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On the origin of cerebrovascular microemboli associated with prosthetic heart valves
T G Mackay1, D Georgiadis, D G Grosset
1Department of Cardiac Surgery (Royal Infirmary), Glasgow, UK.
Insights
Prosthetic heart valves can generate high-intensity Doppler signals, mimicking emboli, even without blood. Modifying valve closure significantly reduces these signals, suggesting mechanical valve function as their source.
Area of Science:
- Biomedical Engineering
- Cardiovascular Ultrasound
- Prosthetic Devices
Background:
- Transcranial Doppler ultrasonography in prosthetic heart valve patients detects signals resembling emboli.
- The origin of these high-intensity Doppler signals in asymptomatic patients remains unknown.
- Investigating prosthetic heart valve function in vitro is crucial for understanding signal generation.
Purpose of the Study:
- To determine if prosthetic heart valves can generate high-intensity Doppler signals independently of blood.
- To investigate the role of mechanical valve closure dynamics in signal production.
Main Methods:
- An in vitro pulse duplicator system was used with a saline solution seeded with microparticles.
- Prosthetic heart valves (Björk-Shiley Monostrut and a tri-leaflet control) were tested under mock-physiological conditions.
- Pulsed wave Doppler ultrasound monitored signals upstream and downstream of the aortic valve, with and without damping mechanical closure.
Main Results:
- Transient high-intensity Doppler signals, indicative of microemboli, were detected downstream from prosthetic valves.
- Signal detection remained consistent at varying distances downstream.
- Damping the closure of the Björk-Shiley valve reduced microemboli signal detection by 80%.
Conclusions:
- Prosthetic heart valves can generate Doppler microemboli signals in the absence of blood components.
- The number of detected signals is directly related to the energy dissipated during mechanical valve closure.
- Mechanical valve dynamics, not emboli, are a likely source of these signals in clinical transcranial Doppler studies.
Abstract:
Application of transcranial Doppler ultrasonography to asymptomatic prosthetic heart valve patients can result in the detection of transient high intensity signals, similar to those induced by the passage of emboli. However, the origin of these signals is unknown. An in vitro study has been undertaken to investigate the capacity of prosthetic heart valves to generate high intensity Doppler signals in the absence of blood. A pulse duplicator, filled with a seeded saline solution, was used to function prosthetic heart valves under mock-physiological conditions. A Björk-Shiley Monostrut valve was mounted in the aortic port while a tri-leaflet control valve was fixed in the mitral port. At stations upstream and downstream from the Björk-Shiley valve, flow was monitored using pulsed wave Doppler ultrasound (Nicolet TC-2000, 2 MHz probe). The effect of damping the harsh closure of the mechanical valve was investigated by applying a thin layer of soft adhesive tape between the valve occluder and outer ring. For all valve configurations, transient high intensity Doppler signals, characteristic of microemboli and similar to those observed in clinical studies of prosthetic heart valve patients, were detected downstream from the aortic port. The number of microemboli signals did not change significantly between sites at 20 cm and 40 cm downstream from the aortic valve. Damping the Björk-Shiley valve closure greatly reduced (by 80%) the number of microemboli signals detected. It is concluded that Doppler microemboli signals can be generated by prosthetic heart valves while functioning in the absence of the formed elements of blood, and that the number of microemboli signals produced depends upon the rate of energy dissipation at valve closure.