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(Circulation. 1997;96:1843-1846.)
© 1997 American Heart Association, Inc.

Articles

Circulating Microemboli in Patients After Aortic Valve Replacement With Pulmonary Autografts and Mechanical Valve Prostheses

Axel Nötzold, MD; Dirk W. Droste, MD; Gunnar Hagedorn; Suliko Berndt, MD; Manfred Kaps, MD, PhD; Bernhard Graf, MD, PhD; ; Hans H. Sievers, MD, PhD

From the Departments of Cardiac Surgery (A.N., S.B., H.H.S.) and Neurology (G.H., M.K.), Medical University of Lübeck; Department of Neurology, University of Münster (D.W.D.); and Department of Cardiology, Klinikum Schwerin (B.G.), Germany.

Correspondence to Prof Dr H.H. Sievers, Klinik für Herzchir-urgie, Medizinische Universität zu Lübeck, Ratzeburger Allee 160, D-23538 Lübeck, Germany.

	Abstract

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Background The pulmonary autograft procedure (Ross) is now considered the gold standard for aortic valve replacement. One of its advantages is the freedom from macroemboli without anticoagulation. Whether this holds true for circulating microemboli, detectable as high-intensity transient Doppler signals (HITS), has not yet been verified.

Methods and Results We investigated 8 patients (2 women, 6 men; mean age, 50.6±17.9 years) after the Ross procedure, 9 patients (3 women, 6 men; mean age, 67.2±9.46 years) after aortic valve replacement with a mechanical valve prosthesis, and 12 young healthy volunteers by unilateral 1-hour recording of the middle cerebral artery on digital audio tape. Patients with extracranial carotid artery disease were excluded by color duplex sonography. During the off-line evaluation, the investigator was not aware of any patient details. No HITS were detected in healthy volunteers (95% confidence interval [CI], 0% to 26.46%). After the Ross procedure, 1 patient had 11 and 1 patient had 1 HITS (95% CI, 3.19% to 65.09%). All recipients of mechanical valves had HITS, ranging from 2 to 84 per hour (95% CI, 66.7% to 100%). Significantly more recipients of mechanical valves exhibited HITS than recipients of pulmonary autografts (P<.05) or control subjects (P<.05).

Conclusions In contrast to mechanical valves, pulmonary autografts are seldom the source of microemboli, confirming the pulmonary autograft as the superior substitute for aortic valve replacement.

Key Words: valves • surgery • embolism • microspheres • cerebrovascular circulation

	Introduction

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High-intensity transient Doppler signals (HITS) are detectable by transcranial Doppler sonography in various diseases with an increased risk of stroke.^{1 2 3 4} The clinical correlation between HITS and risk of stroke for internal carotid artery disease has been suggested in recent publications.^{5 6} The clinical significance of HITS found after cardiac valve replacement with mechanical devices is a source of contro-versy.^{7 8 9 10} Although the clinical relevance of HITS early after valve replacement with mechanical substitutes has not yet been clarified, a possible late neuropsychological impact has to be taken into consideration.

Since the inauguration by Ross,¹¹ the use of pulmonary autografts for aortic valve replacement is widely accepted because the graft exhibits ideal properties regarding hemodynamics and thromboembolism. Whether the known low thrombogenicity of the pulmonary autograft is also reflected by fewer cerebral microembolisms has not yet been verified but may be of interest for the choice of the surgical technique and probably the anticoagulation regime.

Therefore, the aim of this study was the determination of HITS in patients after aortic valve replacement with pulmonary autografts compared with those with mechanical valves and control subjects.

	Methods

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Detection of HITS
The middle cerebral artery was monitored unilaterally with a 2-MHz probe through the temporal bone window for 1 hour. An individual setting for optimal discrimination of HITS from the background spectrum was achieved with a low burst of 3 mm and an ultrasound intensity of 20 mW/cm².¹²

A transcranial Doppler ultrasound device (Multidop X, DWL) containing a 64-point fast-Fourier-transformation processor and displaying the intensity of the received Doppler signals on a graded color scale was used. The Figure shows an example of HITS. The signal was recorded by a digital audio tape recorder (DTC-690, Sony Germany GmbH). The tapes (DM 120, Maxell Europe Ltd) were given numbers and mixed with other recordings randomly. The analysis was performed by one investigator off-line using a special device to introduce the recorded signals into the fast-Fourier-transformation processor by headphone (MDR CD 250, Sony Germany GmbH) and watching the signals on the screen, considering the definitions for embolic events of Spencer,¹ typically visible and audible (click, chirp, and whistle). The code was broken after completion of the analysis. The threshold for accepting HITS was 12 dB. Besides the off-line analysis, multigate embolus detection software (TCD for Multi-Dop X, version 8.0, designed by R. Aaslid, DWL) was used on-line. The distance between the two sample volumes was 5 mm. This software uses the time difference of the signals between the two sample volumes to discriminate HITS from artifacts. HITS detected by the software were rejected if they were not confirmed by the off-line analysis.

View larger version (0K): [in this window] [in a new window]   Figure 1. High-intensity transient Doppler signal (arrow) in a typical transcranial Doppler pattern.

Subjects
All patients and volunteers gave informed consent and received a complete supraaortal Doppler investigation to exclude a significant stenosis of the internal carotid arteries. All participants were in sinus rhythm. The values were obtained under the same conditions, and the analysis was performed by the same investigator.

Control Group
Twelve medical students from 20 and 26 years of age (mean±SD, 21.6±1.9 years) with normal color duplex investigation of their neck and intracranial arteries, without any cardiovascular or cerebrovascular disease, and without any medication participated in the study. There were 6 men and 6 women. The left and right middle cerebral arteries were insonated in 3 men and 3 women.

Patient Groups
Table 1 gives demographics and clinical and operative data for patients with autografts and mechanical valve prostheses. In three patients of the autograft group and four patients of the mechanical valve group, the recording of HITS succeeded from the left; in the remaining patients, from the right middle cerebral artery.

View this table: [in this window] [in a new window]   Table 1. Demographics and Clinical and Operative Data

One patient had a macroembolic event in the early postoperative period caused by an echocardiographically established thrombus in the left atrial appendage after atrial fibrillation with only minor clinical sequelae.

Statistical Analysis
Because of the numerous zero values, nonparametric testing was impossible. The 95% confidence interval (CI) within the binomial distribution was determined for each group. A significance of a difference was rejected if one interval overlapped the other (P.05).

	Results

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Control Group
No HITS were detected (95% CI, 0% to 26.46%).

Pulmonary Autograft Group
One and 11 HITS were found in two patients (95% CI, 3.19% to 65.09%).

Mechanical Aortic Valve Group
All patients had HITS varying from 2 to 84 per hour (95% CI, 66.37% to 100.0%). The difference between the mechanical valve group compared with the control and pulmonary autograft groups is significant (P<.05). Table 2 gives all values.

View this table: [in this window] [in a new window]   Table 2. Number of High-Intensity Transient Doppler Signals Recorded During 1 Hour in Patients and Volunteers

	Discussion

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The detection of HITS in recipients of mechanical heart valves is a well-known phenomenon, but in contrast to the HITS-detection in carotid artery disease, it is not clear whether the HITS represent microembolization of thrombotic material. Some authors⁷ assumed microembolism of local thrombus or local platelet aggregation to be the cause of HITS, whereas other investigators⁸ cannot support this hypothesis because they were unable to find any correlation between the number of HITS and different anticoagulation regimes or antiplatelet medication. Grosset et al¹³ showed a distribution of HITS between the different cerebral arteries corresponding to the flow pattern, providing some evidence that HITS after heart valve replacement are related to the valve. In a recent study on 275 patients, significant differences in the number of HITS between six valve types were found.¹⁴ In this study, the number of HITS after double valve replacement (aortic and mitral) was approximately equal to the sum of the counts in patients with aortic and mitral valve replacement. Müller et al¹⁵ found that the number of HITS decreased in some patients after treatment with 100 mg acetylsalicyclic acid over 4 days but increased in two patients. They discussed that in these two patients, the antiplatelet medication may have led to a partial breakdown of platelet aggregation and thus to more but smaller microemboli. Cavitation occurs in regions of large and rapid pressure declines in the valve-related eddies and is considered a possible source of HITS. It is not possible to detect such bubbles with transcranial Doppler sonography because they have a short lifetime of <10 milliseconds and cannot reach the middle cerebral artery.⁹ Assuming, however, that vaporization of a certain amount of liquid occurs in vacuoles during cavitation, it seems possible that enough dissolved gas becomes free gas, forming gaseous bubbles and thus probably being detectable with transcranial Doppler sonography. However, the reported clinical correlate between HITS and acetylsalicyclic acid¹⁵ supports the hypothesis that HITS may reflect microemboli rather than gaseous emboli.

The patient with 11 HITS per hour after the Ross procedure was operated on while having acute endocarditis with multiple embolic insults preoperatively but had an eventless postoperative course with no sign of recurrence of the endocarditis. The echocardiographic studies on follow-up exhibited normal behavior and morphology of the left-sided cardiac cavities and the aortic and mitral valves. It is possible that a Teflon felt, used for a stable suspension of the commissures, was the source of the detected HITS. The mean follow-up period was considerably longer in the mechanical valve group compared with the pulmonary autograft group. Fresh surgical suture lines and material within the aorta can predispose for thrombus formation until they are overgrown by tissue. With this assumption, the number of HITS would be even lower with time in the autograft group. Therefore, the shorter follow-up period of the pulmonary autograft group does not refute the hypothesis that the valve type causes the difference in the incidence of HITS.

Even under consideration of the above-mentioned restrictions, HITS are detectable in fewer patients after the Ross procedure compared with mechanical valve recipients, comparable to control subjects. Possible explanations might be that the pulmonary autograft is viable and not thrombogeneous and that the pulmonary autograft provides excellent postoperative hemodynamics without significant pressure gradients¹⁶ and therefore produces no eddy regions with large pressure declines. An anticoagulation therapy for patients after the Ross procedure seems not to be advisable because of the low incidence of HITS in these patients. The use of felts for suspension of the commissures should probably be avoided. Whether the occurrence of HITS, especially in patients with mechanical valves, has a potential impact on clinically relevant postoperative cerebral dysfunction remains to be established. The low incidence of HITS in patients with pulmonary autografts adds to the advantages of this surgical technique.

	Acknowledgments

 
We thank Dr Friedrich, Department for Statistics and Biomathematics, University of Lübeck, Germany, for his kind help in the statistical analysis.

Received December 23, 1996; revision received April 17, 1997; accepted April 28, 1997.

	References

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