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BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to a system for measuring the ventricular
volume of a human or animal heart by using impedance plethysmography and
more particularly to a method for matching the sensed length of the
impedance catheter to the length dimension of the ventricle.
II. Discussion of the Prior Art
In the Salo U.S. Pat. No. 4,674,518, which is assigned to the assignee of
the present invention, there is described a method and apparatus for
making instantaneous measurements of the ventricular volume by utilizing
an intracavity electrical impedance catheter. That catheter comprises a
tubular sheath having a plurality of spaced surface electrodes (ring
electrodes) disposed sufficiently near the distal end of the catheter so
that those electrodes effectively span the length dimension of the chamber
whose volume is to be measured. To obtain accurate measurements of
ventricular volume, it is imperative that one be able to locate the
particular pair of sensing electrodes located at the transition between
the ventricular chamber and the atrium or aorta, depending upon which
ventricular chamber is involved. When this has been accomplished, the
volume between each of the electrode pairs may be computed from measured
impedance signals and then these volumes may be summed to yield the total
ventricular volume. Because there is large variability in the length of
the left (and right) ventricle over the total patient population, some
means must be provided for matching the total distance spanned by the
surface ring electrodes on the catheter to the length of the ventricle
being measured.
The aforereferenced Salo U.S. Pat. No. 4,674,518 references the published
work of Jan Baan of the Netherlands. In the work leading to those
publications, a number of catheters with electrodes of different spacing
were manufactured and the appropriate catheter selected under fluoroscopy.
Since the length of the ventricle cannot be accurately estimated without
actually introducing a catheter into the chamber, this technique often
required the introduction of more than one catheter. Either a measuring
catheter with radio opaque markers is positioned in the chamber first and
the desired catheter length determined under fluoroscopy or else one of
the impedance catheters is introduced and later replaced if it proves to
be of the wrong length.
Such a trial and error approach at determining appropriate catheter length
is not only time consuming but increases the risk of damage to the blood
vessels through which the catheter is routed in achieving placement in the
ventricle.
OBJECTS
It is accordingly a principal object of the present invention to provide a
novel method for matching the sense length of an impedance catheter to the
length dimension of the ventricular chamber whose volume is to be
measured.
Another object of the invention is to provide a method of sizing a
ventricular impedance measuring catheter to obviate the need for trial and
error replacements.
Yet another object of the invention is to provide a method of matching the
impedance catheter sensed length to a ventricular dimension so that the
same catheter can be used with patients having marked differences in
ventricular length.
SUMMARY OF THE INVENTION
In accordance with the present invention, the tubular catheter in question
includes a sufficient number of surface electrodes guaranteeing that the
total sense length, i.e., the distance between the most distal sense ring
and the most proximal sense ring will span the largest of ventricles. Such
a catheter may then be used in smaller hearts by determining which sense
ring pairs are in the heart chamber and then using only those ring pairs
in the computation of the total ventricular volume. In accordance with one
technique, an alternating, constant-current voltage signal is applied
between the most distal one and a predetermined more proximal one of the
plurality of electrodes, the more proximal one being known to fall outside
of the ventricular chamber. With such a signal applied, the signals
existing between adjacent pairs of sense electrodes located between the
driven electrodes are compared. The location of the particular pair of
electrodes where the characteristics of the signal therebetween differ
from the characteristics of the signals between more distal ones of
adjacent pairs of electrodes mark the transition between the ventricle and
its associated atrial chamber.
DESCRIPTION OF THE DRAWINGS
Further objects, advantages and features of the invention will become
apparent to those skilled in the art from the following detailed
description of the preferred method, especially when considered in
conjunction with the accompanying drawings in which:
FIG. 1 illustrates an impedance measuring catheter located in a heart;
FIG. 2 shows the same catheter arrangement as in FIG. 1 in a schematic
fashion to better illustrate the principles of the invention;
FIG. 3 is a further schematic diagram helpful in understanding the method
of the present invention;
FIG. 4 is a further schematic drawings illustrative of the invention; and
FIG. 5 is a flow diagram illustrating the steps of the method of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, t here is shown a longitudinal section of the heart.
The left atrium and left ventricle are identified by numerals 10 and 12,
respectively, and the right atrium and right ventricle are identified by
numerals 14 and 16. Shown located in the right ventricle is an impedance
measuring catheter 18 whose distal tip electrode 20 is disposed at the
apex 22 of the heart. The catheter 18 passes through the tricuspid valve
24 and through the right atrium 14 exiting the superior vena cava 26. The
distal end portion of the catheter 18 is provided with a plurality of
surface electrodes which are labeled alphabetically from A through L for
ease of reference. These surface electrodes may be conductive rings and
extending through the lumen of the tubular catheter 18 are the electrical
leads (not shown) for conductively connecting each of the electrodes A
through L individually to an electronics module adapted to be coupled to
the proximal end of the catheter 18 exterior to the body. The electronics
module includes switching means which allow a relatively high frequency,
constant current sinusoidal voltage to be applied across predetermined
ones of the plural electrodes. In a typical application, the sinusoidal
driving signal may be applied across the distal tip electrode A and the
most proximal electrode L.
It should also be mentioned at this point that the length of the catheter
spanned by the surface electrodes exceeds the maximum ventricular chamber
length dimension which one would expect to encounter. Thus, if the
catheter 18 were to be used with a pediatric patient, perhaps only surface
electrodes A through F would reside in the right ventricle of a young
heart. With an adult heart, however, such as illustrated in FIG. 1,
surface electrodes A through H clearly reside in the right ventricle.
To avoid the necessity of fluoroscopic exam with a measuring catheter or
trial and error substitution of impedance measuring catheters of differing
numbers of sense electrodes until one fitting the heart in question is
arrived at, in accordance with the present invention, additional surface
electrodes are included on the impedance catheter so that the total sense
length (the distance between the most distal sense ring and the most
proximal sense ring) will more than span the largest of ventricles. Then,
when using the same catheter configuration in smaller hearts, a
determination is made as to which sense ring pairs are in the ventricular
chamber and then only those ring pairs are utilized in the computation of
the total ventricular volume. For example, in carrying out the present
invention, a catheter having a ring spacing of 0.5 centimeters between
electrodes A and B, and 1.0 centimeter spacings between the remaining
electrodes B to C . . . K to L, the distance from the distal tip 20 to the
most proximal sense electrode K can be varied from 4.0 centimeters to 8.0
centimeters as different combinations of electrodes are selected. As such,
this single catheter can accommodate patients with very small hearts and
very large hearts.
Referring next to the schematic diagram of FIGS. 2 and 3, one technique
used to identify and select those electrodes which are in the particular
patient's ventricle will be explained. A source of a constant current
alternating voltage 28 is schematically illustrated as being connected
between the distal electrode A and the most proximal electrode L. It is to
be understood that this a schematic showing in that the signals would
actually be applied to conductive leads running through the lumen or wall
of the catheter body 18 from its proximal end and terminating at the
appropriate electrodes. Likewise, a series of sense amplifiers 30, 32 and
34 have their inputs connected across adjacent pairs of sense electrodes.
Specifically, amplifier 30 is shown as having its inputs coupled across
surface electrodes B and C, sense amplifier 32 having its inputs coupled
across surface electrodes C and D and sense amplifier 34 with its inputs
coupled across electrode pair D and E. With the voltage source 28
operational, and with the patient's heart beating, a time varying voltage
signal will be developed at the output of each of the amplifiers as
illustrated. As is explained in the Salo U.S. Pat. No. 4,674,518
previously cited, the output signals are directly proportional to the
instantaneous impedance measured between the adjacent electrode pairs. In
that the wave shape of the output signals from each of the sense
amplifiers 30, 32 and 34 are of comparable phase and morphology, it is
known that even the most proximal pair of electrodes, D and E, reside in
the ventricle.
Referring now to FIG. 3, there is illustrated a setup similar to FIG. 2
except that sense amplifier no. 3 has been switched so as to be connected
across a more proximal electrode pair G and H. The outputs from sense
amplifiers 30 and 32 still exhibit a similar phase and shape pattern,
reflecting that the electrode pairs with which those two amplifiers are
coupled still reside in the ventricle. It should be further noted,
however, that the output from the most proximal sense amplifier 34 has a
different phase relationship from the signals obtained at the outputs from
sense amplifiers 30 and 32. In that sense amplifier 34 and sense amplifier
32 share a common surface electrode G, it is known that the ventricular
boundary, i.e., the transition between the right ventricle and the right
atrium falls between the two surface electrodes G and H.
Summarizing, then, one technique for locating the most proximal surface
electrode still residing in a ventricle of unknown dimension is to use the
catheter of FIG. 1 positioned in the ventricular chamber whose volume is
to be measured and then selecting the smallest, closest electrode spacing
on the catheter. In FIG. 2, this corresponds to connecting electrodes B
and C to amplifier 30, C and D to amplifier 32 and D and E to amplifier
34, via an appropriate switching means (not shown). Next, the impedance
waveforms from the most proximal electrode pair (electrodes D and E) are
compared to the signals from the more distal electrodes. If all of the
signals exhibit the same phase relationship and a fairly similar
morphology, it is known that all three electrode pairs reside in the
ventricle. Next, via the aforementioned switching means, amplifier 34 is
successively "walked" up the catheter and at each step the output from the
sense amplifier 34 is compared to the others until the waveform for the
most proximal selected ring pair no longer matches the other two
waveforms. At this point, it is known that both rings of the most proximal
ring pair no longer reside in the ventricle. It is also apparent, then,
that the surface electrode or rings which remain in the ventricle have
been identified.
To avoid the necessity of switching electrode pair outputs to one or more
of the sense amplifiers 30, 32 and 34, an alternative approach is to tie
each electrode pair to its own sense amplifier. Then, when all of the
output waveforms are compared to the most distal one or two waveforms, the
location of the transition between the ventricle and the atrium is
identified. The surface electrodes then identified as being located within
the ventricle can be used in performing the ventricular volume
measurement.
Another approach to identifying the sense length of the ventricular chamber
to be measured is to compute the covariance between the reference signals
(i.e., those signals from the distal ring pairs) and the signals from more
proximal ring pairs. The covariance or cross-correlation between two
measurements, x and y, is defined by:
.sigma..sub.xy =.rho..sub.xy /.sigma..sub.x .sigma..sub.y
where
.rho..sub.xy =correlation coefficient between x and y
.sigma..sub.x, .sigma..sub.y =standard deviations of x and y
A covariance less than some empirically determined threshold would be taken
to indicate that the proximal waveform differs too much from the reference
waveforms and that ring pair would be determined to be outside of the
ventricle. Investigations have shown that a covariance of approximately
0.2 can be used as a threshold.
Alternatively, the signals from the several electrode pairs may be compared
to another physiological signal which is indicative of ventricular
contraction (i.e., ventricular pressure). The phase relationship between
an impedance signal and the other indicator will be different outside of
the ventricle as opposed to within the ventricle. Since the distal
electrode pairs are assumed to be within the ventricle, the most distal
electrode pair which shows a new phase relationship will define the
ventricular boundary. To obtain a pressure measurement, the catheter 18 is
provided with a pressure sensing port 36 communicating with a lumen which
is filled with an appropriate fluid such that pressure variations in the
ventricle are transmitted to a suitable pressure transducer located at the
proximal end of the catheter 18.
Referring next to FIG. 4, a still further approach for identifying the
transition or ventricular boundary is to compute the average resistance or
volume and the variation in each of these values measured between each
electrode pair, from most distal to most proximal, on the catheter 18. As
is illustrated in FIG. 4, a fairly dramatic transition in average volume
occurs at the boundary. A plot of average resistance would show the
opposite relationship to that for volume illustrated in FIG. 4, since
volume is inversely proportional to resistance. In addition, the
resistance (or volume) changes during contraction are smaller in the right
atrium than in the right ventricle. An even more pronounced difference in
the extent of resistance or volume changes during contraction occurs when
the catheter is positioned in the left ventricle with a portion thereof
extending into the aorta.
To more clearly explain the method of the present invention, reference is
next made to the method or process flow diagram of FIG. 5. As indicated in
the flow diagram of FIG. 5, the first step in the method of the present
invention for matching the sense length of an intracardiac impedance
catheter to the actual dimensions of a ventricular chamber (step A)
includes providing a multi-electrode impedance catheter of the type
described herein and inserting same into the ventricular chamber to be
measured such that the spaced surface electrodes thereon span a distance
known in advance to be greater than the expected length dimension of the
ventricular chamber of an adult male. That catheter is routed through the
patient's vascular system in a known fashion in such a way that the distal
end of the catheter resides in the apex of the ventricular chamber whose
volume is to be subsequently measured.
In accordance with step B, an alternating, constant-current, voltage signal
is applied between the most distal one and a predetermined more proximal
one of the plurality of surface electrodes, the more proximal electrode
chosen being known to fall outside of the ventricular chamber in question.
In step C, voltage signals appearing between one or more adjacent pairs of
sense electrodes located sufficiently distal so as to be known to be in
the ventricle are sensed. Such signals are due to the near field effects
caused by the application of the drive signal in step B.
At the same time that step C is carried out, further signals developed
between a more proximally located pair of electrodes is sensed. The
signals sensed in steps C and D are compared in terms of phase and
morphology (step E) and then a determination is made as to whether such
comparison results in a match.
If a match condition prevails, it is established that the proximally
located pair of electrodes used in step D fall within the ventricular
chamber. Step F is then carried out whereby a next pair of electrodes more
proximal than those previously used in carrying out step D are selected
and steps D, E and F are repeated until the "MATCH-?" test fails. With the
particular pair of more proximal electrodes resulting in signals of
differing characteristics from those across surface electrodes known to be
in the ventricle established, the impedance plethysmography method of
measuring the ventricular volume can be carried out using only those sense
electrodes determined by the present invention to be resident in the
ventricular chamber in question.
As already mentioned, the algorithm set forth in the flow chart of FIG. 5
is only one particular way of identifying the sense length of an impedance
measuring catheter disposed within a ventricle. Another method already
described includes the use of a separate sense amplifier for each adjacent
pair of surface electrodes and then observing the outputs from all of the
sense amplifiers to note the one whose output differs from the more distal
pairs. A still further method involves determining the covariance between
distally located surface electrodes known to be in the ventricle and more
proximal electrodes whose location relative to the ventricle is uncertain
to determine if the covariance exceeds a pre-established threshold valve.
Moreover, the impedance sensing technique with electrodes known to be in
the ventricle can be compared in phase to another ventricular physiologic
signal related to chamber mechanics, such as ventricular pressure. Then,
when more proximal impedance measurements are taken and compared in phase
to the intraventricular pressure, the location where a phase change is
noted is indicative of the ventricular boundary.
This invention has been described herein in considerable detail in order to
comply with the Patent Statutes and to provide those skilled in the art
with the information needed to apply the novel principles and to construct
and use such specialized components as are required. However, it is to be
understood that the invention can be carried out by specifically different
equipment and devices, and that various modifications, both as to the
equipment details and operating procedures, can be accomplished without
departing from the scope of the invention itself.
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