Cable termination status detection

Abstract: A method and apparatus for detection of the status of a cable termination. The cable is required to be terminated by being in contact with the body of a patient in a medical environment. The apparatus warns a user if the cable is disconnected from the patient. The status of the termination is established by applying a pulse of electrical energy to the cable and detecting the reflections produced in response to application of the pulse. A characteristic of the reflections is detected and compared with a predetermined characteristic to determine whether the cable is connected to the patient.

Description: (1)Field of the Invention This invention relates to methods of and/or apparatus for detecting the status of a cable termination and has been devised particularly but not solely for use in detecting the status of the connection between a patient and one or more cables from a patient monitoring system such as those monitoring systems used in the healthcare industry. (2)Background Art In medical patient monitoring systems requiring attachment of electrical cables to the body of a patient, it can be of considerable advantage to know if such cables are correctly terminated. A detached cable may not be immediately obvious resulting in time wasting fault diagnosis or a possible life threatening situation. For example, careful control of a physical characteristic such as body temperature is often required for patients such as new-born babies (infants) in hospital environments. One such way of controlling an infant's body temperature is to place the infant in a warmer. The warmer maintains the body temperature of an infant and radiates heat selectively when or as required to maintain the body temperature of the infant at a desired level. The infant's body temperature is measured by the warmer control circuitry using an electric cable having a temperature probe attached to an external surface of the infant's body. It is important that the cable is correctly terminated, that is, that the temperature probe is in secure contact with the infant's body in order that an accurate measurement of the infant's body temperature may be made. It would clearly be advantageous if the warmer control would sound an audible alarm or show a visible alarm if the temperature probe was not securely connected to the infant's body. This is especially important since at present a visual inspection is used to determine if a cable is connected to a patient and this is not always sufficient for the user to determine whether the probe is in fact securely connected. An example prior system which determines the connection status of a probe is disclosed in WO83/01374 to Öberg where a heart activity indicator is provided with a circuit to monitor the contact of electrodes against a patient's body. A small AC signal voltage is applied to the electrode giving rise to an output signal dependent on the "electrode impedance" between the electrode and the patient's body. The output signal is then compared to the applied signal to determine the level of electrode contact. A further prior system is disclosed in US4,294,263 assigned to Air Shields Inc in which a probe (such as a temperature probe) uses optic fibres to convey light impinging on the underside of the probe (ordinarily against the body of the patient) to a photo sensor. This system requires separate "channels" to convey the sensed temperature information (metal wire) and the probe connection information (optic fibre) from the probe to their respective analysis circuitry. BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to provide methods of and/or apparatus for detecting the status of a cable termination which will at least go some way toward overcoming the above disadvantages or which will, at least provide the public with a useful choice. Accordingly, in one aspect, the invention consists in temperature measuring apparatus for measuring a patient's body temperature in a medical environment including a temperature probe having a temperature sensor, an electric cable containing conductors and having a proximal end and a distal end to which said temperature probe is connected, an electrically conductive portion of said temperature sensor adapted to be placed on said patient's body and being in electrical connection with a conductor in said electric cable, said temperature sensor providing a signal indicative of said patient's body temperature to said conductor in said electric cable, there existing a thermal connection of a measurable quality between said temperature sensor and said patient, and means for determining said quality of said thermal connection, characterised in that pulse generating means are provided having an output connected at said proximal end of said electric cable for generating a pulse of electrical energy on said electric cable which is propagated along said electric cable towards a load comprising the patient in thermal contact with said temperature probe, some of said energy of said pulse being reflected by said load as reflected propagating electrical waves having at least one physical characteristic between said proximal and distal ends of said electric cable and some of said energy being absorbed by said load, signal detection means are provided for detecting said at least one physical characteristic of said reflected propagating electrical waves in response to said pulse, said reflected propagating electrical waves having a measurable duration and amplitude, and comparing means are provided to compare said at least one physical characteristic of said reflected propagating electrical waves with a predetermined signal characteristic to thereby provide an indication of the quality of said thermal connection. In a further aspect the invention consists in a method of measuring a patient's body temperature in a medical environment with a temperature probe having a temperature sensor in use attached to said patient, an electric cable having a proximal end and a distal end to which said temperature probe is connected, there existing a thermal connection of a measurable quality between said temperature sensor and said patient and detecting the quality of said thermal connection characterised in that said method comprises the steps of: i) periodically generating a pulse of electrical energy and applying said pulse to the proximal end of said electric cable, ii) detecting a characteristic of reflections resulting from said pulse interacting with a load, comprising the patient in thermal connection with said temperature probe (1), and said proximal end of said electric cable, said reflections having a measurable duration and amplitude, iii) providing the distal end of said cable with a signal indicative of the temperature sensed by said temperature sensor, and iv) comparing said characteristic with a predetermined signal characteristic to determine the quality of said thermal connection. In a further aspect the invention consists in a warmer for warming a patient in a hospital environment, said warmer having heating means for providing heat to said patient, temperature measuring apparatus having a temperature probe including a temperature sensor, an electric cable containing conductors and having a proximal end and a distal end to which said temperature probe is connected, an electrically conductive portion of said temperature sensoradapted to be placed on said patient's body and being in electrical connection with a conductor in said electric cable, said temperature sensor providing a signal indicative of said patient's body temperature to said conductor in said electric cable, there existing a thermal connection of a measurable quality between said temperature sensor and said patient characterised in that means for determining said quality of said thermal connection are provided comprising: pulse generating means having an output connected at said proximal end of said electric cable for generating a pulse of electrical energy on said electric cable which is propagated along said electric cable towards a load comprising the patient in thermal contact with said temperature probe, some of said energy of said pulse being reflected by said load as reflected propagating electrical waves having at least one physical characteristic between said proximal and distal ends of said electric cable and some of said energy being absorbed by said load, signal detection means for detecting said at least one physical characteristic of said reflected propagating electrical waves in response to said pulse, said reflected propagating electrical waves having a measurable duration and amplitude, and comparing means to compare said at least one physical characteristic of said reflected propagating electrical waves with a predetermined signal characteristic to thereby provide an indication of the quality of said thermal connection to said temperature measuring apparatus which provides a visual or audible alarm if the quality of said thermal connection is determined to be below a predetermined quality. The invention consists in the foregoing and also envisages constructions of which the following gives examples only. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 and Figure 2 are diagrammatic elevational views of a section through a probe for use with the present invention, and Figure 3 is a simplified circuit diagram of the apparatus used in accordance with the present invention, and Figure 4 is a graph of amplitude vs time for a waveform produced by the circuitry shown in Figure 3. DETAILED DESCRIPTION Referring to figure 1 a temperature probe is shown generally referenced 1 having a cup constructed from an electrically conductive material such as metal and having a thermally conductive paste 3 therein in which one or more temperature sensors are disposed. The first and second thermosensors 4 and 5 are electrically connected to electronic control circuitry generally referenced 6 by a cable having conductors 7 and 8 and a common conductor 9. The thermosensors are connected to common conductor 9 at point 10 and this common connection is also electrically connected to the metal cup 2 by soldering or clamping for example at point 11. Referring to figure 2, the probe cup of Figure 1 is again shown only in this probe the connection between the common conductor at point 10 and the metal cup at point 11 is made through a capacitor referenced 12. Referring to figure 3 a probe 1 is shown at the distal end of a cable and in the figure the common conductor 9 of the cable is shown together with one of the conductors 7 or 8 or both conductors. For simplicity, only conductors 7 and 9 are shown in Figure 3. At the proximal end of the cable the electronic control circuitry 6 is generally shown within a box having a dashed border. The control circuitry 6 comprises four main sections, being temperature measuring circuitry 13, pulse generating circuitry 14, signal detection circuitry 15 and microprocessor 26 for overall circuit control. Also, connected between the cable terminals 16 and 17 at the proximal end of the cable and the electronic circuitry 6 is capacitor C2 and inductors L1 and L2. The electronic circuitry 6 has a common reference potential 18. The temperature measuring circuitry 13 comprises an operational amplifier A1 configured to provide a gain of unity (that is to act as a buffer) having the non-inverting input connected to inductor L1 and capacitor C2 and having the output connected to transmission gate TG3. Transmission gate TG3 is also connected to Analog to Digital converter 24. The pulse generating circuitry 14 comprises capacitor C1 which is connected between the base of the transistor Q1 and apparatus for producing a transient signal or level shift 20. The emitter of Q1 is connected to the positive or high-potential rail of power supply 22. The reference or low-potential rail of the power supply is connected to a ground rail 18. Connected between the emitter and base of transistor Q1 is resistor R1 and the collector of Q1 is connected to terminal 17 (conductor 9 of the cable). The signal detection circuitry 15 comprises a transmission gate TG1 between terminal 17 and one side of the capacitor C3. The other side of the capacitor C3 is connected to the emitter of transistor Q2. The base of transistor Q2 is connected to positive rail 22 of a D.C. power supply and diode D1 is connected between the base and emitter of Q2. The collector of Q2 is connected to transmission gate TG2 and the capacitor C4 is connected between the collector of Q2 and the ground rail 18. Transmission gate TG2 is also connected to an analog to digital converter 24. If the individual conductors of the cable, connected to a probe which is to be in direct contact with the body, for example the abdomen of the patient, are considered lumped together as shown in Figure 3, and a current pulse is applied at the end remote from the patient (the proximal end), a voltage will propagate the length of the cable. On reaching the distal end of the cable this voltage will in the case of a correctly terminated cable (connected to a patient) be largely dissipated in the principally resistive patient load. However, if the probe cable is unattached at the distal end, this voltage will be substantially reflected back along the cable to the transmitting end. The status or condition of the distal cable end, as to whether it is terminated correctly on the patient or not can be determined by measuring the form of this reflected voltage signal. In a practical application of this principle a cable termination status detection circuit is connected to a patient temperature measuring probe installation for example, in which the probe of figures 1 or 2 is connected to the abdomen of a patient, being held in place by a reflective dot which has an adhesive surface to ensure the probe is held in plan on the skin of the patient. To generate the pulse or pulses required to establish the status of the cable termination, that is the probe connection to a patient, the transient signal represented by wave form 20 is applied and passed through C1 to base of Q1. Conduction between the emitter and collector of Q1 then occurs while C1 is discharged through R1. This gives a short pulse of current across inductor L2, that is between the cable and the ground rail 18. Referring to figure 4 the conduction period of Q1 is indicated by period A-B. The applied current pulse A-B is propagated through the cable to its distal end and some reflection will occur so that reflections will oscillate through the cable at the self resonant frequency of the cable. The ringing oscillation persists for a finite time after the initial exciting current pulse has ended. The oscillation is caused by the voltage signal propagating along the length of the cable and reflecting from the loads at the terminations or ends of the cable. Thus a cable which is in electrical connection with a probe or transducer which is not in contact with the body of a patient will have no load connected to the probe or transducer (the termination) in which the pulse may be dissipated, and the voltage pulse propagating along the length of the cable will be reflected back toward the pulse generating circuitry where it will again be reflected back toward the termination. This oscillation will continue until the losses in the cable and associated components dissipate the energy supplied by the initial pulse. When the probe is in contact with a patient, as required for accurate temperature measurement by the temperature measuring circuitry, the voltage pulse, after propagating along the length of the cable will be largely dissipated in the body of the patient which comprises a resistive load. The voltage pulse travels through the cable conductors 7, 8 and 9, and onto the probe cup 2 through connection 11, and capacitor 12 if it is present, and into the body of the patient. The cable conductors 7, 8 and 9 may be considered lumped together as a single conductor, at the signal frequencies used to test the termination status since they are located adjacent to each other and are capacitively coupled to each other. The reflected signal from the load comprising the patient is minimal since most of the energy of the initial pulse is dissipated in the load, and the oscillation along the cable is therefore reduced considerably compared with the oscillation present when there is a no load condition at the cable termination. The amplitude, duration and shape of the oscillation will vary depending on factors such as the length of the cable and the amplitude, duration, shape and frequent of the initial pulse. Thus if a standard length of cable is supplied and the initial current pulse is of a constant, known form, then no calibration of the detection circuitry is necessary. L1 and C2 form a low pass filter which confines the temperature probe cable oscillations and inhibit their entry into the temperature measurement circuit amplifier A1. Interference from the probe cable oscillations is not of great concern as the temperature measurement phase is distinct from the determination of the status of the probe cable termination. That is, because it takes very little time to determine the status of the probe termination in accordance with the present invention, regular checks upon the status of the cable termination may be made (once every second for example) and the time between status check may be used to provide indications from the thermosensors to amplifier A1 for providing indications for temperature measurement. After the reverse voltage spike referenced B-C in Figure 4, the inductor L2, probe cable 7 and 9 and sensor 1 are oscillating at their natural resonant frequency. At point C, transmission gate TG1 is closed and the diode-transistor pump circuit of C3, D1, Q2 and C4 sums the remaining oscillations referenced C-D in Figure 4. Each transmission gate delivers an output waveform that is a replica of that input to the gate. Each transmission gate is activated by microprocessor 26. The cable oscillations are provided to C3 by TG1 and pass through C3 to the emitter of Q2 which is maintained at a potential of approximately 0.6 volts below rail 22 by D1. The cable oscillations are summed by charge storage in capacitor C4. Each positive oscillation above a predetermined threshold amplitude corresponding to approximately 0.6 volts will forward bias Q2 and current will be supplied to C4. Thus characteristics of the reflected signal indicating its amplitude and duration are detected and these are summed cumulatively. The circuit may be modified to sum one or both of these characteristics. At point D the transmission gate TG1 is opened and TG2 is closed. The voltage across capacitor C4 is then read by the analog to digital converter 24 to provide an indication of the magnitude of the voltage across C4 in digital form. The typical time between points A and D in Figure 4 is usually 3 to 5 microseconds for example. The oscillations C-D represent an un-terminated temperature probe. Therefore when the probe is correctly attached to the patient, the oscillations are reduced in amplitude and number, resulting in a lower voltage across capacitor C4. When the value of the voltage across capacitor C4 is represented in binary form by analog to digital converter 24, the resultant binary representation of the voltage will be much lower and this can be compared with the threshold value corresponding to a predetermined signal characteristic or combination thereof relating to the minimum duration and/or amplitude of the reflected signal. The threshold value is stored in software associated with or included in microprocessor 26 which controls the circuitry described above. The micro-processor controls the circuitry described to sequentially switch, under software control, the transmission gates TG1 to TG3 and process the result provided by A to D converter 24 to indicate an unterminated probe cable by audible and/or visual alarms. The signals indicative of the temperature of the patient are passed from thermo-sensor 4 through cable 7 to buffer A1. When temperature is required to be measured by the control circuitry, gate TG1 is opened and gate TG3 is closed so that the A to D converter converts analog signals representative of the temperature of the patient and presents these to the microprocessor control circuitry in binary form for control of the heat, for example, being supplied to the patient. The advantages of this method of determining correct probe attachment to a patient, are that the function can be completed within a time span of the order of 5 micro seconds and thus can be slotted into a regular temperature measuring routine with minimal or no interference. Also the function need only he repeated at a frequency which will give adequate warning. Once a second is sufficient and the above function can be completed with less than 20 nanowatts of dissipated power. The temperature measuring circuit in which the generating and detection circuitry is installed is totally isolated from ground leakage, that is, the probe is double insulated and is supplied by a dedicated high frequency switching power supply. This isolation from ground does not affect the operation of the generating and detection circuitry. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: Temperature measuring apparatus for measuring a patient's body temperature in a medical environment including a temperature probe (1) having a temperature sensor (4, 5, 13), an electric cable containing conductors (7, 8, 9) and having a proximal end and a distal end to which said temperature probe (1) is connected, an electrically conductive portion of said temperature sensor (4, 5, 13) adapted to be placed on said patient's body and being in electrical connection with a conductor (7, 8) in said electric cable, said temperature sensor (4, 5, 13) providing a signal indicative of said patient's body temperature to said conductor in said electric cable, there existing a thermal connection of a measurable quality between said temperature sensor and said patient, and means for determining said quality of said thermal connection, characterised in that pulse generating means (14) are provided having an output connected at said proximal end of said electric cable for generating a pulse (22) of electrical energy on said electric cable which is propagated along said electric cable towards a load comprising the patient in thermal contact with said temperature probe (1), some of said energy of said pulse (22) being reflected by said load as reflected propagating electrical waves having at least one physical characteristic between said proximal and distal ends of said electric cable and some of said energy being absorbed by said load, signal detection means (15) are provided for detecting said at least one physical characteristic of said reflected propagating electrical waves in response to said pulse (22), said reflected propagating electrical waves having a measurable duration and amplitude, and comparing means (26) are provided to compare said at least one physical characteristic of said reflected propagating electrical waves with a predetermined signal characteristic to thereby provide an indication of the quality of said thermal connection. Temperature measuring apparatus as claimed in claim 1 wherein said at least one physical characteristic comprises an indication of said duration of said reflected propagating electrical waves. Temperature measuring apparatus as claimed in claim 1 wherein said at least one physical characteristic comprises an indication of said amplitude of said reflected propagating electrical waves. Temperature measuring apparatus as claimed in claim 1 wherein said signal detection means (15) comprise summing means which detect said at least one physical characteristic by cumulatively summing an indication of said duration of said reflected propagating electrical waves until said amplitude of said reflected propagating electrical waves is less than a predetermined threshold amplitude. Temperature measuring apparatus as claimed in claim 4 wherein said summing means also cumulatively sum an indicator of both said amplitude and said duration of said reflected propagating electrical waves. Temperature measuring apparatus as claimed in claim 5 wherein said predetermined signal characteristic has a known magnitude and said comparing means (26) compare said predetermined signal characteristic with said summation and indicate that said temperature probe (1) is disconnected from said patient if said summation is greater than said predetermined signal characteristic. Temperature measuring apparatus as claimed in claim 6 wherein said signal detection means (15) comprise energy storage means and signal value conversion means (24), said energy storage means storing energy from said reflected propagating electrical waves for input to said signal value conversion means (24) and said signal value conversion means (24) determining and outputting a numerical value representing the magnitude of signals input thereto and wherein control means (26) control said signal detection means (15) and the state of at least two transmission gate means, at least one of said at least two transmission gate means connected between said temperature probe (1) and said energy storage means and at least one of said at least two transmission gate means connected between said temperature probe (1) and said signal value conversion means (24), and wherein said control means (26) selectively open only one of said at least two transmission gate means at one time to exclusively allow said signal value conversion means to output a value representing either said patient's body temperature or the quality of said thermal connection. A method of measuring a patient's body temperature in a medical environment with a temperature probe (1) having a temperature sensor (4, 5, 13) in use attached to said patient, an electric cable having a proximal end and a distal end to which said temperature probe (1) is connected, there existing a thermal connection of a measurable quality between said temperature sensor (4, 5, 13) and said patient and detecting the quality of said thermal connection characterised in that said method comprises the steps of: i) periodically generating a pulse (22) of electrical energy and applying said pulse (22) to the proximal end of said electric cable, ii) detecting a characteristic of reflections resulting from said pulse interacting with a load, comprising the patient in thermal connection with said temperature probe (1), and said proximal end of said electric cable, said reflections having a measurable duration and amplitude, iii) providing the distal end of said cable with a signal indicative of the temperature sensed by said temperature sensor (4, 5, 13), and iv) comparing said characteristic of reflections with a predetermined signal characteristic to determine the quality of said thermal connection. A method as claimed in claim 8 wherein said step of detecting said reflections characteristic comprises the step of summing an indication of said duration of said reflections until said amplitude is below a predetermined threshold amplitude. A method as claimed in claim 9 including the step of cumulatively summing an indication of both said amplitude and said duration of said reflections. A method as claimed in claim 10 wherein said predetermined signal characteristic has a known magnitude and including the step of comparing said predetermined signal characteristic with said summation and indicating the quality of said thermal connection is poor if said summation is greater than said predetermined signal characteristic. A method as claimed in claim 8 including the step of switching said electric cable between detecting the quality of the thermal connection of said probe to said patient and conducting said indication of the temperature sensed by said temperature sensor. A warmer for warming a patient in a hospital environment, said warmer having heating means for providing heat to said patient, temperature measuring apparatus having a temperature probe (1) including a temperature sensor (4, 5, 13), an electric cable containing conductors (7, 8, 9) and having a proximal end and a distal end to which said temperature probe (1) is connected, an electrically conductive portion of said temperature sensor (4, 5, 13) adapted to be placed on said patient's body and being in electrical connection with a conductor (7, 8) in said electric cable, said temperature sensor (4, 5, 13) providing a signal indicative of said patient's body temperature to said conductor in said electric cable, there existing a thermal connection of a measurable quality between said temperature sensor and said patient characterised in that means for determining said quality of said thermal connection are provided comprising: pulse generating means (14) having an output connected at said proximal end of said electric cable for generating a pulse (22) of electrical energy on said electric cable which is propagated along said electric cable towards a load comprising the patient in thermal contact with said temperature probe (1), some of said energy of said pulse (22) being reflected by said load as reflected propagating electrical waves having at least one physical characteristic between said proximal and distal ends of said electric cable and some of said energy being absorbed by said load, signal detection means (15) for detecting said at least one physical characteristic of said reflected propagating electrical waves in response to said pulse (22), said reflected propagating electrical waves having a measurable duration and amplitude, and comparing means (26) to compare said at least one physical characteristic of said reflected propagating electrical waves with a predetermined signal characteristic to thereby provide an indication of the quality of said thermal connection to said temperature measuring apparatus which provides a visual or audible alarm if the quality of said thermal connection is determined to be below a predetermined quality. A warmer as claimed in claim 13 wherein said at least one physical characteristic comprises an indication of said duration of said reflected propagating electrical waves. A warmer as claimed in claim 13 wherein said at least one physical characteristic comprises an indication of said amplitude of said reflected propagating electrical waves. A warmer as claimed in claim 13 wherein said signal detection means (15) comprise summing means which detect said at least one physical characteristic by cumulatively summing an indication of said duration of said reflected propagating electrical waves until said amplitude of said reflected propagating electrical waves is less than a predetermined threshold amplitude. A warmer as claimed in claim 16 wherein said summing means also cumulatively sum an indicator of both said amplitude and said duration of said reflected propagating electrical waves. A warmer as claimed in claim 17 wherein said predetermined signal characteristic has a known magnitude and said comparing means (26) compare said predetermined signal characteristic with said summation and indicate that said temperature probe (1) is disconnected from said patient if said summation is greater than said predetermined signal characteristic. A warmer as claimed in claim 18 wherein said signal detection means (15) comprise energy storage means and signal value conversion means (24), said energy storage means storing energy from said reflected propagating electrical waves for input to said signal value conversion means (24) and said signal value conversion means (24) determining and outputting a numerical value representing the magnitude of signals input thereto and wherein control means (26) control said signal detection means (15) and the state of at least two transmission gate means, at least one of said at least two transmission gate means connected between said temperature probe (1) and said energy storage means and at least one of said at least two transmission gate means connected between said temperature probe (1) and said signal value conversion means (24), and wherein said control means (26) selectively open only one of said at least two transmission gate means at one time to exclusively allow said signal value conversion means to output a value representing either said patient's body temperature or the quality of said thermal connection.

Inventors: MARTENSON JOHN ERIC, MARTENSON, JOHN ERIC
Applicants: FISHER & PAYKEL, FISHER & PAYKEL LIMITED
Application year: 1993
Main classification: A61B5
IPC codes: A61B5, A61B17, G01K7