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Method and apparatus for continuous pulse contour cardiac outputRelated Patent Categories: Surgery, Diagnostic Testing, Cardiovascular, Measuring Pressure In Heart Or Blood Vessel, Testing Means Inserted In BodyMethod and apparatus for continuous pulse contour cardiac output description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060167361, Method and apparatus for continuous pulse contour cardiac output. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates generally to hemodynamic monitoring devices and methods and particularly to a method and apparatus for monitoring cardiac output using ventricular pressure signals. BACKGROUND OF THE INVENTION [0002] 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. BRIEF SUMMARY OF THE INVENTION [0003] The present invention provides a system and method for monitoring cardiac output estimated from a ventricular pressure signal using pulse contour analysis. In one embodiment, the right ventricular pressure (RVP) signal is acquired from an implantable pressure sensor deployed in the right ventricle. The RVP contour is analyzed to estimate stroke volume (SV) during cardiac systole, from which an estimate of forward flow can be derived. [0004] The system includes a pressure sensor adapted for implantation in the right ventricular chamber for sensing the RVP signal. The system may optionally include an electrode for measuring cardiac electrical signals, such as an intracardiac electrogram (EGM). The electrode and pressure sensor are coupled to an implantable cardiac monitoring device including signal recovery circuitry, processing circuitry, and associated memory for acquiring and storing heart rate (from the EGM) and RVP signal information. The RVP signals are processed by the microprocessor to estimate SV on a beat-by-beat basis when cardiac output monitoring is enabled. The implantable cardiac monitoring device is equipped with telemetry circuitry for communicating with an external programmer. RVP data or estimated SV data in digital units along with heart rate and any other relevant 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 computing an actual flow value using patient-specific calibration, [0005] In an associated method for estimating SV from the RVP signal, landmark points are identified on the RVP contour during the systolic time interval. The start of the systolic time interval is identified by detecting an R-wave event. An analysis window is set upon detecting an R-wave event. The analysis window duration is a predetermined interval set to include the systolic time interval. The RVP signal is acquired and stored in a memory buffer during the analysis window to allow identification of landmark points and computation of the estimated SV. [0006] In one embodiment, the landmark points identified for use in computing an estimated SV are the peak RVP and the estimated pulmonary artery diastolic (ePAD) pressure, which is equal to the RVP signal amplitude at the time of the maximum rate of rise in RVP (dP/dt max). A measure of estimated SV is computed as the difference between peak RVP and ePAD. [0007] In another embodiment, the area under the pressure pulse is approximated using polygonal (e.g. triangular, rectangular, or trapezoidal) shapes whose vertices or sides are defined by various landmarks of the RVP pulse. The vertices and sides of the polygons can be defined by the values or times of occurrence of dP/dt max, dP/dt min, a single global or multiple local pressure peaks during systole, ePAD, RVP at dP/dt min, RVP where the descending RVP contour equals the mean PA pressure, and inflection points between multiple local peaks that sometimes occur on the RVP waveform. There may be other landmarks on the RVP waveform that are useful, besides those listed above. [0008] In yet another embodiment, a pulse contour integral is computed as an estimate of SV. Landmark points are identified on the RVP signal for use as an integration start time and an integration end time. The integration start time corresponds to the start of the systolic ejection interval which is marked by the opening of the pulmonic valve. In one embodiment, integration start time is identified as the time of dP/dt max or ePAD. The integration end time corresponds to the end of the systolic ejection interval, marked by closure of the pulmonic valve. In one embodiment, integration end time is identified as the time that the descending RVP contour equals ePAD. An area defined by the RVP contour during the integration time interval is computed and may be used to derive an estimate of SV. [0009] In some embodiments, the estimated SV computed as the area under the RVP contour during the integration time interval is corrected by a number of correction areas. Correction areas are computed based on landmark points on the RV pressure signal. Correction areas are used to compute a pulse contour integral (PCI) that more accurately represents the pulmonary artery flow contour and accordingly provides a more accurate estimate of SV. [0010] An estimated SV can be used to compute a measure of cardiac output (CO) by dividing the estimated SV by the R-R interval (RRI) on a beat-by-beat basis. The estimated SV and CO will be in digital units. These digital values may be used by the IMD in closed loop control methods for managing device-delivered therapies. SV and CO estimates computed in digital units may be uplinked to an external device and converted to actual volume and flow units using patient-specific calibration values. [0011] In some embodiments, additional hemodynamic monitoring parameters are estimated from the right ventricular pressure signal including mean pulmonary artery pressure (MPAP) and pulmonary vascular resistance (PVR) to provide a set of meaningful hemodynamic diagnostics for use in managing cardiac disease. [0012] Practice of the present invention for estimating stroke volume and cardiac output from a ventricular pressure signal is not limited to using the RVP signal. Use of the RVP signal allows estimation of pulmonary artery pressures at various time points in the cardiac cycle which further allows estimation of flow. However, a left ventricular pressure (LVP) signal may be obtained for use in estimating aortic pressures at various time points in the cardiac cycle which may be further used for estimating flow. Since flow from the left and right ventricles is equal over time, estimation of cardiac output from either left or right ventricular pressure signals can be used. As such, various embodiments of the present invention include approximating an estimated stroke volume as an area under the left ventricular pressure pulse using polygonal shapes whose vertices or sides are defined by various landmarks of the LVP pulse, or computing a pulse contour integral as an estimate of SV based on an integration start time and an integration end time derived from landmark points on the LVP signal. BRIEF DESCRIPTION OF THE DRAWINGS [0013] FIG. 1 is an illustration of an exemplary implantable medical device (IMD) configured to monitor a patient's heart. [0014] FIG. 2 is a block diagram summarizing the data acquisition and processing functions for the IMD shown in FIG. 1. [0015] FIG. 3 is a flow chart summarizing a method for computing an estimated SV from a right ventricular pressure signal for use in CO monitoring. [0016] FIG. 4 shows a RVP signal and illustrates one method for estimating SV from the RVP signal. [0017] FIG. 5 illustrates a method for determining an integration interval for use in computing a pressure pulse contour integral in an alternative method for estimating SV from the RVP signal. [0018] FIG. 6 illustrates a method for computing two correction areas, A2 and A3, used in calculating a RVP pulse contour integral for estimating SV. [0019] FIG. 7 illustrates a method for computing an estimated mean pulmonary artery pressure from the RVP signal. [0020] FIG. 8 illustrates the computation of a third correction area, A4, that may be used in computing a RVP pulse contour integral for estimating SV computed. 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