Surgery – Truss – Pad
Patent
1992-08-03
1995-03-14
Sykes, Angela D.
Surgery
Truss
Pad
128687, 128666, A61B 50205
Patent
active
053968930
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to monitoring apparatus provided with an optical sensor and functioning to analyze pulse frequencies by measuring the blood circulation of a measuring object, such as a part of the human body or an animal body using photoplethysmography (PPG-measuring), said apparatus being of the kind set forth in the preamble of claim 1. The invention also relates to a method of taking such measurements.
Photoplethysmography (hereinafter abbreviated to PPG) has been known to the art for more than 50 years and is applied technically for measuring peripheral blood circulation. The method is primarily used for measuring heart frequencies and blood circulation when performing surgery. The method has many advantages, since, among other things, it is non-invasive and does not subject the patient to any appreciable trauma. Furthermore, the method requires no highly expensive or complicated equipment in order to be put into effect.
When light from a suitable light source impinges on the skin, the light is dampened or attenuated according to the nature of the tissue on which the light impinges. This light attenuation is assumed to be constant. The light also passes through a number of blood vessels and is also attenuated by the blood present. The light is attenuated as a result of a number of complex processes, such as absorption, reflection and different forms of scattering. The PPG-technique is based on the assumption that the more blood that is present in the volume investigated, the more the light is attenuated. This results in two signal components of interest, namely a DC-component which corresponds to the total amount of blood in the investigated volume and an AC-component which signifies pulsation of the blood flow.
In order to study the aforedescribed phenomenon, it is necessary to use a light source, a light detector, amplifying electronics and a display unit, for instance an oscilloscope or a printer.
The light source and the detector may be placed on a respective side of the object on which blood circulation is to be measured, and the detector consequently measures the light transmitted. This technique is, at times, referred to as transmission-photoplethysmography and can only be applied to a few skin surfaces, such as fingers, ear lobes and toes.
A more general method is to place both the light source and the detector in one and the same probe and measure the light reflected. This technique is known as reflection * and is the dominating technique. It has long been understood that the pulsating component or AC-component of the PPG-signal is contingent on changes in blood volume during each heart beat. The greater the volume of blood, the less light will impinge on the detector.
It is obvious, however, that this is not the whole truth. Tests have been carried out in which blood has been allowed to pulsate in rigid glass tubes, where changes in volume are impossible and where solely the flow rate pulsates. A pulsating PPG-signal is also obtained in this case, which can be explained by the detection of changes in orientation of the erythrocytes, which varies during each heart beat.
In summary, there are at least two reasons for the AC-component, namely a change in blood volume and the orientation of the erythrocytes.
A typical PPG-signal has, in the time plane, the form of a blood-pressure curve having the same periodicity as the heart beats. The signal also includes a number of low frequencies.
The present invention is based on the realization that the lower frequencies occur as a result of changes in blood flow caused by the sympathetic nerve system and by respiration, this realization being based on the known fact that the intrathoracic pressure is lowered when breathing-in, or inspiring. This subpressure is utilized to "suck" the venous blood into the atrium and ventricle.
The invention departs from this starting point and assumes that this subpressure causes variations in blood flow in venous plexus and also that it should be possible to detect this variation with the a
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Lindberg Lars-Goran
Oberg Ake P.
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