Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> 1) Modeling the Recording Device: <s> The committee's first report, published in Circulation for June 1953, recommended a terminology and certain conventions for recording ballistocardiograms of the type then in use. However, it soon became apparent that the committee's work was far from complete. Increasing knowledge of ballistic theory soon began to throw light on the relations of records secured by various instruments to one another, and a rapid advance in instrumentation began to provide records related to, but often not identical with, those which had been provided with a standard terminology by the committee in its first report. Accordingly, the committee has continued its labors to provide the rapidly advancing field with a uniform terminology. While they were thus engaged, attention was called to the fact that the designation of spatial axes previously recommended for vector ballistocardiograms differed from that which had been recommended for electrocardiograms by another committee of the American Heart Association. The advantages of... <s> BIB001 </s> Ballistocardiography and Seismocardiography: A Review of Recent Advances <s> 1) Modeling the Recording Device: <s> Abstract The design, construction and evaluation of a linear passive model of the human systemic arterial tree are presented. The performance of this electrical model is compared extensively to its real counterpart in the following areas: magnitude and phase of input impedances, wave travel (amplitude and phase of pressure harmonics) along the aorta, and wave shapes of pressures and flows at different locations. These comparisons demonstrate that the model behaves very much like the real system. A series of refinements in the modeling of a short segment of artery is discussed; although these refinements help to obtain better agreement with reality, none has a major effect on the behavior of the model as measured close to the heart. Reflections play a major role in determining the behavior of the system and occur at all branch points. The largest reflection coefficients are found at the periphery. These reflection coefficients result mainly from the architecture of the arterial tree. It is shown how the nature of the input impedance and wave travel pattern can be explained in terms of these reflections. The input impedance and wave travel in large vessels, for frequencies larger than 2 cps, are largely determined by the characteristics of these vessels themselves and not by the load. This is mainly due to the architecture of the system. Alterations in peripheral resistance affect the input impedance of the system only for very low frequencies; the same holds for wave travel in the aorta: the high frequencies are virtually independent of the peripheral resistance. Some clinical states are simulated and discussed. <s> BIB002
During the early years of ballistocardiographic research, several different instruments were used to measure BCGs, from beds hanging from the ceiling  to tables strongly coupled to ground  . These instruments were giving different records from the same normal subjects. So, efforts were made to model the effect of these instruments on BCG morphology. Limiting ourselves to the head-foot direction the equation giving the components along the y-axis (Fig. 3 , variables defined in Table II ) reads: After sorting and substituting (F int ) y into m sÿc (whereÿ c is the acceleration of center of mass of body): From the above equation, three different classic types of BCGs can be conceived based on the fact that which terms on the left side of the above equation can be neglected. The first is (3) which means that the movement of bed and body is proportional to the movement of the center of gravity. A good approximation of this special case is when the ballistocardiograph is weakly coupled to the environment such as ultralow frequency BCG (ULF-BCG) systems. The second type is when: which represents Nickersons's low-frequency (LF) BCG and the third type is when: which refers to the situation when BCG is strongly coupled to its environment, which were categorized under high-frequency BCG (HF-BCG). In other words, when the resonance frequency of the BCG platform is much higher than heart frequency, then its displacement is proportional to the internal acceleration of body's center of gravity. From this theoretical evaluation, it is clear that very different results will be obtained when one records any one aspect of motion such as displacement or acceleration from each of the three ideal types of ballistocardiographs  . However, there is a fourth category of classical BCGs, which are the direct body recordings based on AHA consensus paper on BCG terminology BIB001 . Direct body BCGs were always criticized for their inconsistencies  . 2) Modeling the Internal Forces: Starr started on BCG modeling, where arteries were segmented into 3-cm long pieces and mass of blood in the aortic segment closest to the aortic valve was multiplied by acceleration, derived from cardiac ejection curve, to calculate force. This was repeated when the blood volume shifted to the next segment  . A more comprehensive model of human systemic arterial tree with distributed properties was constructed in early 1960s by Starr and Noordergraaf  and was improved later on by Westerhof et al. BIB002 . This model was based on the fact that, when using ULF systems, in which the body was free to move in space in the head-foot axis, it was observed that the body moved first footward and then headward during the cardiac cycle. This was explained as a movement to counteract the displacement of the blood mass, that, shortly after the onset of systole, is first driven headward out of the heart to distend the great vessels, and later footward, as the pulse wave spreads peripherally and blood accumulates at a great distance from the heart in the more peripheral vessels. The model divided the arterial tree in 115 segments and calculated the position of the body's center of gravity in the longitudinal direction y c (t), as a function of time, by numerical integration of the products of the excess masses of each segment during the interval t, and the distance y i between the centre of each segment and the reference plane. Noordergraaf's model was successful in quantitatively predicting the amplitudes of ULF BCG waves and in giving an explanation for the origin of the main peaks. The model was verified on the data acquired from an astronaut in MIR station  , where by using the longitudinal BCG recorded in space the model could be used to derive the aortic flow.