NEUROSURGICAL INTENSIVE CARE 1042-3680/94 $0.00 + .20 TRANSCRANIAL DOPPLER ULTRASONOGRAPHY David W. Newell, MD BASIC PRINCIPLES OF of the examination techniques have been pub­ TRANSCRANIAL DOPPLER lished elsewhere and describe the various ULTRASONOGRAPHY methods for obtaining signals from different intracranial arteriesY Various techniques are Transcranial Doppler ultrasonography available for identification of the individual (TCD) is a method introduced by Aaslid et intracranial arteries. The most common is the 1 5 al • to record blood flow velocity in the basal hand-held technique, which utilizes the ex­ cerebral arteries in humans. Early investiga­ aminer's knowledge of the vascular anatomy tors using ultrasound to measure blood flow and vascular interrelationships as well as the concluded that recording from the intracranial characteristic signal generated by the various arteries would be impossible due to the barri­ arteries examined. 1• 15 Additional techniques ers imposed by the human skull. It was, how­ include vessel mapping as well as transcranial ever, possible to evaluate the cerebral circula­ color-coded real-time ultrasonography (color­ 5 10 tion by recording from the carotid arteries in flow) techniques. • the neck, and this application has been refined Transcranial Doppler study makes use of extensively and widely used in clinical medi­ two important principles to measure patho­ cine. The groundwork for the development logical changes in the cerebral circulation. of transcranial applications was made possible First, under conditions of constant flow, the by improvements in Doppler equipment velocity through a vessel will increase in pro­ along with the use of a relatively low fre­ portion to the decrease in cross-sectional area quency (2 MHz) and the introduction of spec­ produced by vessel narrowing. Second, in the tral analysis and pulsed range-gated Doppler. absence of any significant changes in vessel Initial recordings were made of the middle diameter, changes in flow will be directly pro­ cerebral artery through the transtemporal portional to changes in the average cross­ window. It was subsequently recognized that sectional velocity. Changes in the maximal a more complete examination of the basal ce­ flow velocity (V max or spectral outline) will be rebral arteries could be obtained by recording proportional to changes in average velocity in through the orbit via the transorbital route the absence of any turbulence or altered flow and also through the foramen magnum via patterns in the vessel. 6 It is also essential to the transforaminal route. Utilizing these three maintain a fixed probe position in relation to windows, extensive examination of the basal the artery during any calculation of relative cerebral arteries can be accomplished. Details flow changes. From the Department of Neurological Surgery, University of Washington, Seattle, Washington NEUROSURGERY CLINICS OF NORTH AMERICA VOLUME 5 • NUMBER 4 • OCTOBER 1994 619 620 NEWELL 2 54 55 The velocity signal in the basal cranial arter­ neurosurgeons. • • The subsequent ex­ ies, particularly in the middle cerebral artery, tended applications of TCD have made it of can therefore be monitored and used to indi­ interest also to anesthesiologists, intensivists, cate relative flow changes under a variety of and vascular surgeons. Vascular technology circumstances. 57 This information can be used training programs are now including TCD in the intensive care unit setting to assess cere­ methodology in their curricula. Many vascu­ lar laboratories in the United States now offer bral reactivity in response to C02 changes, blood pressure changes, and changes in intra­ TCD as a clinical service. We established a cranial pressure (ICP), as well as during vari­ transcranial Doppler laboratory in the Depart­ ous spontaneous waves in the ICP and in ment of Neurological Surgery at the Univer­ sity of response to certain medications. TCD moni­ Washington in 1986 and have found it essential to have highly skilled personnel with toring can therefore be used to accomplish a background in vascular ultrasound who are many of the objectives that previously re­ trained to perform the examinations. TCD is quired more cumbersome techniques for mea­ a difficult vascular examination and requires surement of cerebral blood flow (CBF). Mea­ training and practice. Maintenance of a high­ surement of CBF following head injury has quality laboratory requires a high volume of been a subject of intense investigation. After examinations as well as constant feedback in the collection of much data, Langfitt and the form of correlation with clinical condition 3 Obrist2 defined some of the potential clinical and radiographic studies including angio­ applications for CBF studies as follows: graphy. 1. To ensure sufficient CBF to meet meta­ Some of the requirements for obtaining con­ bolic demands of the brain. tinuous monitoring tracings were incorpo­ 2. To aid in predicting clinical outcome. rated into the first transcranial Doppler design 3. To assess vasoreactivity in the cerebral by Aaslid. 5 These features included a monitor­ circulation by testing autoregulation and ing headband to ensure probe stability as well as the ability to sample spectral information C02 reactivity. 4. To evaluate the effect of various thera­ and the spectral outline as analog informa­ pies for management of ICP. tion. A variety of devices can be used to record Transcranial Doppler monitoring cannot give quantitative blood flow data and, there­ fore, cannot be used for the first two applica­ tions mentioned. The ability to calculate rela­ tive blood flow changes in response to C02 as well as perfusion pressure changes and the ability to calculate relative blood flow changes to various therapeutic regimens enables TCD to be used for the third and fourth applications mentioned. 42 Advantages over the radioactive xenon CBF method include the fact that TCD provides continuous relative blood flow infor­ mation, can be performed relatively easily us­ ing portable equipment in the intensive care Spectrum Outline unit, and does not involve the use of radioac­ 100 tive isotopes. REQUIREMENT FOR 0 EXAMINATIONS AND MONITORING A The development of TCD has been a natural Figure 1A. Recording from the middle cerebral artery extension of vascular ultrasound. The initial that yields a spectral tracing. The spectral tracing can application of TCD to detect cerebral vaso­ then be assigned a spectral outline that can be used spasm following subarachnoid hemorrhage for monitoring relative changes in blood flow. made it of much interest to neurologists and Illustration continued on opposite page TRANSCRANIAL DOPPLER ULTRASONOGRAPHY 621 continuously either the spectral signal or the as the audio portion using a video recorder. analog signal from the spectral outline. 30• 42 To record relative blood flow changes, the Different recording devices are needed for spectral outline which can be optionally as­ various applications. For example, when per­ signed to the spectral signal can be recorded forming monitoring of emboli, it is most use­ ful to record the entire spectral signal as well using a variety of devices including magnetic tape and strip charts or by converting it to a 150 100 -ABP 50 0 5 10 8 Seconds 150 50 150 200 250 300 c Seconds Figure 18. Simultaneous recordings of arterial blood pressure, bilateral middle cerebral artery velocity, intracranial pressure, and end tidal C02 are illustrated. C. Recording of similar signals as in 8 as a longer trend. ABP = arterial blood pressure; MCA = middle cerebral artery; LMCA = left middle cerebral artery; RMCA = right middle cerebral artery; ICP = intracranial pressure; ETC0 = end tidal C0 • 2 2 622 NEWELL digital signal and using computerized storage setting and that may have an impact on TCD devices. The last option has been refined and velocity values. is now being incorporated into TCD equip­ ment that offers software for continuous mon­ itoring. Computerized storage of digitized sig­ VASOREACTIVITY nals offers trend analyses as well as software to analyze various short intervals that are The ability of the cerebral circulation to un­ commonly used for reactivity testing. The de­ dergo vasoconstriction and vasodilatation in velopment of multi-channel Doppler ultraso­ response to various stimuli has been termed nography has allowed continuous recording vasoreactivity. Many of the currently used of multiple channels, most commonly, both treatments for lowering ICP, such as hyper­ middle cerebral arteries. This feature allows ventilation, mannitol therapy, and barbiturate simultaneous testing of both hemispheres therapy, rely on intact vasoreactivity for their during reactivity testing (Fig. 1). action.11. 12,32-35,45 Conditions leading to hospi­ talization in the neurosurgical intensive care unit such as subarachnoid hemorrhage, intra­ PHYSIOLOGIC FACTORS cranial hemorrhage, or head injury can impair AFFECTING BLOOD vasoreactivity and therefore make it difficult FLOW VELOCITY to control ICP. By monitoring relative ch. nges in CBF velocity, TCD can be used to evaluate Normative data on blood flow velocities in C02 reactivity, autoregulation, and responses 16 42 the basal intracranial arteries of humans have to various medications. • 18• 30• 37, -44 been collected by various investigators and Transcranial Doppler ultrasonography can summarized by Adams et aU Some of the be used to calculate C02 reactivity in the inten­ variables affecting velocities have included sive care unit