Acoustic Characterization of Left Ventricular Assist Device Function

Background: Advanced heart failure, a disease typically resultant to insufficient myocardial contractility, is characterized by reduced cardiac output. Reduced systemic perfusion in patients with advanced heart failure causes end-organ ischemia and dysfunction and ultimately results in loss of functional status. The use of left ventricular assist devices (LVADs), implantable pumps used to supplement cardiac output, has become an increasingly common and effective treatment for advanced heart failure. Although modern continuous flow LVADs improve quality of life and survival over medical management of heart failure, device malfunction remains a common concern. Specifically, thrombus formation within the device, a complication occurring in 8-12% of LVAD patients, is life threatening and often requires prompt surgical intervention. Current imaging modalities cannot penetrate the echo- and radio opaque titanium LVAD housing for clinical detection of LVAD thrombosis. Consequently, indirect markers of hemolysis and heart failure must be used for evaluation of potential LVAD thrombosis. Improved non-invasive methods for assessment of LVAD function are needed to detect device complications. Methods: Sound produced by LVAD operation was recorded with a hand held microphone and uploaded to a personal computer for analysis. Device operation was studied in 70 patients with both axial flow and centrifugal flow LVADs in the operative, inpatient, and outpatient settings. In the laboratory, an axial flow LVAD was implanted in EcoFlex gel and incorporated into a mock-circulatory flow loop. Sound generation under varied pressure, viscosity, and impeller speed rates was studied. Spectral analysis, including peak localization, was used to evaluate samples and to compare acoustic samples. Results: Peak frequency values measured in vivo were found to correlate strongly with both predicted values and in vitro measurements (r>0.999). Plots of the area under the acoustic spectrum curve, obtained by integrating over 50 Hz increments, showed strong correlations between in vivo and in vitro measurements (r>0.966). Device acoustics were related to design. Impeller design and the presence of fixed stators in the blood flow field appeared to generate design-specific acoustic signatures. These acoustic signatures were unique to each of 3 different LVAD manufacturers. The presence of simulated thrombus in laboratory experiments caused reductions in spectral amplitude with preservation of curve morphology. This same trend occurred when spectral tracings were compared for single patients with thrombus who had undergone surgical exchange of their LVAD. Conclusions: This methodology is sensitive to acoustics generated during LVAD operation in the laboratory and clinical environments. Pump acoustics are dependent upon device design and operation parameters. Our results suggest that pump patency may be assessed using serial measurements of a patient’s LVAD acoustics.