CA2241218C - Device for determining a characteristic point in the cardiac cycle - Google Patents
Device for determining a characteristic point in the cardiac cycle Download PDFInfo
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- CA2241218C CA2241218C CA002241218A CA2241218A CA2241218C CA 2241218 C CA2241218 C CA 2241218C CA 002241218 A CA002241218 A CA 002241218A CA 2241218 A CA2241218 A CA 2241218A CA 2241218 C CA2241218 C CA 2241218C
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
- A61B5/026—Measuring blood flow
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
- A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
- A61M60/40—Details relating to driving
- A61M60/497—Details relating to driving for balloon pumps for circulatory assistance
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
- A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
- A61M60/10—Location thereof with respect to the patient's body
- A61M60/122—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body
- A61M60/126—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body implantable via, into, inside, in line, branching on, or around a blood vessel
- A61M60/135—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body implantable via, into, inside, in line, branching on, or around a blood vessel inside a blood vessel, e.g. using grafting
- A61M60/139—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body implantable via, into, inside, in line, branching on, or around a blood vessel inside a blood vessel, e.g. using grafting inside the aorta, e.g. intra-aortic balloon pumps
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
- A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
- A61M60/20—Type thereof
- A61M60/295—Balloon pumps for circulatory assistance
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
- A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
- A61M60/50—Details relating to control
- A61M60/508—Electronic control means, e.g. for feedback regulation
- A61M60/515—Regulation using real-time patient data
- A61M60/523—Regulation using real-time patient data using blood flow data, e.g. from blood flow transducers
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
- A61B5/021—Measuring pressure in heart or blood vessels
- A61B5/0215—Measuring pressure in heart or blood vessels by means inserted into the body
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
- A61M2205/00—General characteristics of the apparatus
- A61M2205/33—Controlling, regulating or measuring
- A61M2205/3331—Pressure; Flow
- A61M2205/3334—Measuring or controlling the flow rate
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- Measuring Pulse, Heart Rate, Blood Pressure Or Blood Flow (AREA)
Abstract
Device for determining a characteristic point in the cardiac cycle, which device 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 from the curve of the blood flow rate D(t) in each cardiac cycle the time at which the incisura point lies. The device can be used for activating an intra-aortal balloon pump (IABP).
Description
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 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 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 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 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 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 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 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 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 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.
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 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 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 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 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 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 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 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 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 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.
Claims (10)
1. A method for determining a characteristic point in the cardiac cycle, said method comprising the steps of: (a) calculating a curve of the blood flow rate D(t) in the aorta from a curve of the arterial blood pressure signal P(t); and (b) determining from the curve of the blood flow rate D(t) in each cardiac cycle the time at which a first local minimum occurs in the curve of the blood flow rate D(t) after the beginning of an ejection phase.
2. The method according to Claim 1, which further comprises delivering a signal when the first local minimum is reached.
3. The method according to Claim 1 or 2, wherein the blood flow rate D(t) lies below a threshold value Dd.
4. The method according to Claim 3, wherein Dd is equal to 10% of the blood flow rate when the blood flow rate from the left ventricle has reached its maximum value.
5. The method according to Claim 4, wherein Dd=O.
6. The method according to Claim 1, which further comprises delivering a signal when D(t)=0 after the beginning of the ejection phase.
7. The method according to any one of Claims 2 to 6, wherein the signal activates an intra-aortal balloon pump (IABP) to inflate.
8. The method according to Claim 1, which further comprises delivering a signal at the beginning of the ejection phase.
9. The method according to Claim 8, wherein the signal activates an intra-aortal balloon pump (IABP) to deflate.
10. The method according to any one of Claims 1-9, wherein calculation of the curve of the blood flow rate D(t) in the aorta from the curve of the arterial blood pressure signal P(t) is based on the Windkessel model.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA2579521A CA2579521C (en) | 1995-12-22 | 1996-12-20 | Device for determining a characteristic point in the cardiac cycle |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL1001979A NL1001979C1 (en) | 1995-12-22 | 1995-12-22 | Device for determining a characteristic point in the heart cycle. |
| NL1001979 | 1995-12-22 | ||
| PCT/NL1996/000490 WO1997024690A1 (en) | 1995-12-22 | 1996-12-20 | Device for determining a charasteristic point in the cardiac cycle |
Related Child Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA2579521A Division CA2579521C (en) | 1995-12-22 | 1996-12-20 | Device for determining a characteristic point in the cardiac cycle |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2241218A1 CA2241218A1 (en) | 1997-07-10 |
| CA2241218C true CA2241218C (en) | 2007-05-29 |
Family
ID=38150438
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002241218A Expired - Lifetime CA2241218C (en) | 1995-12-22 | 1996-12-20 | Device for determining a characteristic point in the cardiac cycle |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2241218C (en) |
-
1996
- 1996-12-20 CA CA002241218A patent/CA2241218C/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| CA2241218A1 (en) | 1997-07-10 |
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