Surgery – Diagnostic testing – Cardiovascular
Patent
1997-09-12
2000-01-04
Nasser, Robert L.
Surgery
Diagnostic testing
Cardiovascular
600504, A61B 500
Patent
active
060104570
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to the derivation of aortic flow waveforms, using non-invasive techniques, in particular, measurement of pressure waveforms in peripheral arteries.
BACKGROUND ART
It is desirable in a wide range of clinical applications to be able to assess the flow velocity waveform in the ascending aorta. Flow velocity may be defined as average volume per second per unit area of the vessel. Flow velocity may be converted to actual volume per second if the vessel's internal diameter is known or determined. For the purposes of this determination, the velocity profile across the vessel may be assumed to be flat--we are here concerned with overall volume flow and rate of flow.
Flow velocity data is useful, for example, in assessing patients presenting with symptoms of cardiac disease, hypertension, and angina pectoris, and in following the response of these patients to treatment. However, techniques in use for assessing these parameters have not been appropriate for routine use. One known invasive technique utilises a probe inserted into an artery. It is also possible to utilise ultrasonic echo flow techniques, however, this has the drawback of using relatively expensive and complex equipment, and requiring a very high level of skill on the part of the operator to produce reliable results.
In a paper, "Computation of aortic flow from pressure in humans using a non-linear, three element model", J. Appl. Physiol. 74(5):2566-2573, 1993, Wesseling et al disclose a method for computing aortic flow from radial pressure waveforms. The calculations described use a Windkessel type model, and whilst some account is taken of age, no account is taken of wave reflection, the timing of wave reflection, nor the changes in wave reflection or impedance which occur with age.
In a paper by Fry DL, "The measurement of pulsatile blood flow by the pressure gradient technique", IREE Transactions on Medical Electronics 6:259-264, 1959, an analog processing arrangement was used to produce a derived flow wave, with criteria imposed relating to the observed characteristics of the system. In particular, the flow wave at the incisura (identified by a flag) was required to approach zero, or pass from positive to negative within 10 ms of the incisura, and flow during diastole is required to be zero or within 3% of zero compared to peak systolic flow, and to show no systematic increase or decrease during diastole.
SUMMARY OF INVENTION
According to a first aspect the present invention provides a method for determining an aortic flow velocity waveform, comprising the steps of: waveform; and diastolic pressures measured at a site comparable to the peripheral site, and thereby calibrating the waveform of (a); ascending aorta using said calibrated blood pressure pulse waveform and a predetermined transfer function; pulse waveform by reference to predetermined values for age dependant impedance modulus and phase difference between the aortic pressure pulse waveform and the aortic flow velocity waveform.
Preferably, a further step e is performed, in which the calculated flow velocity waveform is examined to determine whether the waveform meets predefined criteria, and if it does not, then the assumed age value is varied in a process of iteration until the waveform does meet said criteria.
Preferably, said predefined criteria include requirements that at the time of incisura, (that is, during diastole) the flow is substantially zero, and that after incisura flow remains beneath a predetermined percentage, say 3%, of peak flow relative to zero.
The latter requirement seeks to ensure that computed values correspond to the reality that after incisura, the aortic valve is shut and there is no driving pressure, and accordingly apart from minor effects no positive or negative flow will occur.
Step d is preferably performed by performing Fourier analysis on the derived pressure waveform, calculating the flow wave components corresponding to each frequency component of the pressure wave separately, and combining the res
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"Peripheral Pulse Contour Analysis in Determining Stroke Volume" by H.A.J. Lasance, K.H. Wesseling, C.A. Ascoop.
Chapter 10: "Effect of the Splanchnic Circulation on the Formation of the Arterial Pulse", by Mustafa Karamanoglu, Stimulation, Measurement and Analysis of the Propagating Pressure Pulse in the Human Arterial System, University of New South Wales, Dec., 1992, pp. 196, 209-214.
Nasser Robert L.
PMV Medical Pty Ltd
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