Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-10-19
2003-03-04
Hindenburg, Max F. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S368000, C600S481000
Reexamination Certificate
active
06527728
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates to the field of determining the pulse transit time of a patient or donor where a pulse transit time is measured for pulse waves propagating via the patient's or donor's vascular system and created by his/her heart contractions.
2. Description of the Related Art
A patient's or donor's blood pressure is typically measured by means of an inflatable rubber cuff according to the Riva-Rocci method. This method allows a measurement only at a defined time, at which the pressure of the cuff is varied over a certain period of time. Thus, continuous measurement is limited to time intervals that are determined by the measuring method. A quasi continuous measurement would be associated with a constantly alternating expansion and deflation of the rubber cuff, which would be accompanied by unreasonable stress on the patient.
As an alternative to the non-invasive Riva-Rocci method, there exists a method for determining the pulse transit time, which can also be carried out non-invasively. This method is based on the knowledge that the time that a pulse wave, produced by a heart contraction of a patient or donor, requires to make its way through the vascular system from a first point to a second place is a function of the blood pressure of the person examined. If the time is measured that passes between the occurrence of a heartbeat (detected, for example, by means of an electrocardiograph (EKG)) and the time of arrival of the related pulse wave at an area of the body at a distance from the heart (detected, for example, by an optical sensor on the ear lobe or finger), this pulse transit time represents a direct measure of the patient's or donor's blood pressure. Since the pulse transit time varies from person to person, a calibration by means of an initial Riva-Rocci measurement is necessary. However, a statement on relative changes can be obtained directly from the relative changes in the pulse transit time. The relation between the blood pressure and the pulse transit time is largely linear (Psychophysiology, Vol. 3,86 (1976)). Since one measurement is possible per heart beat, this measuring method represents a semi-continuous blood pressure measurement.
The WO 89/08424 describes a measurement process for determining the pulse transit time by means of an electrocardiograph EKG and an optoelectronic measuring sensor on skin areas with good circulation. However, since the circulation in the skin tissue and thus also the photoelectric profile itself can change over time due to vasomotoric and other adjustments without the blood pressure necessarily having changed, a repeated recalibration should follow the initial calibration according to the Riva-Rocci method, using the measured values of the optoelectronic measuring sensor. In this respect, a constant relationship between the pulse transit time and the blood pressure is assumed for each person. The recalibration serves the purpose of allowing absolute statements about the systolic as well as the diastolic pressure from the photoelectric profiles at later points in time.
Acute emergencies, e.g. during hemodialysis and/or hemofiltration, require careful action. A primary complication during such a hemotherapy is a decrease in blood pressure. The most frequent cause of such an incident is a hypovolemia as a result of an excessively intensive fluid withdrawal. In particular during extracorporeal hemotherapy, it is, therefore, necessary to constantly monitor the blood pressure of a patient or donor in order to recognize possible circulation complications at an early stage.
The EP-A 0 911 044, which is hereby incorporated by reference, describes, among other things, a hemodialysis and/or hemofiltration apparatus, in which a continuous blood pressure monitoring with only a slight negative effect on the patient is made possible by means of a pulse transit time measurement. Using the measurement signal of the pulse transit time, it is possible to recognize critical blood pressure conditions at an early stage and to then inform the staff without delay. If necessary, countermeasures can be carried out automatically on the hemodialysis and/or hemofiltration apparatus, e.g. by infusions or modifying concentrations. This prior art apparatus, like the teaching of the WO 89/08424, assumes a constant relationship between the blood pressure and the pulse transit time. This assumption is not sufficiently accurate in the case of hemotherapies that It change the blood density in particular. In particular due to fluid withdrawal during a hemodialysis and/or hemofiltration treatment, the blood density increases during the course of the treatment (blood density in this case refers to the density of blood as a fluid per se). Since blood density has a direct influence on the pulse wave velocity and thus the pulse transit time, the results are inaccurate measurement values.
SUMMARY OF THE INVENTION
The present invention is based on the technical problem of improving a process and/or a device for determining a patient's or donor's pulse transit time in such a manner that the changes in the blood count are taken into account during the course of time and thus a more precise monitoring of blood pressure is made possible.
According to the teaching of the invention, this problem is solved by means of a process for determining the pulse transit time where a pulse transmit time is measured for pulse waves propagating via the patient's or donor's vascular system and created by his/her heart contractions, in which a value, correlating with the blood density, is determined and then used to calculate from the measured pulse transit time a pulse transit time, for which the influence of blood density is compensated.
The problem is also solved by a device for determining the pulse transit time with means for determining the pulse transit time of pulse waves, which are propagated via the patient's or donor's vascular system and are created by heart contractions, according to which there are means for determining a value, correlating with the blood density, and an evaluation unit that compensates for the influence of blood density on the pulse transit time.
The invention builds on the knowledge that the influence of a variable blood density between the two measurements can be compensated by means of measurements of a value, correlating with the blood density, at the time of a first pulse transit time measurement and at the time of a second pulse transit time measurement. In this manner a compensated first or second pulse transit time can be obtained that is directly comparable with the second or the first pulse transit time, as if it had been measured with constant blood density. In this manner, emergency conditions can be indicated with significantly greater reliability.
The rate at which a disturbance along an elastic, cylindrical, sufficiently long tube spreads in a homogenous fluid, may be expressed (Y. C. Fung, in “Biomechanics Circulation”, 2nd edition, Springer, N.Y., Berlin, 1997, p. 140):
c
=[(
A
/&rgr;)(
dp/dA
)] (1)
where
&rgr;: density of the fluid
A: cross section of the tube
dA: change in cross section
dp: change in pressure in the tube
If equation (1) is assumed to be valid for blood in arteries, this results in equation (2) for blood with a density &rgr;(t
0
) at time t
0
compared to blood with a density of &rgr;(t) at time t at constant blood pressure p(t
0
) for the pulse wave velocity c:
[
c
(
t, p
(
t
0
), &rgr;(
t
))]/[
c
(
t
0
,
p
(
t
0
), &rgr;(
t
0
))]=[&rgr;(
t
0
)/&rgr;(
t
)] (2)
For the pulse transit time PTT, which indicates the passage of the pulse waves at the pulse wave propagation velocity over a defined path L, a similar expression is obtained:
PTT
(
t, p
(
t
0
), &rgr;(
t
))/
PTT
(
t
0
,
p
(
t
0
), &rgr;(
t
0
))=
L/c
(
t, p
(
t
0
), &rgr;(
t
))/
L/c
(
t
0
),
p
(
t
0
), &rgr;(
t
0
))=[&rgr;(
t
)/&rgr;(
t
0
)] (3)
By means
Fresenius Medical Care Deutschland GmbH
Hindenburg Max F.
Jacobson & Holman PLLC
Natnithithadha Navin
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