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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for detecting
myocardial ischemia.
Myocardial ischemia can be defined as a decreased supply of blood to the
heart, and more precisely as an imbalance between the myocardial oxygen
supply and demand. In most clinical situations, the reason for this
imbalance is inadequate perfusion (blood injection) of the myocard (muscle
tissue of the heart) due to obstructions or stenosis (a narrowing) of the
coronary arteries (the arteries that supply blood to the heart). The
ischemia can last only a few seconds or it can persist for minutes or even
hours, causing transient or permanent damage to the heart muscle
(myocardial infarction). Myocardial ischemia is usually accompanied by
chest pain (angina). In some cases, however, it is not accompanied by
pain, or the subject is not aware of the pain, for example, when the
subject is unconscious, and therefore detection of the ischemia must be
made by objective methods rather than by relying on complaints of the
subject.
The most commonly used objective criteria for ischemia detection and
monitoring are the electrocardiographic (ECG) changes at rest or during
effort testing. Ischemia can be demonstrated by the elevation or
depression of the S-T segment, by inversion or other changes in T-waves,
or by changes in the shape or width of the QRS complex. However, sometimes
electrocardiographic changes are not detected because the appropriate
electrocardiograph lead (of the 12 commonly used leads) is not being
monitored. At other times, the electrocardiograph is too sensitive and
reflects changes that have no real significance.
For these reasons, methods other than use of the ECG, are employed to
detect myocardial ischemia. These other methods include:
a. Hemodynamic Changes Associated with Ischemia--subject's blood pressure.
Blood pressure can therefore also be used for continuously monitoring for
myocardial ischemia. This method is commonly used in operating rooms; and
it is good cardiac anesthesia practice to prevent increases and decreases
of blood pressure as much as possible. However, changes in blood pressure
can result from pain or from other reasons; and therefore, changes in
blood pressure alone are unreliable as the primary indicator of ischemia.
Another commonly used hemodynamic parameter is the pressure in the left
atrium. This parameter can be monitored indirectly, for example, by using
a Swann-Ganz catheter which measures the pulmonary-capillary wedge
pressure that is usually equal to the left atrial pressure. Left atrial
pressure can also be measured directly after open heart procedures through
a catheter introduced into the left atrium. In catheterization
laboratories, the left ventricular end diastolic pressure (LVEDP) can be
measured through a catheter introduced through the aorta. Changes in left
atrial pressure usually reflect changes in LVEDP, and ischemia is usually
associated with increased LVEDP. Because of the highly invasive nature of
the pressure measurements of the left atrium, pulmonary-capillary wedge,
or of the left ventricle, these methods are used only in special
situations. It is also important to note that ischemia is not always
associated with increased LVEDP.
b. Two-Dimensional Echocardiography--Important changes in ventricular wall
motions or in ventricular dimensions are associated with ischemia.
Two-dimensional echocardiography, using external transducers, can detect
increased left ventricular end diastolic and end systolic volume. A
trans-esophageal echocardiographic transducer allows continuous detection
and monitoring of changes in ventricular wall motion, and therefore also
enables monitoring of ischemia.
c. Radionuclide Ventriculography--Injection of radioactive marker (Tc-99n
phyrophosphate stanus) that adheres to the myocardial muscle provides a
method for monitoring changes in ventricular wall motion, and therefore
also enables detection of ischemia. This method for the non-invasive
detection of ischemia is used during rest and effort tests.
d. Thalium 201H perfusion scans provide a further method for the selective
and non-invasive monitoring of the blood supply to the heart. Although
radionuclide ventriculography and Thalium perfusion scans can detect
ischemia, they both involve large and expensive instruments, and therefore
these methods are not commonly used for monitoring of ischemia.
An object of the present invention is therefore to provide a new method and
apparatus for detecting myocardial ischemia.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a
method for detecting myocardial ischemia in a subject comprising
monitoring the systemic vascular resistance of the subject, and detecting
when the systemic vascular resistance increases by at least 60% to thereby
indicate the presence of myocardial ischemia.
The systemic vascular resistance (SVR) of a subject is the total peripheral
resistance (TPR) of the subject's cardiovascular system. Measurements of
the subject's systemic vascular resistance, together with other
measurements, are commonly made in order to assess the status of the
subject's cardiovascular system, particularly in monitoring post-operation
recovery of patients. Some of the other measurements presently made in
assessing the status of the subject's cardiovascular system include the
mean arterial pressure (MAP), central venous pressure (CVP), and cardiac
output (CO). All these measurements have the following relationship:
MAP-CVP=CO.times.SVR
This relationship will be recognized as the cardiovascular equivalent of
Ohm's law of electricity, E (voltage)=I (current).times.R (resistance).
A number of techniques are known for measuring systemic vascular
resistance. Particularly good results have been obtained when the method
described in our U.S. Pat. No. 4,429,701, the disclosure of which is
incorporated herein, in its entirety, by reference, is used. This method
is broadly characterized by the following steps:
A. detecting the arterial pressure of the subject and generating in
response thereto a blood pressure signal having a waveform in accordance
with the detected arterial pressure;
B. differentiating the blood pressure signal to produce a dP/dt signal
having a waveform varying in accordance with the rate at which the blood
pressure signal varies;
C. detecting the peak of the dP/dt signal to determine the peak dP/dt;
D. determining a value which is substantially equal to the arterial
pressure at the time of said peak dP/dt; and
E. dividing said latter value by the peak dP/dt signal, which thereby
produces a measurement corresponding to the systemic vascular resistance
of the cardiovascular system.
While we have found that an increase of at least 60% in the systemic
vascular resistance is strongly indicative of myocardial ischemia, we have
also found that most cases of myocardial ischemia are accompanied by an
increase of at least 100% in the systemic vascular resistance, usually
between 100-200%, but sometimes up to 400% and even more.
We have found that myocardial ischemia can be detected accurately when
using only one channel, rather than the two channels described in the
above-cited patent specification. We have also found that the radial
artery is too sensitive, and that best results are obtained when using a
centrally located artery, preferably the femoral artery.
Our invention also provides apparatus for detecting myocardial ischemia in
accordance with the above method.
According to a further aspect of the invention, there is provided apparatus
particularly useful in detecting myocardial ischemia in accordance with an
invasive technique for practicing the above method. The apparatus employs
a flexible catheter tube, which can be inserted into the artery of the
subject, and a micromanometer (pressure transducer) embedded in the outer
face of the catheter tube wall. Best results have been obtained when the
outer face of the embedded micro-manometer is directly exposed to the
blood in the artery, with the inner face of the embedded micro-manometer
covered by the inner face of the catheter tube wall, and when the
micro-manometer is embedded in the distal tip of the catheter tube. There
may also be used a balloon dilatation catheter receivable through the
catheter tube during percutaneous coronary angioplasty (PTCA).
While such an invasive technique has been found to produce best results, it
is conceivable that a non-invasive technique, such as by using pressure
cuffs, may also be used for monitoring the systemic vascular resistance in
order to detect myocardial ischemia in accordance with the above method
and apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention will be apparent
from the following description, with reference to the accompanying
drawings, in which:
FIG. 1 is a block diagram illustrating a preferred apparatus constructed in
accordance with the invention for detecting myocardial ischemia;
FIG. 2 illustrates a flexible catheter tube particularly useful with the
apparatus of FIG. 1; and
FIG. 3 is an enlarged fragmentary view illustrating the use of a coronary
balloon dilatation catheter with the catheter tube of FIG. 2.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIG. 1, a system 1 monitors the systemic vascular resistance
of a subject in accordance with the method and apparatus described in U.S.
Pat. No. 4,429,701 which is incorporated herein by reference. Briefly, the
system includes an arterial pressure detector 2 detecting the
blood-pressure (P) of the subject and generating in response thereto a
blood-pressure signal having a waveform in accordance with the detected
arterial pressure; a differentiating circuit 4 differentiating the
blood-pressure signal (P) to produce a signal (dP/dt) having a waveform
varying in accordance with the rate at which the blood-pressure signal (P)
varies; a peak detector circuit 6 detecting the peak of the dP/dt signal
and producing a corresponding signal (peak dP/dt); a circuit 8 for
determining a value (P.sub.P) which is substantially equal to the arterial
pressure at the time of the peak dP/dt signal; and a divider circuit 10
for dividing the latter value (P.sub.P) by the (peak dP/dt) signal. The
output of circuit 10 is a value which corresponds to the systemic vascular
resistance (SVR) of the subject's cardiovascular system. The systemic
vascular resistance so determined is displayed in a display unit 12.
Circuit 8, which determines a value substantially equal to the arterial
pressure at the time of the peak dP/dt signal, can detect the actual
arterial pressure at the time of the peak dP/dt, or can detect merely the
diastolic pressure, since the diastolic pressure is substantially equal to
the arterial pressure at the time of the peak dP/dt. Further particulars
with respect to the apparatus and method illustrated in FIG. 1 for
measuring the systemic vascular resistance are described in the
above-identified U.S. Pat. No. 4,429,701.
As noted above, we have discovered that when the systemic vascular
resistance increases substantially, by at least 60%, it is strongly
indicative of the presence of a myocardial ischemic episode. Accordingly,
the system illustrated in FIG. 1 includes a circuit 14 which determines
when the systemic vascular resistance value output from circuit 10 has
increased by at least 60%, and when this has been found to be the case, an
indicator 16, such as an alarm, is actuated to indicate the probability of
myocardial ischemia.
As mentioned earlier, while a 60% increase in the systemic vascular
resistance is strongly indicative of myocardial ischemia, in most cases
where myocardial ischemia has been found to exist, the systemic vascular
resistance increased over 100%, usually from 100-200%, but sometimes even
as much as 400% or more. The higher the increase, the greater the
probability that a myocardial ischemic episode exists.
FIGS. 2 and 3 illustrate a device particularly useful for the arterial
pressure detector 2 of FIG. 1 for measuring the arterial blood pressure
(P).
The illustrated device includes a flexible catheter tube 20 insertable into
the artery of the subject. A fitting 22 is carried at a proximal end 20a
of the catheter tube 20 for the insertion of a coronary balloon dilatation
catheter 24 (FIG. 3) using a guide wire 26. The wall at a distal tip 20b
of catheter tube 20 has embedded therein a micro-manometer 28, that is, a
pressure transducer for measuring the blood-pressure and for outputting
the electrical signal P corresponding to the detected arterial pressure.
Electrical leads 30 leading from connectors 32 at the proximal end 20a of
catheter tube 20 to micro-manometer 28 are also embedded within the wall
of the catheter tube 20.
Referring to FIG. 3, micro-manometer 28 is embedded in the wall of catheter
tube 20 so that the outer face of the micro-manometer is directly exposed
to the blood in the artery, and so that the inner face of the embedded
micro-manometer is covered by the inner face of the wall of the catheter
tube 20. Thus, micro-manometer 28 directly senses the blood pressure on
the outer face of the micro-manometer, and generates the electrical signal
P representing a precise measurement of the arterial blood pressure, which
electrical signal is transmitted through leads 30 and connectors 32 to the
differentiating circuit 4 of FIG. 1.
The balloon dilatation catheter 24, illustrated in FIG. 3, is used
particularly when there is an obstruction in the coronary artery. When so
used, the flexible catheter tube 20 is first inserted into the artery to
bring its distal end 20b to the ostium of the coronary artery. The balloon
dilatation catheter 24 is inserted through the catheter tube to the point
of the obstruction, whereupon the balloon is inflated to dilate the
coronary artery at the place of the obstruction. During this procedure,
the arterial pressure is continuously detected by micro-manometer 28 and
is used for continuously monitoring the systemic vascular resistance in
accordance with the above-described method as illustrated in FIG. 1. A
precise measurement of the arterial pressure for this purpose is produced
because the micro-manometer 28 is embedded in the outer face of the
catheter tube 20 so as to be exposed to the blood in the artery. This
permits a more precise detection of the blood pressure, since the
micro-manometer is not significantly influenced by the inflation of the
balloon in catheter 24.
An increase by at least 60% in the systemic vascular resistance of the
subject, as detected by detector 14 in FIG. 1, would be strongly
indicative of a myocardial ischemic episode or condition, and would be
indicated by indicator 16 so as to alert the attendant to take the
appropriate action.
Myocardial monitoring and detection is described above with respect to an
invasive procedure, involving heart catheterization or balloon angioplasty
dilatation of the coronary arteries. While this is a preferred procedure,
ischemia monitoring and detection can also be performed in accordance with
the present invention using a non-invasive procedure, such as by the use
of pressure cuffs. Also, while the described technique monitors the
systemic vascular resistance in accordance with the method and apparatus
described in our U.S. Pat. No. 4,429,701, it will be appreciated that
other methods for monitoring systemic vascular resistance can also be
used.
Other variations, modifications and applications of the invention will be
apparent to those practiced in the field and are within the scope of the
following claims.
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