The cardiac systolic mechanical axis: Optimizing multi-axial cardiac vibrations by projecting along a physiological reference frame

Abstract Multi-axial accelerometers are invaluable for the analysis of anisotropic cardiac vibrations during the first-heart-sound (S1). However, most studies simply consider the multi-axial accelerometer components independently along the different anatomical axes and rarely properly account for this anisotropy. The objective of this study was to evaluate a simple method, intended for chronically implanted medical devices, to identify a physiological reference frame maximizing S1 vibrations in the transverse plane. We attached a tri-axial accelerometer on the tip of a cardiac pacing lead to the chest of 10 human volunteers and implanted it subcutaneously in one pig. The volunteers and pig underwent separate experimental protocols capturing a wide range of conditions. The acceleration components of the tri-axial accelerometer were projected along the identified reference frame and a frame perpendicular to this. These were denoted the cardiac systolic mechanical axis and perpendicular axis, respectively. Results showed that projection along the mechanical axis tended to maximize signal-to-noise ratio and minimize cycle-to-cycle morphological variability. Significant differences were also observed between the mechanical and perpendicular axes projections, demonstrating sensibility to orientation. Along the mechanical axis, signal-to-noise-ratio was up to 21.72 dB greater, and cycle-to-cycle variability was up to 7.47 dB smaller, than along the perpendicular axis. The transverse plane angle defining the reference frame was relatively similar throughout the volunteers (28.5–44.0° from the medial-lateral axis), demonstrating inter-subject homogeneity. Also, it varied little across the different conditions (15.6° mean standard deviation across the volunteers), demonstrating stability. In conclusion, the proposed algorithm optimized signal quality at low computational cost.