The heart's electrical pulse has made possible the modern-day pacemaker, a device that has helped keep millions of human hearts beating. Such invasive devices, however, have proved difficult to use on small, delicate embryonic animal hearts, which some researchers study to learn more about the early stages of heart development, as well as to develop new treatments for disorders.

A team of researchers, however, has announced that they were able to pace embryonic quail hearts—just 40 hours old and less than two millimeters long—with pulses of infrared light, which can be administered without making direct contact with the tiny organ. The work is described in a study published online August 15 in Nature Photonics (Scientific American is part of Nature Publishing Group).

Previous research had shown that individual cardiac cells, called cardiomyocytes, could be stimulated with laser pulses and that some embryonic chick hearts responded to visible light by beating more quickly. But this is the first time, the researchers noted, that whole embryonic hearts had been reliably, successfully paced in vivo with optics.

The researchers, led by Michael Jenkins of the Department of Biomedical Engineering at Case Western Reserve University, used an infrared diode laser that was paired with a light-emitting diode that flashed in tandem so that the timing of invisible infrared pulses could be observed under the video microscope. The laser was emitted from a fiber about 500 micrometers away from the embryo and was trained on a 0.3-millimeter-wide area around the intake of the heart, which at that stage of development has not developed into a four-chambered organ and is still a tube-like structure.

The researchers were able to speed the heart rate to about triple its baseline pacing with the laser pulses (see video below).

Embryonic hearts that had been paced with laser continued beating after the laser pulses stopped, but they operated at a rate slightly faster than their original beat (about 1.44 seconds between beats rather than the original 1.58 seconds). The researchers proposed that this might be due to an increase in calcium ions during the pacing. But the team inspected the hearts and "did not detect evidence of damage," they wrote.

"The mechanism of the observed phenomenon is at this point unclear," the researchers explained. They suggest that it might create a temperature gradient that can stimulate "action potential in excitable tissues," as was also proposed in earlier work on clusters of cardiomyocytes.

"Optical pacing will not only enable a new class of experiments in developmental cardiology, but also may become a useful tool for investigating cardiac electrophysiology, single-cell dynamics and cardiac tissue engineering," Jenkins and colleagues concluded. But although focusing the laser on adult human hearts is an enticing application, much more research remains to be done on its effectiveness and safety.