Medical sensor with amplitude independent output

Details Medical sensor with amplitude independent output Medical sensor with amplitude independent output

1999-07-26

2002-04-09

Nasser, Robert L. (Department: 3736)

Surgery

Diagnostic testing

Measuring or detecting nonradioactive constituent of body...

C600S323000, C600S327000

Reexamination Certificate

active

06370408

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to medical sensors, and in particular to the signals generated for transmission by such sensors.
Non-invasive photoelectric pulse oximetry is an example of a medical sensor which is well known and is described, for instance, in U.S. Pat. No. 4,911,167, incorporated herein by reference. Pulse oximeters typically measure blood flow characteristics including, but not limited to, blood oxygen saturation of hemoglobin in arterial blood. Pulse oximeters pulse light through body tissue where blood perfuses the tissue and photoelectrically sense the absorption of light in the tissue. The amount of light absorbed is used to calculate the amount of the blood constituent being measured.
FIGS. 1A and 1B
together are a block diagram of an oximeter
100
such as the pulse oximeter model N-200 which is commercially available from Nellcor Incorporated, Hayward, Calif., U.S.A.
FIG. 1A
shows the sensor, patient module and analog front end of the oximeter. A patient sensor
110
, for sensing and transmitting the pulsed light, includes a photodetector
112
and a pair of light emitting diodes
114
,
116
(“LED's”). Typically, a first LED
114
emits light having a mean wavelength of about 660 nanometers in the red light range and the second LED
116
emits light having a mean wavelength of about 905 nanometers in the infrared range.
The photodetector
112
detects the red and infrared incident light, producing a current which changes value in response to the changes in the intensity of red and infrared light transmitted in sequence. The photodetector current produced has a small magnitude, typically in the range of 1×10
−9
amps. Because the current generated by the photodetector is so small, the signal is susceptible to inaccuracies caused by noise. In addition, the low current value generated decreases the degree of precision to which the detected signal can be accurately measured. By amplifying the photodetector current, noise susceptibility is decreased and the degree of precision to which the signal may be accurately measured is improved.
The detected current is converted to a voltage signal
122
and amplified by a combined current-to-voltage converter and amplifier
118
in a patient module
124
, which may be separate from sensor
110
. The sensor signal on line
122
from amplifier
118
is provided to an analog front-end circuit
120
which receives the amplified analog optical signal on line
122
from the patient module
124
and filters and processes it. The front-end circuit
120
separates the detected signal into red and infrared analog voltage signals
126
,
128
corresponding to the detected red and infrared optical pulses. The voltage signal on line
122
is first passed through low pass filter
130
to remove unwanted high frequency components and AC coupled through capacitor
132
to remove the DC component and unwanted low frequency components. The signal is then passed through a buffer amplifier
134
to remove any unwanted low frequencies and a programmable gain stage
136
to amplify and optimize the signal level presented to the synchronous detector
138
.
Synchronous detector
138
produces a synchronously-rectified voltage signal, and includes a two channel gating circuit which separates the signal into 2 components, one on line
140
representing the red light transmission and the other on line
142
representing the infrared light transmission. The separated signals on lines
140
,
142
are filtered to remove the strobing frequency, electrical noise, and ambient noise and then digitized by an analog-to-digital converter (“ADC”) section
144
(FIG.
1
B). The digitized signal
146
is used by the microprocessor
148
to calculate the blood oxygen saturation.
It is well known that oxygen saturation may be computed to a useful accuracy by the formula:
oxygen saturation=
AR
2
+BR+C
where:
R
=
AC
R
/
AC
IR
DC
R
/
DC
IR
where AC
R
and DC
R
are respectively the AC and DC components of the red transmissivity signal, AC
IR
and DC
IR
are the AC and DC components of the infrared transmissivity signal, and A, B and C are constants determined by empirical curve fitting against the results of standard blood oxygen measurements. Because the AC and DC components of the red and infrared signals correspond to the maximum and minimum amplitude values of the detected signal, the measured AC and DC signals are critical in calculating the blood oxygen saturation of hemoglobin in arterial blood. The microprocessor
148
uses the maximum and minimum voltages received from the ADC
144
to calculate the blood oxygen saturation level.
Although amplification of the detected current improves the accuracy of the oxygen saturation calculation, the added circuitry necessary for amplification increases system cost, power dissipation and the number of possible sources of errors. The embodiment shown in
FIG. 1
includes amplifiers
118
,
134
,
126
,
128
to amplify the detected signal.
An alternative method and apparatus for measuring blood oxygen saturation which does not require amplification circuitry is needed.
SUMMARY OF THE INVENTION
The present invention provides a medical sensor for detecting a blood characteristic. The sensor includes a transducer for producing an analog signal related to the blood characteristic. The analog signal is converted into a transmission signal which is in amplitude-independent form for transmission to a remote analyzer. The signal is amplitude-independent because the information content of the signal is not affected by changes in signal amplitude. Examples of amplitude independent signals are frequency modulated waveforms and digital pulse trains. In one embodiment of the invention, a current-to-frequency converter converts a signal from a pulse oximeter sensor into a variable-frequency signal which can be transmitted over a transmission line to a remote pulse oximeter.
The transducer and converting means can be integrated onto a single semiconductor chip which can be mounted adjacent to or in the sensor itself. In one embodiment, an automatic gain control (AGC) circuit is connected to the current-to-frequency converter to set the nominal operating frequency of the current-to-frequency converter. Where the sensor is a light detector, a light-to-frequency converter can be used.
Other amplitude independent forms of the signal can be used instead of the frequency-modulated waveform produced by the current-to-frequency converter. A pulse-width modulated signal could be used. Any number of digital transmission techniques can be used, for another example. An advantage of the frequency or digital communication is that it is not amplitude dependent, and is thus relatively noise immune. Thus, the need for a preamplifier next to the sensor, or coaxial cable, can be eliminated. In addition, conversion circuitry in the remote analyzer (such as the pulse oximeter) can be eliminated since the frequency or digital signal could be used directly.
In one embodiment, the converting means, such as a current-to-frequency converter, could be in the remote analyzer itself. This would provide the cost savings advantage of eliminating some circuitry, although not the noise immunity during the transmission to the analyzer.


REFERENCES:
patent: 4759369 (1988-07-01), Taylor
patent: 4947033 (1990-08-01), Kordts
patent: 5961450 (1999-10-01), Merchant
patent: 9221281 (1992-12-01), None

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