Don't expect to hear heartbeats on "Heartsongs." What you hear is music - lovely, lilting melodies on piano. But how it gets from an EKG to a CD is a complex four-stage process. Dr. Goldberger and his collaborators at Beth Israel Hospital obtained digital tape recordings of patients' heartbeats using a Holter monitor, a pocket-size electrocardiogram recorder that can store thousands of consecutive heartbeats over an entire day. Next, using a computer, the researchers measured the precise intervals between the heartbeats, creating a graph of the instant-to-instant changes in heart rate. While your pulse may feel perfectly regular, you actually have a great deal of subtle variability from one beat to the next. These fluctuations are produced by the normal functioning of the involuntary nervous system, which can cause your heart to slow down or speed up. The normal heartbeat, therefore, "is more a dance than a march," says Dr. Goldberger.
The third step in creating these songs is to convert the time intervals between heartbeats into integers. Using a computer program, the intervals are converted into roughly 330 integers per data set. (The team started with 100,000 recorded heartbeats, then calculated the average of every 300 beats, to remove short-term fluctuations caused by movement or breathing.)
The fourth and final step is to take the resulting series of numbers and "map" or translate them into musical notes. Each integer, ranging from 1 to 18, corresponds to a particular note on the diatonic musical scale. The sequence of integers created by the averaged heartbeats generates the melody and the changes in pitch are proportional to the changes in heart rate. Zach Goldberger then chooses the key signature, rhythm and the harmonic accompaniment for each melody. However, the underlying melodic line for each piece remained true to the original heartbeat time series.
According to Dr. Goldberger, the product of these musical mappings raises a fundamental question. Why does the heartbeat, more often than not, create musically pleasing or interesting note sequences, not jarring noise or boring repetition? The answer may lie in the origin of heart rate variability. Dr. Goldberger and others have shown that the resulting complex pattern of heartbeat variations have the mathematical structure of a fractal. The term "fractal" describes objects such as trees and coral formations, which are composed of smaller units resembling the larger scale form - a property called self-similarity.
The musicality of "Heartsongs" raises a further question, says Dr. Goldberger: could the composition of music involve, at least in part, the recreation by the mind of the body's own naturally complex rhythms and frequencies? Perhaps what the ear and brain perceive as pleasing or interesting are variations in pitch that resonate with or replicate the body's own complex (fractal) variability and scaling. "Restoring harmony in the body may be more than a metaphor," says Dr. Goldberger. "It may be that we can learn from music."