Stress is a factor that can severely detriment the results of any human-based experiment or treatment. Symptoms such as heart rate variability (HRV) and pulse oximetry can be used to quantify if someone is stressed or not.
Hyperventilation, or over-breathing that results in lower end-tidal CO2, has been shown to occur in studies where patients are subject to a “stressful” environment
- While the study observed a significantly increased respiratory rate, it also concluded that respiration on its own is not a sufficient indication of stress
Instead, the correlation of increased heart rate variability and respiration rate would allow us to conclude that the patient is undergoing stress, which could be affecting their test scores. For this reason, the team decided to use an electrocardiogram(ECG) and a plethysmographic (PPG) sensor to improve stress monitoring in these research environments.
For this project, a pulse oximeter was used as the PPG sensor, as previous studies that monitor respiratory rate have used a pulse oximeter.
- A pulse oximeter involves the use of a light emitting diode (LED) and a phototransistor placed on either side of the object
- LED emits light and the phototransistor picks up the light and adjusts the output current accordingly
- This enables us to monitor the concentration of oxygen in the blood and thus get a better understanding of the respiratory rate.
There are multiple locations on the body that an oximeter can be placed and still successfully measure blood oxygen concentration. While the most commonly used is the fingertip, some studies have shown that the earlobe may help avoid obstructing the patient’s movement and minimizes the phototransistor’s exposure to ambient light.
To develop the PPG sensor, first a housing for the LED and phototransistor had to be modeled in SolidWorks. It was then 3D Printed and the pieces were assembled together and incorporated with the circuitry.
Currently, the output from the circuit are sporadic peaks. For future work, we are going to perform a Fourier transform on this data in order to obtain a power spectrum that will show us what frequency this person is breathing at. Below is the Arduino code used for the graph below.
We are going to be combing this PPG sensor with a HRV monitor derived from several electrodes that we currently place under the right and left clavicle as well as on the hip. We have plans to make this less invasive, first we will put the ECGs on the patients wrists and ankles but the end goal is to use temporal pulse so that the final device is a band that wraps around the head and connects to the PPG sensor on the ear.
This device finds the peaks in a person’s heart beat and measures the distance between those peaks. This is called heart rate variability and it is an excellent measure for how stressful someone is. The more variable the heart rate, the calmer the person is. Below is the data our device currently outputs.
A quick summary of this device and the background behind it is available on this powerpoint. More information on the PPG sensor and the overall goals of the device are available here.