When I was a cardiology fellow back in the 1980s, I learned about a variety of early tools for evaluating heart health that had been displaced by the modern standards of electrocardiography (ECG, or EKG for the Deutschephiles) and echocardiography. One such technique – ballistocardiography – stuck with me, and may be making a comeback.
Ballistocardiography is based on the observation that the mechanical action of the heart leads to subtle but reproducible movement of the whole body. It is the old “every action has an equal and opposite reaction” maxim in, well, action. We literally shudder a little bit each time the heart ejects blood. Back in the day, researchers compared patterns of that shudder to detect and quantify disorders of cardiac output. As someone who had studied biomedical engineering in college, I thought it was pretty cool that you could non-invasively estimate cardiac output by measuring how much somebody bounced up and down with each heartbeat, even though it had been eclipsed by more accurate and easier to use technology.
By the way, you can easily measure the effect yourself, if you have an analog bathroom scale. Just stand on it as still as you can, and you will notice the needle deflects slightly with each heart beat – as the blood goes “up” out of your heart, your body goes “down” and your weight appears to increase momentarily. More elaborate ways to measure and quantify the effect are, of course, available on YouTube.