Optical Fibre Catheter Pulse Oximeter

Abstract

Apparatus for measuring the oxygen saturation level of blood at an internal measurement site in a human or animal patient by reflectance pulse oximetry, comprising first and second light sources (5, 10), a first optical fibre (30) for transferring light from at least one said light source to the internal measurement site (35), at least one receiver (45), for receiving light from the first and second light sources, at least a second optical fibre (40) for transferring light reflected from the region of the measurement site to the receiver, and means for determining the oxygen saturation level of the blood at the internal measurement site, based on the light produced by the light sources and light received by the receiver. The optical centres of the optical fibres are separated from one another by at least 1 mm at their distal ends. The apparatus can be used in combination with a cranial axis bolt for measuring the oxygen saturation level in the brain tissue of a human or animal patient.

Description

A preferred embodiment of the invention will be further described with reference to the following figures in which:

FIG. 1 shows a schematic diagram of one embodiment of apparatus according to the invention.

FIG. 2 shows a schematic layout of the arrangement at the proximal end of the apparatus of FIG. 1.

FIG. 3 shows a cross sectional view of the distal tips of the optical fibres of the apparatus of FIG. 1.

FIG. 4 shows Cranial access bolt supporting the optical fibres of apparatus according to an embodiment of the invention.

FIG. 5 shows an LED timing cycle suitable for use in the apparatus of FIG. 1.

FIG. 6 shows a preferred arrangement of the positioning of the tips of the optical fibres of the apparatus of FIG. 1, relative to an internal measurement site.

FIG. 7 shows the apparatus of FIG. 4, in position in the skull of a patient.

The apparatus of FIG. 1 comprises a first light source (5), in the form of an LED which emits pulses of red light at a wavelength of 660 nm, and a second light source (10), which is an LED with a wavelength of 850 nm (in the infrared). The light sources (5, 10) are connected to current sources (12, 13) for driving the light source. Optical fibres (15, 20) are connected between light sources (5, 10) and the upper limbs of a “Y”-coupler (not shown, but indicated generally at location 25). The “Y”-coupler connects optical fibres (15, 20) with a single optical fibre (30) for transferring light to the distal end (37) of the device, positioned adjacent an internal measurement site (35) in a human or animal patient.

A further optical fibre (40) is provided for transferring light reflected from the measurement site (35) to a photodiode receiver (45). The optical fibre (40) has an end (47) for positioning near to the measurement site to receive the emitted light. The receiver (45) is connected to a transimpedence amplifier (50), for converting the photocurrent from the receiver to a voltage. The transimpedence amplifier is connected via a further amplifier (55) for amplifying the resultant voltage, to an interface (60), for converting the signal from digital to analogue, and demultiplexing the signal, to allow a value for oxygen saturation to be obtained.

A logic circuit (70) having a system clock and a counter timer is connected to the light sources (5, 10), and the interface (60). The logic circuit (70) can produce a timing cycle for producing a multiplexed signal of light pulses from the light sources (5, 10), and for enabling the interface (60) to demultiplex the signals received by the receiver (45).

Light sources (5, 10), logic circuit (70), current sources (12, 13), photodetector (45), transimpedence amplifier (50), amplifier (55) and interface (60) are housed in a casing (77) which is shown schematically in FIG. 2. The circuit is powered by two 12V lead-acid cells (not shown).

The optical fibres (15, 20, 40) are coupled to the light sources (5, 10) by communications industry-standard SMA connectors to minimise energy losses at each connection.

The optical fibres (30, 40) are connected together so that the optical centres of the distal ends (37, 47) of the optical fibres (30, 40) are separated from each other by 1.46 mm, as shown in FIG. 3. The ends (37, 47) are separated by a spacer fibre (75) of approximately the same diameter as the optical fibres (30, 40). The ends (37, 47) are cleaved and polished to form a flat face to ensure uniform transmission of light out of and into the fibres.

Each optical fibre (15, 20, 30, 40) has an outer diameter of 730 μm and a core diameter of 400 μm. The acceptance angle (θ), i.e. the maximum angle for receiving light, of each fibre is 23°. The fibre is made of hard-clad silica, which is sterilisable and fully biocompatible.

Each of the optical fibres (30, 40) can be threaded through a channel (80, 85) in a cranial access bolt (90) such as the LiCox® IM3 cranial access system manufactured by Integra Neurocare LLC, San Diego, Calif. USA, as shown in FIG. 4. The bolt has three channels, leaving one channel free for a further sensor if required.

Once the fibres (30, 40) are in the correct position in the channels (80, 85), a compression cap (92) can be tightened to create a sterile seal to prevent contamination of internal tissue.

In use, the ends (37, 47) of the fibres (30, 40) are positioned near to the internal measurement site (35) of the patient. The ends (37, 47) can be positioned close to, touching or penetrating the surface of a patient's tissue in order to be positioned correctly for the internal measurement site (35). The optimal distance d, as shown in FIG. 5, between the ends (37, 47), and the internal measurement site (35) is 0<d<s/2 tan θ, where s is the separation between the optical centres of the fibres (30, 40), and θ is the acceptance angle (23° in this case). However, d should preferably be no greater than 2 mm as at distances greater than this, the detected light is partially reflected from the tissue surface (35) and not wholly scattered within the tissue (94) (a phenomenon known as optical shunt).

The counter timer of the logic circuit (70) produces a dedicated pulse train to allow the light sources (5, 10) to pulse sequentially by generating timing signals, which trigger the logic circuit (70), generating the timing cycle shown in FIG. 6. The timing cycle produces a multiplexed signal of sequential of red and infra-red pulses.

The pulses pass down the optical fibres (15, 20, 30) to the internal measurement site (35). Each light pulse is wholly scattered within the tissue of the measurement site (35). A portion of the light from each pulse is then emitted from the measurement site (35), where it passes into optical fibre (40). The light then passes along the optical fibre (40) to the photodiode receiver (45). The photodiode receiver (45) generates a current which is directly proportional to the light intensity measured by the receiver (45). The transimpedence amplifier (50) linearly converts the current into a voltage, which is amplified by the amplifier (55). The amplified voltage passes to the interface (60), where it is sampled by a 16 bit digital-to-analogue converter. The logic circuit (70) gates the data acquisition, synchronising the multiplexed pulses and the acquired data. The resultant signals from the digital-to-analogue converter can then be separated by a demultiplexer into separate signals relating to each red and infra red pulse. The signals are individually filtered to remove high-frequency noise.

The oxygen saturation can be calculated from the ratio (R) of the signals relating to the red and infra red pulses, using the formula:

R=(I_L,R/I_H,R)/(I_L,IR/I_H,IR)

wherein I_L,R and I_H,R are the lowest and highest values respectively of the light intensity detected during the ‘ON’ phases of the red light source, during one cardiac cycle. I_L,IR and I_H,IR are the corresponding values for the light intensity detected during the ‘ON’ phases of the infrared sources.

The oxygen saturation (SpO₂) can be estimated from an empirically derived calibration curve, for example using first order equations.

FIG. 7 illustrates the use of the pulse oximeter of FIG. 1 in combination with a LiCox® cranial bolt, to measure blood oxygenation levels at the surface of the brain. The bolt (90) is screwed through the skull (95) and the dura mater (98). Optical fibres (30, 40) are passed through two of the three channels (80, 85), typically the temperature and oxygen electrode channels, until they are in position in the arachnoid mater (100), which allows measurement of the oxygenation level of the blood in the pia mater (110). The compression cap (92) is then tightened to create the sterile seal, and the blood oxygenation level can be measured.

Claims

1.-8. (canceled)

9. Apparatus for measuring the oxygen saturation level of blood at an internal measurement site in a human or animal patient by reflectance pulse oximetry, comprising: a first light source having a first spectral distribution;a second light source having a second spectral distribution;a first optical fibre having a proximal end adjacent at least one said light source, and a distal end adapted in use to be positioned adjacent said internal measurement site, for transferring light from at least one said light source to the internal measurement site;at least one receiver, for receiving light from the first and second light sources;at least a second optical fibre having a proximal end adjacent the said receiver, and a distal end adapted in use to be positioned adjacent said internal measurement site, for transferring light reflected from the region of the measurement site to the receiver;means for determining the oxygen saturation level of the blood at the internal measurement site, based on the light produced by the light sources and light received by the receiver,wherein the optical centres of the first and second optical fibres are separated from one another by at least 1 mm at their distal ends.

10. Apparatus as claimed in claim 9, wherein the first light source is monochromatic.

11. Apparatus as claimed in claim 9, wherein the second light source is monochromatic.

12. Apparatus as claimed in claim 9, including means for pulsing light from the first and second light sources sequentially along the first optical fibre.

13. Apparatus as claimed in claim 9, wherein the first light source is such as to produce light having a peak emission wavelength of from 630 nm to 760 nm and the second light source is such as to produce light having a peak emission wavelength of from 820 nm to 930 nm.

14. Apparatus as claimed in claim 9, wherein the said measurement site is the surface of the brain, and wherein the apparatus additionally comprises a cranial access bolt for insertion into the skull of the patient, and means for supporting the said optical fibres in the access bolt, such that light from the said light sources is directed towards the surface of the brain, thereby enabling measurement of the oxygen saturation level of blood at the brain surface.

15. Apparatus as claimed in claim 14, wherein the cranial access bolt is adapted to support the said optical fibres, such that the distal ends of the optical are positioned from 0 to 4.0 mm from the surface of the brain.

16. A method of measuring the oxygen saturation level in the brain tissue of a human or animal patient, comprising the steps of inserting the distal ends of the optical fibres of apparatus as claimed in any one of claims 9 to 13 through a cranial access bolt positioned in the skull of the patient; positioning the distal ends of the optical fibres at a distance of from 0 to 4.0 mm from the brain surface;illuminating the brain surface of the patient using the said light sources;and determining the oxygen saturation level of blood at the brain surface from reflected light received at the receiver via the said second optical fibre.