Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2001-10-01
2003-03-25
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S505000, C600S549000
Reexamination Certificate
active
06537230
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus, a computer system and a computer program for determining a cardio-vascular parameter of a patient by thermodilution measurements.
BACKGROUND OF THE INVENTION
The current state of the art in implementing transpulmonary thermodilution measurement are apparatus for injecting a bolus of thermal indicator into a patient's vena cava superior, and measuring the temperature response at a place of the patient's systemic circulation, e.g. patient's arteria femoralis to determine the Thermodilution Curve, i.e. the temperature response as a function of time. From the thermodilution curve, a schematic example of which is illustrated in
FIG. 1
, wherein the abscissa (time axis)
1
is linear and the ordinate (temperature difference axis)
2
is logarithmic, various cardio-vascular parameters can be derived by using computersystems running computer programs, which implement parameter calculations as disclosed in WO 93/21823, the contents of which are included herein by citation, and as set forth briefly below.
The Cardiac Output CO can be determined by algorithms based on the Stewart-Hamilton-equation:
CO
=
V
L
(
T
B
-
T
L
)
K
1
K
2
∫
Δ
T
B
(
t
)
ⅆ
t
where T
B
is the initial blood temperature, T
L
is the temperature of the liquid bolus, which is used as thermal indicator, V
L
is the thermal indicator volume, K
1
and K
2
are constants to consider the specific measurement setup, and &Dgr;T
B
(t) is the blood temperature as a function of time with respect to the baseline blood temperature T
B
. Thermal indicator can either be colder or warmer with respect to blood temperature. To obtain cardiac output, the area under the thermodilution curve has to be integrated.
Other parameters that can be derived from the thermodilution curve
3
as schematically illustrated in
FIG. 1
include the Exponential Decay or Downslope Time DST, i.e. the time the blood temperature difference &Dgr;T
B
(t) takes to drop by the factor e
−1
, the Appearence Time AT, i.e. the time span between bolus injection IT and first appearence of a noticable temperature difference &Dgr;T
B
(t) and the Mean Transit Time MTT.
The Intrathoracic Thermovolume ITTV and the Intrathoracic blood volume ITBV can be determined as follows:
ITTV=CO·MTT
ITBV=a′·GEDV+b′
wherein a′ and b′ are species-specific constants and GEDV is the Global End-Diastolic Volume, which can be determined as follows:
GEDV=CO·(MTT−DST)
An extravascular thermovolume estimate can be determined as the difference between Intrathoracic Thermovolume ITTV and the Intrathoric blood volume ITBV
ETV=ITTV−ITBV
Extravascular thermovolume correlates, if there is no significant perfusion deffect in the lungs (e.g. pulmonary embolism), closely to the degree of Extravascular Lung Water. However, the clinical value of that measurement has not been shown explicitly yet.
A diagram similar to
FIG. 1
is shown in
FIG. 2
illustrating the problem of a baseline drift of the blood temperature. Again, the abscissa (time axis)
11
is linear and the ordinate (temperature difference axis)
12
is logarithmic. The baseline drift is indicated by baseline
14
, the drift being shown excessive for the purpose of illustration. The schematically shown transpulmonary Thermodilution Curves
13
,
15
with the same, constant Cardiac Output result from different boundary conditions. The first Thermodilution Curve
13
has been determined without the presence of a substantial extravascular thermovolume, whereas the second Thermodilution Curve
15
is broader and exhibits a less pronounced blood temperature peak due to the presence of a substantial extravascular thermovolume. The hatched area
16
illustrates the error of the area under the blood temperature curves
13
,
15
and thus the error of the Cardiac Output determined from each curve due to the baseline drift. It is obvious, that determining Cardiac Output from the second Thermodilution Curve
15
will suffer from a significantly larger error due to baseline drift than determining Cardiac Output from the first Thermodilution Curve
13
.
The object of the present invention is therefore to reduce the error in Cardiac Output determination due to a baseline drift, when a substantial extravascular thermovolume is present, and thus improve accuracy and reliability of determining cardio-vascular parameters by thermodilution measurements.
SUMMARY OF THE INVENTION
In order to accomplish the above mentioned object, the present invention provides an apparatus for determining a cardio-vascular parameter of a patient by thermodilution measurements comprising temperature influencing means for provoking an initial local temperature change in the proximity of a first place of a patient's vascular system thus introducing a travelling temperature deviation to patient's blood stream, further comprising a temperature sensor device for measuring the local temperature of patient's blood at a second place of patient's vascular system downstream of the first place, further comprising a computer connected to the temperature sensor device for recording the patient's local blood temperature measured at the second place as a function of time to determine a thermodilution curve, determining an extravascular thermovolume estimate from the thermodilution curve, determining a new initial local temperature change depending on the thermovolume estimate, controlling the temperature influencing means to provoke the new initial local temperature change in the proximity of the first place, determining an improved thermodilution curve, and determining the cardio-vascular parameters from the improved thermodilution curve.
In order to accomplish the above mentioned object, the present invention also provides a computer system comprising first connection means to connect the computer system to temperature influencing means and second connection means to connect the computer system to a temperature sensor device, and accessing means to acces executable instructions to cause the computer system to control temperature influencing means connected to the computer system to provoke an initial local temperature change in the proximity of a first place of a patient's vascular system, thus introducing a travelling temperature deviation to patient's blood stream, to record the patient's local blood temperature measured by a temperature sensor device at a second place of patient's vascular system downstream of the first place as a function of time to determine a thermodilution curve, to determine an extravascular thermovolume estimate from the thermodilution curve, to determine a new initial local temperature change depending on the thermovolume estimate, to control the temperature influencing means to provoke the new initial local temperature change in the proximity of the first place, to determine an improved thermodilution curve, and to determine the cardio-vascular parameters from the improved thermodilution curve.
In order to accomplish the above mentioned object, the present invention also provides a computer program for determining the cardio-vascular parameters of a patient by thermodilution measurements comprising instructions executable by a computer system to cause the computer system to control temperature influencing means connected to the computer system to provoke an initial local temperature change in the proximity of a first place of a patient's vascular system, thus introducing a travelling temperature deviation to patient's blood stream, to record the patient's local blood temperature measured by a temperature sensor device at a second place of patient's vascular system downstream of the first place as a function of time to determine a thermodilution curve, to determine an extravascular thermovolume estimate from the thermodilution curve, to determine a new initial local temperature change
Burger Thorsten
Pfeiffer Ulrich J.
Mallari Patricia
Nasser Robert L.
Nixon & Peabody LLP
Pulsion Medical Systems AG
Studebaker Donald R.
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