Dual-finger vital signs monitor
BACKGROUND OF THE INVENTION 
     The present invention is an apparatus that provides a simultaneous measurement of three primary vital signs of a patient. The Dual-Finger Vital Signs Monitor is a convenient, non-invasive, highly accurate, and reliable instrument that can be used to help safeguard the life of a patient who is in grave danger due to potential cardiac and respiratory failure. 
     Critically ill and seriously injured patients require constant care and attention. Doctors, nurses, and hospital technicians need a continuous flow of information about the many patients under their care. Electrocardiogram (ECG), heart rate, and blood pressure measurements are two primary vital signs that indicate the health of a patient. When these two common indices of wellness fall below normal readings, a patient is usually in distress and requires immediate treatment. Dangerous conditions brought about by a cardio-vascular or pulmonary disease, severe trauma, or drug abuse may bring about a failure of the lungs and heart to supply the bloodstream with life-giving oxygen. Such a potentially fatal deficiency can be detected by continually gauging the amount of hemoglobin in the bloodstream that is carrying oxygen. This third vital sign, which manifests oxygen saturation of the blood, is especially critical because a rapid decline in oxygen in the bloodstream is associated with increased risk of patient mortality. 
     Many devices that measure these three vital signs separately are currently available to the health-care industry. Several conventional systems employ a single flexible membrane which envelops the finger to measure blood pressure or blood oxygen levels. Information about the heart is supplied by separate wire electrodes and leads that measure voltages on the skin of the patient and that are displayed in a familiar electro-cardiogram (ECG). The ECG is a measurement of the changing potentials generated by the electrical activity of the heart. These previous measurements are inconvenient because they require separate instruments and separate cables which connect to the patient. The motion of the patient can also compromise the readings. If the flexible membrane remains constantly pressurized, this constriction can place great stress on the finger and may, eventually, cause tissue damage. These earlier vital sign meters are also plagued by false readings that can cause confusion in a hospital ward. The frequent incorrect reports of patient distress are often attributable to a reliance upon single sensors, which may warn of dangerous conditions based upon spurious signals generated by the inadvertent and undesirable motion of the patient&#39;s finger. 
     The shortcomings of conventional patient monitoring devices has presented a major challenge to designers in the medical instrumentation field. The development of a reliable and highly accurate vital signs monitor that provides a simultaneous measurement of heart rate, blood pressure, ECG, and blood oxygenation would constitute a major technological advance in the health-care industry. The enhanced performance that could be achieved using such an innovative device would satisfy a long felt need within the medical profession and would enable hospital equipment manufacturers to help their customers save substantial expenditures of time and money. 
     SUMMARY OF THE INVENTION 
     The Dual-Finger Vital Signs Monitor disclosed and claimed in this patient application overcomes the problems suffered by previous patient monitors by providing a simultaneous and continuous measurement of three primary vital signs: ECG and heart rate, blood pressure, and blood oxygen saturation. The present invention employs a pair of finger cuffs that each include an electrocardiographic electrode, a first radiation source and detector pair for blood pressure measurement, and a second radiation source and detector pair for blood oxygenation measurement. The electrodes are connected to a voltmeter which displays an electrocardiogram. The first radiation source and detector pairs in each cuff function as a finger-photoplethysmographic pressure sensor. A pair of fluid conduits connect each finger cuff to a pair of blood pressure controllers, which each include a fluid reservoir. The second radiation source and detector pair in each cuff are each connected to a pair of oximeters. All the sensors operate independently. The use of two finger cuffs affords substantial advantages for signal processing and data interpretation. The redundancy of sensors greatly reduces the incidence of false warnings. The two finger cuffs may be inflated and pressed into service in a regularly alternating sequence, so that the blood flow in the patient&#39;s finger is not constricted for a time period that would cause tissue damage. 
     The Applicant&#39;s innovative Dual-Finger Vital Signs Monitor is a reliable, cost-effective, and powerful tool that will enable physicians and nurses to provide higher quality care and to save lives. 
     An appreciation of other aims and objectives of the present invention and a more complete and comprehensive understanding of this invention may be achieved by studying the following description of a preferred embodiment and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph which compares the absorption coefficients of oxygenated and deoxygenated hemoglobin for various wavelengths of light passed through the bloodstream. 
     FIG. 2(a) is a schematic depiction of a conventional ear oximeter. 
     FIG. 2(b) is a schematic illustration of a convention pulse oximeter. 
     FIG. 3 is a schematic cross-section of a finger surrounded by a blood pressure cuff. 
     FIG. 4 presents a plot of volume v. pressure for arterial blood in a finger. 
     FIG. 5 reveals an illustration of a conventional photoelectric plethysmographic chamber around a patient&#39;s finger. 
     FIG. 6 portrays the attenuation of light passing through a patient&#39;s finger. The graph exhibits the separate attenuation components attributable to tissue, venous blood, and arterial blood. 
     FIG. 7 is a schematic diagram of the present invention, the Dual-Finger Vital Signs Monitor. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Previous Methods and Devices 
     FIG. 1 is a comparison 10 of two curves which reveals absorption coefficients at various wavelengths of incident light for deoxyhemoglobin 12 and oxyhemoglobin 14 in the blood. Persons ordinarily skilled in the art of non-invasive blood measurement techniques have exploited the differences between the absorbance of oxyhemoglobin and deoxyhemoglobin to measure the level oxygen in a patient&#39;s bloodstream. 
     FIG. 2(a) depicts one type of conventional oximeter which is based upon the different reflectivity of oxygen-bearing and deoxygenated hemoglobin. Hewlett-Packard&#39;s Multiple-Wavelength Ear Oximeter 16 comprises a light source 18, a lens 19, a beam splitter 20, a narrow band filter wheel 21, a lens 22, a beamsplitter 23, a filter 24, a lens 25, optical fiber bundles 26 and 28 which are placed on either side of a patient&#39;s earlobe 27. A reference path 29 conveys light to a detector 30 which is connected to a signal processor 31. Eight filters are arranged on a rotating wheel 21 so that a set of selected wavelengths of light are sequentially directed to the ear 27 through the fiber optic guides. As the filter wheel 21 rotates, a particular wavelength of radiant energy is first focused on the photodetector 30 through the ear 27, and then to the detector 30 via the reference path 29. The ratio of the transmitted light intensity to the incident light intensity is calculated by signal processor 31, which utilizes equations that are well known in the field to determine the oxygen saturation value. 
     FIG. 2(b) is a schematic illustration of a conventional pulse oximeter. A patient&#39;s finger 32 is surrounded by a finger cuff 33 that includes a red LED 34, a near infrared LED 35, and a detector 36. The LEDs 34 and 35 and the detector 36 are coupled by leads 37 to processing and control circuitry 38 which measures blood pressure based upon the amount of red and near infrared radiation sensed by detector 36. Displays 40 and alarms 42 present the blood pressure information to the user. 
     FIG. 3 is a cross-sectional view 44 of a finger surrounded by the cuff of a blood pressure monitor first described by J. Penaz. The epidermis 46, bone 48, and arteries 50 are shown enveloped within a cuff bladder 52 that is pressurized by a fluid supply 54. A light emitting diode 56 and a detector 58 are located on opposite sides of the bladder 52 are employed to infer the volume of the finger arteries 50, which, in turn, provides a measurement for the patient&#39;s blood pressure. The technique utilized by the Penaz invention is to measure the size of the artery 50 when the blood pressure within it is the same as the external pressure imposed by the inflatable cuff 52 that has been placed around the finger. The LED 56, detector 58, an pressurized fluid supply 54 are connected to a signal processor (not shown) which computes blood pressure based upon pressure in the cuff bladder 52 while the artery is maintained in an unloaded condition. The systolic, mean, and diastolic pressures can all be determined using this method. The systolic pressure is the blood pressure at ventricular systole, i.e., the end of the heart stroke, and is the highest pressure. Diastolic pressure is the pressure at diastole, when the heart is relaxed, and is the lowest pressure. The mean pressure is the average pressure of the arterial pressure during a complete heart cycle. 
     FIG. 4 illustrates the pressure v. volume characteristics 60 of a finger artery. Arterial volume is read along the x-axis and the transmural pressure, the difference between the arterial pressure and the finger cuff pressure, is read along the y-axis. Curve 62 represents the dependence of artery size for a given transmural pressure. Volumes 64, 66, and 68 correspond to pressure levels 70, 72, and 74. When the pressure in the cuff surrounding the finger is held constant, the pressure changes are shown by the graphs 75(a,b,c) bounded by pressure levels 70, 72, and 74. 
     FIG. 5 reveals a schematic diagram showing a patient&#39;s finger 78 within a conventional transparent pressure cuff 80 that is commonly used to measure blood pressure. Cuff 80 includes a light source 81 and a photocell 82. The output of the photocell 82 is fed to a difference amplifier 83, which compares the cuff signal with a preselected first external signal input 84. One terminal of a plethysmogram voltmeter 85 is placed between the difference amplifier 83 and a correcting network 86, which is also connected to a switch 88 and a power amplifier 89. A second external signal is 87 is also fed to the power amplifier 89. An electro-pneumatic transducer 90, which is connected to a fluid-filled conduit 91, receives an output signal from the power amplifier. 
     FIG. 6 exhibits a schematic diagram 96 of waveforms generated by the photoelectric output of a pulse oximeter. Curve 97 represents the pulsatile signal from the spectrophotometric analysis of blood flow in the patient&#39;s finger. Distances 98, 100, and 102 show the amount of attenuation due to absorption by tissue, venous blood, and arterial blood, respectively. Since the components 98 and 100 are constant, oximetry can be performed using only the fluctuating component attributable to the arterial blood. Beer&#39;s Law is employed to calculate the optical density of the light transmitted through the finger. The same methods described above which exploit the different coefficients of absorption for saturated and unsaturated hemoglobin are utilized to determine oxygen levels in the bloodstream. 
     The Dual-Finger Vital Signs Monitor 
     FIG. 7 is a block diagram of the present invention, the Dual-Finger Vital Signs Monitor 104. The patient&#39;s right and left finger 106 and 108 are surrounded by right and left cuffs 110 and 112. Each cuff includes an ECG transducer portion 114 and 120, a pressure transducer portion 116 and 122, and a hemoglobin oxygenation transducer portion 118 and 124. Leads 126 and 128 from ECG transducers are connected to an ECG analyzer and display 130. A pair of fluid conduits 132 and 134 connect the cuffs 110 and 112 to a pair of pressure controllers 144 and 146. Leads 136 and 140 from the pressure detectors 116 and 122 and leads 138 and 142 from pressure LEDs 116 and 122 are also connected to pressure controllers 144 and 146. Amplifiers 148 and 150 are coupled to pressure servos 152 and 154 which automatically maintain the appropriate fluid level in the cuffs 110 and 112. A blood pressure meter 153 is connected to servo 152. Detector leads 156 and 158 from hemoglobin transducers 118 and 124 as well as LED leads 160 and 162 are connected to an oximeter 164 and 166. Pressure controllers 144 and 146 are connected by leads 168 and 172 to processing and control circuitry 176, which is also coupled to oximeters 164 and 166 by leads 170 and 174. Memory 177, a display 178, and an alarm 180 are also linked to processing and control circuitry 176. The alarm 180 may use a buzzer or horn to provide an audible signal, or may incorporate an LED or lamp that supplies a visual indication. 
     The present invention is connected to the patient by placing cuffs 110 and 112 over one finger on each hand. This pair of connections is often referred to as a &#34;Lead One&#34; pair, which connotes that this tandem is the first set of electrodes for ECG procedures. This form of coupling is far simpler and more reliable than the many different connections that must be made when several independent conventional instruments are utilized to provide the same measurements. In the preferred embodiment, display 178 provides a digital array which presents heart rate, blood pressure, and blood gas information. All the sensors within the cuffs operate independently. The use of two finger cuffs affords substantial advantages for signal processing and data interpretation. The redundancy of sensors greatly reduces the incidence of false warnings. 
     The Dual-Finger Vital Signs Monitor provides a non-invasive and reliable device for monitoring patients who may suffer from cardiac or respiratory difficulties. This invention constitutes a major step forward in the field of medical instrumentation. 
     Although the present invention has been described in detail with reference to a particular preferred embodiment, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.