Ballistocardiography


Adam Beehler

In each newsletter, Adam Beehler, Lecture Demonstration Specialist, explains one of his demonstrations. This demo is a fan-favorite done at his demo shows, as well as at the 2008 State of the Department address.

 

Many are familiar with using rockets, toy guns, fire extinguishers, and others to illustrate Newton’s third law. Ballistocardiography is a non-invasive method of measuring the ballistic forces on the heart and body through recoil and impact. Here is what I do. First, I place a frame with a swinging pendulum on top of a board that is free to roll on some rods. One rod has a mirror attached to it with a laser pointer aimed at the mirror. Any small oscillations of the pendulum can readily be seen in the reflected laser beam on the wall. One could now explain the action and reaction forces between the pendulum and the board. Next, I remove the pendulum frame and have a person lie down on the board. The person’s job is to lie perfectly still. Among the inevitable random motion of the person’s body, one can observe the reflected laser beam “beat” with the contractions of the person’s heart. Clearly, the body and board are feeling forces exerted upon them by the heart.

As blood is ejected by the left ventricle, there is an action vector upwards (toward the head), and the reaction impels the body downwards (toward the feet). Next, the blood is projected through the descending aorta, producing an action vector toward the feet that originates the corresponding reaction vector that impels the body upwards.

With its hydraulic pump activity, the heart is capable of impelling a volume of 70-80 ml of blood per beat at considerable speed into the aorta. The body displacements are proportional in amplitude to the volume, speed, and acceleration of ventricular left ejection and inversely proportional to the subject body mass. Although this technique has been known for over 50 years, because the mass of the accelerating blood is small compared to the mass of the body and table (Newton’s second law), errors in measurements were large until modern signal processing techniques came to the rescue.

You can also view this demo, and a complete materials list, here.