Thrombus size and Doppler embolic signal intensity

Cerebrovascular events after cardiovascular surgery: diagnosis, management and prevention strategies

Introduction

Cerebrovascular events after cardiac surgery are among the most serious complications, related to a greater risk of patient mortality. This problem can occur following the formation of gas emboli during open heart surgery.

Aim

To address all the mechanisms that can lead to embolic events after cardiovascular surgery, how to manage them and how to possibly prevent them.

Material and methods

A search of the PubMed database was conducted. We reviewed the clinical literature and examined all aspects to identify the root causes that can lead to the formation of emboli.

Results

Among the studies reviewed, it was found that the main causes include manipulation of the aorta, inadequate deaeration after cardiac surgery, and blood-component contact of extracorporeal circulation. It has been reported that gas emboli can lead to deleterious damage such as damage to the cerebral vascular endothelium, disruption of the blood-brain barrier, complement activation, leukocyte aggregation, increased platelet adhesion, and fibrin deposition in the microvascular system.

Conclusions

Stroke after cardiovascular surgery is one of the most important complications, with a great impact on operative mortality and patient survival. Efforts have been made over time to understand all the pathophysiological mechanisms related to this complication, with the aim of reducing its incidence. One of the goals should be to improve both the surgical technique and the perfusion modality and minimize the formation of air bubbles or to facilitate their elimination during the cardiopulmonary bypass procedure.

Microparticle discrimination using laser feedback interferometry

In this work, we present a method to discriminate between different microparticle sizes in mixed flowing media based on laser feedback interferometry, which could ultimately form the basis for a small, low-cost, real-time microembolus detector. We experimentally evaluated the performance of the system using microparticle phantoms, and the system achieved approximately 45% positive predictive value and better than 98% negative predictive value in the detection and classification of abnormally large particles.

Numerical prediction of thrombus risk in an anatomically dilated left ventricle: the effect of inflow cannula designs

BACKGROUND

Implantation of a rotary blood pump (RBP) can cause non-physiological flow fields in the left ventricle (LV) which may trigger thrombosis. Different inflow cannula geometry can affect LV flow fields. The aim of this study was to determine the effect of inflow cannula geometry on intraventricular flow under full LV support in a patient specific model.

METHODS

Computed tomography angiography imaging of the LV was performed on a RBP candidate to develop a patient-specific model. Five inflow cannulae were evaluated, which were modelled on those used clinically or under development. The inflow cannulae are described as a crown like tip, thin walled tubular tip, large filleted tip, trumpet like tip and an inferiorly flared cannula. Placement of the inflow cannula was at the LV apex with the central axis intersecting the centre of the mitral valve. Full support was simulated by prescribing 5 l/min across the mitral valve. Thrombus risk was evaluated by identifying regions of stagnation. Rate of LV washout was assessed using a volume of fluid model. Relative haemolysis index and blood residence time was calculated using an Eulerian approach.

RESULTS

The inferiorly flared inflow cannula had the lowest thrombus risk due to low stagnation volumes. All cannulae had similar rates of LV washout and blood residence time. The crown like tip and thin walled tubular tip resulted in relatively higher blood damage indices within the LV.

CONCLUSION

Changes in intraventricular flow due to variances in cannula geometry resulted in different stagnation volumes. Cannula geometry does not appreciably affect LV washout rates and blood residence time. The patient specific, full support computational fluid dynamic model provided a repeatable platform to investigate the effects of inflow cannula geometry on intraventricular flow.

The curative effect comparison of two kinds of therapeutic regimens on decreasing the relative intensity of microembolic signal in CLAIR trial

BACKGROUND

Microembolic signals (MESs) are direct markers of unstable large artery atherosclerotic plaques. In a previous study, we found that the number of MESs is associated with stroke recurrence and that clopidogrel plus aspirin more effectively reduce the number of MESs than does aspirin alone. Stroke recurrence is associated with not only the number of MESs but also the size of the MES, which can theoretically be estimated by monitoring the MES intensity via transcranial doppler (TCD). Thus, we compared the effects of clopidogrel and aspirin with aspirin alone on MES intensity using TCD.

METHODS

We recruited 100 patients who experienced acute ischemic stroke or transient ischemic attack (TIA) within 7days of symptom onset. All patients also had large artery stenosis in the cerebral or carotid arteries and the presence of MES as revealed by TCD. The patients were randomized to receive either aspirin or clopidogrel and aspirin for 7days. MES monitoring was performed on days 2 and 7.

RESULTS

Intent-to-treat (ITT) analysis (46 patients in the dual therapy group, 52 patients in the monotherapy group) and per-protocol (PP) analysis (25 patients in the dual therapy group, 31 patients in the monotherapy group) were performed on 98 patients. The primary finding was that the MES intensity was dramatically reduced in the dual therapy group. ITT analysis of the dual therapy group revealed that the MES intensity was 8.04 (0-16) dB before treatment, 0.00 (0-17) dB on day 2, and 0.00 (0-12) dB on day 7 (P=0.000). In the monotherapy group, the MES intensity was 9.00 (0-20) dB before treatment, 8.25 (0-17) dB on day 2, and 7.0 (0-18) dB on day 7 (P=0.577). PP analysis revealed similar results. No severe hemorrhagic complications were detected. The two patients in this study who experienced stroke recurrence were in the monotherapy group.

CONCLUSIONS

Clopidogrel and aspirin more effectively decrease the MES intensity than aspirin alone in patients with large artery stenotic minor stroke or TIA.

An in vitro comparison of embolus differentiation techniques for clinically significant macroemboli: dual-frequency technique versus frequency modulation method

The ability to distinguish harmful solid cerebral emboli from gas bubbles intra-operatively has potential to direct interventions to reduce the risk of brain injury. In this in vitro study, two embolus discrimination techniques, dual-frequency (DF) and frequency modulation (FM) methods, are simultaneously compared to assess discrimination of potentially harmful large pieces of carotid plaque debris (0.5-1.55 mm) and thrombus-mimicking material (0.5-2 mm) from gas bubbles (0.01-2.5 mm). Detection of plaque and thrombus-mimic using the DF technique yielded disappointing results, with four out of five particles being misclassified (sensitivity: 18%; specificity: 89%). Although the FM method offered improved sensitivity, a higher number of false positives were observed (sensitivity: 72%; specificity: 50%). Optimum differentiation was achieved using the difference between peak embolus/blood ratio and mean embolus/blood ratio (sensitivity: 77%; specificity: 81%). We conclude that existing DF and FM techniques are unable to confidently distinguish large solid emboli from small gas bubbles (<50 μm).

Sizing gaseous emboli using Doppler embolic signal intensity

Extension of transcranial Doppler embolus detection to estimation of bubble size has historically been hindered by difficulties in applying scattering theory to the interpretation of clinical data. This article presents a simplified approach to the sizing of air emboli based on analysis of Doppler embolic signal intensity, by using an approximation to the full scattering theory that can be solved to estimate embolus size. Tests using simulated emboli show that our algorithm is theoretically capable of sizing 90% of "emboli" to within 10% of their true radius. In vitro tests show that 69% of emboli can be sized to within 20% of their true value under ideal conditions, which reduces to 30% of emboli if the beam and vessel are severely misaligned. Our results demonstrate that estimation of bubble size during clinical monitoring could be used to distinguish benign microbubbles from potentially harmful macrobubbles during intraoperative clinical monitoring.

New microembolus size estimator for peripheral blood vessels

Several factors affecting the power of Doppler scattered signal and, consequently, microembolus size estimation, may be eliminated when assessing the microembolus size via multiple measurements. A new microembolus size estimator is proposed based on the ratio of microembolus scattering cross-section in two directions and for two emission frequencies. Theoretical considerations indicate that the estimation of size of microembolic elements should be independent of the spatial distribution of the wave intensity, tissue attenuation and hardware factors. The simulation results indicate that this estimation only slightly depends on the material of the microembolus and acoustic properties of blood. The experimental results indicate that the accuracy of median size estimation increases with microembolus size. The measurement error is less than 27% for microemboli with median diameter larger than 360 μm. The method is constrained to the estimation of microembolus size in the vessels of extremities.