The Blood Volume Pulse (BVP) waveform is an important indicator of circulatory function that can be obtained noninvasively through a photoplethysmographic transducer applied to the finger or the earlobe of human subjects. In particular, this signal may be useful in determining the degree of cardiovascular change undergone by a subject through an exercise session.
The photoplethysmographic transducer consists of a matched infrared LED - phototransistor. The complete sensor is small enough to be applied to the finger of a subject using a velcro band or a plastic clip and its use does not require the intervention of trained personnel. The signals obtained have a relatively large amplitude (e.g., over 1 volt variations), which is dependent on the biasing of the infrared pair. These signals can be conditioned to minimize ambient light interference.
We have digitized photoplethysmographic BVP waveforms, measured from the right index finger of normal adult subjects at three different instants: a) Before an exercise session ( at rest); b) Immediately after an 8-minute exercise session (lifting of a 6.6 lb. weight by flexing the right arm) and c) After an 8- minute recovery interval. The signals were digitized with a resolution of 12-bits for a voltage range of -5V to +5V, at a sampling rate of 250 samples/second.
The signals recorded immediately after exercise not only indicate the increased heart rate of the subject but also present characteristic morphological changes that may be due to modifications in the peripheral resistance of regions of the body involved in exercise. Specifically, the signal recorded after exercise has a more symmetrical shape, with a less steep rise and shows a significantly diminished "Dicrotic Notch.". The signals collected after the recovery period tend to be similar to those recorded at rest. This study characterizes BVP waveforms through indices that reflect the changes observed through the exercise session and the recovery period. Three approaches are investigated. First, changes in the BVP waveforms are represented by a single numerical index derived from its normalized histogram. Here modifications introduced by exercise are indicated by a decrease in mid-range values (disappearance of the Dicrotic Notch) and an enhancement of the extreme values (rounding of the waveform). Second, A frequency domain index is derived to reflect the enhancement of the first harmonic of the waveform and the reduction of higher order harmonics that accompanies the disappearance of the Dicrotic Notch after exercise. Finally, conventional parameters like period, amplitude and area under the curve are also computed and their variations observed.
This investigation focuses on the determination of the most appropriate indices extracted from the photoplethysmographic BVP waveforms to reflect the extent of physiological changes induced by exercise. When the best indices are determined, the solution to problems associated with the recording of the signals in a real exercise environment will be attempted through the application of adaptive signal processing techniques. It is important to emphasize that, by measuring the change of parameters in the BVP waveform, the difficulty of calibrating photoplethysmographic BVP measurements is bypassed.