Computerized discrimination of microemboli in extracorporeal circuits

Computerised techniques for detecting gaseous microemboli in blood using pulsed Doppler ultrasound

Pulsed Doppler ultrasound and spectral analysis were used to monitor nonpulsatile blood flow generated in a test rig. Two computerised techniques are described for detecting microemboli in blood by analysis of Doppler blood velocity data (sonagram).

The two ultrasound microemboli indices identify different features in the ultrasound signals to detect microembolic phenomena. Both indices showed significant increases ( p < 0.001) for samples of agitated blood (containing gaseous microemboli) as compared to normal blood injected sequentially into the test rig. The linear relationship demonstrated between data obtained by the two methods ( r = 0.91, p < 0.01) indicates that both are providing similar quantitative information regarding the number of microemboli detected.

These two computerised techniques may be applied to sonagrams obtained from arterial blood flow and thereby provide information regarding the presence of gaseous microemboli in the circulation during cardiopulmonary bypass surgery.

Doppler ultrasound estimation of microbubbles in the arterial line during extracorporeal circulation

A pulsed Doppler ultrasound system was used to analyse microbubble intensity and size in the arterial line during extracorporeal circulation (ECC). Thirty male patients, younger than 70 (range 28-69) years, underwent isolated coronary artery bypass grafting with either a bubble oxygenator (Shiley S-100) without (group 1, n = 10) or with (group 2, n = 10) a depth adsorption arterial line filter (Swank High Flow 6000); or with a membrane oxygenator (Shiley M-2000) without a filter (group 3, n = 10). Mean ECC and aortic crossclamp times were similar in the three groups. Measurements were performed during the initial five minutes of cooling, after 30-40 minutes of ECC and after 10 minutes of rewarming. Microbubble intensity and size did not differ significantly in the three groups at the different intervals. Significantly more and larger bubbles were detected in group 1 (15-150μm) compared to group 2 (< 35μm) (p< 0.001). In group 3 only a minimal number of small bubbles (< 65μm) were observed. An arterial line filter significantly reduced the number and size of microbubbles detected in the arterial line during ECC. A membrane oxygenator was associated with a further reduction of microbubble intensity.

The Artificial Lung Facility at Brown: More Than a Dozen Years With Pierre Galletti

Beginning in 1969, Pierre Galletti and his colleagues established a laboratory at Brown University, which, initially and for many years, was involved with artificial lungs. Various projects budded out of this, including the following: the establishment of standards for oxygenator performance, which anticipated Food and Drug Administration requirements; studies of thrombosis and other pathologies, which could be reduced by design changes; long-term, pumpless arterio-venous (A-V) bypass; early hybrid pancreas and other artificial organs; implantable lungs; and spin off companies. The current report outlines the history, philosophy, collaborations, and major results of the laboratory, as well as how the principal investigators simultaneously moved into research areas outside of artificial lungs in the late 1980s.

Estimate of the maximum absorption rate of microscopic arterial air emboli after entry into the arterial circulation during cardiac surgery

Some clinicians place patients in the Trendelenburg position during aortic unclamping to decrease the incidence of microscopic cerebral air embolism. Experimental studies have shown that use of the Trendelenburg position does not prevent air emboli from reaching the brain. Nevertheless, the position can decrease the velocity at which bubbles approach the brain, giving more time for nitrogen in the bubbles to be absorbed. We applied a validated mathematical model to estimate that the maximum rate of absorption of microscopic air emboli in blood during and after cardiopulmonary bypass (CPB) equals 1.57 microns/s. This rate is very small, since clinically relevant microbubbles have radii 50-1500 microns, and travel from the aorta to the brain within seconds. The result holds for all usual conditions of CPB. We conclude that absorption does not affect the disposition of air introduced into the arterial circulation. Use of the Trendelenburg position cannot decrease neurologic injury from cerebral air embolism by permitting greater bubble absorption.

Experimental aspects of high-intensity transient signals in the detection of emboli

Experimental studies in the 1960s and 1970s demonstrated the high sensitivity of Doppler ultrasound in detecting gaseous bubbles. More recent studies have shown that microscopic air bubbles, as well as glass microspheres as small as 5 mu to 20 mu, cause characteristic high-intensity signals. Recently it has been demonstrated that less echogenic embolic materials such as thrombus, platelet aggregates, and atheroma can also be detected with a high sensitivity. Such "solid," or formed-element, emboli as small as 200 mu to 400 mu can be detected; the lower size limit of detection was due to an inability to make smaller embolic particles rather than to the sensitivity of the detection process itself. Analysis of the Doppler signals provides some information about embolus size and composition, but accurate characterization in clinical practice is not possible using current technology. Studies in experimental models have allowed the detailed description of embolic signals; they appear as a short-duration, frequency-focused increase in intensity, predominantly unidirectional in the direction of flow, and usually contained within the spectral envelope. In contrast, artifacts appear as a bidirectional, high-intensity increase with maximum intensity at low frequencies. These differences have been exploited to develop automatic embolus detection programs, and an off-line version has been successfully validated in an experimental model.

Computerized detection of cerebral emboli and discrimination from artifact using Doppler ultrasound

BACKGROUND AND PURPOSE

Transcranial Doppler ultrasound can detect circulating cerebral emboli. Monitoring of patients with potential embolic sources may allow identification of high-risk patients who can then be selected for prophylactic treatment. However, practical patient monitoring will require automated programs that can detect emboli and differentiate them from artifact.

METHODS

A new off-line algorithm for the detection of emboli, which detects the characteristic relative power increase occurring with an embolus, was evaluated in both an animal model and in patients. (1) In a sheep model, solid embolic materials (thrombus, platelet aggregates, and atheroma) were introduced into the proximal carotid artery while the distal carotid artery or a major branch was insonated. The signals resulting from 77 emboli (mean size, 1.77 mm) were studied and compared with the Doppler signals resulting from artifact. (2) In patients, 100 embolic signals occurring in three patients were analyzed and compared with signals associated with artifact in the same patients.

RESULTS

(1) In the sheep model, emboli resulted in a short-duration, high-intensity signal, but intensity increase alone did not distinguish between emboli and artifact. In contrast, the algorithm discriminated embolus from artifact with a sensitivity of 98.7% and a specificity of 98.0%. (2) In patient studies, embolic signals were differentiated from artifact with a sensitivity of 97.2% and a specificity of 97.0% by the algorithm.

CONCLUSIONS

Using such an algorithm, detection of cerebral emboli and discrimination from artifact are possible with a high sensitivity and specificity. Incorporation of such an algorithm into an on-line system should make prolonged patient monitoring practical.

Transcranial Doppler detection of circulating cerebral emboli. A review

BACKGROUND

The identification of gaseous emboli using Doppler ultrasound was described as early as the 1960s. Recently it has been demonstrated that this method can also detect solid emboli such as thrombi and platelet aggregates. This may make this technology useful in a large number of patients with, or at risk of, embolic stroke.

SUMMARY OF REVIEW

Emboli appear as short-duration, high-intensity signals in the Doppler spectrum. The intensity of the Doppler signal from an artery containing an embolus depends on the density difference between the embolic material and blood. This difference is greatest for gaseous emboli, which are therefore the most easy to detect. Gaseous emboli have been demonstrated during deep-sea diving, and their presence correlates with the occurrence of decompression sickness. Similar signals have been detected during cardiopulmonary bypass. A relation has been demonstrated between the number of emboli detected by transcranial Doppler and a decline in neuropsychological function after cardiopulmonary bypass. Solid emboli such as thrombi and platelet aggregates result in less intense signals than air emboli. Their detection, using Doppler ultrasound, has recently been described in patients with prosthetic heart valves, atrial fibrillation, and carotid artery disease. It may also help in the detection and localization of embolic sources in patients with stroke. Studies in in vitro and in vivo models demonstrate that this technique provides information on the size and type of emboli. Larger emboli produce signals of greater intensity and duration. Practical patient monitoring will require automatic emboli detectors incorporated into the Doppler machine; such programs are being developed.

CONCLUSIONS

Detection of solid emboli using Doppler techniques offers an exciting new diagnostic tool. It has been demonstrated that the technique can detect solid emboli. The prognostic significance of such emboli remains to be determined. It is hoped that the technique will allow detection of patients at high risk of embolic stroke in whom appropriate prophylactic treatment can then be instituted.

When do cerebral emboli appear during open heart operations? A transcranial Doppler study

The transcranial Doppler technique enabled the detection of cerebral air emboli in 10 of 10 patients during open-heart valve operations despite standard deairing procedures. With this technique, the occurrence of emboli in the right middle cerebral artery was followed continuously in patients undergoing aortic or mitral valve replacement. Membrane oxygenators were used. Scattered emboli were observed during the insertion of the aortic cannula, at the start of cardiopulmonary bypass, and after the declamping of the aorta with the heart beating while empty. During the period of aortic cross-clamping, no emboli were detected. Despite careful deairing procedures, the recordings indicated a large amount of emboli during filling of the empty beating heart in all 10 patients. Thus, this study indicates that cerebral emboli in open heart procedures are most likely to occur during the redistribution of blood from the heart-lung machine to the patient when the heart is beginning to eject actively, despite careful standard deairing procedures. Meticulous deairing before declamping the aorta is strongly advocated. In addition, a short period of filling of the beating heart before final closure of the aortic incision or vent may decrease the incidence of cerebral emboli. A concomitant reduction in cerebral blood flow by hyperventilation or anesthetics or both during filling of the empty beating heart may also be beneficial.

Clinical comparison of two devices for detection of microemboli during cardiopulmonary bypass

Detection of gaseous microemboli during cardiopulmonary bypass procedures is important for the clinical evaluation of equipment such as oxygenators and cardiotomy reservoirs. Comparison of published data can be difficult if different detectors are used. Two devices reported in the literature, the Technique Laboratories TM-8 and the Hatteland BD-100, are compared during clinical procedures. The relationship between the outputs of these devices was linear over two ranges, the difference in output amounted to a standard deviation of 11% in the lower range and 38% in the upper range.

A computerized analysis of platelet aggregation detected by ultrasound

The purpose of this study was to determine if computerized image processing could be used to characterize platelet aggregation visualized by ultrasound. Citrated whole blood (WB) or platelet-rich plasma (PRP) was imaged with a 12 mHz transducer before and after the addition of adenosine diphosphate. The images were digitized using a 512 x 512 pixel resolution with 256 levels of gray. Raster interference was eliminated by filtering and aggregates were separated from background by gray level discrimination. The results showed that platelet aggregation in PRP can be described by the number of aggregates counted, their sizes or their gray levels. However, aggregation in WB can be detected only by the sizes or gray levels of the aggregates due to the greater echogenicity of ex vivo WB. This methodology, applied to ultrasonic images of brachial arteries and veins, demonstrates that venous particles are common in patients and normal controls, but arterial particles are more common in patients. Differences in the size and brightness of these particles may have prognostic implications.

The detection of microemboli in the middle cerebral artery during cardiopulmonary bypass: a transcranial Doppler ultrasound investigation using membrane and bubble oxygenators

Twenty-seven patients were examined who were undergoing cardiopulmonary bypass (CPB) surgery with either a bubble oxygenator or a capillary membrane oxygenator. The latter incorporated an arterial filter and bubble trap. A noninvasive Doppler ultrasound technique is described for monitoring irregularities in the Doppler flow signals attributable to gaseous microemboli detected in the middle cerebral artery during CPB. The ultrasound index for detecting gaseous microemboli (MEI) indicated the presence of such microemboli in 22 of the 27 patients during insertion of the aortic cannula. Measurements during CPB showed the MEI ranged from 4 to 39 in the 17 patients with a bubble oxygenator. However, all 10 patients with a membrane oxygenator had an MEI of 0. Varying the gas flow rates in 3 patients with bubble oxygenators showed a change in MEI from 4 +/- 4 (SD) at a flow rate of 2 L/min to 17 +/- 9 at a flow rate of 5 L/min. This observation supports the assumption that the MEI is providing quantitative information regarding the presence of gaseous emboli in the middle cerebral artery.

Integral Pulse Frequency Modulated Ultrasound for the Detection and Quantification of Gas Microbubbles in Flowing Blood

An integral pulse frequency modulated gas microbubble detector is described. When used in conjunction with an electromagnetic blood flowmeter, the ultrasound pulse repetition frequency can be regulated to reduce multiple counting errors due to variations in blood flow rate. A special detector mounting head eliminates problems due to wall curvature and pulsation, permits reliable and reproducible transducer coupling and makes it possible to gate the ultrasound field electronically in order to avoid spurious counts due to crystal and wall artefacts and low intensity regions.

Results obtained from the detector during an in vitro evaluation of bubble oxygenators suggest that the number of arterial line gas microbubbles is less than has been claimed previously and that the total volume of gas liberated in the form of microbubbles may be too small to have any demonstrable primary clinical effects.

Gas bubble detection in fluid lines by means of pulsed Doppler ultrasound

A pulsed Doppler instrument running at 1.5 MHz resonance frequency was used for detection of microbubbles within a recirculating fluid line. Glass beads of nominal 200 microns were injected to get a statistical calibration level of the set-up. This is achieved by identifying the peak intensity of the bubble size histogram which reflected the microsphere size.

Comparison of bubble release from various types of oxygenators

A comparative study of microbubble release from various types of oxygenators was performed using ultrasonic Doppler techniques. Bubble count versus amplitude histograms were calculated to derive the statistical distribution of the relative microbubble sizes. To compare the different oxygenators with respect to differences in microbubble releases, several key parameters as, temperature, liquid flow rate, gas to flow relationship, liquid level within the oxygenator, were altered one at a time to indicate different and oxygenator related sensitivities with respect to variations of the key parameters.

Defibrinogenation as an alternative to heparinization in prolonged extracorporeal circulation

Complications arising from difficulty controlled bleeding and thrombus formation during procedures which require extracorporeal circulation with heparin as anticoagulant motivate the search for better hemostasis and anticoagulant technology. An enzyme (Defibrase), having a specific interaction with fibrinopeptide A such as to cause fibrin depletion in a soluble form, has been proposed as an alternative to heparin. Defibrase, in contrast to heparin, does not affect blood platelet function. Heparin and Defibrase as anticoagulants were compared in 8 h perfusion studies using arterio-venous extracorporeal blood circuits in dogs; the circuitry included pumps, membrane oxygenators and filters serially. Thrombus formation, in the filters and the oxygenators as well as pathology of the perfused animal were investigated. The results suggest that defibrase-like enzymes have potential for enhanced control of hemorrhagic and thrombotic phenomena. Although impeded by a latency of several hours before the full anticoagulant properties are developed in vivo, it appears that enzymatically mediated defibrinogenation of blood may be a valuable alternative to heparinization in extracorporeal circulation procedures.