, we should remember not to be stuck
on a single measurement: what is more important
indeed is to track the trend of measurements in the
patient clinical pathway taking all the indexes and
derived calculations together in order to draft a
complete physiopathological picture of the patient
we are studying and treating.
7.2 Flow Velocity
The velocity of red blood cells owing through
the large vessels of the brain can be detected via
TCD and TCCD ultrasonography. The Doppler
principle was rst described by Christian Doppler
measured velocity real velocity cosine of angleofinciden=´ cce .cos
in 1843 as the frequency shift, the measured difference in frequencies between the original signal
(sent by the ultrasound machine), and the reected
signal. The Doppler frequency shift is directed
proportionally to the ow velocity of red blood
cells and is usually expressed in centimetres per
second.
Flow velocity (FV) can be used as a surrogate descriptor of cerebral blood ow (CBF); it
permits dynamic, non-invasive monitoring with
good temporal and spatial resolution. However,
we must be aware of the two main assumptions
that govern the use of TCD: the constant diameter
of the insonated vessel and an unchanged angle
of insonation [1, 2].
The velocity measured by the probe is
described by the following formula:
q
Therefore, when the angle is 0°, the cosine is
1 and the measured velocity is equal to the real
velocity. At 90°, cosine is 0, and it is not possible
to detect the ow velocity. As a consequence, the
detection of real velocity is limited by anatomical
constraints that derive from the position and the
course of the vessels with respect to the probe.
To counter this, the use of the newer, more
advanced TCCD enables operators to visualise the insonated artery and modify the angle
of insonation for more accurate estimation of
blood velocity. However, as long as the angle
of insonation remains constant, changes in the
detected velocity reect changes in the true velocity and therefore changes in cerebral blood ow.
Another crucial factor that affects the interpretation of TCD velocity is the cross-sectional
area of the insonated vessel since the volume
which passes through a vessel depends on both
the velocity of red cells and the diameter of the
vessel. The diameter of the vessel should, therefore, remain stable during the measurement if the
operator wants to detect a true velocity.
The range of normal ow velocity values for
adults was rst determined by Aaslid etal. [2] in
1982, and was veried during direct intraoperative
Doppler measurements [3]. These values have thus
been adopted as the standard values used by other
authors during the last decades; see Table7.1.
Table 7.1 The table represents the normal range of values of the main brain arteries considering the mean ow
velocity as determined by Aaslid etal. [2]
ARTERY Mean velocity (cm/s)
MCA M1 46–86
ACA A1 41–76
PCA P1 33–64
EICA 37
TICA 60
OPHTHALMICartery 20
CAROTID SIPHON 55
VERTEBRALartery 27–55
BASILARartery 30–57
MCA middle cerebral artery, ACA anterior communicating artery, PCA posterior communicating artery, EICA
extracranial internal carotid artery, TICA terminal internal
carotid artery