Ballistocardiography

Introduction

Ballistocardiography is the method of graphic recording of the stroke volume of the heart for the purpose of calculating cardiac output. The heartbeat results in subtle motion of the body, which in turn causes movement of a suspended supporting structure, usually a special table or bed on which the subject is lying. These movements are recorded photographically as a series of waves. This is called as ballistocardiogram or BCG. The BCG is one of the most sensitive method to measure the force of the heartbeat. An abnormality found on the BCG of an apparently healthy subject of age 40 or younger, may be suggestive of symptomatic coronary disease.

Ballistocardiography is a time-tested, non-invasive technique used for recording the movements of the body occurring synchronously with the heartbeat due to pumping action of the left ventricle. This technique to measure cardiac output has been already superseded by more advanced and precise techniques. However, it is still very useful for teaching cardiac cycle physiology to undergraduate students in practical course as because of its noninvasive nature, clear physiological as well as physiopathological analysis, and practical approach for considering cardiac output problems. The ballistocardiography system is simple, low cost, and easy-to-build. It can also be implemented together with theoretical and practical sessionsincluding cardiac output, cardiac pump activity, anatomy and physiology of the vessel circulation, vectorial composition, and signal transduction. This system can make the cardiovascular physiology easy to understand. It can focus on the study of cardiac output which can otherwise be seen only with the help of computer simulation or echocardiography.

The technique is performed by placing a subject on a bed or light table with great mobility and very low resistance to movement.

With its hydraulic pump activity, the heart is capable of impelling a volume of about 70–80 ml of blood per beat at considerable speed into the aorta. From the physical point of view, this follows Newton’s Third Law of Motion which states that every action force is opposed by a reaction force of equal magnitude and opposite direction. The anatomic arrangement of the exit of blood from the left ventricle to the ascending arch of aorta, carotid branches, and descending segment must be taken into account to understand the cardiac activity. When ventricular ejection occurs and the blood moves through the aortic segments, two main action force vectors are produced. The first is generated in the ascending aortic segment and carotid branches and it has caudo-cephalic direction. The second is a cephalocaudal vector generated due to circulation of the blood in the descending aortic segment. These action force vectors induce reaction force vectors in the body with direction opposite to the blood flow. These reaction vectors move the body first in the cephalocaudal and then in the caudocephalic directions. These main force vectors are only two of several generated during the blood’s path through the aortic arch. This eventually induces a three-dimensional displacement of the body. The body displacements are proportional in amplitude to the volume, speed, and acceleration of left ventricular ejection and inversely proportional to the body mass of the subject. The more is the body mass of subject, the greater is the resistance to its movement.

The technique is performed by placing a subject on a bed or light table with great mobility and very low resistance to movement. A pneumatic mattress or a table suspended by cords is generally used for this purpose. Body movements are transmitted to this table, which is in turn connected to displacement transducers. The transducers allow the recording and indirect estimation of the left-ventricular ejection volume, which is the most important measure of left ventricle function. A clinically useful BCG system requires a series of expensive technical specifications to achieve the needed precision. However, it is possible to build a BCG system at low cost with enough sensitivity to visualize the results with naked eye, for academic purposes.