EmboDop: Insufficient Automatic Microemboli Identification
To the Editor:
We read with great interest the recent article by Drs Markus and Punter1 on the assessment of the EmboDop ultrasound system. Independent evaluation of this device was overdue.
Differentiation between gaseous and solid microemboli in vivo is of utmost importance and would provide exciting new insights to the pathophysiology of stroke. Although the principle of the dual-frequency technique appears striking, the reliability of discrimination reported in the recent article is disappointing. Our own experience with the EmboDop system unfortunately supports these observations.
We performed transcranial Doppler monitoring of the left middle cerebral artery in 22 patients during coronary artery bypass grafting (CABG) using the EmboDop device. Standard machine parameters were used during data recording and offline evaluation. Digital Doppler data were then reevaluated offline by an experienced investigator according to the consensus criteria published in 1998.2 The observer was blinded to the results of the automatic micro embolic signal (MES) detection. A subset of Doppler data was additionally evaluated by a second observer to determine interobserver reliability.
Interobserver agreement was acceptable; 92 of 96 MES (95.8%) were identified concordantly by both observers.
During cardiac surgery the experienced observer (gold standard) detected overall 1256 MES in 22 patients. Only 748 MES (59.6%) were correctly identified and classified as solid or gaseous; 508 MES (40.4%) were overlooked by the EmboDop equipment (false-negative). Furthermore, 718 (solid or gaseous) MES were identified by the machine that could not be verified by the human investigator (false-positive; Table).
The percentage of false-negative MES varied between 19.4% and 73.1% (mean 40.4%; SD=16.2%) among the 22 patients, indicating that this poor recall ratio is not related to individual patients. The Figure indicates that misdiagnoses did not only occur if MES appear in clusters, which might complicate correct identification.
Unambiguous identification of microembolic signals and sufficient artifact rejection are basic requirements before a differentiation between solid and gaseous particles can be performed. In the present study, the EmboDop system as it stands now did not fulfill these requisites. This fact might in part explain and confirm the disappointing findings reported by Drs Markus and Punter.
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.
Ringelstein EB, Droste DW, Babikian VL, Evans DH, Grosset DG, Kaps M, Markus HS, Russell D, Siebler M; for the International Consensus Group on Microembolus Detection. Consensus on microembolus detection by TCD. Stroke. 1998; 29: 725–729.