Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-08-11
2001-05-29
Getzow, Scott M. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
Reexamination Certificate
active
06238350
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of analysing a cardiac signal.
Each beat of the heart generates a sequence of electrical waves called an “ecg (electrocardiogram) complex”, which can be monitored and recorded by apparatus such as an electrocardiograph connected to the body by a set of electrodes attached to the skin. Within each complex separate waves traditionally labelled P,Q,R,S and T are recognised as shown in FIG.
1
.
The interval between one heart beat and the next can be measured between corresponding waves in successive complexes, usually that between the R waves and hence is usually referred to as the “RR interval”. Within each complex itself, other intervals can be considered, particularly the “QT interval” from the start of the Q wave to the end of the T wave.
In the course of the day the heart rate varies in response to the demands of the body and this is reflected in constant variation of the RR interval. As the heart rate increases and the RR interval shortens so, to maintain stable conditions within the heart, the QT interval shortens in sympathy and lengthens again as the heart slows and the RR Interval increases.
Because the maintenance of an appropriate balance between RR and QT interval durations is thought to be important in avoiding serious disturbance of the heart rhythm and possible Cardiac Arrest, there is considerable medico-scientific interest in studying the relationship between RR and QT intervals as the heart rate varies throughout the day in response to therapy.
The importance of this relationship stems from its use to estimate from the QT and RR intervals pertaining at the time a patient's ecg was observed, what the QT interval would be at a “standard” RR interval of 1.0 seconds (the so called “corrected” QT interval, “QTc”). From this measure it can be considered whether the patient's QT interval is “normal” or not.
To define this relationship many workers have measured RR intervals and the corresponding QT intervals from ecg complexes obtained from a given individual at different times of the day to obtain data covering a range of RR intervals. These pairs of values are then plotted as points on a graph of QT vs RR such as that shown in FIG.
2
. From the scatter of experimental points so obtained different workers have used statistical techniques to fit the “best” curves to their particular collections of data so as to obtain empirical “laws” from which to predict the QTc interval corresponding to the QT interval observed at a particular RR interval, RRc.
As early as 1920 Dr Bazett declared that for the population of normal individuals a square root formula of the type QT=QTc (RR/RRc) best fitted his data (Curve (a) in FIG.
2
). Others suggested a cube root formula QT=QTc
3
(RR/RRc) (Curve (b)), while still others have advocated a linear approximation QT=QTc+s (RR−RRc) (Curve (c)) where s is the slope of the line.
Whichever formula is selected to represent the “law” relating QT and RR intervals, it can then be used to estimate the “heart rate corrected” value of QT, namely QTc, from the chosen formula, viz:
QTc
=
QT
(
RR
/
RRc
)
1
/
2
.
.
.
Bazett
square
root
formula
or
QTc
=
QT
(
RR
/
RRc
)
1
/
3
.
.
.
Cube
root
formula
or
QTc
=
QT
(
RR
/
RRc
)
x
.
.
.
An
“
xth
root
”
formula
or
QTc=QT−s. (RR−RRc) using the linear formula
By long familiarity and decades of usage, despite frequent criticism of its inaccuracies the square root Bazett “law” is usually assumed, and “corrected” (QTc) intervals calculated to demonstrate that an individual's QT interval has been affected by influences other than heart rate (RR interval) with the passage of time or change in the individual's physical condition or health.
Since the standard RR interval, RRc, is already chosen (1.0 seconds), QTc can be calculated immediately from the observed values of QT, RR, and either s (linear formula), or “x”, be it equal to ½ (square root formula), or ⅓ (cube root formula) or any other value.
Apparatus (herein referred to as a “Basic QT Analyser”) already exists which can accept a continuous ecg signal detect the ecg complexes, identify the component waves and measure the RR intervals and the QT intervals in each complex, thus generating a continuous stream of successive RR intervals each accompanied by its associated QT interval.
These intervals can be output from such analysing apparatus either in digital form as a sequence of coded numbers, or in analogue form as time varying voltage signals representing the RR intervals as the time function RR(t) and the QT intervals as the time function QT(t).
A complication stands in the way of analysis of the relationship between the ever changing RR interval and the QT interval response. It is found that the QT response to a change in RR(t) is subject to a time delay or “QT lag” which is not fixed but depends on the time course of the change in RR(t). This frustrates the estimation of s, the rate of change of QT(t) with change of RR(t).
SUMMARY OF THE INVENTION
From a first aspect, the present invention provides a method of analysing a cardiac signal, comprising continuously or continually generating values RR(t) and QT(t) representing the RR and QT intervals respectively as functions of time, and operating on one or both of said values so as to compensate for the delay between the change in RR interval and the resulting change in QT interval, to produce output values R(t) and Q(t) respectively. A delay may be introduced into the RR(t) value or an advance introduced into the QT(t) value. The values R(t) and Q(t) may be generated either as analogue electrical signals or as discrete numerical values.
Conveniently, the delay or advance to be introduced is modelled by means of a resistor-capacitor network and preferably the network is adjustable. In a preferred embodiment the resistor-capacitor network has two different time constants which are applied in an adjustable ratio.
From a second aspect, the present invention provides a method of analysing a cardiac signal, comprising continuously generating values R(t) and Q(t) representing the RR and QT intervals respectively as functions of time, and continuously generating S(t), the time function of the slope of the graph of QT interval against RR interval, by operating on the continuously generated values R(t) and Q(t).
The method may include the steps of continuously generating values of &Dgr;Q(t) and &Dgr;R(t), respectively representing the change in the value of Q(t) and R(t) over a time interval &Dgr;t, and continuously determining the quotient &Dgr;Q(t)/&Dgr;R(t)=S(t).
Preferably, however, the method includes the step of cross-correlating the values R(t) and Q(t) to determine S(t). In a particular embodiment, the running average of a regression coefficient S is determined over a moving window of a given duration. Optionally, the running average of a correlation coefficient r is also determined.
The signal S(t) then represents the ever changing slope of the QT(t)/RR(t) relationship, freed from the effects of QT lag because of the compensated Q(t) or R(t) function.
Using this method, at every point in time throughout the continuous analysis, a set of three corresponding values R(t), Q(t) and S(t) is available. In order to give a continuous determination of QTc(t), the time function of the corrected QT interval, the method preferably includes the steps of selecting a formula which is assumed to relate QTc(t) to R(t), the “standard” RR interval RRc, Q(t) and S(t) and continuously generating values of QTc(t) by applying the selected formula. If a linear “law” is assumed, the formula will be
QTc(
t
)=
Q
(
t
)+
S
(
t
) [RRc−
R
(
t
)]
Alter
Getzow Scott M.
Howson & Howson
Reynolds Medical Limited
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