The Committee on Ballistocardiographic Terminology decided in the Scarborough-Talbot Report [48] to keep the terminology of the ballistocardiogram waves proposed by Starr et al. in [8]. The ballistocardiographic waves were hereafter named with the capital letters G through O. The report reproduced a typical signal of each kind of device available in those days. It also compared those ballistocardiograms with simultaneously acquired electrocardiogram, phonocardiogram, and carotid pulse waveforms. Fig. (1) presents the nomenclature used by the abovementioned report. Fig. (2) depicts the acceleration, velocity and displacement ballistocardiograms, simultaneously obtained from a direct body system, having noticeable waveform differences and similarities.

Fig. (1). Depiction of typical displacement records from different ballistocardiographic systems shown to illustrate differences in timing of waves and relationship between them and the electrocardiogram and phonocardiogram. In descending order: electrocardiogram, phonocardiogram, ultra-low frequency ballistocardiograph [4, 5, 10-14], Starr’s ballistocardiograph [8], Nickerson’s ballistocardiograph [9], and Dock’s ballistocardiograph [15-19].

Fig. (2). Depiction of the typical waveforms obtained from a direct body system showing the relationship to the carotid pulse, and electrocardiogram. In descending order: electrocardiogram, carotid pulse, acceleration ballistocardiogram, velocity ballistocardiogram, and displacement ballistocardiogram (adapted from [48]).

Fig. (3). Depiction of the typical waveforms obtained from an ultra-low frequency system showing the relationship to the electrocardiogram. In descending order: electrocardiogram, acceleration ballistocardiogram, velocity ballistocardiogram, and displacement ballistocardiogram.

Fig. (4). Composition showing ballistocardiogram, apex cardiogram, and phonocardiogram (in descending order).

Fig. (5). MB-1 Ballistocardiograph, image courtesy of Nihon Kohden.

Fig. (6). Ballistocardiograms obtained from a chair’s seat and backrest with the device of [126], with some fluctuations due to motion being noticeable. These two measurements spots give emphasis to different waves of the BCG. The backrest sensor highlights the ejection waves. The seat sensor outputs pre-ejection and ejection waves with similar amplitude..

Fig. (7). Prototype described in [58], implementing skin conductivity and ballistocardiogram acquisition and processing.

Fig. (8). Radar BCG system implemented in [162]. Block diagram and signal obtained for a healthy subject.

The Committee on Ballistocardiographic Terminology established in the Scarborough-Talbot Report [48] the common names of these first devices, and presented the most important relations between the waveforms each produced. Starr’s was named “high frequency” [8], Nickerson’s “low frequency” [9], whereas “ultra-low frequency” or “aperiodic” was the name given to the devices developed by Gordon [4], Henderson [5], von Wittern [10], Talbot [11], Burger [12], and Rappaport [14], and “direct body” to the ballistocardiographs developed by Dock [19], Smith [15], Arbeit [16], and Walker [17].

From the results published up to 1956, the Committee was in position to make a quantity of comments regarding the expected waveform delays and relations of the ballistocardiogram. Most of these considerations will be described below, and illustrated by representative waveforms, Fig. (3) incorporates in the exemplifications the output of an ultra-low frequency system.

The ballistocardiogram waves may be separated in three major groups, the pre-systolic (frequently disregarded), the systolic and the diastolic. The I and J waves are also referred to as ejection waves.

Any movement of mass within a body, sustained without constrains, will be reflected by oppositely directed motion of the body itself, conserving the system’s centre of gravity unaffected. This is the primary physical principle of ballistocardiography, Newton’s third law of motion. Such rule implicitly justifies the large number of factors generating artefacts [41-44]. The artefacts observed in ballistocardiography are chiefly related to patient motion, and complementary causes are unavoidable limitations in the device mechanical architecture, for instance sensor bandwidth limitations, or platform resonance. However, leaving device mechanics to the proper section, distortion of the ballistocardiograph output may occur due to any of a number of simple events, from breathing to posture changes, even with a seemingly calm and still patient.

Patient motion is associated with ballistocardiogram baseline drifts, and it is a serious problem, since that dislocating the body greatly exceeds the amount of the naturally vibrating mass the device aims to record. Regarding baseline drifts, interesting observations were made by Scarborough [48], stating that a reasonably flat baseline is often seen in late diastole – when the body’s oscillations are minimal or have ceased – and that, usually, flat baseline is not achieved because the motion of the body and bed-type supports never ceases. Therefore, diminishing patient motion will help stabilize the baseline, although it is unavoidable that some wandering will occur.

The H and J waves are distorted due to halted breathing. Local turbulence in the carotid artery obscures the close relationship between the carotid pressure pulse and the left ventricular waves of the ballistocardiogram. Hence, the amplitude and contour of the J wave may be changed by non-cardiac factors, since the contour of the arterial pressure pulse is profoundly affected by such factors as walls’ compliance and blood’s dynamic viscosity [75]. Apart from changing the ballistocardiogram shape when it is stopped [75], respiration also influences the right ventricular ejection [76, 77]. Consequently, normal breathing modifies the amplitude of the ejection waves, which is visible in practically all publications. Therefore, a healthy subject, without moving, and producing a normal ballistocardiogram, will not be able to produce a beat by beat repeatable pattern, even if he accurately controls his respiration.

Regarding the respiration effect, a simplified scheme of grading the ballistocardiogram has been employed by [32], placing emphasis on respiratory variation, by identifying four grades of ballistocardiographic abnormality:

1. Regularity of complexes is preserved. Inspiration amplitude is normal, but in expiration decreases and varies in definitiveness.
2. Half or more of the complexes are abnormal, mainly in expiration, and the inspiratory amplitude is decreased to some extent.
3. Complexes are identifiable but abnormalities are present in inspiration and expiration phases.
4. All the waves are unidentifiable and of low amplitude.

Smoking also causes changes on the ballistocardiogram, for instance, an elevated percentage of coronary vessel disease patients deform their ballistocardiogram after smoking a cigarette [78]. The abnormalities found in a positive cigarette test are non-specific, but large diastolic waves are commonly observed. Proposed theories pointed at coronary and generalized arteriolar vasoconstriction and direct myocardial effect [74].

The foremost features and physiological results available on the interpretation of ballistocardiographic waves are subsequently presented. Although the ECG is the most well-known electrophysiological signal, phonocardiography was proved to offer better time synchronization to the beginning of mechanical systole and diastole than the electrocardiogram [79]. Therefore, it is the most excellent reference for analysing the physiological relations of the ballistocardiogram waves. The following Fig. (4) depicts a combined image of the synchronisms observed from the ballistic waves with apex cardiography (ACG) and phonocardiography (PCG).

A number of studies [24, 73, 79-91] used PCG together with other techniques and physiological elements to improve the physiological interpretation of the ballistocardiogram, with [91] being an important mark, as it was the first study to report the F wave.

Nevertheless, the cardiovascular analysis regarding the origin of the ballistocardiogram waves is complicated, since there are different studies reporting distinct conclusions. The most critical example is the difference between Gubner et al. [3], a February 1953 Circulation article, and the following month’s publication of a study from Thompson’s team [91]. These studies have significant divergences, given the different devices employed. Both of them are valuable: as Gubner [3] deeply reflects on the major waves while Thompson [91] analyses all the ballistic waves. However, the Committee on Ballistocardiographic Terminology did not comment these divergences in the Scarborough-Talbot Report [48], published in Circulation September 1956.

Ballistocardiography is an accurate indicator of the heart’s age, much better than the chronological age of a person. Furthermore, diseases related to heart contractility are also correctly prognosticated by this signal, long before any symptom manifests. On the other hand it is hard to establish a univocal relation between a disease and the BCG disturbances it induces.

The number of ballistocardiographic patterns considered abnormal, i.e. above the defined grade 1 (as defined in the section Sources of distortion), on a priori normal individuals increases directly with aging, as several studies on numerous groups demonstrate [26, 36, 101-105]. Franco [102] reported that more than 50 percent of ballistocardiograms, of 317 normal individuals (without any evidence of cardiovascular disease, whose ages ranged from 35 to 64), were found abnormal. It is unlikely that Franco’s ballistocardiograph was wrong. The BCG was hinting the development of disease.

A 20-year 211-person follow-up of Starr [106], condenses the previous assertions and introduces quantitative importance to ballistocardiogram analysis, thus deserving an extended citation from the conclusions section: “(…) Hearts contracting with little force at the initial test later suffered from death and cardiac disability, chiefly coronary heart disease, in far greater numbers than those whose hearts contracted strongly. The amplitudes of ballistocardiograms, strongly correlated with the later development of heart disease and also strongly correlated with the ages of the subjects. [The amplitude-age correlation coefficient was 0.74, values of 0.25 being significant for p = 0.01] Hence, the heart tends to weaken and beat with less coordination as it grows older”, concluding “(…) those who later developed heart disease had initial ballistocardiograms that averaged 25 percent smaller than those of their mates who remained healthy for the next 17 years”.

Therefore, those who later developed myocardial disease had, in the initial test, their hearts beating like the ones of much older normal people, that is to say, their hearts “physiologic age” far exceeded their chronologic age.

The above mentioned studies demonstrated that the risk of development of coronary heart disease is strongly dependent upon age, but equally strongly dependent upon the wave form of the ballistocardiogram. Moreover, the likelihood of recurrences, for patients already manifesting coronary heart disease, is predicted by the ballistocardiogram waveform, regardless of the patient’s age.

The amplitude of the ballistocardiographic waves is determined mainly by cardiac output, cardiac force, and the velocity of ejection, explaining the original purpose of Starr of estimating cardiac output [8].

However, one compromising example to this intent is shock, where the arterial blood volume is substantially reduced, while both the velocity of ejection and the aorta blood flow increase. In this case, the ballistocardiogram ejection waves present values of stroke output quite larger than the actual output assessed by the Fick method [107-109].

A significant study, from Moser [34], analysed 100 subjects recovering after myocardial infarction, registered 81 abnormal and 19 normal ballistocardiograms. In this study, the electrocardiogram was also assessed, with 74 subjects of the abnormal BCG group presenting abnormal ECGs as well. On the other hand 17 of the normal BCG group had an abnormal ECG.

Abnormal ballistocardiograms are obtained in case of acute myocardial infarction, angina pectoris, and in asymptomatic coronary artery disease, being an early indicator of future complications [3, 106]. Namely in elders, abnormal ballistocardiograms are an important warning [106].

A study from Dock [104], reported that 92 % of 152 angina patients had an abnormal ballistocardiogram. Relevant results were published by Scarborough in [110], a study including 191 angina patients, reporting that 75.4 % of them had abnormal ballistocardiograms, with increasing incidence in the older age groups.

The study of drug’s effects on cardiac function may be carried out by observing the ballistocardiogram [95, 111], as it was done in the past. In fact, the investigation of a number of agents and situations was performed reporting BCG alterations: digitalis [112], quinidine [113], epinephrine [114], sympatholytic agents [38], visammin [114], nitroglycerin, among other nitrites [115], nicotine [116, 117], as well as surgery, and changes in blood volume [118].

Smoking changes the ballistocardiogram, for instance, patients with coronary vessel disease are likely to present deformed ballistocardiograms after smoking [78], which is induced by nicotine’s effect [116, 117].

The ballistocardiographic modifications that are observed in coronary diseases are not specific of this condition. Other disorders associated with impaired force of ventricular ejection will produce abnormalities of the same kind, such as slurring or significant amplitude decreases of the I and J waves.

The ballistic waves’ investigation appears to be very pertinent regarding coronary artery disease. Despite all the limitations imposed by the exam’s sensitivity, several investigators regard its findings as very worthy [3].

Impressive results were found on the coronary heart disease predicted by the ballistocardiogram, namely regarding the appearance or recurrence of myocardial ischemia [105].

Malfunctioning of heart valves influences the ballistocardiogram, by virtue of their ejection influence, contributing to the development of myocardial failure. Following coronary occlusion, the recording of normal ballistocardiograms indicates that the myocardium’s contractile mechanism is restored [3].

Mitral stenosis decreases the amplitude of the I and J waves, while aortic stenosis and insufficiency increases. However, mitral valvular disease does not produce a specific modification on the ballistocardiogram [3].

Coarctation of the aorta is the medical condition, among the congenital heart defects group, most often prognosticated with the ballistocardiogram, as several different studies corroborate that most cases of coarctation have a diminished or inexistent K wave [25, 29, 30, 119].

Patent ductus arteriosus, Eisenmenger’s complex, hypertension, arteriosclerosis, and interatrial septal defect have also been reported to reveal a diminished K wave [119, 120]. Therefore, this effect is likely to be detected for coarctation of the aorta, but it is not exclusive.

Constrictive pericarditis [62], arteriovenous aneurysm, rheumatic heart disease and congestive heart failure [11], [27], and abnormalities in return flow and cardiac filling in general, affect the diastolic H and L waves, as well as posterior waves. Patients with coronary vessel disease may present resembling deformations in the ballistocardiogram after smoking [78].

Myocardial infarction, angina pectoris and asymptomatic coronary artery disease are likely to induce abnormal ballistocardiographic records [3, 34, 104, 110]. Patients already manifesting coronary heart disease, have the likelihood of recurrences predicted by the ballistocardiogram waveform [106]. The risk of developing coronary heart disease is strongly dependent both on age and the morphology of the ballistocardiogram.

Early ballistic cardiography devices registered the forces exerted in the longitudinal axis of the body. Afterwards some alternative approaches were studied, such as lateral plane [121], and multidimensional vector registration [122-124]. Nevertheless, only the longitudinal measurement technique prevailed to our days. The waves recorded by this method are not pure forces, instead they are the projection of the vector sum of the forces due to heart activity. This vector may reflect forces associated with events in the early systole of the following cycle, thus making unattainable a perfect correspondence between forces, events and waves, especially when heart rate is fast, which favours superimposition of waves. Furthermore, the forces generated are transmitted through the soft tissues and skeleton to the sensing device, so the flexibility of the body structures, together with the natural period of body movement, leads to further modulation of the ballistocardiographic waves.

Starr developed a spring-coupled undamped table, with a natural frequency of 9 Hz [8], later baptized high frequency ballistocardiograph [48]. It did not suffered of baseline wandering due to such slow events as normal respiration, however this table also filtered events related to the cardiac cycle, causing other forms of ballistocardiogram distortion. Nickerson introduced a lower frequency, 1.5 Hz, critically damped table [9], expanding the lower interval of usable spectrum, thus reducing the distortion of cardiac cycle waves, but being susceptible to the baseline variations induced by respiratory movements. The frequency response of this device was an improvement from the previous, albeit not being flat, so the traces wandering was diminished, but far from eradicated. The assumption of unison movement of both the table and the body was questioned by results published in [25], still this device was not overlooked at the time.

Obtaining ballistocardiographic registers directly from the body, without using a suspended table, while the subject is in supine position, was found possible by Nickerson [9] and Hamilton [25]. Both acquired the relative movements of the head’s vertex and a rigid table or where the subject was laid. From this rigid support, Dock and Taubman developed photoelectric and electromagnetic methods for measuring the relative movement between the table and a bar placed in the subject’s shins [18]. Such method was simpler and more economical, while able to reduce distortion, if the signals were properly processed. The photoelectric sensor recorded displacement, whereas the electromagnetic one registered the velocity of body motion, with the shins being an appealing measurement site, as the orthopneic tremor was lower. The natural frequency that a subject, resting in supine position on a hard surface, possesses will change if head and heel support are added, especially due to the latter. It was observed that the type of foot support affects the ballistocardiogram components’ magnitudes, damping and temporal position, being recommended the maximization of natural frequency and damping [44].

A last improvement was the piezoelectric ballistocardiograph introduced by Sheehan’s team [125]. This sensing element, based on electric potential generation when subject to force, was made of barium titanate, with metal foils collecting the electrical charges appearing in the crystal’s faces. Good coupling from the patient and the table to the sensor had to be ensured [44], while the electronic amplifier had to guarantee a frequency response that permitted the ballistic waves to be amplified equally.

During the establishment of ballistocardiography as a valuable exam, and when plenty of the key physiological, clinical and instrumentation aspects were yet undefined, the Japanese company Nihon Kohden already had started the commercialization of its MB-1 device, Fig. (5). This device, built in 1953, is far bigger than the system embedded in a chair, developed in this decade.

After a period of relative abandon, the ballistocardiogram is now being rediscovered. The technological developments of the last century allowed the shrinkage of the huge ballistocardiographs previously described, to such a small size that they are presently inconspicuously embedded in office [58-61] and wheelchairs [62, 126], gathering the signal from their backrest, seat, or both [58-62, 126], as depicted in Fig. (6).

Moreover, analogical and digital signal processing techniques improved the control of baseline wandering due to subject movement and respiration [62, 126, 127]. By means of wavelet decomposition [126], independent [127] and principal component analysis [128], respiration is perfectly isolated, and improvements on the ballistocardiogram are achieved, by identifying the components which are not due to physiological factors.

Subsequently to the modern development of electromechanical film sensors [56, 57] three groups, the authors of [60-62], have been working more actively exploring this new approach to ballistocardiography. These sensors are piezoelectric film, composed of exterior homogeneous surface layers, with an interior composed of a number of thin polypropylene layers with air voids. When external forces are applied on the surface, the thickness and arrangement of the air voids is changed, thus producing movement of electrical charges. A charge amplifier is usually applied to output a voltage proportional to the force exerted [56].

Ballistocardiographic devices almost vanished, and nowadays, many cardiac signals with strong interest and well-established background are available, for instance, electrocardiography, impedance cardiography, and plethysmography. Even so, a manifest interest in developing ballistocardiography exists, due to the fact that this is the cardiac-related signal which, with least cost and simpler technology, can be acquired in a completely unobtrusive way. Current devices permit the acquisition of comprehensive cardiovascular system knowledge, without restricting the user, as they join respiratory and cardiac information, may use motion distortion to quantify the subject’s level of activity [133], and, if a small photoplethysmographic sensor is allowed, also blood pressure estimation [61].

State of the art ballistocardiographic systems implemented in chairs unobtrusively gather additional biological parameters and signals. Embedding skin conductivity [58], or skin temperature [62], sensors in the chair’s armrests, to allow deeper understanding of the patient’s status, as Fig. (7) depicts. When the ballistocar-diograph in not required to be completely inconspicuous, the addition of photoplethysmogram (PPG) and electrocar-diogram (ECG) is of great benefit, because acquiring BCG, ECG and PPG allows computation of pre-ejection period, pulse transit time, and consequently pulse arrival time [61].

Systems based on wheelchairs have as goal the accurate assessment of the subject’s respiratory and heart rate, when in regular movement in his daily activities. Although suffering strongly from artefacts, namely the heart rate obtained from I–I or J–J intervals, both these time features may be estimated, frequently recurring to wavelet decomposition of the signal [62, 126].

Advances in digital ballistocardiographic signal processing are mostly related to the chair-based implementation of BCG. The ballistic waves, gathered from the piezoelectric Emfi sensor, have been subject of insightful investigation [63-70], using neural networks, biortoghonal wavelets, independent component analysis, and innovative automatic clustering methods. These developments aim at an unsupervised segmentation and classification of ballistocardiographic waves, to estimate the abnormality grade of the chair user. The digital signal processing outcome is still limited, due to the signal’s susceptibility to motion artefacts, and their volatile amplitude, because of the degree of freedom the user enjoys.

Component analysis approaches were originally applied in the fields of electroencephalography and magnetic resonance imaging, where the ballistocardiogram is unwanted noise [129, 130]. In these areas numerous methods have been developed to isolate the ballistocardiogram, for instance the dilated discrete Hermite transform [131], among several others available [132], but they have not been transposed yet.

Physiological studies using the latest ballistocardiographic systems are now less concerned with the waves interpretation. Instead they look at expanding the ballistocardiogram use to the field of heart rate and blood pressure variabilities [61, 133]. Heart rate variability (HRV) is both an outcome and a diagnostic parameter for neurological maladies, due to the alterations induced by cardiac pathologies in the cardiovascular control mechanisms [134-137]. Blood pressure variability (BPV) is another parameter which presents significant indications regarding autonomic cardiovascular regulation [137]. In recent years, HRV and BPV have been increasingly studied in order to search correspondences between them and the existence of maladies associated with sympathetic and parasympathetic branches of the autonomic nervous system [58, 135, 138]. Both these variabilities carry prognostic indications in respect to cardiovascular risks [134, 136], and stress estimation from HRV.

Since a truthful assessment of the variabilities requires several minutes of recording, the unobtrusiveness of the ballistocardiograph makes it an ideal substitute for the electrocardiograph. Medical staff is not required to be present during the examination, and the only action to be made by the patient is being sat during a few minutes, hence not affecting his quality of living. Moreover, there are psycophysiological benefits by the use of this kind of method [53], and the frequency of the HRV evaluation may increase to a daily basis.

Considering the preliminary results presented in [61], if digital photoplethysmogram is added to the chair, BPV and systolic blood pressure may also be estimated from the pulse transit time [139, 140], the amount of time the blood takes to go to the periphery after being ejected [141]. Of course that, even if unobtrusiveness is not a requirement, the ballistocardiograph incorporation maintains its interest, as the ballistocardiogram processing in simultaneous with the electrocardiogram and plethysmogram allows the estimation of the pre-ejection period, pulse transit and pulse arrival times, thus having a more insightful characterization of the subject’s cardiovascular control mechanisms.

The chair ballistocardiogram, although an important trend in the present, is not the only approach to ballistocardiography currently under investigation. Special beds are still being patented [142-144], as well as other dissimilar approaches, e.g. a scale resembling device [145], or a more diverging pressocardiograph [146]. The development of sensors for sleep monitoring is the most active topic in embedded vital parameters’ sensing, and a set of solutions have been proposed implementing sensor elements in beds. Generally piezoelectric are employed with the purpose of determining heart and respiratory rates [147-156], but the ballistocardiogram signal itself is not analyzed, only the mentioned biological rates are extracted. However, in polysomnography studies, a number of new approaches and methods are being introduced, which may be interesting to fuse with ballistocardiography information, or apply as new sensors, to cite a few, the oscillosensor of [157], respiratory induced plethysmography [158], thoracic effort detector [159], or the pulse arrival time, an idea presented in the device of [160] and approached in the ballistocardiographic system of [61]. In 2007 one of the newest approaches was published, the implementation of Doppler radar technology to ballistocardiogram acquisition [161], which was recently compared with Emfi sensors using the topology presented in Fig. (8), [162].