US 6,128,526 AGranted

Method for ischemia detection and apparatus for using same

AssigneeMedtronic, Inc.

InventorsRobert Stadler, Shannon Nelson, Lee Stylos, Todd J. Sheldon

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Abstract

In determining whether a patient has ischemia or other conditions discernible in the variation occurring in the ST portion of the electrocardiogram signal, we filter out bad ST change parameters that are not changing at a rate representative of human ischemia ST change parameter rates of change. This can be used for driving therapy systems to alleviate cardiac ischemia. This filtering can be enhanced by using multiple cardiac electrical vectors for the electrogram signal vectors, and using a determination of Axis shift to modify filter parameters and the expected ranges of precursors to the ST change parameter (an ST change variable) to eliminate bad cardiac cycles, that is cardiac cycle information that may be misleading.

Claims38

Dependent claims

A method for use in a medical device which reads electrocardiogram signals and finds values associated with changes manifest in an ST segment of said electrocardiogram signal wherein said method filters an ST change parameter found in data extracted from measured samples of said ST segment at each cardiac cycle determined to be valid to determine whether a time course fluctuation of an ST change identified by said ST change parameter is representative of ischemia, said method comprising a repeated cycle comprising the steps:

a) for a given cardiac cycle, collecting a selected set of characteristic portions of the electrocardiogram signal and sampling specific point locations within said portions,
b) parameterizing each said sampled selected portion by calculating a plurality of characteristic parameter values based on measured values of said samples for each said portion collected in step a, wherein a plurality of point values are determined for an ST segment of said selected portion and a point value is determined for an isoelectric point of said selected portion,
c) calculating a current ST change variable value for said given cardiac cycle using said plurality of point values and said isoelectric point value,
d) determining if said current ST change variable value is within a previously determined expected range for said ST change variable wherein said previously determined expected range is based on previously calculated ST change variables and if said current ST change variable is within an expanded expected range beyond the previously determined expected range, said expected range is then updated based on said ST change variable, and said ST change variable is determined fit for use in determining ischemia and added to a series of ST change variables taken over previous cycles to produce an ST change parameter signal,
e) determining whether said ST change parameter is changing in a manner indicative of a physiologic condition by a step of filtering the frequency of said ST change parameter using a bandpass filter and then producing from the output of said bandpas filter a raw physiologic indicator signal and then passing said raw physiologic indicator signal.

2 more elements

Apparatus for filtering ST change parameter information signals present in a cardiac electrogram signal to eliminate consideration of out of range ST parameter values in using said ST change parameter signals for evaluating a physiologic condition and producing a physiologic condition indicator signal, said cardiac electrogram signal being received by said apparatus through a plurality of electrical leads connected to receive said cardiac electrogram signal in a plurality of electrical vectors, said apparatus comprising:

an electrogram reading device connected to said plurality of electrodes for sensing the amplitude variation in the electrical signal of a heart,
a sampling and digitizing circuit for digitizing samples of said cardiac electrical amplitude signal and to provide an output stream of digitized sample point values representative of said samples,
a V-event detection circuit for generating a V-event signal at its output indicating that a ventricular event has occurred,
a buffer circuit for holding a set of point values that are temporally related to a time in a cardiac cycle when said V-event signal is output from said V-event detection circuit,
a fiducial point determining circuit means for reading the set of point values from said buffer circuit for determining which of said point values is a fiducial point related to said V-event signal,
a subset determining and selecting circuit for determining based at least in part on a temporal relative location of said determined fiducial point to said set of point values which subset of said set of point values to select and selecting them,
a parameterizing processor circuit for producing a set of parameter values related to said set of point values selected and determined by said subset determining circuit wherein at least some of the parameters produced are evaluatable by an ST change parameter processor circuit,
an ST calculating circuit means for calculating a current ST change variable value,
an expected range value recalculating circuit for employing said set of parameter values and said pre-calculated expected ranges to produce new pre-calculated expected ranges and values for a next set of parameter values to be compared with a comparison circuit for comparing said pre-calculated expected ranges with each parameter value from said set of parameter values to determine if it is within its expected range and producing a validity value as an output,
a cardiac cycle validity determining circuit for determining if said validity value is sufficient to employ said set of parameter values for determining a physiologic condition and for recalculating said set of expected ranges by said recalculating circuit and for providing a flag value signal representing whether the determination of cycle validity for a cardiac cycle is sufficient,
an evaluation circuit for evaluating said physiologic condition based on running changes in said current ST Change variable value from those cycles determined to be sufficiently valid,
a filter circuit means for filtering said ST Change variable values so as to produce a physiologic indicator signal.

9 more elements

Dependencies are inferred from the claim text.