The Blood Volume Pulse - Biofeedback Basics

The NeXus BVP sensor

The blood volume pulse (BVP) is widely used as a method of measuring the heart rate and is embedded in lots of heart rate variability (HRV) biofeedback training systems and Apps.

HRV biofeedback training with a BVP or ECG sensor and a respiraton sensor is one of the most widely used approaches with applications in peak performance training in sport, the arts and business in addition to the more often cited stress management.

In this article we look at some of the basic aspects of measuring BVP and what can be done with it - it’s likely that there are some applications you may not have considered.

The BVP measures heart rate based on the volume of blood that passes through the tissues in a localised area with each beat (pulse) of the heart.  Wherever there is easy access to a pulse there is a potential measurement site - but usually the pads of the fingers or the earlobes are commonly used.  

The BVP is often used in biofeedback training situations rather than the ECG which is likely to be preferred in some clinical situation or situations when the subject is moving around a lot. The BVP sensor is easier to apply and sufficient for training applications but less specific than the ECG and subject to some potential measurement errors that need to be avoided.

BVP measurement is obtained by the use of a photoplethysmography (PPG) sensor. This component measures changes in blood volume in the arteries and capillaries that correspond to changes in the heart rate and blood flow.

typical BVP pulse waveform

The PPG sensor detects changes by shining an infrared light, typically via a light-emitting diode (LED), onto the surface of the body. This light is transmitted through the tissues, then backscattered and reflected by the tissue before reaching the photodetector of the PPG sensor. The technology works because red light is selectively absorbed by the hemoglobin of the red blood cells and reflected by other tissues. The amount of light that returns to the PPG photodetector is proportional to the relative volume of blood present in the tissue.

The BVP amplitude is derived from the raw BVP signal and indicates relative blood flow. The measurement units here are sensor specific, in millivolts, and have no further physiological meaning.

With the NeXus Biofeedback systems, good BVP signals result in a signal with 10 -100 millivolts amplitude or more. Signals below 10 millivolt may indicate cold hands and may make it difficult to obtain the heart rate. Make sure the signal is strong enough to detect a clear pulse. This is a relative measure, indicating an increase or decrease of blood flow.

The heart rate (HR) is derived from the raw Blood Volume Pulse (BVP) signal by measuring the inter-beat interval (distance between the peaks of the waveform). A normal heart rate during relaxation usually ranges between 50-70 beats per minute. During stress or sympathetic arousal the HR normally increases.

The image on the right shows a characteristic form of BVP pulse for a healthy adult.

The height (P1) of the pulse defines the measure of relative blood flow. The interval or distance (B) between the peaks (points 2 and 4) defines the heart rate (HR) which can be expressed as a millisecond (MS) inter beat interval or as the number of beats per minute (BPM), also called pulse rate.

BVP, EDA and temperature sensors

Using the BVP, several measures can be derived, like changes in heart rate, peripheral blood flow, tissue engorgement, blood volume amplitude and elasticity of the vascular bed which is the range over which vasoconstriction (narrowing) and vasodilation (widening) of the blood vessels occurs.

Applications
The obvious application for the NeXus BVP sensor is HRV training. This training focuses on balancing the autonomic nervous system. By calming the mind and breathing more slowly the HRV can be varied in a way that has been shown to have many health benefits.  HRV is usually a good sign of cardiac health as it suggests that the heart has flexibility in response to the varying demands of the body. When the HRV is absent or reduced this may be a signal of underlying pathology.

A measure known as SDANN (standard deviation of average normal to normal beat) of less than 50 milliseconds (low HRV) there is fourfold increase in relative risk of death after myocardial infarction compared to those who have a high HRV (SDANN of greater than 100 milliseconds)

Vasoconstriction protocol with a NeXus 10 unit and multiple sensors with Dual display for biofeedback

The shape of the BVP waveform can also be an indicator of cardiovascular variables such as blood pressure because high blood pressure is associated with stiffer arterial walls.

The raw BVP pattern is influenced by

  • the recording location
  • the heart’s left ventricular ejection
  • the elasticity/stiffness of the aorta and arteries

The shape of the BVP can be examined within the Biotrace + software of the NeXus system and used to derive the inter-beat interval, heart rate and pulse amplitude. These measures reflect the relative increase in blood volume caused by the heart contracting.

The elasticity of the vasculature is partially reflected in the magnitude of the dicrotic notch signal (see point 3 with amount V). The depends on the interaction between the initial pressure wave generated when the heart contracts (P1), the arterial stiffness that decreases the pulse transit time and the reflected pressure wave from the peripheral arterial bed (P4)

Loss of arterial wall elasticity is often an indicator of ageing and suggests an increased risk of cardiovascular disease and hypertension in particular. Increasing arterial stiffness decreases the pulse transit time (PTT) - the stiffer the arterial walls the faster the PTT.

In the NeXus Biotrace software there are a number of relevant protocols including vasoconstriction protocols. Using this protocol multiple sensors can be used in a 15 minute or 30 minute session to train the client to have greater control over vasoconstriction and vasodilation. 

Following a baseline recording from the sensors the client is guided with feedback through a number of relaxation and vasoconstriction phases with a report generated at the end. The images shows a client with BVP, temperature, EDA (electrodermal) sensors and respiration sensors (not shown). Data capture and reporting carried out with a NeXus 10 unit.

It is important to recognise that the BVP amplitude is affected by the moment to moment status of the autonomic nervous system and any cognitive/emotional stimulus so for vasoconstriction training these matters need to be managed.

Sample data is shown from a basic BVP measurement session and a Vasoconstriction training session using the NeXus 10, Biotrace software and multiple sensors including GSR/EDA, skin temperature,BVP and respiration.

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