J Thorac Cardiovasc Surg 2008;136:1391-1392
© 2008 The American Association for Thoracic Surgery
Solid and gaseous cerebral microembolization after biologic and mechanical aortic valve replacement: Investigation with multirange and multifrequency transcranial Doppler ultrasound
Justin Nowell, MRCS,
Marjan Jahangiri, FRCS (CTh)
St George's Hospital, London, United Kingdom
To the Editor:
We read with interest the recent article by Guerrieri Wolf and colleagues1
describing their experience of a transcranial Doppler system (Embo-Dop; DWL Elektronische Systeme GmbH, Singen, Germany) to discriminate between gaseous and particulate emboli after aortic valve replacement. The main finding of their study was that solid microemboli occurred more frequently after mechanical aortic valve replacement compared with biological valve replacement at 3 months after surgery. The inability to distinguish the difference between solid and gaseous microemboli has limited the use of standard transcranial Doppler when both coexist, for example, during open carotid endarterectomy. As the authors state, this particular Doppler technique was first assessed by Brucher and Russell2 in a 2-part study comprising both in vitro and in vivo models. The in vivo part used 45 patients with carotid artery stenosis (39 symptomatic), producing a relatively low number of 32 embolic signals, of which 30 were classified as solid.
The assumption in validation studies is that all patients with carotid artery stenosis produce only solid emboli. One arm of a clinical validation study performed by Markus and Punter3 used a cohort of patients with symptomatic carotid artery stenosis to generate 145 presumed solid embolic signals. Of these 145 signals, the Embo-Dop system classified 72 as gaseous emboli. This gave a sensitivity of 50.3% and specificity of 96.5% for detecting solid embolic signals. Markus and Punter concluded that although the Embo-Dop system allowed better discrimination than simple intensity threshold alone, it was not accurate enough for clinical or research studies. Unless the specificity is approximately 100%, a significant number will be classified wrongly as potentially significant solid emboli. We believe that Embo-Dop has to be used with caution in distinguishing gaseous from solid emboli.
The authors comment on no antiplatelet therapy after mechanical versus biological valve replacement. Our group conducted a UK survey of consultant practice and literature review of this subject.4 We concluded that the incidence of thromboembolism after aortic bioprosthesis is highest in the first 3 months but then diminishes. Whether this is related to endothelialization of prosthetic material or specifically to antiplatelet therapy is unknown. Therefore, their observation that the proportion of particulate emboli at 3 months was in the biological valve replacement cohort receiving aspirin but not in the mechanical group receiving only warfarin could indeed be explained by the antiplatelet function of aspirin.
References
- Guerrieri Wolf L, Choudhary BP, Abu-Omar Y, Taggart DP. Solid and gaseous cerebral microembolization after biologic and mechanical aortic valve replacement: investigation with multirange and multifrequency transcranial Doppler ultrasound. J Thorac Cardiovasc Surg 2008;135:512-520.[Abstract/Free Full Text]
- Brucher R, Russell D. Automatic online embolus detection and artifact rejection with the first multifrequency transcranial Doppler. Stroke 2002;33:1969-1974.[Abstract/Free Full Text]
- Markus HS, Punter M. Can transcranial Doppler discriminate between solid and gaseous microemboli? Assessment of a dual-frequency transducer system. Stroke 2005;36:1731-1734.[Abstract/Free Full Text]
- Nowell J, Wilton E, Markus H, Jahangiri M. Antithrombotic therapy following bioprosthetic aortic valve replacement. Eur J Cardiothorac Surg 2007;31:578-585.[Abstract/Free Full Text]
