BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a front view of a blood pressure monitor according to the invention that features a console connected by a cable to a flexible foam pad that includes the substrate;
FIG. 2 shows a schematic view of the blood pressure monitor of FIG. 1 measuring a patient underneath their sternal notch;
FIG. 3A shows a graph of time-dependent electrical, optical, and acoustic waveforms measured with the blood pressure monitor of FIG. 1;
FIG. 3B shows a graph of the time-dependent electrical, optical, and acoustic waveforms shown in FIG. 3A plotted over a relatively short time scale;
FIG. 4 shows an equation used by an algorithm running on a microprocessor within the blood pressure monitor of FIG. 1 to calculate blood pressure;
FIG. 5 shows a graph of VTT* plotted as a function of systolic blood pressure, along with a linear fit to these data, for a single patient;
FIGS. 6A and 6B show, respectively, front and back views of a blood pressure monitor according to the invention wherein the flexible foam pad that includes the substrate connects directly to the console; and,
FIG. 7 is a front view of a disposable, adhesive patch that includes the substrate and connects through a cable to a blood pressure monitor according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a preferred embodiment of a blood pressure monitor 5 according to the invention that includes a console 10 that attaches to a flexible foam pad 16 through a cable 14. The flexible foam pad 16 includes a substrate 15 that supports sensors that measure time-dependent electrical, optical, and acoustic waveforms (shown in FIGS. 3A, 3B). The substrate 15 is preferably a flexible printed circuit board that adheres to the foam pad 16. During operation, the flexible foam pad 16 preferably contacts an area below a patient's sternal notch to measure the various waveforms from a patient. A microprocessor in the console 10 analyzes the waveforms to determine the systolic time intervals, which are then processed with an algorithm and a weighted average to determine the patient's real-time blood pressure.
The flexible foam pad 16 preferably includes three electrodes 18a-c that measure two electrical signals and a ground (or other) signal from the patient. Two of the electrodes 18a, 18c are preferably spaced apart by at least two inches so that, when the flexible foam pad 16 contacts the patient (as shown in FIG. 2), the two electrodes 18a and 18c measure signals that can be processed with a differential amplifier and band-pass filters to determine an ECG-like electrical waveform. The flexible foam pad 16 additionally includes a reflective optical sensor 20 that includes a photodetector and a light-emitting diode (LED) that typically emits green radiation (λ=520-590 nm) to measure a reflective optical waveform representing blood flowing in underlying capillaries. A preferred optical sensor 30 (manufacturer: TAOS, Inc.; part number: TRS1755) includes a green LED light source (567 nm wavelength) and a light-to-voltage converter in a common housing. The flexible foam pad 16 additionally includes a piezoelectric acoustic sensor 22 that detects sounds waves following each of the patient's heartbeat to generate an acoustic waveform. The preferred piezoelectric acoustic sensor is preferably a Condenser Microphone Cartridge (manufacturer: Panasonic; part number: WM-55D103) that detects sounds waves following each of the patient's heartbeats to generate an acoustic waveform, also called a phonocardiogram.
Referring to FIG. 2, during operation the blood pressure monitor 5 is operated so that the console 10 is held in one hand of the patient 30 while the other hand holds the flexible foam pad 16 to the chest. In this way, the pad 16 is proximal to the patient's heart 32, a location that allows it to simultaneously measure optical, electrical, and acoustic activity that follows each heartbeat to generate time-dependent analog waveforms. The waveforms propagate through shielded, co-axial wires in the cable 14 which connect to the console 10 using a bulkhead connector 12. An analog-to-digital converter in the console 10 converts the analog waveforms to digital ones, which the microprocessor analyzes to determine the patient's blood pressure.
FIGS. 3A and 3B show graphs 50, 52 of the time-dependent electrical waveform 60, optical waveform 62, and acoustic waveform 64 in more detail. Each waveform 60, 62, 64 includes time-dependent features that repeat with each heartbeat. For example, the electrical waveform 60 looks similar to a conventional ECG and features a QRS complex featuring a sharp spike that indicates an initial depolarization of the ventricle. Because of its well-defined features, the QRS complex is relatively easy to detect with a computational algorithm, and serves as an effective ‘marker’ that indicates each individual heartbeat. The optical waveform 62 is measured from underlying capillaries in the patient's chest and features a slowly varying pulse that indicates an increase in volume in the capillaries caused by a propagating pressure wave. Finally, the acoustic waveform features two ‘beats’, each representing a collection of acoustic frequencies, that occur with each heartbeat. The first and second beats represent the sounds made following closure of, respectively, the heart's mitral and aortic valves; these are the conventional ‘lub’ and ‘dub’ heard through a stethoscope.
FIG. 3B graphs a portion of the waveforms highlighted by a box 66 of FIG. 3A, and indicates how a microprocessor preferably analyzes the various features of the electrical waveform 60′, optical waveform 62′, and acoustic waveform 64′ to determine a variety of systolic time intervals. These systolic time intervals are then further processed to determine a patient's real-time blood pressure. As described above, the QRS complex in the electrical waveform 60′, which is caused by initial depolarization of the heart muscle, serves as a marker indicating the start of each heart beat. At a later time, the mitral valve opens and blood flows from the heart's left atrium into the left ventricle. The mitral valve then closes, causing the first beat in the acoustic waveform 64′, and the aortic valve opens shortly thereafter. The opening of the aortic valve does not result in a feature in the acoustic waveform 64′ (only closing valves do this), but is assumed to follow within approximately 10 milliseconds after the closing of the mitral valve. The time difference between the onset of the QRS complex and the opening of the aortic valve is called the ‘pre-injection period’, or PEP. Since the technique described herein does not explicitly measure the opening of the aortic valve, but rather the closure of the mitral valve, it is labeled PEP*. Once the aortic valve opens, the heart pumps a bolus of blood through the aorta, resulting in a pressure wave that propagates through the patient's arterial system. The propagation time of the pressure wave is a strong function of the patient's blood pressure, along with their vascular compliance and resistance. When the pressure wave reaches capillaries in the patient's chest, the rise in pressure causes the capillaries to increase in volume with blood, which in turn increases the amount of optical radiation from the LED of the optical sensor 20 that the flowing blood absorbs. The photodetector in the optical sensor 20 detects this as a time-dependent pulse characterized by a relatively sharp rise time and a slower decay, as indicated by the optical waveform 62′. The time difference between the estimated opening of the aortic valve and the onset of the pulse's rise time is the ‘vascular transit time’ (VTT*). Typically the VTT* decreases with higher blood pressure. The second beat in the acoustic waveform 64′ represents the closure of the aortic valve, and the time period separating this from the estimated opening of the aortic valve is called the ‘left ventricular ejection period’ (LVET*). Finally, the onset of the QRS complex and the foot of the plethysmograph is the pulse transit time (PTT*). Note that the transit time essentially represents the time from when the heart begins to beat to when the pressure wave appears underneath the optical sensor 20. To reach this point, the vascular pathway that the pressure wave must travel is somewhat complicated: it extends through the aorta, the subclavian artery, a series of smaller arteries proximal to the patient's ribs, and finally through relatively small capillaries attached to these arteries. The collective length of this pathway explains the relatively long PTT* shown in FIGS. 3A and 3B.
Other properties known to correlate to blood pressure can also be measured from the optical waveform 62, electrical waveform 60, and acoustic waveform 64. For example, as described below in Table 1, the rise and fall times of the optical waveform 62 can meet this criterion, and thus these properties can be measured from the optical waveform 62. In addition, in some cases the optical waveform 62 will include a primary and secondary peak, separated by a feature called the ‘dicrotic notch’. The microprocessor can be programmed to take a second derivative of the waveform to determine the ratio of the primary and second peaks, and this property has been shown to correlate to blood pressure. In addition, variability in the patient's heartbeat, as measured from each of the electrical waveform 60, optical waveform 62, and acoustic waveform 64, can indicate variation in the patient's blood pressure, and can also be processed by the microprocessor. Heart rates from these three waveforms can be calculated and averaged together to yield a very accurate measure of the patient's real-time heart rate.
FIG. 4 shows a semi-empirical equation 100 that describes how blood pressure relates to the different time-dependent properties measured by the blood pressure monitor described in FIG. 1. Specifically, during operation, VTT*, PEP*, LVET*, PTT* waveform properties, and heart rate variability can be measured from underneath the patient's sternal notch with the flexible foam pad 16 and processed with the microprocessor in the console 10 to determine the patient's blood pressure. In general, each of these properties, along with other time-dependent waveform properties, has independently been shown to correlate to blood pressure, typically in a linear relationship following an initial calibration. Table 1, below, lists references that describe these properties and the instrumentation used to measure them. The Table is simply meant to list representative documents, and is not meant to be an exhaustive collection of all documents describing a correlation between blood pressure and STIs or time-dependent waveform properties. Each of the references described in Table 1 are hereby incorporated by reference.
TABLE 1
|
|
relationship between blood pressure and time-dependent properties measured
|
from STIs and other waveform properties.
|
Property
Reference
Instrumentation
|
|
PTT
U.S. Pats. No. 5,316,008; 5,857,975;
ECG and Pulse Oximeter
|
5,865,755; 5,649,543
|
VTT
U.S. Pats. No. 6,511,436; 6,599,251;
Paired Optical and Pressure
|
6,723,054; 7,029,447
Sensors; ICG
|
LVET
‘Short-term variability of pulse pressure
Intra-arterial Catheter
|
and systolic and diastolic time in heart
|
transplant recipients’, Am. J. Physiol.
|
Heart Circ. Physiol. 279, H122–H129
|
(2000)
|
PEP
‘Relationship between systolic time
Intra-arterial Catheter
|
intervals and arterial blood pressure’,
|
Clin. Cardiol. 9, 545–549, (1986)
|
PEP/LVET
‘Systolic Time Intervals in Man’,
Intra-arterial Catheter
|
Circulation 37, 149–159 (1968)
|
PPG Width
‘How does the plethysmograph derived
Pulse Oximeter
|
from the pulse oximeter relate to arterial
|
blood pressure in coronary bypass graft
|
patients’, Anesth. Analg. 93, 1466–1471
|
(2001)
|
PPG Second
‘Assessment of vasoactive agents and
Pulse Oximeter
|
Derivative
vascular aging by the second derivative of
|
the photoplethysmogram waveform’,
|
Hypertension 32, 365–370 (1998)
|
|
FIG. 4 indicates that each of the time-dependent properties correlates with blood pressure according to a function ‘F’ (i.e. F1, F2, F3, F4, F5 and F6), which is typically a linear function characterized by both a slope and y-intercept. The parameter ‘A’ (i.e. A1, A2, A3, A4, A5 and A6) determines the weighting of the function in the blood pressure calculation. Typically the parameters A and F are determined once during an initial calibration period, and then used for all subsequent measurements. For example, during operation the flexible foam pad 16 can be held to the patient's chest, and a button on the console 10 is depressed indicating a calibration is to begin. The pad then measures the time-dependent electrical waveform 60, optical waveform 62, and acoustic waveform 64 and processes them to determine VTT*, PEP*, LVET*, PTT*, any additional waveform properties and heart rate variability. These properties are then stored in non-volatile memory in the console 10. A graphical user interface operating on the console 10 then prompts the user to measure their blood pressure (both systolic and diastolic values) using conventional means, e.g. with a cuff-based device. This can be done at home or in a medical office. The patient then enters the systolic and diastolic values through the graphical user interface and microprocessor stores them in the non-volatile memory. An algorithm operating on the microprocessor performs a simple least-squares fitting routine to determine the slope and y-intercept within each ‘F’ function that relates each property to blood pressure. Weighting parameters ‘A’ are determined prior to any measurements and are loaded into memory during manufacturing.
Once the blood pressure monitor is calibrated, slope and y-intercept values corresponding to each function ‘F’ and weighting factors ‘A’ are stored in memory and are used in subsequent blood pressure calculations along with time-dependent properties measured from the electrical waveform 60, optical waveform 62, and acoustic waveform 64.
In other embodiments the blood pressure monitor 5 is calibrated using pre-set parameters stored in the blood pressure monitor 5 during manufacturing, and is not calibrated using a conventional (e.g. cuff-based) measurement. In this case, for example, clinical studies conducted before manufacturing are used to determine ‘calibrations’ comprising slope, y-intercept, and weighting parameters for specific demographics characterized by biometric parameters such as age, weight, height, gender, and race. After they are determined, these parameters are loaded into non-volatile memory on the monitor during manufacturing. Afterwards, a patient using the blood pressure monitor 5 enters their biometric parameters using the graphical user interface, and an algorithm operating on the monitor analyzes them to determine the appropriate ‘calibration’ to use. The blood pressure monitor 5 uses this ‘calibration’ for all subsequent measurements until the patient enters new biometric parameters.
In another embodiment a ‘universal calibration’, characterized by a single set of slope, y-intercept, and weighting parameters, is determined using clinical studies and stored in non-volatile memory in the blood pressure monitor 5. In this case, the graphical user interface does not include an interface that allows the patient enters biometric or calibration information, and the blood pressure monitor 5 then uses parameters from the ‘universal calibration’ for all subsequent measurements.
In yet another embodiment, the blood pressure monitor 5 may support two or more of the above-mentioned calibration approaches. For example, the blood pressure monitor 5 may have stored in its memory a ‘universal calibration’ and specific ‘calibrations’ characterized by biometric parameters. In addition, the blood pressure monitor 5 may be programmed to accept individual calibrations determined using conventional blood pressure monitors (e.g. cuff-based devices). In this case the graphical user interface is structured so that the patient can easily select the type of calibration to use for each measurement. The patient then proceeds as described above to make each blood pressure measurement.
FIG. 5 shows a graph that describes how a systolic time interval measured using the above-described method correlates to a blood pressure measurement from a conventional cuff-based device. Specifically, the graph plots VTT* as a function of systolic blood pressure for a given patient over a range of blood pressures. As described above in FIG. 4, VTT* varies in a linear manner with blood pressure. When fit with a linear function to determine slope and y-intercept for this time-dependent parameter, the fit correlates with the data with an R value of −0.94. This fitting process, for example, could take place during one of the above-described calibration steps.
FIGS. 6A and 6B show a blood pressure monitor 5′ corresponding to an alternate embodiment of the invention wherein the flexible foam pad 16′ comprises the substrate 15′ and adheres directly to the back surface of the console 10′. The console 10′ includes a display that operates a touch screen panel and graphical user interface. In this case the flexible foam pad 16′ includes all the sensor elements described with reference to FIG. 1, i.e. two signal and one ground electrodes 18a-c′, an optical sensor 20′ featuring a LED and a photodetector, and a piezoelectric acoustic sensor 22′. In this embodiment the flexible foam pad 16′ connects to power, ground, and signal electrical leads in the console 10′ through a flexible tab connector (not shown in the figure), and there is no cable connecting these two components.
During operation, the patient holds the blood pressure monitor 5′ in one hand and gently pressure the flexible foam pad 16′ to their chest so that the display 13′ faces away from the patient. As with the embodiment shown in FIG. 1, the electrodes 18a-c′, optical 20′ and acoustic 22′ sensors measure, respectively, electrical, optical, and acoustic waveforms similar to those shown in FIGS. 3A and 3B. In this embodiment, the patient cannot clearly see the display 13′, and thus the console includes a piezoelectric ‘beeping’ component (not shown in the figure) that beeps when the measurement is complete. At this point the patient removes the blood pressure monitor 5′ from their chest and views the display 13′ the see the blood pressure reading.
FIG. 7 shows yet another embodiment of the invention wherein an adhesive sensor 151 includes a substrate 150 embedded within a flexible foam pad 160. The foam pad 160 attaches to an adhesive backing 224, allowing the system to be temporarily attached to a patient. In this case, the flexible foam pad 160 includes a tab connector 230 that attaches to a detachable cable (not shown in the figure) that connects to a body-worn console (also not shown in the figure). The flexible foam pad 160 also includes a microchip 222 that stores a serial number in a small-scale, non-volatile memory. During a measurement, the body-worn console connects to the microchip 222 through the tab connector 230 and detachable cable to read the serial number that identifies a particular foam pad 160. The body-worn console also includes a user interface wherein a user (e.g., a medical professional, such as a nurse, or the patient) can enter information describing, e.g., the patient. Software running of a microprocessor in the console associates the serial number to the patient's information.
As described with reference to FIGS. 1, 6A, and 6B, the flexible foam pad 160 includes three electrodes 180a-c, an optical sensor 200, and an acoustic sensor 220. The sensors measure electrical, optical, and acoustic information as describe above to determine the patient's blood pressure.
The flexible foam pad 160 described in FIG. 7 can be used to make quasi-continuous measurements from a patient over an extended period of time (e.g., from several hours to several days). In this case, the adhesive backing 224 attaches to the patient so the three electrodes 180a-c, optical sensor 200, and acoustic sensor 220 contact the patient's chest. One end of the detachable cable connects to the tab connector 230, while the other end attaches to the body-worn console. While the cable is attached, the sensors measure electrical, optical, and acoustic waveforms as described above to determine the patient's blood pressure. During periods where it is not necessary to monitor the patient, the detachable cable detaches from the tab connector 230 while the adhesive backing 224 and flexible foam pad stays adhered 160 to the patient. When the detachable cable is reconnected to the tab connector, the console reads a serial number from the microchip 222 to identify the adhesive sensor 150 as well as the patient it is attached to. Once this is complete, the console continues to measure the patient's blood pressure as described above.
Other embodiments are also within the scope of the invention. For example, the console may include wireless systems (e.g. a wireless modem) or serial port (e.g. a USB port) to connect to an Internet-accessible website. Such systems, for example, are described in the below-mentioned references, the entire contents of which are incorporated herein by reference. In other embodiments, short-range wireless systems connect the flexible foam pad and its associated sensors to the console, making the cable unnecessary. In this case, the flexible foam pad and console have matched wireless transceivers and batteries to power them.
In other embodiments, the optical, electrical, and acoustic sensors are included in a circuit board attached to the end of the cable. In this case the circuit board can ‘snap’ into a disposable adhesive sensor, which in turn attaches to the patient. The disposable adhesive sensor typically includes openings for the optical and acoustic sensors so they can contact the patient to measure, respectively, optical and acoustic signals, as well as Ag/AgCl electrodes covered by a solid gel to measure electrical signals. By including the relatively expensive electrical components in the cable, this embodiment minimizes the cost of the disposable component, which is comprised mostly of an adhesive pad and the electrode materials.
In other embodiments, the flexible foam pad can include optical, electrical, and acoustic sensors on one side, and a finger-clip sensor that includes optical and electrical sensors on the opposing side. In this case, during operation, a patient slides their finger into the finger clip sensor to measure optical and electrical signals from one hand. The patient then simultaneously presses the foam pad against their chest so that the optical, electrical, and acoustic sensors measure their respective signals as described above. A cable connecting the flexible foam pad to the console transmits the signals from the patient's hand and chest to the microprocessor, which then processes them as described above to determine systolic time intervals, and particularly PTT, to determine blood pressure.
In other embodiments, the optical, electrical, and acoustic waveforms can be processed to determine other vital signs. For example, relatively low-frequency components of an ‘envelope’ describing both the electrical and optical waveforms can be processed to determine respiratory rate. This can be done, for example, using an analysis technique based on Fourier Transforms. In other embodiments, the substrate can be modified to include light sources (e.g. LEDs) operating in both the red (e.g. λ=600-700 nm) and infrared (λ=800-900 nm) spectral regions. With these modifications, using techniques know in the art, that substrate can potentially measure pulse oximetry in a reflection-mode configuration. In still other embodiments, time-dependent features from the PCG can be analyzed to determine cardiac properties such as heart murmurs, lung sounds, and abnormalities in the patient's mitral and aortic valves.
In other embodiments, the blood pressure monitor can connect to an Internet-accessible website to download content, e.g. calibrations, text messages, and information describing blood pressure medication, from an associated website. As described above, the monitor can connect to the website using both wired (e.g. USB port) or wireless (e.g. short or long-range wireless transceivers) means.
In addition to those described above, a number of methods can be used to calculate blood pressure from the optical, electrical, and acoustic waveforms. These 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. 10/709,01; 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); and 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); and 15) SYSTEM FOR MEASURING VITAL SIGNS USING BILATERAL PULSE TRANSIT TIME (U.S. Ser. No. 11/420,652; filed May 26, 2006).
Still other embodiments are within the scope of the following claims.
Patent Application
20080082004
