Development and validation of implantable sensors for monitoring function of prosthetic heart valves: in vitro studies.

The development of a 'smart' heart valve prosthesis, with the intrinsic ability to monitor thrombus formation, mechanical failure and local haemodynamics and to relay this information externally, would be of significant help to clinicians. The first step towards such a valve is development of the sensors and examination of whether sensor output provides predictive information on function. Custom-made piezo-electric sensors were mounted onto the housing of mechanical valves with various layers of simulated thrombus and bioprosthetic valves with normal and stiffened leaflets. Sensor output was examined using joint time-frequency analysis. Sensors were able to detect leaflet opening and closing with high fidelity for all types of valve. The frequency content of the closing sounds for the mechanical valves contained several peaks between 100 Hz and 10 kHz, whereas closing sounds for the bioprosthetic valve contained energy in a lower frequency range (<1 kHz). A frequency peak of 47 +/- 15 Hz was seen for the normal bioprosthetic valve; this peak increased to 115 +/- 12 Hz for the valve with visibly stiffened leaflets. Total low-frequency (80-3500 Hz) energy content diminished predictably with increasing levels of thrombus for the mechanical valves. Lastly, closing sound intensity correlated well with closing pressure dynamics (dp/dt) (y = 190x - 443; r = 0.90), indicating that the sensors also provide information on haemodynamics. These studies provide initial evidence regarding the use of embedded sensors to detect prosthetic valve function. Efforts to encapsulate these sensors with telemetry into a custom valve are currently underway.