Time and amplitude relationships of the ballistocardiogram in vertical and horizontal direction

The ballistocardiogram (BCG) is the record of the reaction forces of the body due to the mechanical action of the heart and the blood flow in the main arteries. Currently the BCG is commonly recorded in longitudinal direction (head-to-foot) and has been proposed as a complementary tool to estimate hemodynamic changes either alone or simultaneous with an additional cardiac signal used as a reference. However, the relationship between the BCG in different directions has not received much attention. In this work, the vertical and horizontal BCG in seated subjects are analyzed and their amplitude and time relationship are explored. Three different analyses have been performed: each separate BCG, between BCG signals, and between both BCG signals and the electrocardiogram. Time analysis of both BCG signals show that the I wave in the horizontal precedes that in the vertical direction, which contrast to the reported previously in the bibliography. The H wave was usually smaller in the horizontal BCG than in the vertical BCG, or even absent in the former. Two-axis BCG, which is easily recorded in seated persons, can be used as a complementary tool to assess the cardiovascular system.