Dynamic cardiovascular monitor

• US 6,647,287 B1
• Filed: 04/14/2000
• Issued: 11/11/2003
• Est. Priority Date: 04/14/2000
• Status: Expired due to Fees

• Current Assignee: Kimesha May Angell Peel, Omron Healthcare Company Limited (Omron Corporation)
• Original Assignee: Southwest Research Institute
• Inventors: Shinoda, Masayuki, Zhao, Xiao, Peel, Harry Herbert III, Inada, Eiichi, Dodge, Franklin Tiffany
• Primary Examiner(s): LAYNO, CARL HERNANDZ
• Application Number:
  US09/549,611
• Time in Patent Office: 1,306 Days
• Field of Search: 600/513, 600/485
• US Class Current: 600/513
• CPC Class Codes: A61B 5/02125 of pulse wave propagation time
• CPC Class Codes: A61B 5/022 by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers

Abstract

Methods and systems reconstruct and verify aortic blood pressure waveforms from peripheral blood pressure waveform data using mathematical models. The models combine analytical models of pulse wave propagation in the cardiovascular system with empirical models derived from measurements taken from human subjects. When used to reconstruct the aortic pressure of a given subject, the models are adjusted to the subject and the subject'"'"'s physiological state based upon measurements performed on the subject'"'"'s cardiovascular system.

Claims

1. A method that reconstructs the aortic blood pressure of a patient, the method comprising:

measuring a time reference for a start of each blood pressure pulse using an ECG as the pulse leaves the patient'"'"'s aorta root;

measuring brachial blood pressure waveforms using a plethysmograph to produce blood pressure waveforms when blood pressure is held at a constant low level;

measuring a continuous radial or ulnar blood pressure waveform using a tonometer or blood pressure sensor in the artery;

measuring a continuous plethysmographic blood pressure waveform in the patient'"'"'s finger using at least one pulse oximeter;

reconstructing aortic blood pressure waveforms using mathematical models that combine analytical models of pulse wave propagation in the cardiovascular system with an aortic waveform reconstruction model; and

repeatedly adjusting the mathematical models to the patient and the patient'"'"'s physiological state based upon the measurements of the ECG, the plethysmograph and the at least one pulse oximeter to produce a dynamic, patient-specific, reconstructed aortic waveform.

2. The method of claim 1, wherein the plethysmograph is an occlusion cuff of a NIBP monitor.

3. The method of claim 1, wherein the empirical aortic waveform reconstruction model is obtained from measurements performed on a large population of subjects.

4. The method of claim 3, wherein reconstructing aortic blood pressure waveforms is performed by normalizing the empirical aortic waveform reconstruction model using the subjects'"'"' wave-propagation characteristics and other information.

5. The method of claim 4, wherein, reconstructing the aortic blood pressure waveforms includes combining normalized empirical models into a single population average normalized model subsequent to normalizing the empirical aortic waveform reconstruction model.

6. The method of claim 4, wherein normalization is performed using mathematical descriptions of a human vascular system and measurements that account for individual subject'"'"'s variations, non-uniformities, and non-linearities of the cardiovascular system.

7. The method of claim 1, further comprising verifying the dynamic, patient-specific, reconstructed aortic waveform model by using the dynamic, patient-specific, reconstructed aortic waveform model to reproduce the waveform at a first point in the patient'"'"'s vascular system and comparing the reproduced waveform to a waveform measured at the first point.

8. A patient monitoring system that estimates blood pressure at a first site using a blood pressure measurement performed at a second site, the system comprising:

a system controller that controls operation of the patient monitoring system;

a first data storage device, operationally coupled to the system controller, that stores data about blood pressure pulse wave propagation;

a first communication device, operationally coupled to the system controller, by which a user provides information about a patient and the measurement device;

a first blood pressure pulse measurement device, operationally coupled to the system controller, that continuously measures the blood pressure at the second site to produce a first blood pressure measurement;

a first computational device, operationally coupled to the system controller, that constructs a first mathematical relationship using the data stored in the first data storage device to estimate blood pressure at the first site using the first blood pressure measurement;

a first blood pressure measurement modification device, operationally coupled to the system controller, that produces a second blood pressure measurement by removing a mean pressure from the first blood pressure measurement;

a first blood pressure measurement comparator, operationally coupled to the system controller, that compares the estimated blood pressure at the first site with the second blood pressure measurement;

a second communication device, operationally coupled to the system controller, that transfers results of the first blood pressure measurement device, first computational device, first blood pressure measurement modification device and the first blood pressure measurement comparator to a user or a second data storage device;

a systole detector, operationally coupled to the system controller, that detects initiation of a blood pressure pulse;

a third data storage device, operationally coupled to the system controller, that retains the mean pressure removed from the first blood pressure measurement for recombination with the estimated blood pressure at the first site;

an artery occlusion device, operationally coupled to the system controller, that temporarily restricts blood flow in a path of the vascular system between a heart and the second site;

a second blood pressure pulse measurement device, operationally coupled to the system controller, that measures a third blood pressure measurement, or a representation thereof, at, or proximate to, the artery occlusion device;

a third blood pressure pulse measurement device, operationally coupled to the system controller, that measures a fourth blood pressure measurement, or a representation thereof, at a third point that is at, or proximate to, an end of a blood pressure pulse propagation path;

a second blood pressure measurement modification device, operationally coupled to the system controller, that modifies the fourth blood pressure measurement by removing changes in the measurement produced by frequency response characteristics of the third blood pressure pulse measurement device;

a pulse propagation time determination device, operationally coupled to the system controller, that measures a time required for the blood pressure pulse of a given heart beat to travel to the first, third and fourth blood pressure measurement sites from the heart and times required for the blood pressure pulse to travel between each of the first, third and fourth pulse measurement sites;

a second computational device, operationally coupled to the system controller, that uses information from any of the first data storage device, the first blood pressure measurement comparator, the systole detector, the first, second and third data storage devices, the artery occlusion device, the first blood pressure measurement modification device, the first and second blood pressure measurement devices and the pulse propagation time determination device, to construct a computational relationship between the blood pressure measured by the blood pressure measurement device and the blood pressure at the first site;

a third computational device, operationally coupled to the system controller, that uses the estimate of the blood pressure at the first site to produce a second estimate of the blood pressure measurement at the second and third sites;

a fourth computational device, operationally coupled to the system controller, that continuously estimates the blood pressure at the third site from the blood pressure measured at the second site;

a second blood pressure measurement comparator, operationally coupled to the system controller, that continuously compares the blood pressure measured at the third site to the estimated blood pressure at the third site and the propagation time between the first and third sites to produce a measurement indicating whether the first mathematical relationship is accurate; and

an optimizer, operationally coupled to the system controller, that adjusts the first mathematical relationship to improve accuracy with which the system estimates blood pressure at the first site.

9. The system of claim 8, further comprising:

a second blood pressure measurement modification device, operationally coupled to the system controller, and the first blood pressure measurement device, that modifies the first blood pressure measurement by removing changes in the first blood pressure measurement produced by frequency response characteristics of the first blood pressure measurement device; and

a third blood pressure measurement modification device, operationally coupled to the system controller and the third blood pressure measurement device, that modifies the third blood pressure measurement by removing changes in the third blood pressure measurement produced by frequency response characteristics of the second blood pressure measurement device.

10. The system of claim 8, wherein the first, second and third data storage devices are electronic, magnetic or electromagnetic storage devices.

11. The system of claim 8, wherein the systole detector determines a start time of a given pulse using a device for measuring electrical activity of the heart and known empirical relationships of heart rate, blood pressure and patient characteristics to a time period between the given pulse start time and a time of opening of the heart'"'"'s aortic valve.

12. The system of claim 8, wherein the systole detector determines a start time of a given pulse using a device for measuring electrical activity of the heart and sounds produced by closing of the heart'"'"'s valves and a device for determining the opening of the heart'"'"'s aortic valve based upon timing of the given pulse start time and the heart sounds.

13. The system of claim 8, wherein, any of the first, second and third blood pressure measurement devices is a continuous, blood pressure measurement device comprising one of:

a pressure transducer directly inserted into an artery;

a pressure transducer connected to an interior of the artery by a fluid filled tube or catheter; and

a tonometer that indirectly measures pressure in an underlying artery.

14. The system of claim 8, wherein the first blood pressure pulse measurement device is either a high pass filter with a low frequency cutoff below a frequency of blood pressure information contained in the first blood pressure measurement or a device that calculates the first blood pressure pulse measurement as a mean blood pressure using an equation empirically derived from blood pressure measurements performed by the first blood pressure measurement device.

15. The system of claim 8, wherein the combination of the artery occlusion device and the second blood pressure pulse measurement device comprises either:

an automatic non-invasive blood pressure monitor comprised of an air-filled cuff wrapped around the patient'"'"'s limb, a cuff pressure measurement device, a cuff pulse measurement device, and a monitor controller;

or

a combination of a pressurized, fluid-filled occlusion cuff, pressure controller and a separate device for measuring the blood pressure pulse such as a pressure sensing device pressed against the patient'"'"'s skin over an underlying artery, and ultrasonic or electrical impedance measurement device that measures a volumetric change of the underlying artery by introduction of electromagnetic energy into the underlying artery.

16. The system of claim 8, wherein the third blood pressure measurement device measures a shape of a blood pressure waveform to directly or indirectly produce a measurement that is proportional to blood pressure pulse pressure.

17. The system of claim 8, wherein the third blood pressure measurement device measures a shape of a blood pressure waveform directly or indirectly, the third blood pressure measurement device comprising:

a pulse oximeter that produces a continuous measurement of a change in volume of a finger artery produced by a blood pressure pulse;

a finger cuff blood pressure monitor that continuously monitors a change in blood pressure in the finger;

or

one of a finger plethysmograph that uses an inflated cuff or an electrical impedance and strain gauge that continuously measure the volumetric change of the finger artery caused by the blood pressure pulse.

18. The system of claim 9, wherein any of the first, second and third blood pressure measurement modification devices comprises:

a two stage filter, a first stage of the two stage filter being constructed to have a frequency response which is an inverse of a frequency response of the blood pressure measurement modification device in which the two stage filter is used, wherein characteristics of the first stage of the two stage filter are selected from frequency response characteristics based upon information entered in the first communication device that identifies a blood pressure pulse device being used, and, a second stage of the two stage filter is a low pass filter that has a fixed cutoff frequency.

19. The system of claim 8, wherein the pulse propagation time determination device comprises:

a detection circuit that identifies an arrival time of a blood pressure pulse at a given location by identifying a diastolic or minimum point of a blood pressure waveform point of a preceding waveform and subtracting a systolic time from the arrival time at the given location.

20. The system of claim 8, wherein the system controller comprises:

a computational device or electromechanical device that controls operation of the occlusion cuff, first, second and third blood pressure measurement devices, and first, second, third and fourth computational processes according to a predetermined series of actions necessary to produce a reconstructed aortic blood pressure measurement.

21. The system of claim 8, wherein the first computational device comprises:

a first constituent device that computes a natural frequency of an aortic-to-brachial pressure segment of a pulse propagation path from an aortic-to-brachial pulse propagation time measured when the occlusion cuff at the second site is inflated to a pressure above a systolic pressure;

a second constituent device that determines radial-to-finger pulse propagation characteristics from measured radial and finger pulse waveforms and a mathematical relationship that describes a finger-to-radial segment of the pulse propagation path;

a third constituent device that determines brachial-to-radial pulse propagation characteristics from measured brachial and radial pulse waveforms and a mathematical relationship that describes a radial-to-brachial segment of the pulse propagation path;

a fourth constituent device that estimates a damping coefficient of the brachial-to-aortic segment of the pulse propagation path from values of the aortic-to-brachial natural frequency and the pulse propagation characteristics of the finger-to-radial and radial-to-brachial segments;

a fifth constituent device that constructs a mathematical relationship that describes the brachial-to-aortic segment of the propagation path using the natural frequency of the aortic-to-brachial segment and the estimated damping coefficient of the aortic-to-brachial segment;

a sixth constituent device that combines the brachial-to-aortic and radial-to-brachial mathematical relationships to form a radial-to-aortic pulse propagation relationship;

a seventh constituent device that constructs a mathematical relationship that describes the aortic-to-brachial segment of the propagation path using the natural frequency of the aortic-to-brachial segment and an estimated damping coefficient of the aortic-to-brachial segment;

an eighth constituent device that transforms the radial-to-aortic pulse propagation relationship to a time-domain mathematical relationship that produces an estimate of the aortic pulse pressure waveform from a radial pulse pressure waveform measurement; and

a ninth constituent device that transforms the aortic-to-brachial pulse propagation relationship to a time-domain mathematical relationship that produces an estimate of a brachial pressure from an estimate of an aortic pulse pressure waveform.

22. The system of claim 21, wherein, the second computational device comprises:

a tenth constituent device that estimates an aortic blood pressure waveform from at least one of the pressure measurements made by the first blood pressure measurement device using the radial-to-aortic pulse propagation relationship;

an eleventh constituent device that modifies the retained first blood pressure measurement mean pressure using a mathematical relationship that correlates damping coefficients of individual segments of the pulse propagation path to a mean pressure decrease from the aorta to the first blood pressure measurement site;

an adder that adds the modified mean pressure produced by the eleventh constituent circuit to a series of estimated aortic pulse pressures;

a synchronizer that synchronizes the estimated aortic pressure to the estimated time of systole;

a first constituent display that displays the estimated aortic pressure in a form suitable for visual assessment; and

a twelfth constituent device that determines and displays the estimated aortic blood pressure parameters including but not limited to the systolic, mean, diastolic pressures and the rate of change of pressure during systole.

23. The system of claim 22, wherein the third computational device comprises:

a thirteenth constituent device that computes estimates of the brachial blood pressure waveform corresponding in time to the brachial waveforms obtained from estimates of the aortic pressure waveform using the aortic-to-brachial relationship;

a fourteenth constituent device that adjusts the relative timing of the estimated brachial waveform to the measured brachial waveform such that minimums of the estimated and brachial waveforms are synchronous; and

a second constituent display that displays and retains the estimated brachial waveform and measured brachial waveform for visual assessment.

24. The system of claim 23, wherein the fourth computational device comprises:

a fifteenth constituent device that continuously computes estimates of a finger blood pressure waveform using a measured radial waveform and the radial-to-finger pulse propagation relationship; and

a sixteenth constituent device that adjusts relative timing of the estimated finger blood pressure waveform to the measured finger blood pressure waveform such that the minimums of the estimated and measured finger blood pressure waveforms are synchronous.

25. The system of claim 24, wherein the first blood pressure measurement comparator comprises:

a seventeenth constituent device that computes estimates of a brachial blood pressure waveform corresponding in time to the brachial waveforms obtained from estimates of the aortic pressure waveform using the aortic-to-brachial relationship to produce an estimated aortic blood pressure error as a worst case estimate of estimated aortic blood pressure error;

an eighteenth constituent device that computes a root mean square difference between the measured brachial waveform and the estimated brachial waveform to produce a root square error as a measure of waveform fidelity;

a third constituent display that displays the estimated aortic blood pressure error and root mean square error for visual assessment;

a nineteenth constituent device that compares the estimated aortic blood pressure error and root mean square error to predetermined limits stored in one of the first, second or third data storage devices;

a first constituent indicator that informs a user that an accuracy of the estimated aortic pressure is outside predetermined limits based on the comparison performed by the nineteenth constituent device; and

a first constituent re-calibration initiator that initiates a re-calibration of the radial-to-aortic blood pressure reconstruction device.

26. The system of claim 25, wherein the second comparison device comprises:

a twentieth constituent device that computes a difference in amplitude of the measured finger blood pressure pulse waveform and the estimated finger blood pressure pulse waveform as a first index of a state of the patient'"'"'s vascular system;

a twenty-first constituent device that computes a root mean square difference between the measured finger blood pressure pulse waveform and the estimated finger blood pressure pulse waveform as a second index of the state of the patient'"'"'s vascular system;

a twenty-second constituent device that retains the finger blood pressure pulse waveform measurement amplitude mean difference and the root mean square difference made at a time of calibration of the aortic blood pressure reconstruction device;

a twenty-third constituent device that compares a most recent finger blood pressure pulse waveform measurement amplitude mean difference and root mean square difference to the retained from the last calibration to a set of limits stored in at least one of the first, second and third data storage devices;

a second constituent indicator that informs a user that an accuracy of the estimated aortic pressure is outside the predetermined limits based on the comparison performed by the twenty-third constituent device; and

a second constituent re-calibration initiator that initiates a re-calibration of the radial-to-aortic blood pressure reconstruction device.

27. The system of claim 26, wherein the optimization device comprises:

a twenty-fourth constituent device that compares the mean amplitude difference and root mean square difference of the estimated brachial pressure waveform and measured brachial waveform to predetermined accuracy standards stored in at least one of the first, second or third data storage devices;

a twenty-fifth constituent device that adjusts the brachial-to-aortic mathematical relationship if the mean amplitude difference and RMS difference of the brachial-to-aortic waveform do not meet the accuracy standards;

a twenty-sixth constituent device that reconstructs the radial-to-aortic and aortic-to-brachial mathematical relationships and re-computes the aortic blood pressure and brachial blood pressure waveforms;

a constituent controller that controls the twenty-fourth constituent device, twenty-fifth constituent device and twenty-sixth constituent device to operate until the mean amplitude difference and root mean square differences of the brachial-to-aortic waveform meet the predetermined accuracy standards or achieve a minimum error; and

a third constituent indicator that informs the user that the accuracy of the estimated aortic pressure is outside the predetermined limits.

28. A method for estimating a patient'"'"'s aortic pressure waveform comprising:

constructing an aortic pressure estimation mathematical relationship by creating a time domain equation obtained by taking an inverse transform of a radial-to-aortic transfer function constructed by dividing a radial-to-finger transfer function, computed from measurements of the patient'"'"'s radial and finger blood pressure waveforms, by a product of three second order transfer functions, each transfer function representing a segment of a blood pressure pulse propagation path, and each transfer function incorporating parameters determined for the patient by numerical analysis of the measurement of the patient'"'"'s radial and finger blood pressure pulse waveforms, and

calculating a patient'"'"'s aortic pressure waveform by using said time domain equation.