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This invention relates to oximeters which measure levels of blood
oxygenation and, in particular, to a plethysmograph system for pulse
oximetry.
A pulse oximeter measures the oxygen level of blood by transmitting two
different wavelengths of light through a portion of a subject's body where
arterial blood is flowing. Conveniently this may be a finger or earlobe.
The light which has been transmitted through the body is detected by a
photodetector, which produces a current that is a function of the
pulsatile blood flow. The current produced in response to each wavelength
of light is measured, and these measurements may be combined by well-known
algorithms such as Bier's Law to produce a quantification of the oxygen
content of the blood.
Since the sensor used in the measurement is an electro-optic device, it can
respond to interfering signals from the other electrical and optical
energy sources. The sensor must respond to changes in light transmissivity
through the body. These physiological effects contain frequency components
in the DC to 50 Hz band. However, it is desirable that the sensor not
respond to ambient light. Accordingly, the plethysmograph system should
reject ambient light while detecting physiological signals in the
bandwidth of interest.
A second category of sources of interference is other electrical apparatus.
Other electrical devices in hospitals, such as electro-surgical
instruments, can generate radio frequency signals that a plethysmograph
system can pick up. It is desirable then to minimize the sensitivity of
the system to interfering signals from sources of this nature.
A known technique for eliminating the interfering signals described above
is to drive the light sources by a signal having a frequency which is not
present in artificial light or characteristic of other medical
instrumentation. Received signals are then passed through a bandpass
filter to reject signals outside the band of interest, and the filtered
signals are then detected by an envelope detector. While effective for
rejecting unwanted signals, the energization of the light sources in
alteration by the driving signal mandates that the detector be
synchronized with the driving signal for correct demodulation. As the
following discusson will show, this arrangement requires undesired
widening of the receiver bandwidth, or electrical connections which
complicate electrical isolation of the light sources and optical sensor.
In accordance with the principles of the present invention, the response of
a plethysmograph system to interfering signals is reduced through
modulation of the sensor light sources. The light sources are each
modulated with a characteristic that distinguishes received signals from
each other and that can be distinguished from ambient light contributions
to the detected signal. The deodulation is performed over selective
bandwidths which further immunizes the system against radio frequency
interference.
In the drawings:
FIGS. 1-3 illustrate spectra resulting from use of the waveform of FIGS.
1a, 1b, 2a, 3a and 6 in frequency multiplexing in accordance with the
principles of the present invention;
FIG. 4 illustrates a bandpass filter response for the waveforms of FIGS. 2
and 3;
FIG. 5 illustrates a back-to-back configuration of LEDs;
FIG. 7 illustrates a preferred embodiment of the present invention; and
FIG. 8 illustrates waveforms used to explain the arrangement of FIGS. 7a-7b
.
In a conventional pulse oximeter sensor a light emitting diode (LED) is
used as the light source which transmits light through tissue. Use of an
LED is desirable due to its dependability, low voltage requirement, and
narrow optical bandwidth of light emission. In accordance with the
principles of the present invention, the LED is switched on and off at a
frequency which is substantially higher than the frequency range of
ambient light (DC) and the physiological signals of interest (DC to 50
Hz). A photodetector receives the transmitted light which further contains
a component representative of pulsatile blood flow, the physiological
signal, and also receives any ambient light present. The photodetector
signal is passed by a bandpass filter which is tuned to a significant
component frequency of the switched excitation signal and exhibits a
bandwidth similar to that of the physiological signal. The narrow
bandwidth enables the system to reject interfering signals at frequencies
outside the filter passband, included the substantially constant (DC)
component resulting from detection of ambient light. The filter output is
a sinusoidal wave, amplitude modulated with the physiological signal. An
amplitude demodulator is employed to recover the physiological signal.
The frequency multiplexing technique is a significant improvement over the
commonly employed technique of time division multiplexing. In time
division multiplexing the LED is similarly switched on and off, and the
photodetector signal received when the LED is off, which is caused by
ambient light, is subtracted from the signal received when the LED is on.
However, due to the need to preserve the phase relationship between the on
and off states, the receiver bandwidth must extend from DC to above the
excitation signal frequency. Thus, the receiver employing time division
multiplexing is responsive to wideband noise over this full bandwidth.
Referring now to FIG. 1, the spectrum of a square wave F1 of FIG. 1a is
shown. The spectrum is seen to consist of only odd harmonics of square
wave F1, i.e., F1 3F1, 5F1, etc. If the square wave F1 is modulated with a
square wave F0, shown in FIG. 1b, the result is the modulated waveform
F0.times.F1 shown in FIG. 2a. This modulated waveform has a spectrum shown
in FIG. 2. The spectrum of FIG. 2 consists of the same odd harmonics of
the F1 square wave, each with upper and lower sidebands spaced at odd
harmonics of F0 from F1, i.e. F1-F0, F1+F0, etc. Neither the harmonics of
F1 nor the modulation sidebands occur at frequencies which are even
harmonics of F1.
In pulse oximetry it is necessary to use LEDs of two wavelengths in order
to gather signal components which can be used to compute blood
oxygenation. Conventionally, one LED transmits light at a red wavelength,
and the other LED transmits light at an infrared (IR) wavelength. It is
convenient to connect the two LEDs in an oximeter sensor in a back-to-back
configuration as shown in FIG. 5, allowing either LED to be selectively
energized by reversing the applied current and requiring only two
connecting conductors. If a differential voltage drive is used, capacitive
coupling of the LED drive signals to the detector circuitry, the cable of
which is generally in close proximity to the LED conductors, can be
minimized. In accordance with the principles of the present invention, one
LED will be driven by a signal with the spectrum shown in FIG. 2. The
second LED in the sensor is switched by a square wave F2 of a second
frequency, which is modulated by the F0 square wave of FIG. 1b. The result
of this modulation is the F0.times.F2 waveform shown in FIG. 3a. This
waveform has a spectrum as shown in FIG. 3. The spectrum shows the odd
harmonics of F2 and and 3F2, each with upper and lower sidebands spaced at
odd harmonics of F0 from F2.
Since the spectrum of FIG. 2 has no components at F2 and the spectrum of
FIG. 3 has no components at F1, two bandpass filters can be used to
separate the F1 and F2 signal components from the received signal. FIG. 4
shows the responses of two filters that may be used to separate the two
desired signals. A bandpass filter centered at F1 will respond to the
transmission of light from the LED modulated by the F0.times.F1 waveform,
and a bandpass filter centered at F2 will respond to the transmission of
light from the LED modulated by the F0.times.F2 waveform. Each filter must
have a bandwidth of at least twice the bandwidth of the physiological
signal, that is, two times 50 Hz=100 Hz, since this information is
contained in sidebands of the center frequency. The filter must be narrow
enough to exclude the nearest modulation sidebands of the F0 square wave,
which are F0 above and below the respective center frequencies of the
filters. This is representatively shown in FIG. 4a, which is an expansion
of a portion of the spectrum of FIG. 2. This spectrum shows the center
frequency F1 of the bandpass filter and the filter bandwidth in the range
indicated by the bracket. The F1-F0 and F1+F0 sidebands are outside the
filter passband, and the physiological information signals, indicated as
PI, are sidebands of the center frequency and contained within the
passband.
The excitation signal waveforms of FIGS. 2a and 3a are not suitable for use
by back-to-back configured LEDs, shown in FIG. 5. This is because the
times that the LEDs are on are time coincident, a physical impossibility
when the LEDs are so connected. FIG. 6 shows waveforms that exhibit the
spectral characteristics of FIGS. 2 and 3 while illuminating only one LED
at a time. The square waves F1, F2, and F0 of FIGS. 6a-6c are combined to
produce the excitation waveforms of FIGS. 6d and 6e. Specifically, the F0
square wave is used to modulate the F1 square wave such that an excitation
pulse is produced each time F0 and F1 are coincidentally high. This
produces the excitation waveform F0.times.F1 shown in FIG. 6d. The inverse
of the F0 square wave, F0, is used to modulate the F2 square wave to
produce the excitation waveform F0.times.F2 shown in FIG. 6e. Thus, the
modulating F0 waveform interleaves the F1 and F2 excitation signals such
that there is no time when the two LEDs must be simultaneously turned on.
Referring to FIG. 7, the modulation and demodulation section and sensor of
a pulse oximeter constructed in accordance with the principles of the
present invention are shown. In order to minimize electrical hazards to
the patient, the sensor electronics are electrically isolated from the
electronics of the processor by three transformers T1, T2 and T3. To
energize the sensor electronics a 28.8 kHz clock signal is supplied by a
source 20 of clock signals to a terminal 12. The 28.8 kHz clock signal
switches a transistor 14, which drives the primary winding P3 of
transformer T3. A 7.6 volt reference potential is connected to the other
end of the primary winding P3 to provide a DC voltage +V for transistor 14
and amplifiers 76, 78, and associated circuits.
The 28.8 kHz signal is transformer coupled to the secondary winding S3 of
the transformer T3, which is center-tapped to the isolated ground of the
sensor electronics. A resistor 30 is coupled to one end of the secondary
winding S3 and provides a 28.8 kHz clock reference signal .PHI. for the
sensor electronics. Rectifying diodes 32 and 34 are coupled to opposite
ends of the winding S3 to produce a DC supply voltage +6Vi for the sensor
electronics. The rectified supply voltage +6Vi is filtered by a capacitor
36 and stabilized by a Zener diode 38, and is applied at various points to
the sensor electronics.
The 28.8 kHz reference signal .PHI. is applied to the input of a three
stage binary counter 40 and to the clock inputs of J-K flip-flops 42 and
44. These digital elements cooperate to produce the modulated waveforms
which energize LEDs 62 and 64 by way of drive transistors 50 in accordance
with the present invention. The counter 40 changes state on the
positive-going transitions of the .PHI. signal and produces square waves
at its outputs which are sequentially divided by two. FIG. 8 shows
waveforms occurring during one cycle of LED energization. The 28.8 kHz
reference signal .PHI. is shown at the top of the FIGURE. The output
signal at the output Q0 of the first counter stage, a 14.4 kHz signal, is
shown immediately below in the FIGURE, followed by the 7.2 kHz waveform at
the Q1 (second stage) output and the 3.6 kHz waveform at the Q2 (third
stage) output. The Q0, Q1, and Q2 output waveforms are all seen to switch
on positive-going transitions of the .PHI. signal.
The Q2 output of the counter 40 is coupled to the reset input of J-K
flip-flop 42, and the Q0 output of the counter is coupled to the J and the
K inputs of the flip-flop 44. The J and K inputs of flip-flop 42 are
coupled to the +6Vi supply voltage, and both flip-flops will accordingly
toggle under predetermined conditions. The reset input of flip-flop 44 is
coupled to the Q output of flip-flop 42. The J-K flip-flops change state
on negative-going clock signal transitions.
Consider first the Q output of flip-flop 44, which is to produce a 7.2 kHz
waveform as shown at the bottom of FIG. 8. At the beginning of the LED
energization cycle both flip-flops 42 and 44 are reset. The Q output of
flip-flop 42 is high, and this high signal at the reset input of flip-flop
44 permits the flip-flop 44 to be toggled. The first falling edge of the
clock signal .PHI. at time t.sub.0 will not toggle the flip-flop 44
because the Q0 signal at its J and K inputs is low. However, at time
t.sub.1 the Q0 signal is high, and the negative-going edge of the clock
signal .PHI. will toggle the flip-flop 44 to its set condition. At time
t.sub.2 the flip-flop will not change state because the Q0 signal is again
low. But at time t.sub.3 the Q0 signal is again high, and the clock signal
.PHI. toggles the flip-flop 44 to its reset condition. This toggling of
flip-flop 44 produces the waveform shown at the bottom of FIG. 8 at the Q
output of flip-flop 44, and the inverse at the Q output.
During the time that flip-flop 44 is being toggled, flip-flop 42 is
inhibited from switching by reason of the low Q2 signal at its reset
input. This condition ends at time t.sub.4 when the Q2 signal goes high,
whereafter the flip-flop 42 may be toggled. Flip-flop 42 is to produce a
14.4 kHz waveform, interleaved in time with the 7.2kHz pulses of flip-flop
44, as shown in the penultimate line of FIG. 8.
At time t.sub.5, the clock signal .PHI. toggles flip-flop 42 to its set
condition. The flip-flop 44 will not set at this time because the Q0
signal is low. When flip-flop 42 is set, the low signal at its Q output
holds flip-flop 44 in its reset condition. At time t.sub.6 the clock
signal .PHI. toggles flip-flop 42 to its reset state. Although the Q0
signal is high at this time, the flip-flop 44 cannot be set because the
low Q signal of flip-flop 42 holds flip-flop 44 in its reset condition
during the transition of the clock signal .PHI.. The simultaneous clocking
of the flip-flops by the clock signal .PHI. sets up a controlled race
condition whereby the clock signal .PHI. cannot toggle flip-flop 44 at the
moment of the clock transition by reason of the low signal still at the
reset input of flip-flop 44.
At time t.sub.7 the flip-flop 42 is toggled again as it was at time t.sub.5
and at time t.sub.8 the flip-flop 42 is toggled to its reset state as it
was at time t.sub.6. The flip-flop 44 does not switch at these later times
for the same reasons that applied at times t.sub.5 and t.sub.6. Finally at
time t.sub.9 the Q2 signal goes low. Flip-flop 42 is once again inhibited
and the cycle repeats.
The Q output of flip-flop 42 is coupled to the base of drive transistor 52,
and the Q output is coupled to the base of drive transistor 58. The Q
output of flip-flop 44 is coupled to the base of drive transistor 56 and
the Q output is coupled to the base of drive transistor 54. The +6Vi
supply voltage is applied to the collectors of transistors 52 and 56,
which are source transistors for the drive current to LEDs 62 and 64. The
collectors of transistors 54 and 58 are coupled to the isolated ground of
the sensor electronics so that these transistors may sink LED current. The
emitters of transistors 52 and 54 are coupled to each other and to a
connector for the LEDs. The emitters of transistors 56 and 58 are coupled
to each other and to another LED connector. The back-to-back coupled LEDs
62 and 64 may thus be detachably connected to the respective joined
emitters.
In operation, when the Q output signal of flip-flop 42 goes high to drive
one of the LEDs with a modulated 14.4 kHz waveform, the Q output signal
turns on transistor 52 to provide a current path to the anode of LED 64
and the cathode of LED 62. Transistor 54 is turned off at this time by the
high signal from the Q output of flip-flop 44, and transistor 56 is turned
off by the low Q signal of flip-flop 44. The low Q signal at the Q output
of flip-flop 42 turns on transistor 58 at this time, and transistor 58
will thus sink the current provided by transistor 52. The flow of current
thus is from the +6Vi supply, through transistor 52, the LEDs and
transistor 58 to the isolated ground. This direction of current flow will
forward bias LED 64, turning it on, and will reverse bias LED 62 and keep
it off. LED 64 is accordingly illuminated at the modulated 14.4 kHz rate.
In a similar manner, when the Q output of flip-flop 44 goes high to drive
LED 62 with the modulated 7.2 kHz waveform, transistor 56 turns on to
source current to the anode of LED 62 and the cathode of LED 64.
Transistors 52 and 58 are not conducting at this time by reason of the low
and high signals at the Q and Q outputs of flip-flop 42. The low signal at
the Q output of flip-flop 44 turns on transistor 54 to sink current from
the LEDs. This path of current will forward bias LED 62 into conduction
and reverse bias LED 64, thereby illuminating LED 62 at the 7.2 kHz rate.
The light emitted by the LEDs passes through the tissue of the patient and
is received by a photodiode 60. Photodiode 60 is also detachably connected
to the sensor electronics by a connector. The photodiode 60 is energized
by application of the +6Vi supply voltage to one side of the connector,
with the other side of the connector providing a DC path through the
series coupled primary windings P1 and P2 of transformers T1 and T2 and a
resistor 70 to the isolation ground. The voltage supply to the photodiode
connector is filtered by a capacitor 71.
The photodiode 60 produces an alternating signal in response to the light
pulses produced by the LEDs 62 and 64. The alternating signal has two
components modulated by physiological information: a 7.2 kHz component
developed by the light pulses from LED 62, and a 14.4 kHz component
developed by light pulses from LED 64. These two frequency components are
separated by transformers T1 and T2. A capacitor 72 is coupled across the
secondary winding S2 of transformer T2 to form a tuned circuit resonant at
7.2 kHz. A capacitor 74 is coupled across the secondary winding S1 of
transformer T1 to form a tuned circuit resonant at 14.4 kHz. Thus, the
composite alternating signal from the photodiode 60 is applied to the
primary sides of the two transformers, but their secondary tuned circuits
are responsive only to the frequency components corresponding to their
respective resonant frequencies. In the preferred embodiment the bandwidth
of each tuned circuit is approximately 60 Hz to respond to those signals
in the physiological band of interest while providing immunity to
out-of-band interference. The transformer coupling provides DC isolation
between the sensor electronics and the processor electronics.
The two tuned circuits are coupled to the noninverting inputs of respective
amplifiers 76 and 78. The amplifiers have gain determining resistors 84,
86, 92, and 94 coupled to provide negative feedback, and the two
amplifiers are DC biased by resistors 80 and 82, coupled between the +V
voltage supply and processor ground. The resistor network also provides a
DC reference to the side of each tuned circuit opposite the inputs to the
amplifiers. The amplifier 76 provides amplified 7.2 kHz signal components
and physiological information signals at its output, and the amplifier 78
provides amplified 14.4 kHz signal components and physological information
signals.
The amplified signal components are then demodulated by amplitude
demodulators 100 and 102 to recover the physiological information. The
28.8 kHz clock signal is divided by a divider 22 to produce a 14.4 kHz
mixing signal for demodulator 102, thereby enabling detection of the
amplitude modulated physiological information signals from LED 64. The
signal provided by divider 22 is again divided by two by divider 24 to
produce a 7.2 kHz reference signal for demodulator 100. This enables
demodulation of the amplitude modulated physiological information signals
from LED 62. The demodulated information signals, termed RED and IR in the
drawing, may then be further filtered to remove the mixing signals and
transmitted to the oximeter processor for calculation of the level of
blood oxygenation.
It is seen that the arrangement of FIG. 7 provides the modulated LED drive
signals on the DC isolated (sensor) side of transformer T3. Separation of
the two desired signal components is done through the tuning of
transformers T1 and T2, which likewise provide DC isolation for the
sensor. It may be appreciated that if the states of the LED drive signals
(specifically Q2) were known on the processor side of the transformers, a
single demodulator could be used to demodulate the received signals in a
time division multiplexing manner. However, coupling this information back
to the demodulator would undesirably require a further transformer. The
arrangement of FIG. 7 preferably provides all signal requirements and DC
isolation with only three transformers. Insofar as the processor side is
concerned, transformer T3 provides an energization signal and a free
running clock signal to the isolated sensor electronics. The LED drive
signals are modulated in asynchronism with respect to the processor side
of the system, and LED wavelength discrimination is performed by the
resonant secondaries of transformers T1 and T3. No other decoding or
discrimination between the isolated sections of the arrangement is
required.
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