Ann Thorac Surg 2004;77:1615-1621
© 2004 The Society of Thoracic Surgeons
Original article: cardiovascular
High-intensity transient signals due to prosthetic valve obstruction: diagnostic and therapeutic implications
Ryoichi Hiratsuka, MDa,
Shuji Fukunaga, MDa*,
Eiki Tayama, MDa,
Kazuyoshi Takagi, MDa,
Koichi Arinaga, MDa,
Takahiro Shojima, MDa,
Hideki Teshima, MDa,
Shigeaki Aoyagi, MDa
a Department of Surgery, Kurume University School of Medicine, Kurume, Japan
Accepted for publication October 2, 2003.
* Address reprint requests to Dr Fukunaga, Department of Surgery (2), Kurume University School of Medicine, 67 Asahi-machi, Kurume 830-0011, Japan
e-mail: fukunaga{at}sankoukai.net
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Abstract
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BACKGROUND: High-intensity transient signals (HITS) can be detected by transcranial Doppler ultrasonography (TCD) in patients carrying a mechanical prosthetic valve. The HITS counts and a frequency analysis were evaluated in patients with prosthetic valve obstruction in the aortic position.
METHODS: Simultaneous echocardiographic, cineradiographic, and TCD evaluations for a St. Jude Medical valve were performed in 108 patients. All patients were asymptomatic and had no significant stenosis of the carotid artery. The HITS were identified according to criteria established by consensus of the International Cerebral Hemodynamics Symposium.
RESULTS: The HITS counts in 69 patients with normal prosthetic valve function were 2.2 ± 4.4, and the counts in 39 patients with prosthetic valve obstruction (group D) were 8.3 ± 10.8. This difference was significant (p = 0.0002). In 2 patients of group D who had a greater rate of less than 400 Hz HITS, which were produced by solid microemboli, thrombolysis resulted in a mitigation of restricted leaflet movement equal to or greater than 10o. The total number of HITS decreased and the rate of less than 400 Hz HITS also markedly decreased after thrombolysis in these 2 patients. On the other hand, 4 patients who obtained no improvement of leaflet movement by thrombolysis had lower rates of less than 400 Hz HITS than did the 2 patients.
CONCLUSIONS: These results suggest that measurement of HITS counts is useful for detection of prosthetic valve obstruction, and that a frequency analysis of HITS may be valuable to clarify the cause of the obstructed prosthetic valves.
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Introduction
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The St. Jude Medical (SJM) valve (St. Jude Medical, St. Paul, MN) is now one of the most widely used cardiac valve prostheses in the world because of its excellent hemodynamic performance and extremely low incidence of valve-related complications [1–4]. Among the valve-related complications, prosthetic valve obstruction is a rare but serious complication [5, 6]. We have demonstrated that prosthetic valve obstruction resulting from restricted leaflet movement is more frequently found on the SJM valve in the aortic position than was previously thought, even in asymptomatic patients [7, 8], and have emphasized that a combination of cineradiography and echocardiography is extremely useful for an accurate and detailed diagnosis of this problem [7, 9].
High-intensity transient signals (HITS) can be detected by transcranial Doppler ultrasonography (TCD) in patients carrying a mechanical prosthetic cardiac valve [11, 12]. Although detection of HITS represents microembolization to the brain, the clinical implication of HITS is still unclear. Previous studies [12, 13] have indicated that HITS are produced by both fibrin and platelet microemboli and by gaseous microbubbles originating in cavitations occurring around prosthetic valves. On the other hand, it is well known that obstruction of a mechanical prosthetic valve generally leads to acceleration of the velocity of blood flow across the prosthesis and secondary thrombus formation around the prosthetic valves [7, 10]. Therefore, we hypothesized that a relatively large number of HITS should be detected in patients with prosthetic valve obstruction compared with patients with normally functioning prosthetic valves. However, there have been no previous reports to assess the usefulness of measurement of HITS for detecting prosthetic valve dysfunction.
In this paper, we describe the number of HITS and a frequency analysis of HITS in patients with prosthetic valve obstruction in the aortic position, and discuss the diagnostic and therapeutic implications of measurement of HITS for prosthetic valve dysfunction.
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Patients and methods
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As of the end of 1998, 605 patients had undergone aortic valve replacement (AVR) or aortic and mitral valve replacement (DVR) with a SJM valve at our hospital. Among the 605 patients, 116 patients who visited our outpatient department simultaneously underwent cineradiography and echocardiography to assess the prosthetic valve function of the SJM valve in the aortic position. Informed consent was obtained from all patients included in this study. All patients were clinically asymptomatic and had no significant stenosis (> 50%) of the carotid artery by ultrasonography. According to our previous study [7–9], 8 patients were excluded from this study because of obscure diagnosis of normal or abnormal prosthetic valve motion and function estimated by cineradiography and echocardiography. As the result, 108 patients form the basis of this study. There were 62 male and 46 female patients whose ages ranged from 19 to 81 years (mean, 65.0 ± 11.9) at the time of this study. Of the 108 patients, 71 had a SJM valve in the aortic position alone, and the remaining 37 patients had SJM valves in both the aortic and mitral positions. Heart rhythm was sinus in 86 patients and atrial fibrillation in 22, and history of cerebrovascular events was confirmed in 6 patients. The evaluations by TCD, echocardiography, and cineradiography were performed at a mean interval of 87.6 ± 37.6 months (range, 10.2 to 235.1) after surgery. Clinical characteristics and hematologic data of these patients are summarized in Table 1.
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Operative techniques and postoperative anticoagulant therapy
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Standard techniques of cardiopulmonary bypass and valve replacement were used for all operations. A SJM standard valve was inserted in the supraannular position, and was fixed perpendicular to the interventricular septum in all patients.
Postoperative anticoagulant therapy, consisting of warfarin and a platelet inhibitor, was usually instituted within 48 hours after the operation, and the thrombotest level was kept at about 10% to 20%, or the international normalized ratio (PT-INR) was maintained at more than 1.6.
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Transcranial Doppler study
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The transcranial Doppler study was performed using a Companion Micro System (Nicolet, Madison, WI) with 2.0-MHz pulsed Doppler for 15 minutes. The probe was placed at the right temporal region in the supine position, and HITS were measured in the right middle cerebral artery. Settings of the Doppler machine were as follows: window overlap 44%, power 110 mW, sample volume 10 mm, and sweep 2 seconds. Doppler gain was as low as possible, so that the background middle cerebral artery signal appeared pale blue. These settings were used for all studied patients. The ultrasound identification criteria used for HITS detection basically followed the recent consensus by the International Cerebral Hemodynamics Symposium, and were as follows: signals with a characteristic harmonic sound (called snap, chirp, and moan), unidirectional signals within the advancing velocity spectrum, and those visible on the screen. Also, the HITS were of short duration (< 0.3 seconds) and occurred randomly within the cardiac cycle, with an intensity equal to or more than 9 dB above the background Doppler blood velocity spectrum. After recording of the TCD, all data were analyzed by two physicians (E.T., H.T.) who were masked to profiles of the patients.
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Cineradiography and echocardiography
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The previously described technique was used for cineradiography and echocardiography [7, 9]. In cineradiography, the opening angle and closing angle (in degrees) were measured as the angle between the two leaflets in the fully open and closed positions, respectively (Fig 1).
Our previous study [9] demonstrated that the opening angle of normally functioning aortic SJM valves was 11.0o ± 0.4o and the closing angle was 120.2o ± 0.7o for 19- to 25-mm valves and130.2o ± 0.4o for 27- to 31-mm valves, and that the opening angle of normally functioning mitral SJM valves was 11.0o ± 0.4o and the closing angle was 120.6o ± 0.9o for 19- to 25-mm valves and 130.7o ± 1.2o for 27- to 31-mm valves.
In Doppler echocardiography, the velocity profiles across the SJM valve were obtained by placing the transducer in the apical position. Peak transvalvular velocity was measured in meters per second, and peak pressure gradient (in millimeters of mercury) was calculated using the modified Bernoulli equation. Valve area was obtained by means of the continuity equation.
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Hematologic examinations
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All patients were examined complete blood count and prothrombin time with calculated international normalized ratio at the time of evaluation for prosthetic valve function.
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Statistical analysis
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Data are expressed as the mean ± standard deviation. Statistically significance of the differences was analyzed by the unpaired Student's t test for normally distributed data and by the 
2 test for frequency distributions.
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Results
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Cineradiography
The leaflets and leaflet movement of the SJM valve were both plainly observed by cineradiography, and measurement of the opening and closing angles was possible in all patients. The opening angle of the aortic SJM valve measured on cineradiograms ranged from 10o to 55o, and the closing angle ranged from 117o to 132o. Although the closing angle of the leaflets corresponded closely with the manufacturer's in vitro data and our previous data [9], restricted leaflet movement for opening was found to varying degrees. The opening angle was equal to or less than 14o (
14o) in 69 patients (Fig 2, A)
and was equal to or greater than 20o (
20o) in the other 39 patients (Fig 2, B). The mean opening angle in the former group was 11.8o ± 1.4o (range, 10o to 14o) and that in the latter was 26.9o ± 10.4o (range, 20o to 55o), as shown in Table 2.
This difference in the opening angle was statistically significant (p < 0.0001). However, no restricted leaflet movement of the SJM valve in the mitral position was observed in any of the 37 DVR patients (the opening angle, 12.5o ± 1.5o [range, 10o to 14o], and the closing angle, 132.9o ± 1.9o [range, 130o to 138o]).
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Fig 2. A St. Jude Medical valve showing normal leaflet movement (A) and restricted leaflet movement (B).
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Based on these results, we divided the 108 patients into two groups: group N included the 69 patients with opening angles of equal to or less than14 o; and group D consisted of the 39 patients with opening angles of equal to or greater than 20o.
Echocardiography
An echocardiographic examination ruled out the presence of intracavitary thrombosis in all patients. Doppler echocardiography for the aortic SJM valves revealed that the peak velocity in group N was 2.32 ± 0.5 m/s, and the mean pressure gradient was 12.5 ± 6.1 mm Hg. The peak velocity and the mean pressure gradient in group D were 2.89 ± 0.48 m/s, and 18.4 ± 6.7 mm Hg, respectively, and were significantly higher than those in group N (p < 0.0001). Similarly, the effective valve area was also significantly smaller in group D (1.11 ± 0.31 cm2 than in group N (1.42 ± 0.40 cm2, as shown in Table 2. Since no significant difference was seen in prosthetic valve size between the two groups, these results indicated that prosthetic valve function in group N was normal and that that in group D was obstructed.
Transcranial doppler
Of the 108 patients, HITS were detected in 60 (55.6%). No HITS were observed in the remaining 48 patients during a 15-minute period.
In group N, HITS counts were detected in 31 (44.9%) of the 69 patients and HITS counts measured during 15 minutes were 2.2 ± 4.4 (range, 0 to 19), as a whole, as shown in Table 3.
Group N included 50 patients with isolated AVR and 19 patients with DVR. In the 50 patients with isolated AVR, the mean HITS counts during 15 minutes were 1.9 ± 3.9 (range, 0 to 19), and HITS counts in the 19 patients with DVR were 3.3 ± 4.5 (range, 0 to 16), as shown in Table 3. Although a greater number of HITS was detected in DVR patients, no significant differences (p = 0.2514) in HITS counts were seen between AVR patients and DVR patients.
Of the 69 patients in group N, 52 had a SJM valve 23 mm or larger in the aortic position and the remaining 17 patients had a prosthetic valve of 21 mm or less. The HITS counts in patients with a large valve and in patients with a small valve were 2.2 ± 4.2 (range, 0 to 19) and 2.2 ± 3.9 (range, 0 to 16), respectively (Table 3). There were no significant differences (p = 0.8437) in HITS counts by valve size.
At the time of TCD, the intensity of anticoagulant therapy was equal to or more than 1.6 of the international normalized ratio (PT-INR) in 42 of the 69 patients, and less than 1.6 of PT-INR in the remaining 27 patients. The HITS counts in patients with equal to or greater than 1.6 of PT-INR were 1.8 ± 3.2 (range, 0 to 11), and those in patients with less than 1.6 of PT-INR were 3.1 ± 4.7 (range, 0 to 19), as shown in Table 3. There was no significant difference (p = 0.1842) in HITS counts between the two patient groups by the values of PT-INR.
In group D, of the 39 patients, 21 had received isolated AVR and 18 had undergone DVR, and HITS were detected in 29 (74.4%) of the 39 patients. In these 39 patients, the mean HITS count was 8.3 ± 10.8 (range, 0 to 45). Compared with the HITS count of 2.2 ± 4.4 in group N, HITS counts were significantly greater (p = 0.0002) in group D, as shown in Table 4.
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Table 4. High-Intensity Transient Signals (HITS) Counts and Frequency Analysis of High-Intensity Transient Signal in Patients of Group N and Group D
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According to the above data, the sensitivity and specificity of the HITS measurement to diagnose prosthetic obstruction were 74.4% and 55%.
A frequency analysis of HITS was performed in every patient in whom HITS were detected. In group N, the total number of HITS was 161 in the 69 patients. Among these HITS, high-frequency HITS equal to or greater than 400 Hz accounted for 89.4% of the total number of HITS, and the remaining 10.6% were low-frequency HITS less than 400 Hz, as shown in Table 4. The total number of HITS counted in group D was 323. Of the 323 HITS, 83.3% were equal to or greater than 400 Hz and 16.7% were less than 400 Hz. The rate of the low-frequency HITS was greater in group D than group N, and this difference was statistically significant (p = 0.003).
A frequency analysis of HITS during thrombolytic therapy was performed. Of the 39 patients in group D, 6 received thrombolytic therapy with a recombinant human tissue plasminogen activator for treatment of prosthetic valve obstruction [13]. Mitigation of restricted leaflet movement by more than 10o (opening angle, from 40o to 27o and from 36o to 25o) was obtained in 2 of the 6 patients. In these 2 patients, HITS counts decreased from 11 and 20 to 7 and 14, respectively, after thrombolytic treatment. A frequency analysis of the HITS demonstrated that less than 400 Hz HITS accounted for 35.5% of the total number of HITS before thrombolysis and the proportion of the less than 400 Hz HITS decreased to 4.8%, respectively, by thrombolysis (Table 5). One (the second) of the 2 patients received replacement of the malfunctioning SJM valve 5 days after thrombolysis because the opening angle of this patient was still restricted by 25o, and the hemodynamic performance of the valve was unsatisfactory. During the operation, no thrombi were found but excessive pannus formation involving pivot guard systems of the SJM valve was observed on the inflow aspect of the valve. In the other 4 patients in whom the improvement of leaflet motion was less than 10o, the ratio of less than 400 Hz HITS was low (10.7%) compared with the former 2 patients, and HITS counts revealed almost no changes before and after thrombolysis (from 28 HITS to 23 HITS), as shown in Table 5.
Hematologic examinations
As shown in Table 1, no significant differences were seen in any of the hematologic data except for platelet counts between group N and group D; however, platelet counts remained within normal range in both groups. There was also no significant difference in data from these hematological examinations between AVR patients and DVR patients.
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Comment
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Prosthetic valve obstruction such as valve thrombosis and pannus formation is a rare but serious complication [5, 6]. Almost 50% of obstructed valves have been diagnosed at autopsy [15]; therefore, accurate and prompt diagnosis of the prosthetic valve obstruction is essential. In addition to a careful physical examination, various diagnostic techniques such as echocardiography, phonocardiography, cineradiography, and cardiac catheterization have been used to evaluate prosthetic valve function [9]. Among these diagnostic techniques, we have mainly employed Doppler echocardiography and cineradiography [7, 9]. Echocardiography provides visualization of the prosthetic valve leaflets, prosthetic valve function, and hemodynamic information about pressure gradients and valve areas; however, a relatively wide variation among Doppler data obtained in SJM valves of the same size has been observed. In addition, an overlap has been also frequently found between measured pressure gradients and valve areas obtained in normally functioning SJM valves and those obtained in malfunctioning SJM valves [7, 9]. Conversely, cineradiography gives information about leaflet movement of prosthetic valves, which have radiopaque leaflets, but restricted leaflet movement of bileaflet prosthetic valves on cineradiograms does not always indicate prosthetic valve obstruction [7, 9, 16]. Thus, we have recommended a combination of echocardiography and cineradiography for an accurate and detailed diagnosis of obstruction of the prosthetic valves [7].
The present and previous studies [7] have demonstrated that the SJM valves that show restricted leaflet movement on cineradiograms are hemodynamically obstructed, even though the patients are asymptomatic. Based on these results, we first assessed leaflet movement and hemodynamic performance of the SJM valves by echocardiography and cineradiography in all of the patients. Then, we performed TCD evaluations in the 69 patients who had normal prosthetic valve function (group N) and the 39 patients who had some degree of prosthetic valve obstruction (group D). This is the most important and significant point that differentiates our study from other similar studies [11, 12].
According to the study by Hanzawa and associates [12], HITS counts varied with the anatomic position of the prosthetic valve implanted and two kinds of HITS, gaseous microbubbles and solid microemboli, have been generally detected in patients with normally functioning SJM valves. In the present study, HITS were observed in 60 (55.6%) of the 108 patients and the number of HITS was 2.2 ± 4.4 in group N during a 15-minute period. Hanzawa and colleagues [12] indicated that HITS counts were 4 ± 6 in15 minutes in patients with the SJM valve in the aortic position. Georgiadis and colleagues [11] reported that a mean of 8.5 HITS (range, 3.8 to 13.8) was counted in 60 minutes. Although the HITS counts in these two studies are slightly different from our data, they are very close to the HITS counts we obtained in the present study. Variables that could affect measurement of HITS counts, such as the number of prosthetic valves implanted, valve size, the intensity of anticoagulation, cardiac rhythms, or the duration of valve implantation have been listed by many investigators [11, 13, 17, 18]. However, positive correlations between HITS counts and these variables have not been generally established [11, 13, 17, 18]. In our study, the HITS counts in group N were not significantly different whether the prosthetic valve was present in the aortic position alone or in both the aortic and mitral positions, whether the valve size was large or small, or whether the intensity of anticoagulant therapy was strong or weak. Differences in operative technique and method used to measure HITS, such as the valve orientation in the aortic or mitral positions [18], machines per se, and settings of the Doppler machines employed for detection of HITS, or the duration of valve implantation [11, 13, 17], probably contribute to the variations seen in reported results.
Generation of HITS is certainly related to cardiac valve implantation, particularly mechanical valve implantation, because they are absent before surgery and in patients with valve repair [11–13, 17, 18]. However, detection of HITS in patients with obstruction of a prosthetic valve has never been previously evaluated. In this study, we have demonstrated for the first time that HITS counts were significantly greater (p = 0.0002) in patients with obstructed valves (group D) than in patients with normally functioning valves (group N). Two kinds of HITS, gaseous microbubbles and solid microemboli, have been detected in patients with SJM valves [12, 19]. Hanzawa [20] has emphasized that through frequency analysis of these HITS it is possible to differentiate microbubbles and solid microemboli, and has demonstrated that microbubbles produce equal to or greater than 400 Hz HITS and that solid microemboli produce less than 400 Hz HITS. In our experience, the ratio of the less than 400 Hz HITS was low (about 10%) in group N, and higher in group D (16.7%). This difference was statistically significant (p < 0.0001). Moreover, 2 patients who had relatively high rates of less than 400 Hz HITS showed a decrease in the total number of HITS and the rate (32.1%) of less than 400 Hz HITS also markedly decreased (5.8%) with thrombolytic therapy. Particularly, in 1 of these 2 patients, the cause of restricted leaflet movement was not valve thrombosis but pannus formation. Compared with these 2 patients, the ratio of less than 400 Hz HITS was remarkably low (10.7%) before thrombolysis in the 4 patients who had unsuccessful thrombolysis.
These data suggest that excess generation of gaseous microbubbles resulting from the increased velocity of the blood flow across the obstructed SJM valves, on the one hand, and of solid microemboli originating from secondary thrombus formation around the SJM valves, on the other, contributed to the detection of a large number of HITS in group D. This study also demonstrated that a frequency analysis of HITS may help to clarify the detailed pathologic mechanism of prosthetic valve obstruction, ie, valve thrombosis or pannus formation, and may be valuable for selection of the optimal treatment of prosthetic valve obstruction. In patients with a high ratio of less than 400 Hz HITS, thrombolytic therapy is probably effective for improving leaflet movement and hemodynamic performance because it eliminates the contribution of secondary thrombus formation to further progression of restricted leaflet movement.
Study limitation
The HITS were detected in 60 (55.6%) of the 108 patients but no HITS were observed in the remaining 44.4% of patients, even in patients with obstruction of the prosthetic valve; and an overlap of HITS counts was observed between group N and group D. Moreover, the sensitivity and the specificity of the HITS measurement to diagnose prosthetic obstruction were low. This fact indicates that measurement of HITS counts and a frequency analysis of HITS do not by themselves always give an accurate diagnosis of prosthetic valve obstruction. In addition, we unfortunately were only able to confirm the true cause of prosthetic valve obstruction in 1 patient of group D. Therefore, further studies and clinical experience are needed to clarify the usefulness of measurement of HITS counts and frequency analysis of HITS for the accurate diagnosis of prosthetic valve obstruction and its pathologic mechanism.[14]
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References
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|---|
- Aoyagi S., Oryoji A., Nishi Y., Tanaka K., Kosuga K., Oishi K. Longterm results of valve replacement with the St. Jude Medical valve. J Thorac Cardiovasc Surg 1994;108:1021-1029.[Abstract/Free Full Text]
- Czer L.S., Chaux A., Mattloff J.M., et al. Ten-year experience with the St. Jude Medical valve for primary valve replacement. J Thorac Cardiovasc Surg 1990;100:44-55.[Abstract]
- Kratz J.M., Crawford F.A., Jr, Sade R.M., Crumbley A.J., Stroud M.R. St. Jude prosthesis for aortic and mitral valve replacement: a ten-year experience. Ann Thorac Surg 1993;56:462-468.[Abstract]
- Arom KV, Nicoloff DM, Kersten TE, Northrup WF III, Lindsay WG, Emery RW. Ten years experience with the St. Jude Medical valve prosthesis. Ann Thorac Surg 1989:831–7
- Aoyagi S., Fukunaga S., Suzuki S., Nishi Y., Oryoji A., Kosuga K. Obstruction of mechanical valve prostheses: clinical diagnosis and surgical or nonsurgical treatment. Surg Today 1996;26:400-406.[Medline]
- Deveri E., Sareli P., Wisenbaugh T., Cronje S.L. Obstruction of mechanical heart valve prostheses: clinical aspects and surgical management. J Am Coll Cardiol 1991;17:646-650.[Abstract]
- Aoyagi S., Nishimi M., Kawano H., et al. Obstruction of St. Jude Medical valves in the aortic position: significance of a combination of cineradiography and echocardiography. J Thorac Cardiovasc Surg 2000;120:142-147.[Abstract/Free Full Text]
- Aoyagi S., Nishimi M., Tayama E., et al. Obstruction of St. Jude Medical valves in the aortic position: a consideration for pathogenic mechanism of prosthetic valve obstruction. Cardiovasc Surg 2002;10:339-344.[Medline]
- Aoyagi S., Higa Y., Matsuzoe S., et al. Obstruction of the St. Jude Medical valve: diagnostic and therapeutic values of cineradiography. Thorac Cardiovasc Surg 1993;41:357-363.[Medline]
- Renzulli A., Luca D., Caruso A., Verde R., Galzerano D., Cotrufo M. Acute thrombosis of prosthetic valves: a multivariate analysis of the risk factors for a life threatening event. J Eur Cardiothorac Surg 1992;6:412-421.
- Georgiadis D., Braun S., Ulmann F., et al. Doppler micro-embolic signals in patients with two different types of bileaflet valves. J Thorac Cardiovasc Surg 2001;121:1101-1106.[Abstract/Free Full Text]
- Hanzawa K., Ohzeki H., Moro H., et al. Clinical significance of high intensity transient signals (HITS) by transcranial Doppler sonography in patients with St. Jude Medical heart valve. Neurosonology 1999;12:7-12.
- Milano A., D'Alfonso A., Codecasa R., et al. Prospective evaluation of frequency and nature of transcranial high-intensity Doppler signals in prosthetic valve recipients. J Heart Valve Dis 1999;8:488-494.[Medline]
- Teshima H., Hayashida N., Nishimi M., et al. Thrombolytic therapy with tissue plasminogen activator for the treatment of nonstructural malfunction of bileaflet cardiac valve prostheses. Artif Organs 2002;26:460-466.[Medline]
- Murphy D.A., Levine F.H., Buckley M.J., et al. Mechanical valves: a comparative analysis of the Starr-Edwards and Bjork-Shiley prostheses. J Thorac Cardiovasc Surg 1983;86:746-775.[Abstract]
- Aoyagi S., Kawara T., Fukunaga S., et al. Cineradiographic evaluation of ATS open pivot bileaflet valves. J Heart Valve Dis 1997;6:258-263.[Medline]
- Muller H.R., Burckhardt D., Casty M., Pfisterer M.E., Buser M.W. High intensity transcranial Doppler signals (HITS) after prosthetic valve implantation. J Heart Valve Dis 1994;3:602-606.[Medline]
- Kleine P., Perthel M., Hasenkam J.M., Nygaard H., Hansen S.B., Laas J. High-intensity transient signals (HITS) as a parameter for optimum orientation of mechanical aortic valves. Thorac Cardiovasc Surg 2000;48:360-363.[Medline]
- Georgiadis D., Mackay T.G., Kelman A.W., Grosset D.G., Wheatley D.J., Lees K.R. Differentiation between gaseous and formed embolic materials in vivo: application in prosthetic heart valve patients. Stroke 1994;25:1559-1563.[Abstract]
- Hanzawa K. Clinical significance of frequency analysis of high-intensity transient signals (HITS) by TCD in cardiopulmonary bypass. Neurosonology 1999;12:64-70.
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Abstract
Introduction
