Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR DETZRMIJING A CHA,RACTERISTIC POINT IN THE
CARDIAC CYCLE
The invention relates to a device for
determining a characteristic point in the cardiac
cycle. Such a device can be used for, for example,
activating an intra-aortal balloon pump (IABP).
An IABP contains, inter alia, an intra-aortal
balloon (IAB), which can be inserted, for example, into
the aorta of a patient with a poorly functioning heart,
and a pumping device.
In each cardiac cycle the IAB is inflated by
means of the pumping device after the end of an
ejection phase of the left ventricle of the heart, and
is deflated again before the commencement of the
following ejection phase.
The pumping action of the heart is improved
in this way, and there is an improvement in the blood
supply to the coronary artery.
For good functioning of the IABP it is of
great importance for the IAB to be inflated and
deflated at the correct times in the cardiac cycle. In
particular, the correct choice of the time at which the
IAB is inflated is of very great importance.
If the IAB is inflated too soon, the pumping
action of the heart is reinforced to a lesser extent,
or the pumping action can even be adversely affected,
because the prematurely inflated TAB causes a flow
resistance in the aorta during the ejection of the left
ventricle which is still occurring at the time.
If the time selected is too late, the
functioning of the IABP is also less effective. A lover
volume of blood is then pumped through the IAB, and the
coronary artery and the vascular bed undergo a high
perfusion pressure for only a short period of time.
The times for inflating and deflating the IAB
can be set manually by an expeLienced person at fixed
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times in the cardiac cycle on the basis of the electro-
cardiogram (ECG) of the heart. A disadvantage of this
is that when there is a gradual acceleration or
slowing-down of the cardiac cycle the set times deviate
increasingly from the desired times, and therefore have
to be reset repeatedly. It is also impossible to make
allowances for an irregular cardiac cycle, and in
particular the setting of the time at which the IAB is
inflated is not performed sufficiently accurately.
The end of the ejection phase and the
accompanying closure of the aortic valve are themselves
indicated accurately by the occurrence of a dip in the
arterial blood pressure signal P(t). This dip is also
known as the incisura point.
US-5,183,051 discloses a device by means of
which an attempt is made to determine the incisura
point by looking for the dip in the curve of the
arterial blood pressure signal P(t) within a previously
defined period of time. However, the period of time may
be incorrectly defined and, besides, the device does
not work in the case of a damped blood pressure signal,
because in that case the incisura point is not
accoaipanied by a clear blood pressure change.
A further disadvantage of this device is that
it is still not possible to make allowance for an
irregular cardiac cycle. while patients in whom an IABP
is used generally have an irregular cardiac cycle.
Moreover, the use of the device for activating an IABP
is not mentioned at all in US-5,183,052.
A device which detects the incisura point in
the curve of the arterial blood pressure signal P(t) is
proposed in IEEE Transactions on Biomedical Engineering
1990, 37(2), pp. 182-192. However, it is possible that
this device may interpret irregularities in the curve
of the arterial blood pressure signal as the incfsura
point, which upsets the functioning of the IABP.
US-4,809,681 discloses a device for
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activating an IABP which determines from the ECG the
point at which the IAB must be deflated. However, it is
not possible to determine the incisura point using the
device.
Sakamoto et al., ASAIO Journal 1995, pp. 79-
83, discloses a device which forecasts the position of
the incisura point in a cardiac cycle from.the ECG by
calculating the length of the ejection phase from the
period of time of the previous heartbeat. This device
is still inaccurate.
The object of the invention is to provide a
device for determining a characteristic point in the
cardiac cycle vhich does not have the above-mentioned
disadvantages.
Surprisingly, this is achieved by the fact
that the device according to the invention comprises
means for calculating the curve of the blood flow rate
D(t) in the aorta from the curve of the arterial blood
pressure signal P(t) and determining the time (ti) at
which the incisura point lies from the curve of the
blood flow rate D(t) in each cardiac cycle.
The time at which the incisura point lies can
be determined instantaneously from the curve of D(t),
so that the moment the incisura point is reached in a
cardiac cycle the IABP can be activated by the device
at precisely the correct moment (in real time).
A further advantage of the device according
to the invention is that only the curve of the arterial
blood pressure need be measured, which is a simple
procedure.
The device according to the invention can be
used for many purposes. For instance, the device is
suitable for use in activating heart-function-
supporting equipment. The device is preferably used for
activating an IABP. It is also possible to use the
device as part of a monitoring system which determines
the duration of the ejection phase of a heartbeat and
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relates the latter to the total period of the
heartbeat. The haemodynamic condition of a patient can
be folloved using such a monitoring system. This can be
carried out starting from the arterial blood pressure
signal or from the pulmonic blood pressure signal.
The device preferably has means for deliver-
ing a signal at the moment when the incisura point is
reached. By means of said signal, the IABP, for
example, is put into operation in order to inflate the
IAB. The means which the device according to the
invention comprises for calculating the curve of the
blood flov rate D(t) from the curve of the arterial
blood pressure signal P(t) can be a calculating device,
for example a computer, a microprocessor or a digital
calculating machine. The calculating device in this
case is loaded vith a calculation program for
calculating the blood flow rate D(t) from the arterial
blood pressure signal P(t).
The calculation program can be based on one
of the models known to the person skilled in the art
for calculating the blood flow rate D(t) from the
arterial blood pressure signal P(t).
Examples of such models are given in IEEE
Transactions on Biomedical Engineering 1985, 32(2), pp.
174-176, Am. J. Physiol. 1988, 255 (Heart Circ.
Physiol.), H742-H753 and in WO 92/12669.
Very good results are obtained if the
calculation program is based on the Windkessel model,
as also described in the abovementioned literature. In
a suitable embodiment the Windkessel model is based on
three elements, namely a characteristic input
resistance, Rao, an arterial compliance, Cv, and a
peripheral resistance, Rp.
The characteristic input resistance, Rao,
represents the flow resistance experienced by the
heart. The arterial compliance, Cv, represents the
ability of the aorta and the arteries to store a
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particular volume of blood as the result of elastic
expansion. The peripheral resistance, Rp, represents
the resistance of the vascular bed.
The values used for the elements in the Wind-
kessel model are known from the literature. Suitable
values are known from, for example, Am. J. Physiol.
1988. 255 (Heart Circ. Physiol.), H742-H753.
Very good results are achieved if account is
taken of the dependence of the elasticity of the aorta
on the current blood pressure, as described in WO
92/12669.
An advantage of the calculation program based
on the Windkessel model is that the values for the
elements in the model used in the calculation do
influence the absolute value of the calculated blood
flow rate, but the position of the incisura point
depends only to a very small extent on the values used
for the elements. It is therefore not necessary to know
the values of the elements very well for a particular
patient in order to obtain good results from the
calculation of the incisura point.
The position of the incisura point can be
determined very accurately from the curve of the blood
flow rate D(t) calculated in this way, while said point
is difficult to determine from the curve of the
arterial blood pressure signal P(t).
zt is therefore possible for the first local
minimum which occurs in the curve of the blood flow
rate D(t) after the beginning of the ejection phase of
the left ventricle to be determined. This point
represents the incisura point.
The minimum in the blood flow rate D(t) can
be determined according to one of the calculation
methods knovn for it. For instance, it is possible in
each case to compare three successive values in the
curve of the blood flow rate D(t) with each other. If
the condition D(t-dt)>D(t)<D(T+dt) is met, the minimum
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is reached at time t. If dt is selected at a
sufficiently low level, the minimum can actually be
detected virtually at the moment when it is reached,
and at that moment a signal can be generated and
supplied to the IABP. It is preferable for dt to be
less than 0.02 second, more preferably less than 0.01
second, and still more preferably less than 0.005
second.
Even more accurate results are obtained if
the occurrence of the local minimum is also subject to
the condition that at the moment when the local minimum
is reached the blood flow rate is situated below a
specific threshold value Dd. This ensures that
reflections in the blood pressure signal which can
occur during the ejection phase of the left heart
ventricle are not detected as the incisura point. The
threshold value can be equal to, for example, 10% of
the value of the blood flow rate, while the blood flow
rate from the left ventricle has reached its maximum
value. The calculation program can be set up in such a
way that the threshold value is recalculated after each
cardiac cycle. The threshold value is preferably
selected so that it is equal to zero, Dd=O. This
threshold value is reached just before the incisura
point occurs, so that the chance of a reflection being
detected wrongly as an incisura point is very small.
In order to obtain an accurate calculation of
D(t) when the latter is just above the zero value, it
is important to know the correct value for Rp. This
value can also be calculated from Rao and Cw using the
Windkessel model, by assuming that the total quantity
of blood which flows into the Windkessel compliance in
a heartbeat, or even taken over a number of heartbeats,
also flows out of it again. Rao and Cv as such can be
estimated accurately for a patient if sex and age are
known.
It is also possible for the incisura point to
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be determined at the time that D(t)=C, after the
beginning of the ejection phase. This ensures that a
signal can already be given to the IABP just before the
aorta valves close, and if an inertia occurs during the
inflation of the IAB, the IAB can be inflated when the
valves are actually closing.
The beginning of the ejection phase of the
left heart ventricle can be determined from the ECG,
from the curve of the arterial blood pressure signal
P(t), or from a combination of the two. The way in
which this can be carried out is knovn to the person
skilled in the art.
The time at which the beginning of the
ejection phase is reached can be transmitted to the
device according to the invention, vhich uses the time
to"activate the device according to the invention for
the calculation of the next incisura point. A signal
for deflating the IABP can also be supplied at that
time to the IABP.
A yet further improved device according to
the invention is obtained if the device has a filter
for filtering high-frequency noise out of the blood
pressure signal.
This ensures that irregularities in the curve
of the blood pressure signal are filtered out, vith the
result that the chance of the device detecting the
incisura point at an incorrect time is reduced even
further. This is important in particular if the device
has to function in an environment where its proper
functioning can be interfered with by the frequent
occurrence of electromagnetic waves.
The device according to the invention is
preferably connected to a pressure recorder for
measuring the arterial blood pressure, which pressure
recorder is attached to the IAB. In this way a blood
pressure signal P(t) is supplied to the device and is
measured in the aorta, directly behind the heart. This
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reduces even further the chance of the blood pressure
signal having irregularities which are incorrectly
identified by the device as an incisura point. Another
advantage of this is that the measured pressure signal
is slowed down little, if at all, but properly reflects
the current stage of the cardiac cycle.
The invention is explained in greater detail
with reference to the drawing, without being restricted
thereto.
Fig. 1 gives an example of the curve of a
measured arterial blood pressure P(t). P(t) is plotted
in millimetres mercury pressure (mmHg) on the y-axis,
and the time t is plotted in seconds (sec) on the x-
axis.
Fig. 2 shows a diagram of a Windkessel model.
Fig. 3 gives a diagram of a calculation
program f or calculating the curve of the blood flow
rate D(t) from the curve of the arterial blood pressure
signal P(t), based on the Windkessel model of Fig. 2.
Fig. 4 gives the curve of the blood flow rate
D(t) calculated from the curve of the measured arterial
blood pressure signal P(t) according to Fig. 1, using
the calculation program from Fig. 3. D(t) is plotted in
millilitres per second (mi/sec) on the y-axis, and the
time is again plotted on the x-axis.
In Fig. 1 the time a in the curve of the
arterial blood pressure signal P(t) is the point at
which an ejection phase of the left ventricle begins.
This is the time at which the IAB must be deflated.
Further, at this time a signal can be supplied to the
device according to the invention, in order to start up
a calculation cycle for determining the typical point
in the cardiac cycle. The time a can be determined from
the ECG, from the arterial blood pressure signal P(t),
or from both.
The time b is the time at which the incisura
point is reached and the heart valves close. This is
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the time at which the IAB must be inflated. It can be
seen clearly that the incisura point is manifested only
in the form of an unsharp local minimum in the curve of
the arterial blood pressure signal P(t).
At the time a' the cardiac cycle ends, and
the ejection phase of the following cardiac cycle
begins. The time interval a-b is also known as the
systolic phase. The time interval b-a' is also known as
the diastolic phase.
Fig. 2 gives a diagram of a simple Windkessel
model containing the following elements: a
characteristic input resistance, Rao (1), an arterial
compliance, Cw (2), and a peripheral resistance, Rp
(3). Further, the aorta valves are modelled by means of
an. ideal diode (4) which closes after D(t) becomes
negative in a cardiac cycle.
Fig. 3 gives a diagram for a calculation
program for calculating the incisura point.
In step 1, indicated by (1) in Fig. 3, the
beginning of the ejection phase (point a) is detected,
for example from the ECG. So long as the beginning of
the ejection phase has not yet been detected, D(t)=0 is
assumed. If the beginning of the ejection phase is
detected, one proceeds to step 2 (2).
During step 2 the curve of the blood flow
rate D(t) is calculated from the measured pressure
signal P(t), for example by means of the equation:
(1 + Rao/Rp).Iao + Rao.Cw.Iao = Pao/Rp + Cw.Pao
(lao and Fao are the first-order derivatives according
to time of lao and Pao)
As soon as D(t)<O the program starts to look
for the first local minimum in the D(t), for example by
in each case comparing a series of at least three
successive points in the curve of D(t).
When the first local minimum, the incisura
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point, is reached, a signal is sent to the IABP, and
step 1 (1) starts again.
From the curve of the blood flow rate in Fig.
4, calculated from the curve of the arterial blood
pressure signal according to Fig. 1 by means of the
calculation program from Fig. 3, the incisura point can
be seen clearly as a sharp local minimum.
The diode (4) from Fig. 2:nakes D(t) equal to
zero during the diastolic phase.
