Pulse Transit Time
This tutorial will show you how to use two Pulse Sensors to determine Pulse Transit Time.
Pulse Transit Time [PTT] is a measurement of the time it takes for the heart pulse wave to travel throughout your body. As it is widely applied, it measures the time between the R wave of an Electrocardiogram [ECG], which is the 'moment' of heart beat, against the pulse wave recorded with a Photoplethysmogram [PPG] which is, in fact, our Pulse Sensor. In the case of this example, we will NOT be using an ECG. Instead, we will use two Pulse Sensors placed on different parts of the body: the earlobe and the fingertip. The whole idea behind measuring a PTT is to gain some insight into the internal pressures of the cardio system and has been shown to approximate Blood Pressure.
We do not pretend that the Pulse Sensor is a medical device, and we offer this tutorial for educational and entertainment purposes only.
If you do not already have Arduino and our Pulse Sensor Playground library installed on your computer, go here and follow the instructions on how to set that up.
Once you're set up with Arduino and Processing, open up Arduino and select the PTT example sketch
File > Examples > PulseSensor Playground > PulseSensor_PTT
Upload the sketch to your Arduino board, and connect the two Pulse Sensors to your Arduino as shown in the fritzing diagram above.
The Arduino and Processing sketches assume that the Pulse Sensor plugged into Analog pin 0 (Sensor #1) is closer to your heart than the Pulse Sensor plugged into Analog pin 1 Sensor #2). For accurate PTT readings from this code, please make sure that you have Sensor #1 connected to a body part that is closer to your heart than Sensor #2.
In the image of the Processing sketch above, I have Sensor #1 attached to my earlobe, and Sensor #2 attached to my fingertip. Using a tape measure, I find that my earlobe is about 12 inches (30cm) from my heart, and my fingertip is about 36 inches (90cm) from my heart. The difference between the two is 24 inches (60cm), and the Pulse Transit Time value is 16mS. So that means that it took my pulse wave 16mS to travel 24 inches. Pulse Transit Time! If the PTT changes up or down, that can be an indication of change in my relative blood pressure.
Here's some links to scholarly papers investigating PTT measurement using PPG (Pulse Sensor is a PPG).