Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-05-22
2003-05-27
Hindenburg, Max F. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C128S900000
Reexamination Certificate
active
06569103
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
It 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 IAB 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 experienced 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.
U.S. Pat. No. 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 accompanied 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 U.S. Pat. No. 5,183,051.
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 incisura point, which upsets the functioning of the IABP.
U.S. Pat. No. 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.
SUMMARY OF THE INVENTION
The object of the invention is to provide a device for determining a characteristic point in the cardiac cycle which 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 followed 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 delivering a signal at the moment then 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 flow 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 with 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, Cw, and a peripheral resistance, Rp.
The characteristic input resistance, Rao, represents the flow resistance experienced by the heart. The arterial compliance, Cw, 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 Windkessel 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.
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Hoeksel Sebastiaan Adrianus Alphonsus Petrus
Jansen Josef Reinier Cornelus
Schreuder Johannes Jacobus
Amster Rothstein & Ebenstein
Arrow International Investment Corp.
Astorino Michael
Hindenburg Max F.
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