A proof-of-concept study has shown that mechano- and photoplethysmography (MPG and PPG) can be combined in conventional oximeter sensors. According to the research, the combination could offer a new approach to signal quality assessment and possibly even new physiologic information.
What’s more, said Christian L. Petersen, PhD, piezoelectric elements are inexpensive, making this a viable addition to clinical oximeter sensors.
“Our findings show that a piezoelectric transducer can register mechanical blood pulsations inside a typical PPG finger boot sensor concurrently with the optical PPG measurements,” said Dr. Petersen, director of technology development, Pediatric Anesthesia Research Team, at the University of British Columbia, in Vancouver. “This opens the way for a new type of measurement. For example, the relative timing of the two waveforms could potentially be used to detect changes in peripheral blood pressure.”
As Dr. Petersen reported at the Society for Technology in Anesthesia 2017 annual meeting, the plethysmogram (PG), which was formerly measured by mechanical displacement, is now typically captured in a finger PPG. The PPG is sensitive to motion artifacts, he noted, and an accelerometer on the sensor can be employed to estimate signal quality (Electronics 2014;3:282-302).
With this prototype, however, the researchers considered an alternative. “Having a piezoelectric transducer mounted inside the sensor finger boot generates a concurrent mechanical PG that can be used to detect motion through correlation with the PPG,” Dr. Petersen said.
Sensors Correlate Well
For this study, Dr. Petersen and his colleagues inserted a brass disc with ceramic piezoelectric coating into a regular PPG finger boot. The voltage generated from finger pulsatile blood engorgement was sent to an oscilloscope through a high-input impedance preamplifier and filter. The PPG sensor was connected to a standard pulse oximeter, and the timing of the oscilloscope scan was chosen to be comparable to the PPG waveform readout on the pulse oximeter. Simultaneous measurements were performed with the pulse oximeter and the oscilloscope.
According to Dr. Petersen, under static conditions, the output from the two sensors correlated well, and it was possible to obtain a strong signal from both sensor elements simultaneously. Nevertheless, he said, positioning of the mechanical sensor is important, with the strongest signal emanating from the fingertip.
“The readings from the mechanical sensor were more sensitive to vibrations than the optical readings, as expected,” he said. “If you don’t care about the optical signal and place the piezo sensor in the best position, you can get a very large-amplitude mechanical signal from the sensor. We could easily get 150 millivolts of signal out of the sensor, and it’s a very clean plethysmogram. It’s inverted, but it does have all the structure that you’d expect from a regular plethysmogram.”
However, as Dr. Petersen reported, it is possible to obtain a very high-quality MPG and PPG at the same time.
“You could mount the piezo discs behind the optical elements so that the precision of both sensor types are identical,” he said. “By using two discs connected in series, you would also double the mechanoplethysmogram effect.”
Moreover, because these two measurements are not identical, it’s possible to obtain additional information about the quality of the PPG and examine other effects.
“We envision using the MPG to gate the PPG,” Dr. Petersen said. “The MPG does not require energy to measure, so in a device that is battery operated, you could use this concept to turn on the sensor’s light-emitting diodes only when you need them. This could lead to substantial power savings in a wearable tech configuration.”
Finally, there is also the potential for new physiologic information, as the phase lag between MPG and PPG is related to peripheral blood pressure.
Proof of Principle
“This new sensor type could add a new dimension to your pulse oximetry,” Dr. Petersen said. “These piezo elements are so inexpensive that you could do this modification without adding much cost to the sensors. … Clearly, there are potential advantages, both in a clinical context and in the context of fitness-wearable technology.”
Kirk Shelley, MD, PhD, professor of anesthesiology and division chief of ambulatory anesthesia at Yale School of Medicine, in New Haven, Conn., expressed interest in the possibility of new physiologic information and asked Dr. Petersen whether he had plotted MPG versus PPG to understand the loop that might develop.
“It’s still early days,” Dr. Petersen said. “We’ve just made a proof of principle. The next step will be to try to get a synchronized measurement of the two waveforms and see how they compare in some detail.”
“The PPG slope should be faster than the MPG slope because the system is, in effect, pressurizing the garden hose and therefore changing the pressure,” Dr. Shelley predicted. “I love that it’s all noninvasive,” he added. “Fantastic work.”