13 TRANSCRANIAL DOPPLER - National TRANSCRANIAL DOPPLER ... application remains a goal shared by...
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13 TRANSCRANIAL DOPPLER
Peter J. Kirkpatrick and Kwan-Hon Chan
The study of large numbers of head-injured patientshas identified clinical and radiological features whichindicate the severity of the initial cerebral trauma.(Born et al., 1985; Jennett et al., 1979; Marshall et al.,1991; Miller, 1985; Miller, 1992). In some individuals,secondary neuronal injury follows the primary injury(Chan et al., 1992a; Gopinath et al., 1994; Kirkpatrick etal., 1995; Miller, 1986; Robertson et al., 1989). Thesesecondary insults may, in part, account for the poorpredictive value of early clinical findings. Untilrecently, the nature of secondary neuronal injuryfollowing severe head trauma has remained elusive.However, modern methods for monitoring variousphysical and biochemical parameters indicate thatpotentially adverse episodes can be detected (Boumaet al., 1992; Chan, Dearden and Miller, 1992; Chan,Miller and Dearden, 1992; Chan, Miller and Piper,1992; Chan et al., 1992a, b; 1993; Cruz et al., 1991; Cruz,1993; Czosnyka et al., 1994a; Jones et al., 1993;Kirkpatrick et al., 1994a, 1995, 1996; Kirkpatrick,Czosnyka and Pickard, 1996) and that some areimportant in terms of prognosis. Low cerebral bloodflow (CBF) values in the first few hours after injury,and profound cerebral hypoxia are events whichpredict a poor outcome (Bouma and Muizelaar, 1990;Chan et al., 1992a; Gopinath et al., 1994). Episodes ofhypoxia, hypotension and reduced cerebral perfusiondue to high levels of intracranial pressure (ICP) areimportant factors that reduce cerebral blood flow andoxygen delivery (Siesjo, 1992). Cerebral ischemiaensues if compensation by increased oxygen extrac-tion is incomplete. Since histological ischemic changesare found in the brains of up to 80% of patients dyingfrom head injuries, the provision for monitoringcerebral ischemic episodes appears important, and hasreceived wide attention (Kirkpatrick, Czosnyka andPickard., 1996). The reliable detection and quantifica-tion of such episodes has, however, proven difficult.
The depth and duration of secondary cerebralischemic episode varies, and recent evidence indicates
that such episodes may last only a few minutes (Cruz,1993; Kirkpatrick, Czosnyka and Pickard, 1996;Robertson et al., 1989). The need for real-time meas-urements is therefore apparent. The imaging methodsfor estimations of CBF and oxygenation using contrastagents and isotopes (xenon-enhanced CT, SPECT andPET) are invasive, and may require transfer of criticalpatients to specialized facilities (Bouma and Muize-laar, 1990; Choksey et al., 1991; Gonalaves et al., 1994;Marion, Darby and Yonas, 1991; Meixensberger, 1993;Obrist and Wilkinson, 1990). Most importantly, theyonly allow isolated estimations which will misstransient secondary episodes. Their routine use in theintensive care of head-injured patients is clearlyinappropriate. In response, monitors that allow thereal-time estimation of cerebral pathophysiologicalparameters have evolved and include the continuousmeasurement of ICP, arterial blood pressure (BP),cerebral perfusion pressure (CPP = BP ICP), and theindirect estimation of global cerebral oxygenation bymeasuring jugular venous oxygen saturation (SjO2)using jugular venous oximetry. Episodes of low CPPand SjO2 are now recognized as being detrimental(Chan et al., 1992a; Czosnyka et al., 1994a; Gopinath etal., 1994) although the exact values at which cerebralischemia occurs probably varies with time andbetween individuals (Chan, Miller and Dearden,1992). The continuous measurement of CBF for clinicalapplication remains a goal shared by many.
Transcranial Doppler (TCD) provides a means ofmeasuring relative changes in CBF by observing bloodflow velocity (FV) in basal cerebral arteries (Aaslid,Markwalder and Nornes, 1982). The method doesrequire a certain degree of technical expertise, but isnon-invasive, relatively inexpensive and provides real-time information with high temporal resolution. TCDcan be used for measuring flow velocities from severalvessels of the circle of Willis (Aaslid, 1986; Figure 13.1),but most published data refer to the middle cerebralartery (MCA). This vessel has a favorable orientation(see below), is readily accessible to TCD insonation andprovides the most reliable flow velocity signal with a
Head Injury. Edited by Peter Reilly and Ross Bullock. Published in 1997 by Chapman & Hall, London. ISBN 0 412 58540 5
244 TRANSCRANIAL DOPPLER
high signal-to-noise ratio. Further, the MCA deliversapproximately 7080% of the ipsilateral carotid arteryblood flow and can therefore be considered to reflectblood flow to the majority of the ipsilateral cerebralhemisphere. Bilateral real-time estimations are nowavailable using a dual-channel facility. Most commer-cial machines provide analog signals with provision fordigital logging, making on-line analysis of the FVsignal possible and the collection of data on bedsidecomputers convenient.
Before discussing the application of TCD in thehead-injured patient, attention to the theoretical lim-itations of the technique and their relevance inpractical terms is necessary.
For a detailed description of TCD theory, the readeris referred to the excellent descriptions provided byrespected workers in the field (Newell and Aaslid,1992a, b).
13.2 The theory of TCD sonography
13.2.1 PHYSICAL PRINCIPLES
The shift in frequency of a wave when either thetransmitter or the receiver are moving with respect tothe wave propagating medium was described byDoppler in 1843 and is accordingly known as theDoppler effect. The difference in frequency is knownas the Doppler shift.
In a pulsed ultrasound Doppler instrument, thesame transducer is used for both transmitting andreceiving wave energy. The moving blood acts as areflector, first receiving the transmitted ultrasoundwave from the transducer and then reflecting theultrasound wave back toward the transducer.
The simplified formula for the Doppler shift ( f)from the moving blood with a velocity v is:
f = 2 f0v/c
where f0 and c are the frequency and velocity of theemitted ultrasound wave respectively.
Insonation of the basal vessels of the circle of Williswas made possible by the development of a highfrequency pulsed Doppler technique designed topenetrate skull bone. (Aaslid, Markwalder and Nornes,1982). The frequency necessary for transcranial Dop-pler (TCD) applications is in the order of 2 MHz.
It is seldom that blood within a vessel is movingdirectly toward or away from the transducer. Moregenerally, it will be moving in a direction at an angleof insonation to the ultrasound beam. The blood flowvelocity measured using TCD is thus dependent onthe angle of insonation, and can vary according totechnique. For the middle cerebral artery (MCA) theerror is small and of the order 3%, since the directionof proximal segment of the MCA is such that, ifextrapolated, it would meet the pterional bone at anear right angle (Figure 13.1).
Figure 13.1 Diagram showing the different anatomical locations of the basal cerebral arteries from which Doppler signalsare obtained. Flow detected from the middle cerebral artery (middle trace) is towards the probe and is shown as a positive(above zero baseline) waveform. This is in contrast to the normal flow direction in the anterior cerebral artery, which is awayfrom the probe (negative flow upper trace). The posterior cerebral artery also produces a positive waveform.
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The TCD recorded velocity varies with the realblood velocity according to the cosine of the angle ofinsonation. For angles of insonation between 0 and30, the observed velocity varies between 0.87 and 1.0of the true velocity. The anatomical limitations fortranstemporal insonation of the MCA are such thatsignal capture is only possible at narrow angles. Thusthe observed velocity is a close approximation of truevelocity at a typical depth of insonation of 56 cmfrom the scalp surface (1 cm less for children). Otheranterior circulation basal vessels have a less favorableorientation, hence variation in FV derived from themis much more dependent on the technique of insona-tion (Figure 13.1).
13.2.2 RELATIONSHIP BETWEEN INTRACRANIALMCA BLOOD FLOW VELOCITY AND CBF
TCD measures the velocity of red blood cell movingwithin a vessel. It has a single dimension per unit oftime (usually quoted in cm/s). In contrast, CBF is athree-dimensional measure of volume of blooddelivered per unit of cerebral tissue per unit of time(usually quoted in ml/100 g/min). The two para-meters are quantitatively different; hence the correla-tion between absolute values of FV and CBF is poor.However, provided the cross-sectional area of theinsonated blood vessel remains constant, the CBF andFV should vary directly with one another. The MCAdiameter appears to be relatively constant underchanging conditions of BP and carbon dioxide tensionin the normal brain, as demonstrated in healthyvolunteers (Aaslid et al., 1989; Dahl et al., 1989; Newellet al., 1994) thus good correlations between relativechanges in FV and CBF have been reported experi-mentally (Barzo et al., 1991; Czosnyka et al., 1994c;Richards et al., 1995) and in clinically stable patients(Bishop et al., 1986; Romner et al., 1991). However inunstable patients with acute brain injury, the MCAdiameter may be altered by changes in ICP and theeffects of vasospasm, reducing the reliability of therelationship between relative CBF and FV (Kontos,1989).
Some workers have indicated that the relationshipbetween FV and CBF varies according to the level ofCBF, with a stronger correlation occurring when CBFwas low (less than 20 ml/100 g/min; Halsey, McDowelland Gelmon, 1986). These observations may reflectdifferent states of autoregulation and a changing MCAdiameter, although pressure autoregulati