Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-03-01
2003-06-24
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
active
06584336
ABSTRACT:
BACKGROUND OF THE INVENTION
Oximetry is the measurement of the oxygen level status of blood. Early detection of low blood oxygen level 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 oximetry system consists of a sensor applied to a patient, a pulse oximeter, and a patient cable connecting the sensor and the pulse oximeter.
The pulse oximeter may be a standalone device or may be incorporated as a module or built-in portion of a multiparameter patient monitoring system, which also provides measurements such as blood pressure, respiratory rate and EKG. 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, the pulse oximeter may display the patient's plethysmograph, which provides a visual display of the patient's pulse contour and pulse rate.
SUMMARY OF THE INVENTION
FIG. 1
illustrates a prior art pulse oximeter
100
and associated sensor
110
. Conventionally, a pulse oximetry sensor
110
has LED emitters
112
, typically one at a red wavelength and one at an infrared wavelength, and a photodiode detector
114
. The sensor
110
is typically attached to an adult patient's finger or an infant patient's foot. For a finger, the sensor
110
is configured so that the emitters
112
project light through the fingernail and through the blood vessels and capillaries underneath. The LED emitters
112
are activated by drive signals
122
from the pulse oximeter
100
. The detector
114
is positioned at the fingertip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues. The photodiode generated signal
124
is relayed by a cable to the pulse oximeter
100
.
The pulse oximeter
100
determines oxygen saturation (SpO
2
) by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor
110
. The pulse oximeter
100
contains a sensor interface
120
, an SpO
2
processor
130
, an instrument manager
140
, a display
150
, an audible indicator (tone generator)
160
and a keypad
170
. The sensor interface
120
provides LED drive current
122
which alternately activates the sensor red and IR LED emitters
112
. The sensor interface
120
also has input circuitry for amplification and filtering of the signal
124
generated by the photodiode detector
114
, which corresponds to the red and infrared light energy attenuated from transmission through the patient tissue site. The SpO
2
processor
130
calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on that ratio. The instrument manager
140
provides hardware and software interfaces for managing the display
150
, audible indicator
160
and keypad
170
. The display
150
shows the computed oxygen status, as described above. The audible indicator
160
provides the pulse beep as well as alarms indicating desaturation events. The keypad
170
provides a user interface for such things as alarm thresholds, alarm enablement, and display options.
Computation of SpO
2
relies on the differential light absorption of oxygenated hemoglobin, HbO
2
, and deoxygenated hemoglobin, Hb, to determine their respective concentrations in the arterial blood. Specifically, pulse oximetry measurements are made at red and IR wavelengths chosen such that deoxygenated hemoglobin absorbs more red light than oxygenated hemoglobin, and, conversely, oxygenated hemoglobin absorbs more infrared light than deoxygenated hemoglobin, for example 660 nm (red) and 905 nm (IR).
To distinguish between tissue absorption at the two wavelengths, the red and IR emitters
112
are provided drive current
122
so that only one is emitting light at a given time. For example, the emitters
112
may be cycled on and off alternately, in sequence, with each only active for a quarter cycle and with a quarter cycle separating the active times. This allows for separation of red and infrared signals and removal of ambient light levels by downstream signal processing. Because only a single detector
114
is used, it responds to both the red and infrared emitted light and generates a time-division-multiplexed (“modulated”) output signal
124
. This modulated signal
124
is coupled to the input of the sensor interface
120
.
In addition to the differential absorption of hemoglobin derivatives, pulse oximetry relies on the pulsatile nature of arterial blood to differentiate hemoglobin absorption from absorption of other constituents in the surrounding tissues. Light absorption between systole and diastole varies due to the blood volume change from the inflow and outflow of arterial blood at a peripheral tissue site. This tissue site might also comprise skin, muscle, bone, venous blood, fat, pigment, etc., each of which absorbs light. It is assumed that the background absorption due to these surrounding tissues is invariant and can be ignored. Thus, blood oxygen saturation measurements are based upon a ratio of the time-varying or AC portion of the detected red and infrared signals with respect to the time-invariant or DC portion:
RD/IR
=(Red
Ac
/Red
DC
)/(
IR
AC
/IR
DC
)
The desired SpO
2
measurement is then computed from this ratio. The relationship between RD/IR and SpO
2
is most accurately determined by statistical regression of experimental measurements obtained from human volunteers and calibrated measurements of oxygen saturation. In a pulse oximeter device, this empirical relationship can be stored as a “calibration curve” in a read-only memory (ROM) look-up table so that SpO
2
can be directly read-out of the memory in response to input RD/IR measurements.
Pulse oximetry is the standard-of-care in various hospital and emergency treatment environments. Demand has lead to pulse oximeters and sensors produced by a variety of manufacturers. Unfortunately, there is no standard for either performance by, or compatibility between, pulse oximeters or sensors. As a result, sensors made by one manufacturer are unlikely to work with pulse oximeters made by another manufacturer. Further, while conventional pulse oximeters and sensors are incapable of taking measurements on patients with poor peripheral circulation and are partially or fully disabled by motion artifact, advanced pulse oximeters and sensors manufactured by the assignee of the present invention are functional under these conditions. This presents a dilemma to hospitals and other caregivers wishing to upgrade their patient oxygenation monitoring capabilities. They are faced with either replacing all of their conventional pulse oximeters, including multiparameter patient monitoring systems, or working with potentially incompatible sensors and inferior pulse oximeters manufactured by various vendors for the pulse oximetry equipment in use oat the installation.
Hospitals and other caregivers are also plagued by the difficulty of monitoring patients as they are transported from one setting to another. For example, a patient transported by ambulance to a hospital emergency room will likely be unmonitored during the transition from ambulance to the ER and require the removal and replacement of incompatible sensors in the ER. A similar problem is faced within a hospital as a patient is moved between surgery, ICU and recovery settings. Incompatibility and transport problems are exacerbated by the prevalence of expensive and non-portable multiparameter patient monitoring systems having pulse oximetry modules as one measurement parameter.
The Universal/Upgrading Pulse Oximeter (UPO) according to the present invention is focused on solving these performance, incompatibility and transport
Ali Ammar Al
Crothers Don
Dalke David
Diab Mohamed K.
Goldman Julian
Knobbe Martens & Olson Bear LLP.
Masimo Corporation
Winakur Eric F.
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