fig1 , 2 a , and 2 b show a body - worn sensor system 20 , according to the invention , featuring a lightweight , low - power processing module 5 connected to a flexible sensor assembly 15 for measuring blood pressure . the body - worn sensor system 20 includes three separate small circuit boards ( shown in more detail in fig3 a , 3 b ) within the processing module 5 , all of which are contained within a plastic housing 21 . the processing module 5 connects to the sensor assembly 15 which includes a multi - sensor array 2 , electrodes 4 a , 4 b , 4 c . the sensor assembly 15 connects to a pulse oximetry circuit 8 that , in turn , connects to a finger - worn pulse oximetry module 41 . the sensor assembly 15 includes a male electrical connector 3 that mates with a corresponding female connector 26 on the processing module 5 . the processing module 5 operates using two aa batteries 9 a , 9 b or equivalent rechargeable batteries . during a measurement , the body - worn sensor 20 is worn on the patient &# 39 ; s arm 45 , and the sensor assembly 15 connects to electrodes 4 a , 4 b using a shielded flex cable 10 . the flex cable 10 typically includes a flexible , polyimide substrate with embedded conductive traces ( typically made of metal or conducting ink ) that can easily and comfortably wrap around the patient &# 39 ; s arm . in addition to the conductive traces , the cable typically has pads that optical components in the multi - sensor array can solder to . it typically features a flat , rectangular surface . the electrodes 4 a , 4 b adhere to the patient &# 39 ; s skin to measure unique electrical signals . the same flex cable 10 connects to a multi - sensor array 2 that measures an optical waveform . during a measurement , both optical and electrical signals pass to an amplifier / filter circuit within the processing module 5 , and from there through separate channels to the analog - to - digital converter . the serial connector 3 also includes a shielded electrical connector 18 that receives an electrical lead 13 that connects to a third electrode 4 c positioned on the patient &# 39 ; s chest . the three electrodes 4 a , 4 b , 4 c form a proxy for an einthoven &# 39 ; s triangle configuration , and are used to measure a single - lead ecg . a secondary shielded electrical connector 19 connects to an acoustic sensor , not shown in figure , to measure a respiratory rate from the patient . the sensor assembly 15 further connects to a pulse oximetry circuit 8 through a separate flex cable 6 . the pulse oximetry circuit connects to a pulse oximetry sensor 41 through a cable 12 . a soft wristband 40 holds the cable 12 in place . to measure optical waveforms , the multi - sensor array 2 includes three optical modules 80 , 81 , and 82 that collectively measure an optical waveform , or ppg , from the patient . use of the three optical modules 80 , 81 , 82 increases both the signal - to - noise ratio of the optical waveform , as well as the probability that the waveform is measured from an artery , as opposed to a capillary bed . typically an optical waveform measured from an artery yields a ptt that correlates better to blood pressure . the pulse oximetry sensor 41 measures a second optical waveform which can be processed along with the optical waveform measured with the multi - sensor array 2 to determine vtt . each optical waveform features a time - dependent ‘ pulse ’ corresponding to each heartbeat that represents a volumetric change in an underlying artery caused by the propagating pressure pulse . the electrodes 4 a , 4 b in the sensor assembly 15 feature metal snaps 11 a , 11 b to secure disposable electrode patches , not shown in figure , that attach to the patient &# 39 ; s arm and chest . the disposable electrode patches typically feature a metal contact coated with an ag / agcl thin film , a solid or liquid gel component that interfaces to the patient &# 39 ; s skin , and an adhesive component . in an alternate embodiment , these materials are embedded directly in the sensor assembly 15 ( i . e . the assembly does not include metals snaps or disposable electrode patches ) to form the electrode . the electrode materials generate electrical signals that , once processed , form the electrical waveform . the electrical waveform includes a sharp peak corresponding to the qrs complex . ptt is calculated for each heartbeat by measuring the time difference between the peak of the qrs complex and the foot ( i . e . onset ) of the optical waveform . this property is then used as described below to determine the patient &# 39 ; s blood pressure . the process for measuring blood pressure using a multi - sensor array is described in the following co - pending patent application , the entire contents of which are incorporated herein by reference : multi - sensor array for measuring blood pressure ( u . s . ser . no . 12 / 139 , 219 ; filed jun . 13 , 2007 ). the optical modules within the multi - sensor array 2 typically include an led operating near 570 nm , a photodetector , and an amplifier . alternatively the array can include one or more discrete leds and one or more discrete photodetectors . this wavelength is selected because it is particularly sensitive to volumetric changes in an underlying artery when deployed in a reflection - mode geometry , as described in the following co - pending patent application , the entire contents of which are incorporated herein by reference : system for measuring vital signs using an optical module featuring a green light source ( u . s . ser . no . 11 / 307 , 375 ; filed feb . 3 , 2006 ). 570 nm is also particularly effective at measuring optical waveforms from a wide range of skin types featuring different levels of pigmentation . use of this wavelength is described , for example , in the following technical paper , the contents of which are incorporated herein by reference : ‘ racial differences in aortic stiffness in normotensive and hypertensive adults ’, journal of hypertension . 17 , 631 - 637 , ( 1999 ). a preferred optical module is the trs1755 manufactured by taos inc . of plano , tex . ( www . taosinc . com ). typically , three optical modules are used in the multi - sensor array 2 to increase the effective optical field and , consequently , the probability that an underlying or proximal artery is measured . this in turn increases both the strength of the optical signal and its signal - to - noise ratio . operating in concert , the three sensors collectively measure an optical waveform that includes photocurrent generated by each optical module . the resultant signal forms the optical waveform , and effectively represents an ‘ average ’ signal measured from vasculature ( e . g ., arteries and capillaries ) underneath or proximal to the sensor 2 . the above - described system determines the patient &# 39 ; s blood pressure using ptt , and then corrects this value for vi using algorithms described in the following patent application , the entire contents of which are incorporated herein by reference : vital sign monitor for cufflessly measuring blood pressure corrected for vascular index ( u . s . ser . no . 12 / 138 , 199 ; filed jun . 12 , 2008 ). specifically , it is well know that a patient &# 39 ; s arteries stiffen with biological age . this property can thus be used to estimate the patient &# 39 ; s vascular stiffness . when used with a ptt - based measurement of blood pressure , which depends strongly on vascular stiffness , biological age can therefore reduce the need for calibration and increase the accuracy of the blood pressure measurement . the accuracy of the measurement can be further improved with vi , which serves as a proxy for a ‘ true ’ age of the patient &# 39 ; s vasculature : patients with elastic arteries for their age will have a vi lower than their biological age , while patients with stiff arteries for their age will have a vi greater than their biological age . the difference between vi and the patient &# 39 ; s biological age can be compared to a pre - determined correction factor to improve the accuracy of a ptt - based blood pressure measurement . in an alternate embodiment , the body - worn sensor system 20 can be integrated with a conventional blood pressure cuff and used to perform a blood pressure measurement called the ‘ composite technique ’, as described in the following patent application , the entire contents of which are incorporated herein by reference : vital sign monitor measuring blood pressure using optical , electrical , and pressure waveforms ( u . s . ser . no . 12 / 138 , 194 ; filed jun . 12 , 2008 ). referring to fig2 a , the body - worn sensor system 20 is designed to wrap around the arm of an average patient . the dimensions of the body - worn sensor ( in inches ) are as follows : as shown in fig3 a , 3 b , and 4 , to minimize size , the processing module 5 is constructed using three circuit boards : a main circuit board 14 and analog board 25 are disposed horizontally , and are connected by a power regulating board 24 , which is disposed vertically . during a measurement , an electrical current is drawn from the aa batteries 9 a , 9 b through positive 27 a , 28 a and ground 27 b , 28 b battery terminals connected to the power regulating board 24 . the main circuit board 14 houses the data - processing circuit 101 and microprocessor 34 and controls the sensor assembly 15 . as described above , the sensor assembly includes three electrodes 4 a , 4 b , 4 c and a multi - sensor array 2 that includes three optical modules 80 , 81 , and 82 . each optical module includes an led 85 , 86 , 87 operating near 570 nm , and a photodetector 90 , 91 , 92 that detects reflected radiation at this wavelength . during operation , the main circuit board 14 receives signals from the analog board 25 , which processes the optical and electrical signals directly from the sensor assembly 15 . each optical and electrical signal is amplified by an amplifier / filter circuit 16 using separate amplifier and filter circuits . this generates analog optical and electrical signals , which are is then digitized with an analog - to - digital converter 32 . the analog - to - digital converter 32 is typically a separate integrated circuit ( manufacturer : texas instruments ; part number : ads8344nb ) that digitizes the waveforms at rates typically between 250 - 1000 hz with 16 - bit resolution . such high resolution is required to adequately process the optical and electrical waveforms and generate an accurate ptt value . the data - processing circuit 101 is programmed with computer code that controls the body - worn sensor &# 39 ; s various functions . the computer code runs on a high - end microprocessor 34 , typically an arm 9 processor ( manufacturer : atmel ; part number : at91sam9261 - cj ) contained in a conventional ball grid array package . once digitized , the optical and electrical waveforms can be stored in memory 75 . the pulse oximetry sensor 41 is in direct communication with the pulse oximetry circuit 8 , and includes separate leds 95 , 96 operating near , respectively , 650 nm and 950 nm , and a photodetector 94 . the pulse oximetry circuit 8 determines a pulse oxygenation value from a patient , and connects directly to the data processing circuit 101 . a preferred pulse oximeter module is provided by spo medical ; part number : pulseox 7500 ™. the processing module 5 communicates using a short - range wireless transceiver 7 that transmits information through an on - board ceramic antenna 67 to a matched transceiver in a remote device . the short - range wireless transceiver can be a bluetooth ® transceiver 7 , or alternatively a wireless transceiver that operates on a wireless local - area network , such as a wifi ® transceiver . the processing module can also use a usb connection 65 to communicate with external devices or recharge the aa batteries . fig5 a and 5b show a patient wearing the body - worn sensor system 20 , 20 ′ in wireless communication 50 , 50 ′ with a personal computer 55 or handheld display component 56 . the personal computer 55 or handheld display component 56 is in further communication through a wireless interface 51 , 51 ′ with a wireless network 70 , 70 ′ that connects to the internet 71 , 71 ′. the handheld display component 56 is highly portable and can be easily removed from a docking station 150 . a number of additional solutions can be used to calculate blood pressure from ptt measured as described above . such method are described in the following co - pending patent applications , the contents of which are incorporated herein by reference : 1 ) cuffless blood - pressure monitor and accompanying wireless , internet - based system ( u . s . ser . no . 10 / 709 , 015 ; filed apr . 7 , 2004 ); 2 ) cuffless system for measuring blood pressure ( u . s . ser . no . 10 / 709 , 014 ; filed apr . 7 , 2004 ); 3 ) cuffless blood pressure monitor and accompanying web services interface ( u . s . ser . no . 10 / 810 , 237 ; filed mar . 26 , 2004 ); 4 ) vital sign monitor for athletic applications ( u . s . ser . no . ; filed sep . 13 , 2004 ); 5 ) cuffless blood pressure monitor and accompanying wireless mobile device ( u . s . ser . no . 10 / 967 , 511 ; filed oct . 18 , 2004 ); 6 ) blood pressure monitoring device featuring a calibration - based analysis ( u . s . ser . no . 10 / 967 , 610 ; filed oct . 18 , 2004 ); 7 ) personal computer - based vital sign monitor ( u . s . ser . no . 10 / 906 , 342 ; filed feb . 15 , 2005 ); 8 ) patch sensor for measuring blood pressure without a cuff ( u . s . ser . no . 10 / 906 , 315 ; filed feb . 14 , 2005 ); 9 ) patch sensor for measuring vital signs ( u . s . ser . no . 11 / 160 , 957 ; filed jul . 18 , 2005 ); 10 ) wireless , internet - based system for measuring vital signs from a plurality of patients in a hospital or medical clinic ( u . s . ser . no . 11 / 162 , 719 ; filed sep . 9 , 2005 ); 11 ) hand - held monitor for measuring vital signs ( u . s . ser . no . 11 / 162 , 742 ; filed sep . 21 , 2005 ); 12 ) chest strap for measuring vital signs ( u . s . ser . no . 11 / 306 , 243 ; filed dec . 20 , 2005 ); 13 ) system for measuring vital signs using an optical module featuring a green light source ( u . s . ser . no . 11 / 307 , 375 ; filed feb . 3 , 2006 ); 14 ) bilateral device , system and method for monitoring vital signs ( u . s . ser . no . 11 / 420 , 281 ; filed may 25 , 2006 ); 15 ) system for measuring vital signs using bilateral pulse transit time ( u . s . ser . no . 11 / 420 , 652 ; filed may 26 , 2006 ); 16 ) blood pressure monitor ( u . s . ser . no . 11 / 530 , 076 ; filed sep . 8 , 2006 ); 17 ) two - part patch sensor for monitoring vital signs ( u . s . ser . no . 11 / 558 , 538 ; filed nov . 10 , 2006 ); and , 18 ) monitor for measuring vital signs and rendering video images ( u . s . ser . no . 11 / 682 , 177 ; filed mar . 5 , 2007 ). other embodiments are also within the scope of the invention . for example , the system is not limited to three optical modules . additional optical modules could be added to further strengthen the magnitude of the optical waveform . also , the optical modules within the multi - sensor array are not limited to the ‘ linear ’ form factor shown in fig1 . the modules , for example , may be placed in a circular configuration , may be offset from one another , or may be fashioned in a random distribution to irradiate a relatively large area of underlying skin . such a configuration may be desirable for patients with a darker pigmented skin . in other embodiments , additional electrodes may be added to strengthen the electrical waveform .
a system is described that continuously measures a patient &# 39 ; s blood pressure over a length of time . the system features a sensor assembly featuring a flexible cable configured to wrap around a portion of a patient &# 39 ; s arm . the flexible cable features a back surface that includes at least two electrodes that are positioned to contact the patient &# 39 ; s skin to generate electrical signals . it additionally features an optical sensor that includes at least one light source and at least one photodetector . these components form an optical sensor that is configured to generate an optical signal by detecting optical radiation emitted by the at least one light source and reflected from a blood vessel underneath the patient &# 39 ; s skin .