Heart activity is a valuable indicator the individual's overall activity level, for example, heart rate accelerations occur in response to exercise, loud noises, sexual arousal, mental effort[Fri86]. Lower heart rate level is generally associated with a relaxed state or a state of experiencing pleasant stimuli[Fri86]. However, heart rate also accelerates upon inhalation and decelerates on exhalation. To assess an individual's stress response in real time this arrythmia should also be modeled.
In the initial pilot, heart rate was measured using the BVP and sampled at 20Hz. To detect heart rate, an algorithm was developed to detect the rising edge of the signal a threshold. The inter-beat interval (IBI) was calculated as the time between detected rising edges, and the inverse of the IBI was recorded as the heart rate for that interval. An example of the BVP signal and the result of this algorithm are shown in Figure 8.