By taking advantage of photoplethysmographic technology, smartphone cameras were used to measure respiratory rate, according to Jesus Lazaro and associates, writing in Physiological Measurement.

The researchers used the iPhone 4S, iPod 5, and the HTCV One M8 to demonstrate the viability of this technique by collecting smartphone-camera-acquired photoplethysmographic (SCPPG) signals from 30 healthy volunteers enrolled in controlled respiration experiments. Photoplethysmography, the same technology used in pulse oximeters, illuminates the skin and measures changes in light absorption, which in turn correlate with changes in blood volume. Lazaro et al point out that “Respiration also modulates the morphology of the PPG signal. Inspiration can lead to a reduction in tissue blood volume, and this lowers the amplitude of the PPG signal. This reduction in tissue blood volume is generated by two different mechanisms: a reduction of cardiac output, and a reduction of intra-thoracic pressure. Variations in amplitude of the PPG signal have been used to obtain respiratory information.”

During the experiment, subjects were instructed to place their right index finger over the flash so that the camera could record the signal. The signals “were extracted from an average 50 x 50 pixel region of the green video signal at each frame. The reason for using the green band is that there is high absorption by hemoglobin in the green range.”

Lazaro and colleagues concluded that the mobile phone approach to collecting respiratory rate data “has the potential to be widely-accepted by the general population and can facilitate the capability to measure some of the vital signs using only the subject’s fingertip.”

Of course, Lazaro et al weren’t the first to demonstrate the value of mobile technology to measure respiratory rate. In the past, researchers have shown that devices like Google Glass can calculate both heart rate and respirations.