Summary
Measurement of the cardiac output by dilution of a thermal indicator is considered theoretically under both generalized and specific conditions. The practical aspects of the measurement show that only a part of derived equations is available for the calculation of flow from recorded thermodilution curves. A real dilution process, as it is detected, deviates from the theoretically required conditions in more respects.
The first major source of variability is the time-proportional instead of volume-proportional detection. The volume-averaging of the indicator within succesive systolic volumes and/or the efficient longitudinal distribution of the indicator may keep these errors at a low level.
The second major source of variability are derivations from the required mixing conditions.
The possibilities of approximating the theoretical requirements in conditions in vivo depend on the hemodynamic conditions and on the arrangement of the measurement.
Zusammenfassung
Die theoretischen Voraussetzungen für die Bestimmung des Herzzeitvolumens mit der Thermodilutionsmethode unter allgemeinen und besonderen Bedingungen werden erörtert. Bei der Auswertung von Thermodilutionskurven zur Bestimmung des Herzzeitvolumens sind nur einige der theoretisch ableitbaren Formeln benutzbar. Der tatsächliche Verdünnungsvorgang, so wie er registriert wird, weicht in mehrfacher Hinsicht von den theoretisch zu fordernden Bedingungen ab:
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1.
die Registrierung des Indikators erfolgt proportional der Zeit und nicht proportional dem Volumen. Durch die Erzeugung einer konstanten gleichmäßigen Abkühlung über mehrere systolische Volumina und einer wirksamen Indikatorverteilung über den gesamten Durchfluß kann dieser Fehler auf ein Minimum reduziert werden;
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2.
es bestehen Abweichungen von der erforderlichen homogenen Durchmischung von Indikator und Blut.
Die Möglichkeiten einer Annäherung der theoretischen Voraussetzungen an die Bedingungen in vivo hängt von den jeweiligen hämodynamischen Verhältnissen und der verwandten Meßmethodik ab.
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Pávek, K., Karovic, K. & Boška, D. Measurement of cardiac output by thermodilution: Theoretical considerations and practical aspects. Archiv für Kreislaufforschung 58, 228–240 (1969). https://doi.org/10.1007/BF02119445
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DOI: https://doi.org/10.1007/BF02119445