Understanding Sampling Rates in Biomedical Signal Recording: EDA and PPG Perspectives
Electrodermal Activity (EDA) Sampling Rates
Electrodermal activity, or EDA, is a measure of the electrical conductance of the skin, which varies with its moisture level. As a slow-changing signal, EDA is often captured at lower sampling rates than other physiological data. For general purposes, a rate of 70-100 samples per second is recommended to capture the signal correctly. However, for capturing the quick, phasic components of EDA, which are fleeting and tied to stimuli, literature suggests a minimum of 200 Hz.
Empatica, a recognized name in wearable technology, offers a device that captures EDA data at a 4Hz sampling rate, which has garnered FDA approval for clinical use. This rate is deemed sufficient for the device's application in long-term monitoring and research, suggesting a tailored approach that considers practical aspects like battery life and data management alongside clinical utility.
Photoplethysmography (PPG) Sampling Rates
PPG is a technique used to detect blood volume changes in the microvascular bed of tissue, with a variety of applications in monitoring cardiovascular health. Sampling rates for PPG can be as high as 1000 Hz, as in some research contexts, which allows for detailed analysis and robust data collection.
Studies show that a reduced sampling rate of PPG, even down to 50 Hz, can still yield accurate time and frequency indices for pulse rate variability analysis, provided the right fiducial points are used. For wearable devices, which balance the need for data accuracy with other considerations, 64 Hz has been determined to be an optimal rate for capturing heart rate variability (HRV) metrics, although some metrics may require a higher rate.
Comparison of Devices and Implications
The diversity in sampling rates across biomedical devices for EDA and PPG signal recording reflects the specialized needs of different applications. Devices like the PulseOn wrist wearable or specialized PPG platforms choose their sampling rates to optimize for their primary function, whether for everyday consumer health tracking or for detailed clinical research.
For example, the Empatica device, which samples EDA at 4Hz and is approved by the FDA, is designed for long-term monitoring where extreme precision is less critical than battery life and data storage efficiency. This makes it ideal for studies spanning several hours or days without the need for frequent recharging or data offloading. On the contrary, high-stakes research scenarios that analyze the phasic reactions of EDA or the intricate details of PPG waveforms may employ devices with sampling rates exceeding 200 Hz, ensuring that even the most transient events are captured.
The selection of a sampling rate thus becomes a strategic decision that affects not just the fidelity of data, but also the practical aspects of device deployment. A higher sampling rate will yield more detailed information but at the cost of increased power consumption and data storage requirements. Conversely, a lower rate enhances the device's longevity and simplifies data management at the expense of temporal resolution. This trade-off is evident in the range of available devices, with sampling rates as low as 25 Hz suitable for some health monitors and as high as 1 kHz for devices used in acute research settings.
Ultimately, the choice of sampling rate is governed by a matrix of factors including the intended use-case, the nature of the signal being recorded, user compliance, device form factor, and the technological constraints of power and memory. By carefully considering these parameters, biomedical engineers can tailor devices to the nuanced demands of both clinical and research environments, ensuring that each device serves its purpose with the appropriate balance of accuracy and efficiency.
The selection of a sampling rate is a critical decision that impacts the quality and utility of biomedical data. While higher sampling rates provide more detailed data, they may not always be necessary or practical for all applications. For EDA, lower rates like the FDA-approved 4Hz used by Empatica may suffice for clinical monitoring, whereas higher rates are preferred for research requiring finer temporal resolution. In contrast, PPG recording generally necessitates higher rates, particularly for applications demanding high accuracy in pulse variability measurements.
Biomedical engineers, researchers, and clinicians must consider their specific needs when choosing a device, balancing the demand for data precision against the constraints of real-world applications.