Transit-Time Ultrasound Volume Flowmetry
Transit-time ultrasound flowmetry gives the surgeon quick, direct (real-time), quantitative measurements of pulsatile and average flow by applying a perivascular flowprobe around an exposed major cerebral vessel. The measurements either prompt clipping readjustment or they confirm preservation of flow and avoid unnecessary clip readjustment. Used with an acoustic couplant such as saline or cerebrospinal fluid, the probe does not constrict the vessel or require any direct contact with it. Measurements take less than five minutes per surgery and rarely require significantly more than routine dissection. The technology cannot discern residual aneurysm remnants. Flow in perforators or small vessels must be derived by measuring flow in larger branches.
Intraoperative Angiography
Angiography presents a visual (anatomical) image of a region of the circulatory system and is used both for intraoperative assessment of vessel patency and for assessment of residual aneurysm. Its invasiveness carries a certain risk. The time it takes to perform an intraoperative angiogram (20-30 min) may exceed the threshold for irreversible ischemia. Consequently, clip repositioning based on intraoperative angiography may be too late to avert a stroke. It is also impractical, for it requires the ready availability of an angiography team and equipment. When compared to conventional post-op angiograms, the quality of imaging is inferior and can lead to false positives or negatives.
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Fig. 1: Graph demonstrates that volume flow will decrease during a Grade II & III stenosis (75% occlusion), as flow velocity is spiking before dropping during a Graft IV stenosis (90% occlusion). (Adapted from Spencer P, Reid, JM, “Quantification of Carotid Stenosis with Continuous-Wave (C-W) Doppler Ultrasound,” Stroke 1979; 10(3) 326-330.)
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Microvascular Doppler Sonography
Microvascular Doppler uses a pen-tip sensor applied against the wall of a vessel exposed during surgery to hear how fast the blood is moving. It measures velocity, not volume flow. As demonstrated in Fig. 1, it is difficult to differentiate the degree of stenosis between robust and poor flow in a nonocclusive vessel compromise. Grade I stenosis exhibits the same flow velocity as Grade V. The velocities in a Grade II stenosis are similar to the velocities in Grade IV. Although velocity and Doppler profile analysis may also be used to assess the degree of potential flow compromise, it can be cumbersome and difficult, as the measurement is qualitative, not quantitative.
Electrophysiological Monitoring
Electrophysiological monitoring uses evoked somatosensory electrophysiological potentials (SSEP) of the cerebral cortex to avert cerebral ischemia during aneurysm surgery. This technology is labor intensive, influenced by anesthetic technique, prone to false negatives, dependent on vascular territory of interest and provides no anatomic information regarding specific vessel compromise.
Fluorescein Dye and Indocyanine Green (ICG) Video Angiography
Fluorescein angiography images vessels with light following intravenous administration of fluorescein dye. The image quality is problematic and there is potential for systemic adverse reactions. Fluorescence angiography Is a one-time procedure and cannot be repeated after a clip readjustment.
Similar to fluorescence angiography but using indocyanine green (ICG) as an indicator, this modality can only assess vessels directly visible to the surgeon through the operating microscope field. Therefore, vessels obscured by anatomy of hidden aneurysm remnants will not be readily identified. ICG video angiography can be repeated following clip reapplication, but there is a required delay of ten or
more minutes between injections. Nonocclusive but flow-limiting vessel compromise may be underestimated.
Observation of Pulse
“A pulse” only indicates that an artery connects to the heart. If there is a partial occlusion, the proximal pulse will increase, the distal pulse will decrease. Determining arterial obstruction from the pulse is a tenuous art form at best, but may provide the experienced surgeon with qualitative information in cases where the vessel can be palpated.
View Comparison Chart of Modalities >
Reference
Amin-Hanjani, Meglio, G., Gatto, R., Bauer, B.S., Charbel, F.T., “The Utility of Intraoperative Blood Flow Measurements in Aneurysm Surgery Using an Ultrasonic Perivascular Flow Probe,” Department of Neurosurgery, Univ. of Illinois at Chicago. 2005. Pre-publication (3041AH)
Last Updated 7/2005
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