Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-03-28
2002-02-19
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S526000, C600S505000
Reexamination Certificate
active
06348038
ABSTRACT:
The present invention relates to an improved method and apparatus for the measurement of cardiac output and in particular to an improved method and apparatus which has a rapid rate of response and good noise rejection.
Cardiac output is an important haemodynamic variable which is defined as the volume of blood that is pumped by the heart per minute.
Blood pressure was first measured in 1750. Since at least 1904 (Erlanger and Hooker, Bull. John Hopkins Hosp. 15:179) it has been suggested that the arterial pulse pressure could be regarded as a rough index to the stroke volume of the heart and, in combination with the heart rate, could provide the cardiac output. This approach was found to be simplistic and has been surpassed by other methods.
Kouchoukos et al. (Estimation of stroke volume in the dog by a pulse contour method, Circ. Res., vol. 26, 5:611-23, 1970) used a method that uses the systolic area to determine stroke volume. The systolic area is the area between the blood pressure and end diastolic pressure during systole. Since 1970 there have been many modifications to the systolic area technique of pulse contour analysis. For example, correction factors such as age, height and weight have been added as well as factors to allow for the changing compliance of the arteries and reflections of the pressure wave from the peripheral circulation.
Even after the correction factors were introduced the results were still not reliable. Currently, pulse contour analysis is not routinely used by clinicians despite the importance of cardiac output. One of the major shortcomings of these methods is their reliance upon measuring morphological features of the blood pressure waveform. In particular, the position of the dicrotic notch, which signifies the closure of-the aortic valve, must be found in order to measure the systolic area. During surgery and intensive care the dicrotic notch may not be detectable or it may be mimicked by other minor waves superimposed upon the pressure waveform.
U.S. Pat. No. 5400793 describes another method for determining the stroke volume from aortic blood pressure in a subject. The method uses a simulation model of the aorta as a transmission line, including a pressure volume relationship for the aorta that is known in the art and supplemented with a Windkessel compliance.
In essence, the pressure recorded in the aorta is used to determine the characteristic impedance of the transmission line model. The simulation is then performed and the parameters of the Windkessel are adapted until the flow calculated in the model is consistent with the pressure in the aorta. The flow is then integrated over the period of systole. Ideally, this method requires a high fidelity transducer positioned in the aorta. Although a method of correcting a pressure measurement in a peripheral artery is mentioned, this method cannot be used with the poor frequency response given by most pressure transducers now routinely in clinical use: in the presence of noise an “anti-resonance filter” cannot recover the information that is lost by the poor quality of these transducers.
None of the aforementioned methods of measuring cardiac output explicitly account for the frequency response of the transducers now in routine clinical use. Lambert et al. (Pressure measurement in diagnostic and therapeutic cardiac catheterisation, eds. Pepine et al., Williams and Wilkins, Baltimore, 283-97) found that with added extension tubes, the response of some measuring systems is accurate (to within 5%) only for frequencies less than 2 Hz.
Finally, Hamilton (The physiology of cardiac output, circulation 8: 527, 1953) suggested that cardiac output could be derived from a patient's blood pressure pulse height following calibration by another device.
It is now accepted in the art that all existing pulse contour methods require calibration for improved accuracy. The present invention is also intended to be used with a calibration device, for example a thermodilution or indicator dilution method. An indicator dilution method is described, for example, in WO93/09427. The method as described in WO93/09427 is highly repeatable and only one single point calibration is required to give the cardiac output. It will be understood, however, that the method of the present invention may be used without calibration in order to show trends in or directions of change of the cardiac output of a patient.
In our co-pending application WO97/24982 we have described an improved method for measuring cardiac output using pulse contour analysis. A non-linear transformation is used to correct for the changing characteristics of the arterial system with pressure and autocorrelation is then used to derive the cardiac output. Although this technique is an improvement over the prior art methods discussed above, there is still a need for a further improved method.
The method described in WO97/24982 was an empirical finding and has given good results in patients undergoing cardiac surgery. The present invention gives results that are numerically similar under normal conditions. However, the frequency response of the pressure measurement system is explicitly accounted for. It is also based upon a stronger theoretical framework which will allow modification to the method to be assessed more easily.
Accordingly, in a first aspect the present invention provides a method for the measurement of cardiac output in a patient, which method comprises the steps of:
(i) recording and storing the arterial blood pressure waveform of a patient from a blood pressure monitoring device over a period of time;
(ii) subjecting the data obtained in step (i) to Fourier analysis in order to obtain the modulus of the first harmonic;
(iii) determining the nominal stroke volume from the modulus of the first harmonic obtained in step (ii) and data relating to the arterial blood pressure and the heart rate; and
(iv) obtaining the nominal cardiac output and/or the systemic vascular resistance from data obtained in step (iii).
Step (ii) above is preferably achieved by identifying a period of the waveform obtained in step (i) that contains at least one beat.
Arteries generally have non-linear properties. The above method assumes that the compliance does not vary significantly within the range of blood pressures that occur during a single beat. The compliance at the corresponding mean arterial pressure is preferably used. Alternatively, the blood pressure may be transformed during an initial step which linearises the blood pressure with respect to the arterial compliance.
In a second aspect the present invention provides a method for the measurement of cardiac output in a patient, which method comprises the steps of:
(a) recording and storing the arterial blood pressure waveform of a patient from a blood pressure monitoring device over a period of time;
(b) subjecting the waveform obtained in step (a) to a non-linear transformation that corrects for the variation of the characteristics of the arterial system with pressure;
(c) subjecting the data obtained in step (b) to Fourier analysis in order to obtain the modulus of the first harmonic;
(d) determining the nominal stroke volume from the modulus of the first harmonic obtained in step (c) and data relating to the heart rate and optionally the arterial blood pressure; and
(e) obtaining the nominal cardiac output and/or the systemic vascular resistance from data obtained in step (d).
In step (b) the non-linear transformation preferably linearises the pressure with respect to the arterial compliance.
Step (c) is preferably achieved by identifying each beat of the waveform obtained in step (b) that contains at least one beat.
In both aspects of the present invention the heart rate may be determined, for example, using an autocorrelation method as described in WO97/24982, Fourier analysis, filtering techniques on the pressure waveform and/or edge detection or any other suitable technique. The same data is preferably used to determine the nominal stroke volume and the heart rate.
In both aspects of the present invention t
Band David Marston
Fox Linton Nicholas William
Fox Linton Robert Anthony
O'Brien Terence Kevin
Bacon & Thomas
Monitoring Technology Limited
Nasser Robert L.
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