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blood pressure calibration system, 'PhysioCal' [Wesseling 1995], Auto
calibration or 'AutoCal' is used to define and maintain the correct diameter
at which the finger artery is clamped. Photoplethysmography, the volume-
clamp method, and the AutoCal algorithm are addressed in this chapter.
Photoplethysmography
Photoplethysmography (PPG) is an optical measurement technique that is
widely used to detect blood volume changes in vascular beds in a variety
of tissues. A standard photoplethysmograph is a simple device consisting
of a light source to illuminate the tissue, and a light detector to detect
small changes in light intensity associated with variations blood volume.
For the Human NIBP finger cuff s, the light source is a LED (Light Emitting
Diode) emitting infrared light. The light detector is an infrared photodiode.
PPG is used in a variety of diff erent, non-invasive clinical applications
including: pulse oximetry, vascular diagnostics and blood pressure
measurement. For most PPG applications, the light source utilizes visible
red, near-infrared or infrared wavelengths. These wavelengths are optimal
for interactions with biological tissues for the following reasons:
1. The optical properties of water: Water, the main constituent of
tissues, absorbs light very strongly in ultraviolet and longer infrared
wavelengths. Melanin strongly absorbs the shorter wavelengths of
light. There is, however, an 'optical window' in water's absorption
spectrum that permits visible red and shorter, infrared wavelengths
to pass – this allows the measurement of changes in blood volume
with perfusion at these wavelengths.
2. The isobestic point for hemoglobin: Oxyhemoglobin and
deoxyhemoglobin diff er significantly in their absorption spectra,
except at their 'isobestic' wavelengths, at which the two species
have the same molar absorptivity. In the near-infrared range close
to 805 nm, PPG signals should be largely unaff ected by changes in
oxygen saturation [Gordy and Drabkin, 1957].
3. Depth of tissue penetration: At a given light intensity, diff erent
wavelengths vary in their depths of tissue penetration [Murray and
Marjanovic, 1997].
Plethysmograph
The most commonly examined plethysmograph waveform is the
peripheral pulse, which is synchronized with each heartbeat. The
waveform component that varies with each pulse is oft en referred to as
the 'AC' component, and this has a frequency of around 1 Hz, depending
Human NIBP Nano Owner's Guide
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