Abstract:
A monitoring system for monitoring the health status of a patient by performing measurements such as skin temperature, blood flow, blood constituent concentration, and pulse rate at the finger of the patient. The monitoring system has an inner ring proximate to the finger as well as an outer ring, mechanically decoupled from the inner ring, that shields the inner ring from external loads. Measurements are performed in accordance with a protocol that may be preprogrammed, or may be modified on the basis of real-time data or by command from a remotely located medical professional.

Description:
The present application claims priority from U.S. Provisional Application Ser. No. 60/130,774, filed Apr. 23, 1999, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention pertains to a device for monitoring the health status of a patient and, more particularly, for isolating such an apparatus from external disturbances. 
     BACKGROUND OF THE INVENTION 
     The present invention is an improvement upon finger-ring sensors such as those described in U.S. Pat. No. 5,964,701, issued Oct. 12, 1999, which incorporated herein by reference. 
     One of the most difficult problems in implementing a sensor that may be worn on the body is the issue of eliminating signal artifacts due to motion of, or forces exerted upon, the sensor. A further problem is the inflexibility of preprogrammed operating protocols. 
     SUMMARY OF THE INVENTION 
     In accordance with preferred embodiment of the invention, there is provided a monitoring system for monitoring the health status of a patient. The monitoring system has an inner ring characterized by a first mass. The inner ring is proximate to a finger of the patient and has at least one sensor coupled to the inner ring for providing a signal based on at least one of skin temperature, blood flow, blood constituent concentration, and pulse rate of the patient. The monitoring system also has an outer ring characterized by a second mass. The outer ring is coupled to the finger of the patient. The monitoring system has an electronics module disposed on the outer ring for processing the signal provided by the sensor and a flexible electrical coupling for conveying the signal from the sensor to the electronics module in such a manner as to maintain mechanical decoupling of the inner and outer rings. 
     In accordance with further embodiments of the invention, the pressure of the inner ring against the finger of the patient may be adjustable within a specified range, including by means of a material having a stiffness that is a nonlinear function of extension. The monitoring system may also have a light source and a detector for monitoring a characteristic of arterial blood flow within the finger, and a control loop for regulating the intensity of light emitted by the light source. Regulation may be in response to a ratio of signal to noise in the detector or to a signal generated by the temperature sensor. 
     In accordance with yet further embodiments of the invention, a health monitoring system may be provided that has at least one sensor coupled to a ring proximate to a finger of the patient for providing a signal based on at least one of skin temperature, blood flow, blood constituent concentration, and pulse rate of the patient, and an electronics module disposed on the ring for processing the signal provided by the sensor, wherein the electronic module includes a protocol scheduler for specifying a schedule of physiological measurements. The monitoring system may have a transponder for transmitting physiological data based on the sensor signal to a remote station unit and a receiver for receiving protocol scheduling commands from the remote station unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will more readily be understood by reference to the following description taken with the accompanying drawings in which: 
     FIG. 1 a  is a cross-sectional diagram of an isolating ring sensor design in accordance with preferred embodiments of the present invention; 
     FIG. 1 b  illustrates the effect of an external force on the isolating ring sensor of FIG. 1 a ; 
     FIG. 2 depicts a perspective view of an embodiment of the ring sensor device of FIG. 1 a;    
     FIG. 3 shows an experimental configuration, in axial cross-section, for evaluating the operation of an isolating ring sensor in accordance with an embodiment of the present invention; 
     FIG. 4 shows the response of a prior art ring sensor in the face of applied external forces; and 
     FIG. 5 shows the response of an isolating ring sensor, in accordance with an embodiment of the present invention, in the face of applied external forces. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention provide improvements upon finger-ring sensors of the kind described in U.S. Pat. No. 5,964,701, and may also be applied to other sensors worn on the body for monitoring any of a variety of physiological parameters, including, without limitation, skin temperature, electrical impedance, pulse, blood constituent concentration, and/or blood flow. In accordance with preferred embodiments of the invention, sensor data is transmitted to a computer through a wireless communication link and the patient status is analyzed continually and remotely. Any trait of abnormal health status and possible accidents may be detected by analyzing the sensor data. A sensor worn as a finger ring sensor has particular advantages such as that such a sensor may be worn by the patient at all times, hence the health status may be monitored 24 hours a day. For purposes of the present description, the sensor will be referred to, without limitation, as a ring sensor, and the sensing modality, again without limitation, will be described in terms of a photoplethysmographic device for measuring a pulse using optical elements such as infrared light-emitting diodes (LEDs) and photodiodes. 
     In a preferred embodiment of the invention, the ring sensor is a miniaturized telemetered ambulatory monitoring device in a ring configuration that combines the technology of pulse oximetry with microelectronics and wireless communication technologies. This device optically captures the pulsation and the oxygen saturation of the arterial blood flow of the patient and transmits the signals to a personal computer in a wireless manner. The light emitted from LED traverses human -tissue, and reaches the photodetector, with some of the photons absorbed by tissue including the blood. Since the near infrared (NIR) absorption coefficient of blood exceeds that of the other intervening tissue, the intensity of received light depends on the amount of the blood in the path through the tissue. As a result, when the digital arteries and the capillaries in the finger expand by the pumping activity of the heart, the intensity of light received by the detector decreases. Conversely, the measured light intensity becomes higher when the arteries and the capillaries contract. Such a ring sensor can be worn by the patient twenty-four hours a day at home. Real-time, continuous monitoring with the ring sensor allows not only for emergency detection of an abrupt change of the patient health condition but also for long-term monitoring of vital signs of otherwise difficult and noncompliant patients such as demented elderly people. 
     Sensors such as the ring sensor, however, are inevitably susceptible to a variety of motion and ambient light artifacts. For example, in a highly accelerated motion of the patient, the inertia may cause the optical sensor unit to move or slide on the skin surface, and, as a result, the optical sensor measurement may be distorted or even ruined completely. Even a static external force may cause a similar distortion of the measurement due to the relative displacement of the sensor to the finger. Additionally, ambient light may be another major source of measurement artifacts. These kinds of external disturbances can seriously degrade the quality of measurement of the ring sensor. The optical method of measuring pulse is particularly prone to external disturbances, both mechanical and optical. As the signal detected by the optical sensor is amplified thousands of times, any small disturbance on the sensor will result in significant change of the amplified signed and eventually degrade the measurement. 
     Referring now to FIG. 1 a , a ring sensor, denoted generally by numeral  10  is shown in cross-section. An inner ring  12 , proximate to a finger  14  of the subject, includes a sensor unit, denoted generally by numeral  16 . Inner ring  12 , and thus sensor unit  16 , are decoupled from most of the inertia of the device. The decoupling may be achieved, in preferred embodiments of the invention, by having two rings, namely inner ring  12  and outer ring  18 , which are mechanically independent to each other. By putting the optical sensor unit  16  on one of the rings while a circuit board and battery module  20  is on the other ring, it is possible to protect sensor unit  16  from the influence of the most of any inertia force to which the ring  10  is subjected. 
     The problem caused by external static forces can also be solved by same approach. The ring device  10  comprises two rings, with a first ring  12  enclosed in the second outer ring  18 . Inner ring  12  floats inside outer ring  18  so that static displacement of the outer ring  18  does not substantially influence inner ring  12 . The problem of ambient light can be also alleviated by this design since outer ring  18  works as an optical seal for sensor unit  16  on the inner ring  12 . 
     In accordance with preferred embodiments of the invention, inner ring  12  is basically a thin band that carries optical sensor unit  16  which, in turn, includes one or more LEDs  22  and one or more photodiodes  24 . Inner ring  12  is made of light material such as plastic or acrylic, or even a rubber band. A rubber or latex band is preferable in that it is flexible and compliant. From the analysis of finger models, it has been found that giving a certain pressure on the optical sensor on the skin may increase the amplitude of the measured signal, resulting in a higher signal to noise ratio. Additionally, using a compliant material for the inner ring may advantageously contribute to holding the finger firmly. 
     The second part of ring device  10  is the outer ring  18  that carries the circuit board and battery module  20 . Additionally, module  20  may include a radio-frequency or other transponder or transmitter for transmitting signals conveying physiological data to a station unit  28 , either elsewhere on the person of the patient or in the vicinity-of the patient. Second part  18  is made of a stiffer material such as metal or PVC so that it can sustain the circuit board and the batteries. It also works as a mechanical shelter against external forces. First part  12  is put into the second part  18  such that the first part  12  can float inside the second part  18 . Sensor unit  16  is mechanically coupled to the first part  12  and is electrically connected to the circuit board  20  on the second part  18  by means of a few flexible, thin wires  26  that are long enough that the rotation of outer ring  18  does not substantially influence the inner ring  12 . Wires  26  are the only connection between the first part  12  and the second part  18 . With this configuration, the first part carrying the sensor unit is virtually de-coupled from the movement of the second part, and unconstrained by the influence of the movement of the second part. 
     FIG. 1 b  illustrates the effect of an external force applied in direction  30  on device  10 . When an external force is applied to outer ring  18 , the relative position of the inner ring  12  to the outer ring  18  changes, however the relative position of the finger  14  to the sensor unit  16  that is attached on the inner ring  12  does not change. It is preferred that the wires  26  that connect the circuit board  20  and the sensor unit  16  electrically be thin and long enough that when the outer ring  18  rotates by any kind of external torque, the inner ring  12  does not rotate substantially although the outer ring  18  may rotate to some extent. 
     In preferred embodiments of the invention, the miniaturized sensor unit  16  is attached on the internal ring  12  whose mass is almost negligible with respect to the total mass of the device. Typically the ratio of the mass of the outer ring to the mass of the inner ring is at least an order of magnitude. Sensor unit  16  preferably includes a small circuit board (5 mm×5 mm×0.8 mm) that contains two light emitting photodiodes and a photodetector, although other embodiments such as an integrated optical unit are within the scope of the present invention. The main circuit board and batteries comprising module  20  are relatively heavy and bulky sit on the external ring  18  which may also be referred to as a housing. The main circuitry is composed of many small element, which may be of surface mount style or bare die form. Resistors and capacitors are either wire-bonded or glued with conducting epoxy. Integrated circuits may be in die forms and wire-bonded on gold pads on the circuit board. Even though small and light components are used, the total mass of this circuit board is not negligible. In addition. button type batteries used for providing power to the circuit have relatively large mass. In a preferred embodiment of the invention, these components sit on the surface of the outer ring. 
     Inner ring  12  and the outer ring  18  are connected only by a few wires  26  that are flexible and thin. As a result, the inner ring and the outer ring are virtually isolated mechanically. The outer ring is subject to external forces including direct forces and inertial forces, and it may move or rotate around the finger. However, the inner ring is not substantially influenced by the movement of the outer ring. Moreover, the inner ring floats inside the outer ring. When a person wears the device, both inner ring  12  and outer ring  18  are worn simultaneously, but there is no direct mechanical connection between the two rings except a few thin wires  26  for signal exchange. Thus, any external force applied to the device applies substantially only to the outer ring, and the force is sustained mostly by the parts of the outer ring that are in contact with the finger, The external force-does not substantially influence the contact point between the finger and the internal ring since the two rings are virtually de-coupled. As a result, the measurement from the sensor unit can be kept stable even in the presence of external forces applied to the outer ring. 
     The isolating ring sensor, in accordance with embodiments of the present invention, may provide the additional advantage of optical isolation since outer ring  18  may serve to block the penetration of ambient light such that the sensor measurement is not influenced significantly by such light. 
     The pressure with which inner ring  12  is held against finger  14  is known to affect the level of signal in the case of certain measurements. For example, pressure on the outside of blood vessels reduces the stiffness of blood vessel walls, thereby increasing the amplitude of pulse measurement signals, for example. On the other hand, a trade-off exists in that pressure must, not be increased to such an extent as to occlude the free flow of blood within the vessel. Pressures for sensor ring applications have been found advantageously to lie in the range of 10-30 mm Hg. In accordance with an embodiment of the invention, a preferable nonlinear characteristic of tension in ring  12  as a function of extension of the ring is provided by fashioning the ring from an elastomer. In particular, it is desirable that the tension saturate with extension. A preferable material for providing that property is a elastomeric weave such as used in the elastic bands of clothing. 
     Other factors effect blood flow in the extremities of a mammal, and, more particularly in the finger of a human subject. One such factor is low ambient temperature which causes a reduction in blood flow in the extremities. To compensate for temperature effects, in accordance with an embodiment of the invention, the intensity of light emitted by LED  22  is increased to maintain an acceptable level of signal-to-noise. To that end, a temperature sensor  8  may be provided as part of circuitry  20  to allow for automatic compensation of LED intensity. Alternatively, in accordance with further embodiments of the invention, a signal-to-noise level ratio may be specified, either in hardware or in software, and the LED level may be adjusted by a control loop to maintain the specified ratio or to optimize the signal level within specified constraints which may include power utilization, for example. 
     FIG. 2 depicts a perspective view of ring sensor device  10 , in accordance with a preferred embodiment of the invention. Outer ring  18  is divided into two pieces  32  and  34  for ease of wearing by the patient. Module  20  containing the signal processing electronics and transmitter is part of outer ring piece  34 . 
     To verify new designs in accordance with an embodiment of the present invention, the measurement obtained from the new dual-ring sensor was compared with that of an old ring sensor. The old ring sensor uses just a simple ring made out of aluminum, which is only one piece. The circuit board is attached on the outer surface of the ring and the sensor unit is attached inner surface of the same ring. Referring now to FIG. 3, an experiment was conducted giving an external static force on the ring  40  at various positions. Digital arteries  42  are shown for angular orientation about axis  44  of finger  14 . 
     Initially, an external pressure was applied on the point of angle θ=0°, and the corresponding photoplethysmograph was measured. The same experiments were conducted with θ=90°. θ=180°, and θ=270°, respectively. The photoplethysmographs from the experiments of the plain (old) ring sensor are shown in FIG. 4, and the results of the new ring are shown in FIG.  5 . 
     In the case of the old ring composed of one piece, the measurement varied significantly as a function of the external forces. With an external force at an angle of θ=0° or 270°, the photoplethysmograph is clear with large amplitude. It is known from analysis of finger models that the signal becomes stronger with a certain pressure on the sensor unit. In the experiment with an external force at 0° or 270°, the amplitude of signal is actually larger than the case with no external force due to the pressure applied on the sensor unit. However. when the external force is applied at 90°, the amplitude of the signal is significantly reduced and the pulses are hard to detect. In this configuration, the pressure applied on the sensor unit is almost zero, and even a small air gap can exists between the optical sensor and the skin, which will seriously degrade the measurement. In the case of θ=180°, the air gap becomes even larger and we can hardly recognize the pulses. Actually this is the worst configuration among the four cases. 
     Referring now to FIG. 5, the photoplethysmographs graphs of a new ring sensor, in accordance with a preferred embodiment of the present invention, show much better results. Because of the reason explained above, the signal is most clear and the amplitude is large with θ=0° or 270°. However, even with an external force applied at θ=90° or 180°, the photoplethysmographic signal is still clear and it is not difficult to identify the pulses. Even at the worst configuration which is θ=180°, the amplitude of signal is relatively small, but is clearly reflecting the human pulse to the extent that is enough to identify the pulse. With this new ring design, there exists no air gap between the optical sensor and the skin even though the outer ring will move from its original position significantly. 
     The ring sensor apparatus described herein may be operated, in accordance with embodiments of the invention, in a variety of modes. It is recognized that there are inherent trade-offs between accuracy of measuring any physiological parameters and power consumption by the sensor electronics and thus battery charge lifetime. Under some episodic conditions, frequent and accurate monitoring of vital signs may be essential, such as if the patient is undergoing some cardiac distress. Under routine conditions, however, less frequent monitoring will allow for longer operation between battery maintenance operations. 
     Therefore, various flexible contingent measurement protocols may be provided. For example, a feedback mode provides for modification of the measurement schedule based on real time data. A protocol scheduler  6  (shown in FIG. 1 a ) is provided, in software or hardware, either on board the ring or at remote station unit  28 , to initiate a specified time series of scheduled physiological measurements. Protocol scheduler  6  may be responsive to a command packet uploaded to the ring from a remotely located medical professional or may respond to variations in locally monitored signal-to-noise or to monitored physiological parameters that are programmed to cause a modification of the measurement protocol. 
     In accordance with an alternate embodiment of the invention, protocol scheduler  6  provides a medical professional at station unit  28  with a menu of physiological monitoring protocols and allows, via bidirectional communications with the sensor ring, for selection by the medical professional of a preferred mode of vital sign measurement. 
     The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.