The pulse is strongest in the arteries. A strong pulse is required to get a PPG of large amplitude from which the respiratory rate induced variations can be discerned. Traditionally, medical practitioners have palpated arteries such as the carotid or the radial to estimate pulse. The really experienced ones can also figure out problems with the left ventricle from the pulse itself. There are two main arteries in the wrist, the radial and the ulnar arteries. They are named based on their anatomic location in the forearm.

Images of actual palpation give further clues towards the sensor position on the wrist.

After deciding a suitable position, the method of mounting on skin has to be decided. The sensor breakout could be house in a 3D printer enclosure with a hole through which the sensor package on the PCB is inserted. The enclosure can be covered with black silicone (from 3M) to interface to the skin acting as a vibration dampener to remove motion artifacts due to sensor motion. The motion artifacts introduced into the pulse signal itself cannot be corrected for. From a design standpoint, we can only expect that respiratory depression due to opioids is also accompanied by cessation of unnecessary motion.