Julia L. Smith

Prevention of postoperative thrombotic stroke after carotid endarterectomy: The role of transcranial Doppler ultrasound

PURPOSE To determine the incidence of particulate embolization after carotid endarterectomy (CEA), the effect of dextran-40 infusion in patients with sustained postoperative embolization, and the impact of transcranial Doppler (TCD) monitoring plus adjuvant dextran therapy on the rate of postoperative carotid thrombosis. METHODS Prospective study in 100 patients who underwent CEA with 6-hour postoperative monitoring using a TCD that was modified to allow automatic, intermittent recording from the ipsilateral middle cerebral artery waveform (10 minute sample every 30 minutes). An incremental dextran-40 infusion was commenced if 25 or more emboli were detected in any 10-minute period. RESULTS Overall, 48% of patients had one or more emboli detected in the postoperative period, particularly in the first 2 hours. However, sustained embolization that required Dextran therapy developed in only five patients. In each case, the rate of embolization rapidly diminished. CONCLUSIONS A small proportion of patients have sustained embolization after CEA, which in previous studies has been shown to be highly predictive of thrombotic stroke. Intervention with dextran reduced and subsequently stopped all the emboli in those in whom it was used and contributed to a 0% perioperative morbidity and mortality rate in this series.

Interpretation of Embolic Phenomena During Carotid Endarterectomy

BACKGROUND AND PURPOSE Air and particulate emboli are a major source of morbidity during carotid endarterectomy (CEA); however, amplitude overload and poor time resolution have restricted the ability of transcranial Doppler ultrasound to differentiate between the two. METHODS We have now overcome these two limitations by (1) rerouting embolic signals away from the audio frequency amplifier to avoid amplitude overload and (2) substituting the Wigner distribution function for the fast Fourier transform to improve time and frequency resolution. Thus, we can now accurately determine embolic duration and embolic velocity, the product of which is the sample volume length (SVL). This measurement represents the physical distance over which an embolic signal can be detected. The underlying hypothesis was that air reflected more ultrasound and would therefore be detected over a greater SVL. RESULTS The median SVL (interquartile range) for 75 in vitro air emboli was 1.97 cm (range, 1.70 to 2.35) compared with 0.27 cm (range, 0.16 to 0.43) for 185 particulate emboli detected during the dissection phase of CEA. Off-line analysis on an additional 560 embolic signals detected during different phases of CEA suggested that 46 of 143 (32%) of emboli immediately after shunt insertion were particulate, as were 19 of 33 (58%) occurring during shunting, 28 of 78 (36%) after restoration of flow in the external carotid artery, 23 of 251 (9%) after restoration of flow in the internal carotid artery, and 55 of 55 (100%) of those emboli detected during the early recovery phase. CONCLUSIONS This development provides objective physical criteria upon which embolus characterization (particulate/air) can be based. This could have major implications for future patient monitoring with respect to modification of surgical technique and pharmacological intervention.

On-table diagnosis of incipient carotid artery thrombosis during carotid endarterectomy by transcranial Doppler scanning

We present a case where transcranial Doppler ultrasound monitoring of a carotid endarterectomy enabled us to detect the incipient thrombosis of the operated artery before reversal of anesthesia. The use of transcranial Doppler ultrasound monitoring in carotid endarterectomy has the potential to detect this complication before serious neurologic damage has occurred and therefore reduce the morbidity and mortality rates associated with the operation.

Quality control during endovascular aneurysm repair: monitoring aneurysmal sac pressure and superficial femoral artery flow velocity.

Purpose: To use intraoperative aneurysmal sac pressure measurement and flow monitoring of the superficial femoral artery (SFA) to ensure complete exclusion of the aneurysm from the circulation. Methods: A 5F catheter was positioned in the aneurysmal sac of 15 consecutive patients undergoing endovascular aortomonoiliac aneurysm repair between February and September 1997. The catheter was connected to an external pressure transducer allowing pressure monitoring throughout the operation and for 24 hours postprocedurally. Flow velocity was monitored in the contralateral SFA by insonation with a 2-MHz Doppler ultrasound probe. Results: No technical defect was observed in the deployment of 10 endografts, which demonstrated marked reduction in sac pressure and good flow in the lower limb. The mean aneurysm pressure dropped from 123 to 57 mmHg after graft insertion. In 5 cases, monitoring detected problems during the endograft procedure. In 3, incomplete stent deployment was detected by a failure of sac pressure to fall following stent inflation and by the presence of flow in the contralateral femoral artery. In the other 2 cases, a distal endoleak was detected by direct injection of contrast into the sac. Conclusions: Measuring aneurysm pressure in combination with SFA Doppler flow monitoring can detect complications of endovascular aneurysm repair.

A Comparison of Four Methods for Distinguishing Doppler Signals From Gaseous and Particulate Emboli

BACKGROUND AND PURPOSE Many reports in the medical literature have proposed methods of differentiating between gaseous and particulate emboli detected with the use of transcranial Doppler ultrasound. The purpose of this study was to compare the previously published methods with our own sample volume length (SVL) parameter to assess the accuracy of each method in classifying emboli. METHODS A pure source of gaseous and particulate emboli was obtained from in vitro and in vivo studies, respectively, and recorded onto digital audiotape for off-line analysis. In total, 100 gaseous emboli and 215 particulate emboli were analyzed to measure four embolic parameters, namely, embolic duration, embolic velocity, relative signal intensity increase (measured embolic power [MEP]), and SVL of the embolic signal (= Duration x Velocity). Receiver operator characteristic analysis was used to assess the optimum threshold for each parameter to differentiate between particulate and gaseous emboli, and levels of sensitivity and specificity were calculated. RESULTS Embolic duration and velocity produced the poorest levels of sensitivity and specificity compared with the MEP and SVL parameters. The optimum thresholds for embolic duration and velocity were 35 ms and 1 m/s, respectively, which produced a sensitivity (specificity) of 85.1% (87%) and 87% (67%), respectively. The optimum MEP and SVL thresholds were 30 dB and 12.8 mm, respectively, which produced a sensitivity (specificity) of 86.5% (95%) and 93% (97%), respectively. The SVL and MEP parameters were compared statistically (chi 2) at chosen specificity values of 90%, 95%, 97%, 99%, and 100%, which showed that the SVL sensitivities were statistically greater than MEP sensitivities (P < 0.01). CONCLUSIONS SVL is the best parameter for differentiating between gaseous and particulate emboli but needs to be calculated with the use of a high-temporal-resolution spectral analyzer to measure embolic duration and velocity.

Differentiation between emboli and artefacts using dual-gated transcranial Doppler ultrasound

It is well documented that transcranial Doppler ultrasound has the ability to detect cerebral emboli. During intraoperative patient monitoring studies, many signals due to artefact (probe motion, patient movement or surgical manipulation) are also detected and can be difficult to distinguish from genuine embolic events. We have constructed a Doppler system that can simultaneously range-gate at two separate depths, in order to test the hypothesis that it should be possible to distinguish between emboli and artefact by comparing the signal from the two separate regions within the vessel. The classification algorithm is based on the principle that emboli propagate with blood motion (whereas artefacts do not) and thus will be detected sequentially at different depths along the insonated cerebral artery. One hundred thirty-eight (presumed) embolic and 170 artefact signals were analysed. The median (interquartile range) gate separation was 10.01 mm (7.41-10.78 mm). The time delay between detection of embolic signals in the two channels was 11.04 ms (6.24-16.41 ms, but was only 0.08 ms (-0.48(-)+0.64 ms) for artefact (p < 0.0001). Dual-gated Doppler ultrasound is a conclusive and independent method that differentiates emboli from artefact. Incorporation of this system for long-term monitoring may eliminate the need for an experienced observer to be present.

Processing Doppler ultrasound signals from blood-borne emboli

Abstract Analysing Doppler ultrasound signals from blood-borne emboli has been hindered by two basic problems. Firstly, the ratio of the signal levels from blood and emboli is greater than the dynamic range of conventional Doppler instruments used to detect such emboli. This causes amplitude overload, which must be eliminated before accurate analysis of embolic data can be performed. Secondly, the temporal resolution of fast Fourier transforms (FFT) usually used to analyse Doppler signals, is insufficient to quantify accurately such short duration signals. This paper describes methods to overcome these two problems. The Doppler signal is recorded from close to the front-end of the velocimeter, and a Wigner distribution analyser is used to provide Doppler spectra with both high temporal and frequency resolution. The resulting sonographic display allows quantitative measurement of embolic events to be made.

Analysis of the frequency modulation present in doppler ultrasound signals may allow differentiation between particulate and gaseous cerebral emboli

The purpose of this study was to investigate the incidence of frequency modulation in Doppler signals from cerebral emboli and to seek possible explanations for its occurrence. Signals from 200 particulate emboli and 200 presumed gaseous emboli were studies. The Doppler signals were visualised in the time domain and were classified into three main types. Type I signals contained no modulation, type II signals showed gradual frequency changes and type III showed a rapid change evident in only a small percentage of the entire signal. Type I signals were observed from 71.5% of particulate emboli but only 19% of gaseous emboli (X(2)) = 111, p < 0.001). Type II signals were found in 28.5% of particulate emboli and 38% of gaseous emboli (X(2)) = 4.06, p < 0.05). The most surprising and significant finding was that 43% of gaseous embolic signals were categorised as type III signals compared with 0% of signals from particulate emboli (X(2)) = 109, p < 0.001). The finding that known particulate emboli appear never to produce rapid frequency modulation may provide a basis for differentiating between gaseous and particulate emboli.

Signals from dual gated TCD systems: Curious observations and possible explanations

Dual gated transcranial Doppler (TCD) systems have the potential to distinguish definitively embolic events in the cerebral circulation from artefact. While developing such a system, three curious observations were made and are the subject of this article. The first was that the velocity of propagation of an embolus in an artery as calculated from the ultrasonic Doppler shift was, in many cases, dramatically different from the velocity calculated using a time of flight method. The second observation was that the durations of embolic signals measured in two gates were often very different, despite the received gates being identical in length. The third observation was that some emboli detected in the deeper gate did not appear in the more superficial gate, and, more surprisingly (as has recently been reported by another group), some emboli that had not previously appeared in the deeper gate appeared in the more superficial gate. It is hypothesised that all of these effects can be explained in terms of the geometry of the middle cerebral artery in relation to the interrogating ultrasound beam. None of these effects should detract from the usefulness of TCD, but will need to be widely appreciated if dual gated measurements of cerebral emboli are to be interpreted correctly.

Time domain analysis of embolic signals can be used in place of high-resolution Wigner analysis when classifying gaseous and particulate emboli

The sample-volume length (SVL) of an embolic signal has previously been used to differentiate between gaseous and particulate emboli and has been calculated using high-resolution Wigner analysis. Although successful, this method of analysis is not widely available to other groups using transcranial Doppler ultrasound (TCD) to classify emboli. The SVL of embolic signals can also be calculated using time domain analysis, which is a far simpler method and potentially available to all TCD users. The aim of this study was to compare the SVL of embolic signals calculated using Wigner analysis and time domain analysis to assess whether or not time domain analysis can replace Wigner analysis to classify emboli. In total, 215 particulate and 100 gaseous emboli were recorded onto digital audiotape and analysed off-line. Two SVLs for each embolic signal were calculated by measuring embolic duration and velocity in the time domain and with Wigner analysis. Receiver operator characteristic (ROC) curves were plotted to assess the optimum SVL threshold for each method, and levels of sensitivity and specificity were defined. The optimum SVL threshold using Wigner analysis was 1.28 cm, yielding 93% sensitivity and 97% specificity. Using time domain analysis, the optimum threshold was 1.12 cm, yielding 90% sensitivity and 96% specificity. The methods were compared statistically (chi2) using their optimum thresholds, and were found not to be statistically different for classifying particles p=0.283) or gaseous emboli (p=0.700). This study has shown that the SVL of embolic signals, used to differentiate particulate from gaseous emboli, can be calculated more simply in the time domain, which yields as accurate results as calculating the SVL using Wigner analysis.