Three-dimensional ballistocardiographic responses to changes of posture

In an assessment of cardiovascular performance in eighteen young healthy adults, ballistocardiograms were recorded in three mutually perpendicular directions in both the supine and the seated postures, and in the head-foot direction when standing. From these recordings three-dimensional vector ballistocardiograms were obtained which were found to be consistent over repeated measurements (r = 0.81 to 0.88 for the three components when supine) and sensitive to the functional changes imposed by changes of posture. It was observed that in the antero-posterior ballistocardiogram recorded in the supine position there was, in the IJK complex, an additional peak and trough that was not present in any other directional, or postural, ballistocardiogram. This extra wave is held to reflect the directional change of blood flow in the arch of the aorta due to its sagittal curvature around the trachea and vertebral column. From these three-dimensional recordings the functional mechanical axis of the heart was determined for both the supine and seated positions. although similar these axes were not collinear, there being an 8 degrees greater tilt in both the rightward (29 degrees: 21 degrees) and the posterior (28 degrees: 20 degrees) direction in the seated position.