Blood Volume Pulse

Blood volume pulse (BVP) is the change in volume of blood over a given period of time. BVP can be affected by heart rate, heart rate variability (HRV, which is the interval between heart beats) and respiration rate. Certain emotion can trigger the release of hormones, such as epinephrine and norepinephrine, which will increase blood flow to bring more oxygen to the muscles. Blood volume can also change due to widening or contraction of blood vessels. These emotion can then be interpreted using BVP as the blood flow will be affected.

How is BVP monitored?

BVP can be monitored using photoplethysmography (PPG) which is a non-invasive technique that relies on light absorption and reflection. It can give important data on the cardiovascular system of the patient/user. It is also relatively cheap but is only used on certain skin areas. The signals detected will form a wave which represent the change in blood volume. This wave will also be proportional to the heart rate.

Beer-Lambert's Law

Beer-Lambert’s law is one of the laws used to help measure the data we need. It describe the relationship between a monochromatic incident light and the absorbance by the medium. This can be expressed using an equation:

where I=transmitted intensity, Io=initial intensity, ε(λ) =the extinction coefficient (absorptive of the absorbing substance at a specific wavelength, c the concentration of the substance which is constant, and d the optical path length through the medium.

A low voltage, high intensity infra-red light emitting diode is used as a light source and a photo sensor to detect and receive the light. The sensor converts the optical signal to a voltage signal. This signal has a DC component and also an AC component. The AC component contains the data about the heart rate and blood volume. It is filtered by both a low-pass filter and high-pass filter (or just a band-pass filter). A low-pass filter cuts off wave with a frequency higher than the cutoff frequency (a frequency determined by the resistance and capacitance using the equation:

R = Resistance, C = Capacitance

This is a low-pass filter graph representation of what happens when the frequency exceeds the cut-off frequency.

A high-pass filter is like the opposite of a low-pass filter such that it cuts off wave with a frequency lower than the cutoff frequency. The equation used to calculate the cutoff frequency is the same. By passing the initial wave through these two filters, it will smooth out a wave. In practice, the filters reduces the amplitude of the wave as it approaches and pass the cutoff frequency (as shown in the graph above). There will likely also have a shift in the phase of the wave as the filter will cause a slight delay. The DC component of the signal will be removed by the filters. A microprocessor-based system process these data and sends it to an external computer to analyze it.

Modes of PPG

This diagram shows the logical steps of how PPG get the signal from the photodetector and converts it to an AC/DC signal.

Figure 2 From John Allen (2007). Photoplethysmography and its application in clinical physiological measurement. Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne NE7 7DN, UK

There are 2 different mode for PPG sensor: reflection and transmission. Transmission is when the light source and photo sensor is at opposite end of the tissue (e.g. source on one side of the earlobe and sensor on the other). The amount of light received by the sensor depends on the light absorption of the blood, tissues, and bone and the decreased intensity of the light that passed through is detected. Reflection is when the light source and sensor is on the same plane close to each other on the skin surface (e.g. both source and sensor on the chest). This will usually require a shield between them to prevent the infra-red light to directly enter the sensor. The sensor will detect the scattered light instead of the light passing through.

This shows the two possible ways of using PPG with the different modes: reflection and transmission.

The diagram above compares the different signals measure from different parts of the body along with a signal detected using ECG.

The graph on the right shows how the signals detected can be used to meassure the heart rate.

Figure 3 From John Allen (2007). Photoplethysmography and its application in clinical physiological measurement. Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne NE7 7DN, UK

Limitations of PPG and BVP

PPG does have its disadvantages and limitations. For example, the measurements can be affected by the method of attachment of the instruments, how much pressure is being exerted from the instruments, the subject posture and room temperature. PPG is also limited to certain parts of the body.

BVP alone is hard to use as an indicator to specific emotions as certain emotions have similar effect. However, experiments were done which shows that BVP generally increases with the induction of fear and decreases with sadness. BVP can also be hard to measure in a natural environment where data is more reliable.

Figure 3 From Erik Peper, Rick Harvey, I-Mei Lin, Hana Tylova, Donald Moss (2007) Is There More to Blood Volume Pulse Than Heart Rate Variability, Respiratory Sinus Arrhythmia, and Cardiorespiratory Synchrony?. Association for Applied Psychophysiology & Biofeedback.