HK1008726B - Device for determining the fill level of a blood circulatory system - Google Patents

Description

The invention relates to a device for determining the filling state of a blood circulation, in particular the global end-diastolic heart time volume GEDV, the intrathoracic blood volume ITBV, the pulmonary blood volume PBV, the extra-vascular pulmonary fluid volume EVLW and/or the global cardiac function index CFI, whereby the intrathoracic thermovolume ITTV and the pulmonary thermovolume PTV are determined by thermodilution.

In the intensive care unit diagnosis and treatment of critically ill patients, the cardiac time volume CO and the circulatory volume are important parameters.

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The thermo-dying process has the disadvantage of requiring the injection of a relatively expensive dye, and the light and heat sensitivity of the dye means that the injectable dye solution must always be prepared for injection, i.e. at least once a day.

The fibre optic thermistor catheter used in this method is very expensive as a single-use product and the associated measuring apparatus is relatively expensive due to the use of a reflection photometer to measure the concentration of the dye.

The fibre optic dye measurement can also be affected by problems with optical measurement, for example by deposition on the optical eye of the measuring catheter, dislocation of the measuring catheter, etc. Finally, due to the size of the fibre optic thermistor catheter, the thermo dye dilution measurement is only possible in the femoral artery.

The purpose of the invention is therefore to specify a device of the type described above which can determine the filling state of a blood circulation without dye dilution only by means of thermal dilution.

This task is solved by the training specified in claims 1 and 2 respectively.

Particularly preferred designs and further training of the device of the invention are the subject of claims 3 to 10.

The device of the invention is thus based on deriving a parameter from the thermodilution curve representing the sum of smaller mixing volumes without the largest mixing volume. These volumes essentially correspond to the end-diastolic cardiac volumes. The corresponding parameter is therefore simply called global end-diastolic cardiac volume (GEDV). The GEDV is obtained, for example, from the difference between total mixing volume ITTV and the largest mixing volume PTV or as a function of the time of onset AT. In conjunction with other results of the evaluation of the thermodilution curve and specific relationships, further parameters for the assessment of the circulatory system can be obtained.

The following illustration describes in more detail the known thermo-dying process and particularly favourable examples of the device of the invention. Fig. 1 a schematic representation of a conventional measuring device for the thermo-dye process,Fig. 2 a schematic representation of the various values to be determined according to the state of the art,Fig. 3 the characteristic course of the dilution curves used to determine the desired volume values, in particular the thermodilution curves, andFig. 4 a schematic representation of the various values to be determined with the device of the invention.

In practice, the thermo-dying procedure involves injecting a chilled Indocyanate green solution into the central venous system of a patient. Since the Indocyanate green dye immediately binds to plasma proteins, it remains in the blood vessel system (intravasal marker) at least during the first heart-lung passage. The cold is first transported analogously to the dye from the right ventricle of the heart into the lungs.

The arterial thermodilution curve TD is used to calculate the cardiac time volume C.O.TDart, the apparent time AT, the mean transit time MTDTDart and the exponential waste time DSTTDart. The arterial dye dilution curve DD only determines the time parameters MTTDDart and DSTDDart. These curves are shown in Fig. 3 of the corresponding figure.

The standard Stewart-Hamilton formula is used for calculating the cardiac time volume CO in its adaptation for thermodilution.

The human cardiopulmonary system is a series of individual mixing volumes with respect to the dilution of the indicator, as shown schematically in Fig. 2. The total volume passed through an indicator is calculated from the product of the cardiac time volume C.O. and the mean transit time MTT. The volume of the largest mixing chamber is given by the product of C.O. and DST.

The arterial thermodilution curve can be used to determine the intrathoracic thermal volume ITTV and the pulmonary thermal volume PTV, since the lungs are the largest volume of mixture for the cold of the injection: $\begin{array}{c}\begin{array}{c}{\text{ITTV = C.O.}}_{\text{TDart}}{\text{x MTT}}_{\text{TDart}}\\ {\text{PTV = C.O.}}_{\text{TDart}}{\text{x DST}}_{\text{TDart}}\text{.}\end{array}\end{array}$

The arterial dye dilution curve can be used to calculate the intrathoracic blood volume ITBV and the pulmonary blood volume PBV, since the largest volume of the dye is in the pulmonary vascular system: $\begin{array}{c}\begin{array}{c}{\text{ITBV = C.O.}}_{\text{TDart}}{\text{x MTT}}_{\text{DDart}}\\ {\text{PBV = C.O.}}_{\text{TDart}}{\text{x DST}}_{\text{DDart}}\text{.}\end{array}\end{array}$

The extra-vascular pulmonary fluid volume (EVLW) can be equated to the extravascular cold-spread volume (EVV) in the lungs based on the literature. $\begin{array}{c}\begin{array}{c}{\text{EVLW = ETV}}_{\text{MTT}}\text{= ITTV - ITBV}\\ {\text{und EVLW = ETV}}_{\text{DST}}\text{= PTV - PBV.}\end{array}\end{array}$

A conventional measuring system for the thermo-dying process is shown in Fig. 1. For example, such a measuring system is marketed under the name COLD Z-021 manufactured by Siemens AG, Erlangen. For further details, refer to the relevant data sheet. As shown in Fig. 1, in addition to the above-mentioned COLD Z-021 system 1, the entire measuring system includes a thermodyl injector 2, a temperature sensor 3, a pulmonary arterial catheter 4, a catheter separator 5, an optical module 6 and a femoral fiber optic thermistor catheter 7.

The patient is usually placed with a pulmonary catheter and a femoral artery fiber optic thermistor catheter. The refrigerated dye solution is injected either by hand or by an injection machine after a measurement on the measuring device has been started by pressing a button. During the injection, the temperature of the injected product is measured by the injection temperature sensor. The measuring device registers the dye and thermodynamic curves by the connected fiber optic thermistor catheter placed arterially in the patient. The above methods determine the WST values for C.O., PTV, PTVB, PBV, ITTV and ETT. The WST is used to determine the error of measurement and when the ETTVM is not given within the limits of the ETT.

In the device according to the invention, unlike the thermo-dying process, the desired values are obtained only by thermal dilution without dye dilution measurement.

The time course of the baseline blood temperature can be determined before the cold liquid is injected, and the baseline for the thermodilution curve extrapolated from this using the method of least squares error, or the baseline blood temperature can be used towards the end of the measurement to determine the baseline.

This baseline determination is intended to correct errors, e.g. in error correction of intoxication, and to exclude effects from physiological fluctuations such as bodily functions, fever, etc., and from inaccuracies in the infusion regimen.

The analysis is no longer initiated by pressing a button on the device, but is carried out automatically by analysing the output signal of an injection flow sensor. The time course of this output signal leads to the conclusion that there is an injection and its uniformity is required for the measurement purpose. The flow sensor can also be used as the injection temperature sensor if it is designed to heat up at rest. This can be achieved by a temperature equal to the ambient temperature, for example room temperature or body temperature, or by heating the sensor by means of the measuring current.

To determine the mean MTT, the exact time of injection is necessary.

The test is assumed to be able to perform the indicator injection, which is normally made up of 10 ml of ice-cold glucose solution, evenly. If an injection machine is used, a uniform injection is ensured. The injection temperature curve recorded during the injection is examined for its shape. The measurement is rejected if the curve is irregular, i.e. if it has two or more minima. If the criteria for a correct injection are met, the temperature of the injection during the injection are determined from the time course of the injection, the starting and ending points of the injection and the minimum temperature.

Since bolus injection is not theoretically feasible, the start and end times of the injection are used to construct a corrected injection time, i.e. a starting time for the evaluation of the mean transit time MTT, which is, for example, approximately halfway between the start and end times of the injection, and then the values for CO, ITTV and PTV are calculated from this in the above way, which is well known.

The global end-diastolic volume of the heart GEDVTDart is calculated from the arterial thermodilution curve taken as new for the heart specific filling state parameters according to the following equation: ${\text{GEDV}}_{\text{TDart}}{\text{= ITTV}}_{\text{TDart}}{\text{- PTV}}_{\text{TDart}}\text{.}$

The global end-diastolic volume of the heart GEDVAT TDart can also be determined by the following equation: ${\text{GEDV}}_{\text{AT TDart}}{\text{= a x (AT - VOL}}_{\text{TDart}}\text{) + b}$ wherein ${\text{AT - VOL}}_{\text{TDart}}{\text{= C.O.}}_{\text{TDart}}{\text{x AT}}_{\text{TDart}}$ and C.O.T. This is the cardiac time volume, ATT. This is the time of onset and b are species-specific values.

The values a, b can be determined by comparing the GEDV obtained by another known method, e.g. the thermo-dye method, with the determined AT-VOLTDart for the species concerned.

They are between 1.0 and 3.0 ml for a and between +/- 20% of the normal ITBV value (approximately 900 ml/m2 body surface area for humans) for b.

These values are approximately a = 2.0 and b = -200 for humans, measured in ml.

The intrathoracic blood volume ITBVTDart can then be calculated using the following equation: ${\text{ITBV}}_{\text{TDart}}{\text{= a' x GEDV}}_{\text{TDart}}\text{+ b',}$ where a' and b' are also species-specific values, i.e. between 1.0 and 2.0 for a' and +/- 15% of the normal ITBV value. For humans, a' = 1.22 and b' = 109 (n = 89) and for pigs, a' = 1.33 and b' = 13.3 (n = 145), where the unit of measurement is ml. These values are determined by comparison with values obtained by another method, as are the values for a and b.

This generally means that there is a firm species-specific allocation between ITBVTDART and GEDVTDART, and individual and pathological differences can be neglected.

The pulmonary blood volume PBVTDart is then calculated from the arterial thermodilution curve as: ${\text{PBV}}_{\text{TDart}}{\text{= ITBV}}_{\text{TDart}}{\text{- GEDV}}_{\text{TDart}}\text{.}$

The extravascular pulmonary water volume EVLWTD can then be determined from the arterial thermodilution curve using one of the following two equations: $\begin{array}{c}\begin{array}{c}{\text{EVLW}}_{\text{TD}}{\text{= ETV}}_{\text{MTT TD}}{\text{= ITTV}}_{\text{TD}}{\text{- ITBV}}_{\text{TD}}\text{, und}\\ {\text{EVLW}}_{\text{TD}}{\text{= ETV}}_{\text{DST TD}}{\text{= PTV}}_{\text{TD}}{\text{- PBV}}_{\text{TD}}\text{.}\end{array}\end{array}$

Finally, as another recent parameter, the global cardiac function index (CFI) is calculated as follows: ${\text{CFI}}_{\text{TDart}}{\text{= C.O.}}_{\text{TDart}}{\text{/GEDV}}_{\text{TDart}}\text{.}$

The device of the invention may be a single arterial thermodilution catheter, which may have a small electrically insulated thermistor at its distal tip and a lumen for blood drawings and measurements of arterial blood pressure, and may be so small that it can be easily placed in the radial artery.

The above measuring device is much cheaper than the known thermo-dying method because of its much simpler instrumentation, and it does not require any complicated technology, especially pulmonary analysis catheters.

In general, the injection can be done via a central venous catheter, the placement of the temperature probe via an arterial radial catheter.

The measuring and evaluation apparatus connected shall be so designed as to have the units of measurement necessary to determine the desired values according to the above equations or shall be so designed and/or programmed in the form of a microprocessor.

Claims (10)

1. Device for determining the fill level of blood circulation by means of thermodilution, having a thermodilution measuring arrangement which comprises a thermodilution injector, a temperature sensor and a control and evaluation apparatus, at which the output values of the temperature sensor and further measuring parameters lie and which from the thermodilution curve forms the intrathoracic thermovolume ITTV_TDART and the pulmonary thermovolume PTV_TDart according to the following equations: $\begin{array}{c}\begin{array}{c}{\text{ITTV}}_{\text{TDART}}{\text{= C.O.}}_{\text{TDart}}{\text{x MTT}}_{\text{TDart}}\text{and}\\ {\text{PTV}}_{\text{TDart}}{\text{= C.O.}}_{\text{TDart}}{\text{x DST}}_{\text{TDart,}}\end{array}\end{array}$ in which C.O._TDart is the cardiac output, which is determined according to the Standard-Stuart-Hamilton formula in its adaptation to thermodilution. MTT_TDart is the mean transit time and DST_TDart denotes the exponential fall time, characterised in that the control and evaluation apparatus comprises a differential unit, at which the values ITTV_TDart and PTV_TDart that have been formed lie and the output value ITTV_TDart - PTV_TDart of which is the global end diastolic volume of the heart GEDV_TDart, which represents the sum of the cardiac chamber volumes without the volume of the lungs.
2. Device for determining the fill level of blood circulation by means of thermodilution, having a thermodilution measuring arrangement which comprises a thermodilution injector, a temperature sensor and a control and evaluation apparatus with a processor, at which control and evaluation apparatus the output values of the temperature sensor and further measuring parameters lie and which from the thermodilution curve forms the appearance time AT_TDart and the cardiac output C.O._TDart, characterised in that in the processor the cardiac output C.O._TDart is multiplied by the appearance time AT_TDart, the result is multiplied by a characteristic value a, a characteristic value b is added and that result is read out as global end diastolic volume of the heart GEDV_ATTDart, which represents the sum of the volumes of the heart without the volume of the lungs, wherein in the evaluation apparatus there are stored lists of species-specific characteristic values a and b, which the processor accesses during the evaluation.
3. Device according to claim 1 or 2, characterised in that lists of species-specific characteristic values a' and b' are stored in the evaluation apparatus and a processor is provided which multiplies an accessed value a' and the value GEDV_TDart and adds an accessed value b' to form the intrathoracic blood volume ITBV_TDart.
4. Device according to claim 3, characterised in that the evaluation apparatus contains a subtraction unit, at which the values ITBV_TDart and GEDV_TDart lie and the output value of which is the pulmonary blood volume PBVT_TDart.
5. Device according to claim 3, characterised in that the evaluation apparatus contains a subtraction unit, at which the values ITTV_TDart and ITBV_TDart lie as input values and the output value of which represents the extravascular volume of lung fluid EVLW_TDart.
6. Device according to claim 4, characterised in that the evaluation apparatus contains a subtraction unit, at which the values PTV_TDart and PBV_TDart lie as input values and the output value of which represents the extravascular volume of lung fluid ELVW_TDart.
7. Device according to claim 1, characterised in that the evaluation apparatus contains a division unit, at which the cardiac output C.O._TDart and the value GEDV_TDart lie as input values, and the output value of which represents the global cardiac function index.
8. Device according to any one of the preceding claims, characterised by an injected substance flow sensor, the initiation of a measurement being controlled by way of an analysis of the time characteristic of its output signal.
9. Device according to claim 8, characterised in that an injected substance temperature sensor for use also as flow sensor is designed so that it becomes warm when the injected substance is at rest.
10. Device according to claim 9, characterised in that the injected substance temperature sensor is warmed by being heated by means of a measuring current.