Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2001-05-15
2004-01-27
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S324000, C600S336000
Reexamination Certificate
active
06684090
ABSTRACT:
BACKGROUND OF THE INVENTION
Oximetry is the measurement of the oxygen status of blood. Early detection of low blood oxygen is critical in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximeter typically provides a numerical readout of the patient's oxygen saturation, a numerical readout of pulse rate, and an audible indicator or “beep” that occurs in response to each pulse. In addition, a pulse oximeter may display the patient's plethysmograph waveform, which is a visualization of blood volume change in the illuminated tissue caused by pulsatile arterial blood flow over time. The plethysmograph provides a visual display that is also indicative of the patient's pulse and pulse rate.
A pulse oximetry system consists of a sensor attached to a patient, a monitor, and a cable connecting the sensor and monitor. Conventionally, a pulse oximetry sensor has both red and infrared (IR) light-emitting diode (LED) emitters and a photodiode detector. The sensor is typically attached to a patient's finger or toe, or a very young patient's patient's foot. For a finger, the sensor is configured so that the emitters project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode is positioned at the fingertip opposite the fingernail so as to detect the LED transmitted light as it emerges from the finger tissues.
The pulse oximetry monitor (pulse oximeter) determines oxygen saturation by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor. The pulse oximeter alternately activates the sensor LED emitters and reads the resulting current generated by the photodiode detector. This current is proportional to the intensity of the detected light. The pulse oximeter calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The pulse oximeter contains circuitry for controlling the sensor, processing the sensor signals and displaying the patient's oxygen saturation and pulse rate. A pulse oximeter is described in U.S. Pat. No. 5,632,272 assigned to the assignee of the present invention.
SUMMARY OF THE INVENTION
FIG. 1
illustrates the standard plethysmograph waveform
100
, which can be derived from a pulse oximeter. The waveform
100
is a display of blood volume, shown along the y-axis
110
, over time, shown along the x-axis
120
. The shape of the plethysmograph waveform
100
is a function of physiological conditions including heart stroke volume, pressure gradient, arterial elasticity and peripheral resistance. The ideal waveform
100
displays a broad peripheral flow curve, with a short, steep inflow phase
130
followed by a 3 to 4 times longer outflow phase
140
. The inflow phase
130
is the result of tissue distention by the rapid blood volume inflow during ventricular systole. During the outflow phase
140
, blood flow continues into the vascular bed during diastole. The end diastolic baseline
150
indicates the minimum basal tissue perfusion. During the outflow phase
140
is a dicrotic notch
160
, the nature of which is disputed. Classically, the dicrotic notch
160
is attributed to closure of the aortic valve at the end of ventricular systole. However, it may also be the result of reflection from the periphery of an initial, fast propagating, pressure pulse that occurs upon the opening of the aortic valve and that precedes the arterial flow wave. A double dicrotic notch can sometimes be observed, although its explanation is obscure, possibly the result of reflections reaching the sensor at different times.
FIGS. 2-4
illustrate plethysmograph waveforms
200
,
310
,
360
that display various anomalies. In
FIG. 2
, the waveform
200
displays two arrhythmias
210
,
220
. In
FIG. 3
, the waveform
310
illustrates distortion corrupting a conventional plethysmograph
100
(FIG.
1
).
FIG. 4
shows a filtered waveform
360
after distortion has been removed through adaptive filtering, such as described in U.S. Pat. No. 5,632,272 cited above.
FIG. 4
illustrates that, although the waveform
360
is filtered, the resulting pulses
362
have shapes that are distorted in comparison to the pulses illustrated in FIG.
1
.
A desirable feature of pulse oximeters is an audible “beep” tone produced to correspond to the patient's pulse. Conventionally, the beep is triggered from recognition of some aspect of the plethysmograph waveform shape. Such a waveform-triggered beep may indicate an arrhythmia, like those displayed in
FIG. 2
, but may also generate false pulse indications as the result of motion-artifact or noise induced waveform distortion, as illustrated in
FIGS. 3 and 4
. This characteristic results because both distortion and arrhythmias result in anomalies in the plethysmograph waveform shape on which this beep mechanism is dependent. Alternatively, the beep can be triggered from a time base set to the average pulse rate. Signal processing can generate an average pulse rate that is resistant to distortion induced error. A pulse beep based on average pulse rate is relatively insensitive to episodes of distortion, but is likewise insensitive to arrhythmias.
An example of the determination of pulse rate in the presence of distortion is described in U.S. Pat. No. 6,002,952, filed Apr. 14, 1997, entitled “Signal Processing Apparatus and Method,” which is assigned to the assignee of the current application and incorporated by reference herein. Another example of pulse rate determination in the presence of distortion is described in U.S. patent application Ser. No. 09/471,510, filed Dec. 23, 1999, entitled “Plethysmograph Pulse Recognition Processor,” which is assigned to the assignee of the current application and incorporated by reference herein.
One aspect of the present invention is a processor having a decision element that determines if the waveform has little or no distortion or significant distortion. If there is little distortion, the decision element provides a trigger in real-time with physiologically acceptable pulses recognized by a waveform analyzer. If there is significant distortion, then the decision element provides the trigger based synchronized to an averaged pulse rate, provided waveform pulses are detected. The trigger can be used to generate an audible pulse beep that is insensitive to episodes of significant distortion, but is capable of responding to arrhythmia events.
Another desirable feature for pulse oximeters is a visual indication of the patient's pulse. Conventionally, this is provided by an amplitude-versus-time display of the plethysmograph waveform, such as illustrated in FIG.
1
. Some monitors are only capable of a light-bar display of the plethysmograph amplitude. Regardless, both types of displays provide a sufficient indication of the patient's pulse only when there is relatively small distortion of the plethysmograph waveform. When there is significant distortion, such as illustrated in
FIG. 3A
, the display provides practically no information regarding the patient's pulse.
Yet another desirable feature for pulse oximeters is an indication of confidence in the input data. Conventionally, a visual display of a plethysmograph waveform that shows relatively small distortion would convey a high confidence level in the input data and a corresponding high confidence in the saturation and pulse rate outputs of the pulse oximeter. However, a distorted waveform does not necessarily indicate low confidence in the input data and resulting saturation and pulse rate outputs, especially if the pulse oximeter is designed to function in the presence of motion-artifact.
Another aspect of the current invention is the generation of a data int
Ali Ammar Al
Breed Divya S.
Kiani Massi E.
Novak Jerome J.
Knobbe Martens Olson & Bear LLP
Kremer Matthew
Masimo Corporation
Winakur Eric F.
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