Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-07-16
2004-05-04
Bennett, Henry (Department: 3742)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S480000, C600S485000, C600S500000, C600S473000, C600S475000, C600S477000
Reexamination Certificate
active
06731967
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to non-invasive and implantable (i.e., invasive) plethysmography methods and devices. The present invention more particularly relates to methods and devices for monitoring volume changes in a limb or tissue segment of a patient. The present invention also relates to methods and devices that calculate blood oxygenation levels.
2. Background Art
Plethysmography is a generic term referring to a variety of techniques for monitoring volume changes, for example, volume changes of the lungs due to respiration, or of blood vessels of a limb or tissue segment. When applied to measurements of blood volume, changes occur in a pulsatile manner with each beat of the heart as blood flows in and out of a portion of the body. The study of vascular activity by fluid displacement methods dates hack to at least 1890. More contemporary techniques include strain gauge, pneumatic, impedance, doppler, and photoelectric plethysmography. A plethysmography device produces a waveform that is similar to an arterial pressure waveform. The waveform is useful in measuring pulse velocity and indicating arterial obstructions.
FIG. 1
illustrates an exemplary plethysmograph
100
, which includes a waveform
102
produced by a plethysmography device. For timing reference, an electrocardiogram (ECG) signal
104
is illustrated. Waveform
102
provides a measure of the volume of the arterial vasculature. A measure of arterial pulse amplitude is derived from it. A few tens to a few hundreds of milliseconds after the QRS complex, the plethysmography voltage reaches a minimum and starts to increase. This is due to the increasing blood volume in the arterioles as the systolic pulse reaches the periphery. The delay is influenced by the distance that the sensor is placed from the heart. It requires approximately 100 msec for the waveform to reach its maximum. The excursion from minimum to maximum represents the arterial pulse amplitude. During diastole, the recoil of the elastic arterial vessels continues to force blood through the capillaries, so that blood flows through the capillary bed throughout the entire cardiac cycle.
A photoplethysmography device (PPG) (also called a pseudoplethysmography or photoelectric plethysmography device) includes a light detector and a light source. The PPG utilizes the transmission or reflection of light to demonstrate the changes in blood perfusion. Such devices might be used in the cardiology department or intensive care department of a hospital or in a clinic for diagnostic purposes related to vascular surgery. A photoplethysmography device is also referred to, herein, simply as a plethysmography device.
An exemplary circuit
200
A for a conventional photoplethysmography device is shown in FIG.
2
A. An exemplary mechanical arrangement
200
B for a conventional photoplethysmography device is shown in FIG.
2
B. In these examples, the light source is a light-emitting diode (LED)
202
, although in alternative models an incandescent lamp can be used as the light source. The light detector in this example is a photoresistor
204
excited by a constant current source. Changes in light intensity cause proportional changes in the resistance of the photoresistor. Since the current through the photoresistor is constant in this example, the resistance changes produce varying analog voltage (V
out
—
analog
) at the output terminal. In order to be useful, this varying analog voltage (V
out
—
analog
) typically must be converted to a digital signal (V
out
—
digital
) using an analog to digital converter (A/D)
206
. Other known light detectors include photo diodes, photo transistors, photo darlingtons and avalanche photo diodes. Light detectors are often also referred to as photo detectors or photo cells.
Light may be transmitted through a capillary bed such as in an ear lobe or finger tip. As arterial pulsations fill the capillary bed the changes in volume of the blood vessels modify the absorption, reflection and scattering of the light. Stated another way, an arterial pulse in, for example, a finger tip, or ear lobe, causes blood volume to change, thereby changing the optical density of the tissue. Therefore, the arterial pulse modulates the intensity of the light passing through the tissue. Light from LED
202
is reflected into photoresistor
204
by scattering and/or by direct reflection from an underlying bone structure. Such a PPG does not indicate “calibratable” value changes. Thus, its usefulness is generally limited to pulse-velocity measurements, determination of heart rate, and an indication of the existence of a pulse (e.g., in a finger). Additionally, a conventional PPG provides a poor measure of changes in volume and is very sensitive to motion artifacts.
It is noted that photoplethysmography devices may operate in either a transmission configuration or a reflection configuration. In the transmission configuration, the light source (e.g., LED
202
) and the photodetector (e.g.,
204
) face one another and a segment of the body (e.g., a finger or earlobe) is interposed between the source and detector. In the reflection configuration, the light source (e.g., LED
202
) and photodetector (e.g.,
204
) are mounted adjacent to one another, e.g., on the surface of the body, as shown in FIG.
2
B. If the photoplethysmography device is incorporated into an implantable cardioverter defibrillator (ICD) or other implantable therapy device or monitor, and thus implanted, then the light source (e.g., LED
202
) and light detector (e.g.,
204
) can be mounted adjacent to one another on the housing (e.g., can) or header of the ICD, as disclosed in U.S. patent application Ser. No. 09/543,214, entitled “Extravascular Hemodynamic Sensor”, filed Apr. 5, 2000, which is incorporated herein by reference in its entirety.
In a conventional photoplethysmography device (e.g.,
100
A), a constant average optical power is delivered by the optical source (e.g., LED
202
) and plethysmograph information (e.g., waveform
102
shown in
FIG. 1
) is determined based on time varying optical power incident on the detector (e.g., photoresistor
204
). This approach is not optimal for many reasons, some of which are discussed above and others of which are discussed below.
First, providing a constant average optical power does not allow for power consumption to be minimized. This may not be a concern if the plethysmography device is a non-invasive device, and thus, can receive power from an relatively inexpensive and inexhaustible power supply. However, if the plethysmography device is an implantable device (or part of an implantable device), as is the case in many embodiments of the present invention, the device is likely powered by a battery that is not easily accessible. For example, if the plethysmograph is incorporated into an ICD, pacemaker, or other implantable therapy device or monitor, then the battery of the device could be used to power components (e.g., LEDs, amplifiers) of the plethysmography device. Typically, invasive surgery is required to replace the ICD or pacemaker when its battery nears depletion. Accordingly, there is a need to minimize power consumption of the components of the plethysmography device. This holds true for any implantable plethysmography device, whether or not it is incorporated into an ICD or pacemaker.
Second, the dynamic range (e.g., linear range) of a photodiode, or any other type of photoresistor or photodetector, is limited. The upper limit is usually a function of the saturation point of the detector and/or detector amplifiers. The lower limit is typically a function of environmental and/or circuit noise. When any of these components are operating outside of its dynamic range, the accuracy and integrity of the information being obtained (e.g., plethysmography waveform
102
) is adversely affected. Accordingly, there is a need to operate a plethysmography device within the dynamic range of its components. The criticality of this need is increased when the plethysmography device i
Bennett Henry
Dahbour Fadi H.
Mitchell Steven M.
Pacesetter Inc.
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