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A method to distinguish between gaseous and solid cerebral emboli in patients with prosthetic heart valves

^a Division of Cardiac Surgery, Department of Surgery, University of Ottawa Heart Institute, Ottawa, Ontario K1Y 4W7, Canada
^b Division of Cardiac Anesthesia, Department of Anesthesia, University of Ottawa Heart Institute, Ottawa, Ontario K1Y 4W7, Canada
^c Department of Respiratory Therapy, University of Ottawa Heart Institute, Ottawa, Ontario K1Y 4W7, Canada

Received 6 February 2008; received in revised form 4 September 2008; accepted 9 September 2008.

^_* Corresponding author. Address: University of Ottawa Heart Institute, Division of Cardiac Surgery, Room H-4403, 40 Ruskin Street, Ottawa, Ontario K1Y 4W7, Canada. Tel.: +1 613 761 4263; fax: +1 613 761 4392. (Email: Rrodriguez{at}Ottawaheart.ca).

Background: The difficulty of distinguishing solid from air emboli using transcranial Doppler has limited its use in situations where both types of emboli can occur, such as in mechanical heart valve patients. To make transcranial Doppler clinically useful, a method must be found to distinguish benign air bubbles from the more damaging solid particulates. Since inhalation of 100% oxygen reduces the amount of air bubbles in mechanical heart valve patients, the ultrasonic features of the remaining emboli would be characteristic of solid particulates. Objective: We determined the accuracy of the signal relative intensity measured with transcranial Doppler to distinguish between gaseous and non-gaseous emboli in mechanical heart valve patients examined during room air and 100% oxygen. Embolic signals detected in patients with bioprosthetic valves examined during 100% oxygen comprised the source of solid particulates. Methods: Embolic signals were detected during room air (n = 141) and 100% oxygen (n = 45) from 17 mechanical valve patients at two Doppler examinations (4 h and 4 days after surgery). Solid embolic signals (n = 31) from seven patients with bioprosthetic valves were identified with 100% oxygen within the first 4 h after surgery. Frequency plots and receiver operating characteristic curves assessed signal intensity differences between mechanical and bioprosthetic valve groups during 100% oxygen and the efficacy of the relative intensity for differentiating gaseous from solid emboli. Results: Administration of 100% oxygen during transcranial Doppler examination in mechanical heart valve patients decreased the count of embolic signals compared with room air (p = 0.006). The embolic signals of mechanical heart valve patients breathing 100% oxygen showed lower relative intensities compared with those during room air. The distribution of the signal relative intensity between mechanical and bioprosthetic valve groups during 100% oxygen was similar. A 16 dB cut-off threshold achieved the best accuracy for differentiating non-gaseous from gaseous emboli (sensitivity: 60%; specificity: 82%; area: 0.721; p < 0.0001). Conclusions: The use of a signal intensity cut-off offers adequate discrimination of the embolic composition in mechanical heart valve patients. Future studies evaluating prophylactic treatments of thrombosis in these patients should assess the predictive value of this intensity threshold and their potential association with outcome indicators and procoagulant markers.

Key Words: Heart valve prosthesis • Intracranial embolism • Ultrasonography • Doppler • Transcranial