MONITORS OF CEREBRAL BLOOD FLOW
Transcranial Doppler Monitoring
Transcranial Doppler (TCD) ultrasound is an easily applied, direct,
noninvasive measurement of CBF. TCD is a diagnostic tool that has found application
as a monitor in the operating room relatively recently.[201]
TCD uses sound waves to measure the velocity of blood flowing in the basal arteries
of the brain. Sound waves are transmitted through the relatively thin temporal bone
( Fig. 38-21
). When these
sound waves come in contact with blood, they are reflected off the red blood cells
through the brain and skull to a detector. The velocity of the sound waves reflected
to the surface is changed because the blood cells themselves are in motion toward
or away from the sound wave detector. This phenomenon is known as the Doppler
shift and is directly related to the velocity and flow direction of the
blood cells. The velocity of the blood cells is faster during systole and slower
during diastole. The blood in the center of the lumen moves faster than the blood
near the vessel wall. A spectrum of flow velocities is produced. This spectrum
resembles the shape of the waveform produced by the pulse oximeter or an arterial
pressure transducer ( Fig. 38-22A
).
TCD measurements of flow velocity are most commonly and easily made in the middle
cerebral artery but may also be measured in other arteries, including the anterior
cerebral, anterior communicating, posterior cerebral, posterior communicating, and
basilar arteries.
Two assumptions must be made for TCD-measured blood flow velocity
to have a direct relationship to CBF. First, blood flow velocity is directly related
to blood flow only if the diameter of the artery where the flow velocity is measured
and the measurement angle of the Doppler probe remain constant. Second, CBF in the
basal arteries of the
Figure 38-21
Schematic of transcranial Doppler ultrasonography. Sound
waves are transmitted through the relatively thin temporal bone and reflected from
red blood cells moving in the basal arteries of the brain.
Figure 38-22
A, Normal transcranial
Doppler (TCD) tracing from the middle cerebral artery at a depth of 55 mm. B,
TCD tracing from a patient with cerebral vasospasm. Notice the very high flow velocities
beyond the ability of the machine to quantify. C,
TCD pattern from the middle cerebral artery in a case of brain death. Notice the
brief systolic inflow of blood followed by flow reversal in diastole.
brain must be directly related to cortical CBF. Neither of these assumptions has
been proved scientifically. The TCD has not been validated in a large series of
intraoperative cases against established monitors of cerebral flow such as the EEG
or against direct cortical CBF measurements.
The major reported intraoperative use of the TCD involves testing
adequacy of CBF while the carotid artery is cross-clamped during carotid endarterectomy.
Studies comparing measured CBF with the velocity in the middle cerebral artery have
not shown a good correlation between the two measurements.[214]
The period of carotid cross-clamping is only a relatively small portion of the time
that the patient with carotid disease is at risk for ischemia, and most strokes occur
at other times during or especially after surgery. Few data are available about
the nature and degree of acceptable TCD changes during the remainder of the operation.
Normal variations in blood flow velocities during surgery in patients without cerebrovascular
disease appear to be large (Pashayan AG, Mahla ME: unpublished data). With increasing
clinical experience with TCD during carotid endarterectomy, it has become one of
the accepted monitors of CBF during carotid surgery, especially with respect to the
detection of emboli. Preoperative TCD screening may be useful in defining which
asymptomatic patients are at higher risk for stroke and may benefit from prophylactic
surgery.[215]
Surgeons using TCD monitoring have
been able to alter their technique and reduce intraoperative microembolization.[216]
TCD is becoming increasingly accepted as useful for the detection of emboli after
completion of arterial repair, and studies are appearing that demonstrate the utility
of TCD in detecting an abnormally high rate of postoperative embolization after carotid
surgery.[217]
[218]
[219]
[220]
[221]
These patients have been shown to be at higher risk for postoperative cerebral ischemia,
and the implication is that excessive postoperative emboli counts merit urgent re-exploration
of the arterial repair.
TCD is also used during CPB to detect air or particulate emboli
during cannulation, during bypass, when weaning from bypass, and during decannulation.
[222]
Several small clinical studies have suggested
that TCD monitoring is useful in identifying portions of the procedure with the highest
risk for microemboli, potentially allowing alterations in the procedure to decrease
microembolic events. The number of microemboli has been suggested to be predictive
of postoperative neurologic dysfunction; however, most studies are relatively small,
and the clinical utility of TCD emboli counts is still undefined.[223]
[224]
[225]
[226]
[227]
Ease of application and interpretation of
the characteristic spectra produced by emboli make this a very attractive monitor
for cardiac surgery, but outcome data proving utility are still lacking.
TCD has been used in the intensive care unit to document the presence
and severity of cerebral vasospasm after subarachnoid hemorrhage. As the major cerebral
arteries narrow, flow velocity within the lumen must increase if blood flow is to
be maintained (see Fig. 38-22B
).
Mean flow velocities of more than 120 cm per second seem to correlate well with
angiographic vasospasm,[228]
[229]
and the TCD is increasingly used for noninvasive, early detection of vasospasm, allowing
therapy to be initiated before the onset of clinical symptoms.[230]
[231]
[232]
[233]
[234]
The TCD has also been used in the intensive care unit as an aid
to the diagnosis of brain death. Studies have demonstrated a characteristic blood
flow velocity pattern in patients who are clinically brain dead (see Fig.
38-22C
).[235]
TCD studies are easily
performed at the bedside and can be used to determine whether definitive studies
documenting brain death (which may require transporting the patient) need to be performed.
Further studies documenting the sensitivity and specificity of this test in the
diagnosis of brain death are needed before this test can be used as a sole criterion
for brain death.
