Method of determining the stroke volume and the cardiac output o

Details Method of determining the stroke volume and the cardiac output o Method of determining the stroke volume and the cardiac output o

1993-09-20

1995-03-28

Sykes, Angela D.

Surgery

Truss

Pad

128691, 128692, 128713, A61B 5029

Patent

active

054007933

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a method of determining the stroke volume and the cardiac output of the human heart from the pulse-type bloodstream pressure signal derived from the aorta and consisting in each case of a systolic and diastolic period. It is known in practice to calculate the stroke volume and the cardiac output with the aid of a thermodilution determination by injecting cold liquid into the blood stream and measuring it downstream at regular intervals.
In another known method, a pulse contour method or prescription is used to determine the stroke volume V.sub.s and the cardiac output or heart minute volume Q from a pressure signal p(t) measured in the human aorta. In this connection, the stroke volume is the volume of blood ejected by the heart in one contraction or stroke. A typical value is, for example, 70 cm.sup.3, but this quantity can vary from stroke to stroke. The cardiac output is the volume of blood which the heart pumps in a unit time of one minute. In this connection, a typical value is 5 l/min, and this volume can typically vary between 2 and 30 l/min.
In the past relatively simple relationships have been assumed between the measured aorta pressure signal p(t) and the mean aorta flow q(t). In this connection, the starting point is the pressure signal since said pressure can be measured relatively easily and well, but the flow cannot. In practice it has been found that such a relationship or "model" is much more complicated than was first assumed. This has emerged, in particular, in hospitals, where this so-called pulse contour calculation has resulted in frustration in the case of seriously ill patients.
In practice, usually a so-called "Windkessel" or air-receiver model is assumed, the aorta being conceived as a single compliance (FIG. 1b). In this case a windkessel--a container partially filled with liquid and with gas--which can absorb the surges in flow and buffer them in the gas bubble and which is incorporated downstream of a pulsatile pump is meant. The volume of blood ejected by the heart pump is largely received in the aorta or windkessel and partially flows away through the peripheral vascular regions of the various organs which branch off from the aorta. In the period in which the heart pump does not eject a volume of blood, i.e. the diastolic period T.sub.d, the outward flow from the windkessel continues to be fed to the peripheral vascular regions. The aorta does not then contain any gas but the aorta wall is elastic and this elastic vascular wall fulfils the same function as the windkessel.
This concept contains various factors which are neglected. Firstly, the aorta is much longer than wide. In fact, the pressure wave generated by the heart requires a time of 0.1 to 0.3 sec. to reach the end of the aorta and then approximately the same time to return to the heart, whereas the entire expulsion period of the heart (the systolic period or systole, T.sub.s) lasts only 0.2 to 0.4 sec. At the beginning of the output flow, only a small portion of the compliance is therefore available for buffering the outward flow. However, still more important is the fact that, after two times the propagation or transit time of the pressure wave, twice the capacity is available, albeit with a doubling of the initial pressure wave amplitude. In addition, a second neglect which is generally made is that the windkessel is linear, that is to say a doubling of the stored stroke volume is accompanied by a doubling of the pressure. In reality it has been known for a number of years from the work of G. J. Langewouters et al. in J. Biomechanics 17, pages 425-435 (1984) that the relationship between pressure p and volume V is strongly nonlinear in accordance with an arctangent function (FIG. 3a). In addition, said arctangent function is dependent on the age and sex of the patient. Finally, the drainage from the aorta to the peripheral vascular zones is not concentrated at one location but distributed along the aorta. That is to say, it takes a little time before the increas

REFERENCES:
patent: 4632125 (1986-12-01), Webler et al.
patent: 4841981 (1989-06-01), Tanabe et al.
patent: 5183051 (1993-02-01), Kraidin et al.
patent: 5211177 (1993-05-01), Chesney et al.
"The Static Elastic Properties . . . ", Langewouters et al. pp. 425-435, 1984.
"Use of a Simulation Model For . . . ", Brubakk, pp. 697-706, Nov. 1978.
"Estimation of Beat-to-beat stroke . . . ", DeLoskey et al. pp. 707-714, 1978.
"Ecoulements pulses dans les . . . ", Flaud et al., pp. 869-882, Nov., 1974.
"Beat-by-Beat Stroke Volumn from . . . ", Guier et al., Jul. 1974.

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