1. Field of the Invention
The present invention relates to a process and devices for detecting the instant of injection and for determining the duration of injection in hemodynamic monitoring by means of thermodilution.
2. Description of the Prior Art
The measurement of hemodynamic parameters, for example the cardiac output, is largely performed at present either by means of pulmonary arterial or transcardiopulmonary thermodilution (Pfeiffer U. J., Knoll R. (1993): Process for Determining a Patient's Circulatory Fill Status. U.S. Pat. No. 5,526,817) or else by means of thermo-dye-dilution (Pfeiffer, U. J., Backus G., Blumel G., Eckart J., Muller P., Winkler P., Zeravik J., Zimmermann G. J. (1990): A Fiberoptics-Based System for Integrated Monitoring of Cardiac Output, Intrathoracic Blood Volume. Extravascular Lung Water, O.sub.2 Saturation, and a-v Differences. Practical Applications of Fiberoptics in Critical Care Monitoring, Springer Verlag, 114-125). In these processes, a defined volume of an indicator substance which is as cold as possible, for example glucose or saline solution, is injected. The instant of injection into the body is registered by means of an extracorporeal temperature sensor which is integrated directly in the injection lumen.
At the same time, the thermodilution measurement is started by means of a thermosensor, which in the case of pulmonary arterial measurement is located in the distal lumen of the pulmonary artery catheter in the Arteria pulmonalis or, in the case of transcardiopulmonary measurement, in the tip of a catheter lying in the Arteria femoralis or in the Aorta abdominalis. By plotting the thermodilution curve, the cardiac output can be calculated, for example by means of the Stewart-Hamilton method.
The special aspect of the transcardiopulmonary method is the additional determination of a number of cardiovascular parameters, in particular for assessing the output status, for example by the intrathoracic blood volume. For the calculation of these parameters, knowledge of the characteristic times of the indicators, in particular the mean transit time and exponential fall time, is required. To be able to calculate these exactly, the instant of injection, the mean passage time of the injectate and the duration of injection must in turn be accurately measured, which is accomplished by means of the curve plotted using the extracorporeal temperature sensor (cf. FIG. 1, which reproduces the injection curve profile with a known injectate temperature sensor system; in contrast to this, FIG. 2 shows the injection curve profile with a sensor system according to the invention set out below).
As a function of the temperature difference between ambient air and injectate, the value T.sub.inj, required for correct measurements, is calculated using additionally determined correction factors.
A major disadvantage of the existing technique is that injectate of a temperature deviating from room temperature was required for optimum measurements in order to determine exactly the instant of injection and the duration of injection, since the volume in the customary extracorporeal injectate temperature sensor housing is essentially at room temperature. To be able to detect the instant at which injection starts and to be able to calculate the duration of injection from the temperature profile, a clear temperature difference between the fluid at the sensor before injection and the injectate is required.
For this reason, it must be ensured that the injection solution is available in a well cooled state at any time. This means additional work also for the nursing staff in intensive care units and in operating rooms. In addition, measurements often do not proceed absolutely smoothly, with the result that injectate taken out of cooling too early may already have warmed up again by the time it is used; the same problem arises if a number of measurements are carried out at short intervals one after the other.
The use of cooling sets, which can be installed at the patient's bed, does offer the advantage of an injectate cooled for a certain time directly at the patient, but again brings about considerable disadvantages due to increased work, for example to obtain fresh ice for the cooling box, and due to the costs additionally incurred.