Derivation of flow contour from pressure waveform

The present invention provides a system and method for estimating a blood flow waveform contour from a pressure signal. An arterial or ventricular pressure signal is acquired from a pressure sensor. Landmark points are identified on the pressure waveform that correspond to features of a flow waveform. In one embodiment, the landmark pressure waveform points correspond to the onset of flow, the peak flow, and the end of the systolic ejection phase. The landmark pressure waveform points define a contour that approximates the flow contour. Beat-by-beat flow contour estimation can be performed to allow computation of flow-related hemodynamic parameters such as stroke volume or cardiac output for use in patient monitoring and/or therapy management.

FIELD OF THE INVENTION

The present invention relates generally to hemodynamic monitoring devices and methods and particularly to a method and apparatus for deriving a blood flow contour from a pressure waveform.

BACKGROUND OF THE INVENTION

Implantable hemodynamic monitors are available for monitoring right ventricular pressure chronically in an ambulatory patient. Patients with congestive heart failure (CHF) have elevated cardiac filling pressures and reduced cardiac output. A major treatment objective is to lower filling pressures while maintaining adequate cardiac output. Therefore, from a hemodynamic monitoring perspective, it is advantageous to monitor both filling pressures and measures of cardiac output.

Chronic pressure monitoring in ambulatory patients using chronically implantable pressure sensors has been realized. However, direct monitoring of flow chronically in an ambulatory patient has not been realized clinically. Pressure measurements alone do not account for variations in vascular impedance, which changes in response to varying physiological conditions and is time-varying over the cardiac cycle. Variations in vascular impedance will affect the forward arterial flow produced by developed pressure in the ventricles. Pressure pulse contour cardiac output methods have been developed for estimating flow from arterial pressure signals, however, such methods generally require frequent calibration, particularly after a suspected change in hemodynamics.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a system and method for estimating a blood flow waveform contour from a pressure signal. The estimated flow contour is useful for estimating forward flow (cardiac output) and useful in monitoring changes in vascular impedance or other hemodynamic parameters.

In one embodiment, the system includes a pressure sensor adapted for implantation in an anatomical location suitable for acquiring a ventricular or arterial pressure signal. The pressure sensor is coupled to an implantable cardiac monitoring device having control circuitry, in the form of a microprocessor, and associated memory for acquiring and storing pressure signals. The device will typically include a cardiac electrogram (EGM) sensing circuit to allow for the detection of the onset of cardiac cycles or the R-wave. R-wave detection can be used for timing pressure data acquisition during the desired portion of the cardiac cycle, for example during the systolic phase and in particular the systolic ejection phase, or during the diastolic phase and in particular the early diastolic filling phase. The pressure signals are processed by the microprocessor to estimate the flow contour on a beat-by-beat basis when flow monitoring is enabled. The implantable cardiac monitoring device is equipped with telemetry circuitry for communicating with an external programmer. Pressure and flow data may be uplinked to the external programmer and further processed by a microprocessor included in the external programmer or transferred to another computer for further analysis.

In other embodiments, the system includes an implantable pressure sensor and an ECG or EGM sensing circuit interfaced with an external monitoring device. The external monitoring device includes a microprocessor and associated memory for storing the pressure and ECG/EGM data for processing pressure signals to estimate the flow contour.

In still other embodiments, high-fidelity pressure signals are obtained from external pressure sensors positioned for sensing an arterial pressure signal and coupled to an external monitoring device having processing circuitry for receiving and processing the pressure signals.

In an associated method for deriving a flow contour from a pressure signal, landmark points are identified on the pressure signal contour that correspond to features of the flow contour. The flow contour is observed to be substantially unchanged in response to clinical interventions that change afterload, preload, or cardiac contractility. The pressure signal can be substantially altered in response to the same intervention, however, selected points on the pressure waveform can be identified which correspond in time to features of the flow contour. The flow contour can be approximated by defining a contour based on landmark points on the pressure signal that correspond to features of the actual flow contour.

In one embodiment, the landmark points identified for use in estimating the flow contour from an arterial pressure signal include the onset of the pressure rise, a first pressure peak, and the time of the dicrotic notch. These landmark pressure waveform points correspond to the onset of flow, the peak flow, and the end of the systolic ejection phase, respectively. The three landmark points define a triangle which can be used to approximate the flow contour.

In another embodiment, an arterial flow contour is derived from a ventricular pressure waveform. In one example, landmark points identified from a right ventricular pressure waveform that correspond to features of the pulmonary artery flow contour include the pressure amplitude at the time of the peak rate in pressure rise (dP/dt max), an early shoulder in the pressure waveform that corresponds to an inflection point in the first derivative of the pressure signal, and the pressure amplitude at the time of dP/dt min. These landmark points correspond to the onset of flow, the peak flow, and the end of ejection and define a triangle that can be used to approximate the flow contour.

Alternative embodiments can include substitution or addition of selected landmark points corresponding to actual flow contour features. The estimated flow contour may be used for estimating stroke volume, forward flow or cardiac output, vascular resistance, characteristic impedance, wave reflection, contractility, or other hemodynamic parameters of interest that normally rely on flow measurements.

DETAILED DESCRIPTION

FIG. 1is an illustration of an exemplary implantable medical device (IMD)100connected to monitor a patient's heart120. IMD100may be configured to integrate both monitoring and therapy features, as will be described below. IMD100collects and processes data about heart120from one or more sensors including a pressure sensor and an electrode pair for sensing cardiac electrogram (EGM) signals. IMD100may further provide therapy or other response to the patient as appropriate, and as described more fully below. As shown inFIG. 1, IMD100may be generally flat and thin to permit subcutaneous implantation within a human body, e.g., within upper thoracic regions or the lower abdominal region. IMD100is provided with a hermetically-sealed housing that encloses a processor102, a digital memory104, and other components as appropriate to produce the desired functionalities of the device. In various embodiments, IMD100is implemented as any implanted medical device capable of measuring the heart rate of a patient and a ventricular or arterial pressure signal, including, but not limited to a pacemaker, defibrillator, electrocardiogram monitor, blood pressure monitor, drug pump, insulin monitor, or neurostimulator. An example of a suitable IMD that may be used in various exemplary embodiments is the CHRONICLE® monitoring device available from Medtronic, Inc. of Minneapolis, Minn., which includes a mechanical sensor capable of detecting a pressure signal. In a further embodiment, IMD100is any device that is capable of sensing a pressure signal and providing pacing and/or defibrillation or other electrical stimulation therapies to the heart. Another example of an IMD capable of sensing pressure-related parameters is described in commonly assigned U.S. Pat. No. 6,438,408B1 issued to Mulligan et al. on Aug. 20, 2002.

Processor102may be implemented with any type of microprocessor, digital signal processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA) or other integrated or discrete logic circuitry programmed or otherwise configured to provide functionality as described herein. Processor102executes instructions stored in digital memory104to provide functionality as described below. Instructions provided to processor102may be executed in any manner, using any data structures, architecture, programming language and/or other techniques. Digital memory104is any storage medium capable of maintaining digital data and instructions provided to processor102such as a static or dynamic random access memory (RAM), or any other electronic, magnetic, optical or other storage medium.

As further shown inFIG. 1, IMD100may receive one or more cardiac leads for connection to circuitry enclosed within the housing. In the example ofFIG. 1, IMD100receives a right ventricular endocardial lead118, a left ventricular coronary sinus lead122, and a right atrial endocardial lead120, although the particular cardiac leads used will vary from embodiment to embodiment. In addition, the housing of IMD100may function as an electrode, along with other electrodes that may be provided at various locations on the housing of IMD100. In alternate embodiments, other data inputs, leads, electrodes and the like may be provided. Ventricular leads118and122may include, for example, pacing electrodes and defibrillation coil electrodes (not shown) in the event IMD100is configured to provide pacing, cardioversion and/or defibrillation. In addition, ventricular leads118and122may deliver pacing stimuli in a coordinated fashion to provide biventricular pacing, cardiac resynchronization, extra systolic stimulation therapy or other therapies. IMD100obtains pressure data input from a pressure sensor that is carried by a lead such as right ventricular endocardial lead118. IMD100may also obtain input data from other internal or external sources (not shown) such as an oxygen sensor, pH monitor, accelerometer or the like.

In operation, IMD100obtains data about heart120via leads118,120,122, and/or other sources. This data is provided to processor102, which suitably analyzes the data, stores appropriate data in memory104, and/or provides a response or report as appropriate. Any identified cardiac episodes (e.g. an arrhythmia or heart failure decompensation) can be treated by intervention of a physician or in an automated manner. In various embodiments, IMD100activates an alarm upon detection of a cardiac event. Alternatively or in addition to alarm activation, IMD100selects or adjusts a therapy and coordinates the delivery of the therapy by IMD100or another appropriate device. Optional therapies that may be applied in various embodiments may include drug delivery or electrical stimulation therapies such as cardiac pacing, resynchronization therapy, extra systolic stimulation, neurostimulation.

FIG. 2is a block diagram summarizing the data acquisition and processing functions appropriate for practicing the invention. The functions shown inFIG. 2may be implemented in an IMD system, such as IMD100shown inFIG. 1. Alternatively, the functions shown inFIG. 2may be implemented in an external monitoring system that includes sensors coupled to a patient for acquiring pressure signal data. The system includes a data collection module206, a data processing module202, a response module218and/or a reporting module220. Each of the various modules may be implemented with computer-executable instructions stored in memory104and executing on processor102(shown inFIG. 1), or in any other manner.

The exemplary modules and blocks shown inFIG. 2are intended to illustrate one logical model for implementing an IMD100, and should not be construed as limiting. Indeed, the various practical embodiments may have widely varying software modules, data structures, applications, processes and the like. As such, the various functions of each module may in practice be combined, distributed or otherwise differently-organized in any fashion across a patient monitoring system. For example, a system may include an implantable pressure sensor and EGM circuit coupled to an IMD used to acquire pressure and EGM data, an external device in communication with the IMD to retrieve the pressure and EGM data and coupled to a communication network for transferring the pressure and EGM data to a remote patient management center for analysis. Examples of remote patient monitoring systems in which aspects of the present invention could be implemented are generally disclosed in U.S. Pat. No. 6,497,655 issued to Linberg and U.S. Pat. No. 6,250,309 issued to Krichen et al., both of which patents are incorporated herein by reference in their entirety.

Data collection module206is interfaced with one or more data sources207to obtain data about the patient. Data sources207include any source of information about the patient's heart or other physiological signals. Data sources207include an ECG or EGM source208that provides cardiac electrical signals such as P-waves or R-waves used to monitor the patient's heart rhythm. Data sources207further include a pressure sensor210for obtaining a pressure signal from which a flow contour will be approximated according to methods described in detail below.

Pressure sensor210may be deployed in an artery for measuring an arterial pressure signal or in the left or right ventricle for measuring a ventricular pressure signal. In some embodiments, pressure sensor210may include multiple pressure sensors deployed at different arterial and/or ventricular sites to provide multiple pressure waveforms for use in estimating a flow contour. Pressure sensor210may be embodied as the pressure sensor disclosed in commonly assigned U.S. Pat. No. 5,564,434, issued to Halperin et al., hereby incorporated herein in its entirety.

Data sources207may include other sensors212for acquiring physiological signals useful in monitoring a cardiac condition such as an accelerometer or wall motion sensor, a blood gas sensor such as an oxygen sensor, a pH sensor, or impedance sensors for monitoring respiration, lung wetness, or cardiac chamber volumes. The various data sources207may be provided alone or in combination with each other, and may vary from embodiment to embodiment.

Data collection module206receives data from each of the data sources207by polling each of the sources207, by responding to interrupts or other signals generated by the sources207, by receiving data at regular time intervals, or according to any other temporal scheme. Data may be received at data collection module206in digital or analog format according to any protocol. If any of the data sources generate analog data, data collection module206translates the analog signals to digital equivalents using an analog-to-digital conversion scheme. Data collection module206may also convert data from protocols used by data sources207to data formats acceptable to data processing module202, as appropriate.

Data processing module202is any circuit, programming routine, application or other hardware/software module that is capable of processing data received from data collection module206. In various embodiments, data processing module202is a software application executing on processor102ofFIG. 1or another external processor to implement the process described below in conjunction withFIG. 3. Accordingly, data processing module202processes pressure signals for estimating a flow contour, as described more fully below.

In an exemplary embodiment, processing module202receives data from pressure sensor210and EGM data from EGM sensing electrodes208from data collection module206and interprets the data using analog or digital signal processing techniques to approximate a flow contour on a beat-by-beat basis. The estimated flow contour can be used further to compute an estimated stroke volume, cardiac output, vascular resistance or other hemodynamic monitoring parameters of interest that require a measure of blood flow. The estimated flow contour and/or intermediate computational results may be stored in memory204, which may correspond to hardware memory104shown inFIG. 1, or may be implemented with any other available digital storage device.

When a change in hemodynamic function based on the estimated flow contour and/or other hemodynamic signals is detected, processing module202may trigger an appropriate response. Responses may be activated by sending a digital message in the form of a signal, passed parameter or the like to response module218and/or reporting module220.

Reporting module220is any circuit or routine capable of producing appropriate feedback from the IMD to the patient or to a physician. In various embodiments, suitable reports might include storing data in memory204, generating an audible or visible alarm228, producing a wireless message transmitted from a telemetry circuit230. Reports may include information about the estimated flow contour or hemodynamic parameters derived from the flow contour, pressure measurements derived from the pressure signal, heart rhythm, time and date of data collection, and any other appropriate data. In a further embodiment, the particular response provided by reporting module220may vary depending upon the severity of the hemodynamic change. Minor episodes may result in no alarm at all, for example, or a relatively non-obtrusive visual or audible alarm. More severe episodes might result in a more noticeable alarm and/or an automatic therapy response.

When the functionality diagramed inFIG. 2is implemented in an IMD, telemetry circuitry230is included for communicating data from the IMD to an external device adapted for bidirectional telemetric communication with IMD. The external device receiving the wireless message may be a programmer/output device that advises the patient, a physician or other attendant of serious conditions, e.g., via a display or a visible or audible alarm. Information stored in memory204may be provided to an external device to aid in diagnosis or treatment of the patient. Alternatively, the external device may be an interface to a communications network such that the IMD is able to transfer data to an expert patient management center or automatically notify medical personnel if an extreme episode occurs.

Response module218is any circuit, software application or other component that interacts with any type of therapy-providing system224, which may include any type of therapy delivery mechanisms such as a drug delivery system, neurostimulation, and/or cardiac stimulation. In some embodiments, response module218may alternatively or additionally interact with an electrical stimulation therapy device that may be integrated with an IMD to deliver pacing, extra systolic stimulation, cardioversion, defibrillation and/or any other therapy. Accordingly, the various responses that may be provided by the system vary from simple storage and analysis of data to actual provision of therapy in various embodiments. Any therapy provided may be controlled or adjusted in response to a hemodynamic change observed as a change in flow estimated from a pressure signal or in response to a combination of physiological signals acquired by data sources207. Drug dosage may be adjusted according to episode severity, for example, or electrical stimulation parameters can be adjusted in response to observed deterioration in hemodynamic measures.

The various components and processing modules shown inFIG. 2may be implemented in an IMD100(FIG. 1) and housed in a common housing such as that shown inFIG. 1. Alternatively, functional portions of the system shown inFIG. 2may be housed separately. For example, portions of the therapy delivery system224could be integrated with IMD100or provided in a separate housing, particularly where the therapy delivery system includes drug delivery capabilities. In this case, response module218may interact with therapy delivery system224via an electrical cable or wireless link.

FIG. 3is a flow chart summarizing a method for estimating a flow contour from a pressure signal. Pressure signal acquisition for flow monitoring may be enabled upon detecting predetermined triggering events, on a scheduled basis, or manually by a clinician, patient or other caregiver using an external device. When flow monitoring is enabled, the EGM/ECG signal is sensed at step255to allow detection of the onset of a cardiac cycle based on a detected R-wave event at step260. A detected R-wave event may be an R-wave detected on an EGM/ECG signal but could alternatively be a pacing event or other electrical signal appropriate for marking the start of a cardiac cycle. The present invention is not limited, however, to the use of an EGM/ECG signal or pacing signal for detecting the start of a cardiac cycle. Other physiological signals could be substituted from which an approximation of the start of the cardiac cycle may be made. In one alternative embodiment, a pressure signal may be used to detect the start of the cardiac cycle. For example, a predetermined threshold crossing of amplitude or dP/dt may be detected as an R-wave related event and used as the starting point of a cardiac cycle for the purposes of the present invention.

Once an R-wave event is detected at step260, the pressure signal is acquired for a predetermined interval of time during which analysis of the pressure signal will be performed for estimating the corresponding flow contour. The pressure signal data is stored in a memory buffer to allow the signal to be analyzed during the analysis window defined by the predetermined interval of time. The analysis window is defined such that the systolic portion of the cardiac signal is included to allow the systolic phase of the flow contour to be approximated. In one embodiment, pressure signal data is stored in a memory buffer for about 500 msec following an R-wave event detection. Anomalous cardiac cycles associated with arrhythmias, premature contractions, or noise may be rejected.

At step270, a number of landmark pressure points are identified from the pressure signal during the analysis window. As will be described in detail below, the amplitude or time of the landmark pressure points will be used for defining a geometric boundary or area for use in approximating a flow contour at step275. The flow contour is estimated on a beat-by-beat basis and results may be statistically analyzed over time to obtain baseline, mean, trends, ranges or other statistical parameters of the beat-by-beat flow contour estimate.

At step280, any of a number of hemodynamic parameters that require a measure of flow can be computed using the estimated flow contour. Such parameters include, for example, stroke volume (SV), cardiac output (CO), vascular resistance, characteristic impedance, contractility, and wave reflection. In one embodiment, the estimated flow contour is used to compute an estimated beat-by-beat stroke volume (SV) measurement using the area defined by the estimated flow contour. Using heart rate information obtained from the EGM/ECG signal, the SV measurements can be used to compute an estimate of cardiac output (CO). The estimated CO may be computed on a beat-by-beat or interval basis allowing trends in CO to be determined by data processing circuitry202(FIG. 2). Changes in estimated CO detected by data processing circuitry202can be responded to appropriately by response module218(FIG. 2). Thus, estimated flow contour data may be used in a closed-loop control algorithm for controlling therapy delivery. Estimated flow contour data may additionally or alternatively be stored in IMD memory for uplinking to an external device for offline review and analysis by a clinician.

Upon uplinking to an external device, the estimated flow contour data provided in digital units may be converted to units of volume. CO may then be determined in units of volume per unit time, and other hemodynamic parameters computed using the flow contour estimate may be converted to appropriate physical units. The estimated flow contour values are converted to units of volume by multiplying by a calibration gain and/or adding a calibration offset value. The calibration values are determined for a given pressure sensor and may be individualized for a given patient. In some embodiments, non-linear calibration factors may be used for converting estimated flow contour digital values to actual values in units of volume.

FIG. 4is a time-based plot of an arterial pressure signal and a flow signal and illustrates one method for estimating a flow contour from the arterial pressure signal. In this example, a pulmonary artery pressure (PAP) signal10is acquired for use in approximating a pulmonary flow waveform18. A recording of the pulmonary flow signal18is shown for the purpose of illustrating the usefulness of landmark pressure points determined from the PAP signal10in approximating the flow contour, however, during practice of the invention the flow waveform18is not obtained during monitoring procedures. The flow waveform18may be obtained during calibration procedures for determining calibration constants that can be used to convert digital units to units of volume if desired.

The pulmonary flow contour resembles a triangle30. The triangle30is defined by three landmark points on the flow contour, the onset of flow20, the peak flow22, and the end of flow24at the end of the systolic ejection duration (ED)26. This flow contour changes little in response to interventions that alter the pressure contour. By identifying landmark points on the PAP waveform10that correspond to the three points20,22and24defining flow contour triangle30, a pressure triangle28may be defined that approximates the flow contour.

The onset of PAP development12coincides with the onset of pulmonary flow20. The peak PAP14corresponds with the peak flow22. The time16of the dicrotic notch17in the PAP waveform10corresponds to closure of the pulmonic valve at the end of the systolic ejection duration26, marking the end of flow24. A pressure triangle28is defined by the landmark pressure points12,14and16and can be used to approximate the pulmonary flow contour. As such, characteristics of pressure triangle28may be used to derive flow-related data for patient monitoring purposes. In one example, the area of the triangle28may be computed as an estimate of stroke volume:
SVest=0.5*(Pp*ED)

wherein Pp is the magnitude of the peak PAP14and ED is the ejection duration determined as the time difference between the onset12and end16of the PAP waveform10.

FIG. 5shows a right ventricular pressure waveform and illustrates a method for estimating the pulmonary artery flow contour from a ventricular pressure waveform. Landmark points are selected on the right ventricular pressure (RVP) waveform40that correspond to features of the pulmonary flow waveform. The first derivative dP/dt41of the RVP signal and the third derivative d3P/dt343are also shown to illustrate methods for identifying landmark points on the RVP waveform40.

In the embodiment shown, a first landmark point48is the point on the RVP waveform40at the approximate time of pulmonic valve opening and the onset of pulmonary artery flow. The first landmark point48is identified as the point on the RVP waveform at the time of the maximum rate of RVP rise, or the peak of the first derivative (dP/dt)42.

The second landmark point50is a point on the RVP waveform at the first shoulder of the RVP waveform40. The first shoulder on the RVP waveform40defining the second landmark point50has been observed to correspond in time to peak pulmonary artery flow. The first shoulder on the RVP waveform40defining the second landmark point50can be identified as the as the point on the RVP waveform40at the time of the first peak of the third derivative (d3P/dt3)44that occurs after dP/dt max42.

A third landmark point52corresponds approximately to the time of pulmonic valve closure at the end of the ejection phase and the end of forward flow in the pulmonary artery. In one embodiment, the third landmark point52is identified as the point on the descending portion of the RVP waveform40that is equal to the RVP amplitude at the first landmark point48. Having identified three landmark points48,50and52on the RVP waveform, which correspond in time to the onset of pulmonary artery flow, peak pulmonary artery flow, and the end of pulmonary artery flow, the pulmonary flow contour can be approximated as a triangle54(dashed line) defined by the three landmark points48,50, and52. The base of the triangle54is parallel to the absisca.

An alternative triangle55(dash-dot line) can be defined using alternatively defined landmark points50,56, and57. Landmark point50is defined as described above as the point on the RVP waveform40occurring at the time of the first peak of the third time derivative (d3P/dt3)44that occurs after dP/dt max42. Landmark point50corresponds to the first shoulder of the RVP waveform40. The second landmark point56is defined as the point occurring on the RVP waveform at the time of the minimum dP/dt46and approximates the time of pulmonic valve closure and the end of flow. The third landmark point57is defined as the point occurring on the ascending portion of the RVP waveform40equal to the RVP amplitude at landmark point56. The triangle55has a base parallel to the absisca.

In another alternative embodiment, a triangular estimation of the flow contour can be derived as a triangle having a base that is not parallel to the absisa. The base of the triangle (not illustrated inFIG. 5) is defined by the landmark point48on the RVP waveform occurring at the time of dP/dtmax42and by the landmark point56on the RVP waveform corresponding to the time of dP/dtmin46. The third point of the triangle is defined by landmark point50on the RVP waveform40at the first shoulder of the waveform40.

As can be seen by the above examples, a triangular estimation of a flow contour includes selecting three landmark points which are defined by a pressure amplitude derived from the pressure waveform at a time that corresponds in time to a feature of the flow contour being estimated. The landmark points selected may or may not fall exactly on the pressure waveform as in the case of landmark point52of triangle54and landmark point57of triangle55. The landmark points are defined by: 1) a pressure derived from the pressure waveform at a selected time corresponding to the onset, peak, or end of flow or another feature of the flow contour, and 2) a time corresponding to a feature of the flow contour. The landmark points can be represented in a Cartesian coordinate system as (p,t) where the pressure amplitude, p, is plotted along the y-axis and the time, t, is plotted along the x-axis.

The time used to derive the pressure amplitude of a landmark point and the time at which the landmark point is applied for defining a triangular estimation of the flow contour may not be the same in some embodiments, resulting in a landmark point that does not fall exactly on the RVP waveform40. In one example, a landmark point may be defined by a pressure amplitude equal to the RVP amplitude at the time of dP/dtmax42and by a time of dP/dtmin46. Such a point will not fall exactly on the RVP waveform40, but could be used in defining an estimated flow contour.

In alternative embodiments, other pressure signals can be acquired for approximating a other flow contours. For example, left ventricular pressure, aortic pressure, or other arterial pressure signals can be acquired for estimating an arterial flow contour. Having an estimate of the flow contour, a number of useful applications can be implemented for assessing a patient's hemodynamic status. As described previously, the estimated flow contour can be used to estimate stroke volume by computing an area defined by the estimated contour. The beat-by-beat stroke volume is computed from the estimated pulmonary flow contour using the following equation for computing the area of the pressure triangle54:
SV=0.5*(RVPshoulder−RVPonset)*(Tonset−Tend)

wherein RVPshoulder is the RVP amplitude at the second landmark point50, RVPonset is the RVP amplitude at the first landmark point48, Tonset is the time of the first landmark point48and T(end) is the time of the third landmark point52. In the example of pressure triangle55shown inFIG. 5, RVPshoulder is the RVP amplitude at the second landmark point50, RVPonset is the RVP amplitude at the first landmark point49, Tonset is the time of the first landmark point49and T(end) is the time of the third landmark point56. In other embodiments, the appropriate values are selected for suitably computing an area of the estimated flow contour as an estimate of SV.

Knowing an estimate of stroke volume, flow or cardiac output, can be computed using a measured heart rate. Other hemodynamic parameters that normally require a flow measurement can be computed using the estimated flow contour such as vascular resistance, characteristic impedance, ventricular contractility, wave reflectance.

FIGS. 6A and 6Bshow a comparison of pressure derived flow contours and actual flow contours for control conditions and after Dobutamine infusion. InFIG. 6A, a control RVP waveform60and a control pulmonary artery flow waveform62obtained during a canine study are shown. The control pulmonary artery waveform62is seen to be approximately the shape of a triangle64. Extraction of landmark points68,70and72, corresponding in time to the onset, peak and end of pulmonary artery flow, respectively, allows triangular estimation66of the flow contour using the method described in conjunction withFIG. 5.

InFIG. 6B, the pressure and flow response to Dobutamine infusion is shown. The pulmonary artery flow waveform76retains the generally triangular contour78but is characterized by a higher peak associated with increased flow in response to Dobutamine infusion. In contrast, the RVP contour74is altered considerably compared to the normal RVP contour60shown inFIG. 6A. However, the landmark points82,84, and86, which correspond in time to the onset, peak and end of flow, can be extracted from the pressure waveform74using the method described above in conjunction withFIG. 5. The landmark points82,84, and86define a triangular contour80. The estimated flow contour for the Dobutamine response provided by triangle80is increased in height compared to the estimated triangular flow contour66for control (FIG. 6A), reflecting the increase in peak flow observed in the Dobutamine response flow waveform76. Changes in the RVP contour74, in particular the heightened second shoulder85, which is expected to be related to an increased afterload and increased wave reflection, do not affect the triangular estimated flow contour80based on the extracted landmark points82,84and86. Thus, a flow contour estimate can be derived from a pressure waveform, independent of afterload and preload related changes to the pressure contour, using extracted points corresponding in time to key features of an actual flow waveform.

FIG. 7is a time-based plot showing the beat-by-beat tracking of the actual measured peak pulmonary artery flow by the estimated peak flow during Dobutamine infusion in a canine study (n=9). The estimated peak flow is computed from the triangular estimation of the pulmonary artery flow contour using RVP signals. The measured pulmonary artery flow90presented an increase in peak flow in response to Dobutamine infusion in all subjects. The estimated peak flow92demonstrates that the actual measured flow90is well-tracked by the triangular flow contour estimation method.

FIG. 8is a plot of beat-to-beat stroke volume estimated using the flow contour method described herein and the actual measured stroke volume during Dobutamine infusion in a canine study (n=9). Pulmonary flow contour estimation was performed using a right ventricular pressure waveform according to the method described above in conjunction withFIG. 5. The area of the triangular flow contour estimated according to extraction of the three landmark points corresponding to the onset, peak and end of pulmonary artery flow was computed as the estimated stroke volume. The estimated stroke volume96tracked well with the Dobutamine response of the actual measured stroke volume96in all subjects.

In the exemplary results shown, universal calibration constants were used for all 9 subjects in the study. Individually determined calibration constants can be expected to yield a more accurate estimate of actual stroke volume. The linear regression of the beat-to-beat relationship of the estimated and measured stroke volumes can be used for determining calibration values (a linear gain and offset) for converting the estimated stroke volume in digital units to actual units of volume (ml).

In the exemplary embodiments described herein, the flow contour is approximated as a triangle using a set of three landmark points derived from the pressure waveform. In other embodiments, the flow contour may be approximated using other geometric shapes or combinations of geometric shapes or functions defined by multiple landmark points derived from the pressure waveform. It is further recognized that the process of identifying landmark points on the pressure waveform may involve the analysis of other physiological signals. For example, identifying the time of a particular event during the cardiac cycle at which a landmark pressure point is to be identified may involve sensing events from a wall motion, acoustical, or other mechanical sensor of cardiac function.

The examples provided herein relate to estimation of the flow contour during the systolic portion of the cardiac cycle. However, the methods provided by the present invention may be put to practice for estimating a flow contour during the diastolic portion of the cardiac cycle. For example, landmark points may be identified on a ventricular pressure waveform during diastolic filling to estimate a flow contour corresponding to early diastolic filling, commonly referred to as E-waves during Doppler ultrasound measurements of blood flow.

Thus, a system and method have been described which provide an estimation of a blood flow contour derived from a pressure signal. Aspects of the present invention have been illustrated by the exemplary embodiments described herein. Numerous variations for estimating a flow contour from pressure signals may be conceived by one having skill in the art and the benefit of the teachings provided herein. The described embodiments are intended to be illustrative of methods for practicing the invention and, therefore, should not be considered limiting with regard to the following claims.