Acoustic spectral analysis: a new method for thrombus detection in the HeartWare HVAD pump

Objectives: Pump thrombosis is a severe and life-threatening complication in patients supported by rotary blood pumps. Early detection and treatment is crucial to prevent further damage caused by hemolysis or low pump output. Analysis of the sound generated by the running HVAD pump reveals thrombus formation independently of other clinical or technical signs.

Methods: In a mock loop the sound emitted by a HeartWare HVAD was recorded using a data acquisition device combined with a laptop. The acoustic spectra calculated by runtime-FFT were investigated.

Measurements were taken at various rotational speed settings and different flow values. The spectra recorded with a clean rotor (O) were compared with those of two different artificial thrombi adhering to the rotor made from blots of silicone glue. One thrombus was placed at one of the four flow channels, impairing the blood flow through this flow channel and causing an imbalance of the rotor (A). The other artificial thrombus replicated the commonly observed fibrin layer on one of the hydrodynamic bearing planes (B).

Results: Analysis of the clean pump (O) showed the existence of characteristic frequency peaks at the rotational speed of the pump (fundamental frequency - 1st harmonic), the twofold (2nd harmonic) and the fourfold frequency (4th harmonic), which corresponds with the frequency at which the blades of the rotor pass the outflow volute. For better comparison the amplitudes of the 1st and 2nd harmonic are normalized to the 4th harmonic, which has the highest intensity after eliminating the underlying noise level.

In both test series with artificial thrombi (A, B) the existence of a sound peak with the threefold frequency (3rd harmonic) was found. This finding was independent of the rotational speed and the created flow.

Conclusion: A thrombus adhering to the rotor of the HVAD pump causes an imbalance which leads to an eccentric rotation. Every time the deflected part of the rotor passes one of the three driving solenoids of the stator it will be drawn to the center by the magnetic forces thus creating the 3rd harmonic frequency peak.

Analysis of the acoustic spectrum emitted by HVAD pumps can be used as an independent method for detecting thrombotic deposits on the rotor. Pump thrombosis thus can be recognized before clinical signs such as hemolysis or increased power consumption occur. The success of lysis therapy in these cases can be monitored without delay.