The arterio-venous difference technique is now well established in the study of organ and tissue metabolism. This technique requires samples to be obtained of the arterial blood supplying and the venous drainage from a tissue, together with a measurement of the blood flow through the tissue. The technique is most appropriate when the arterial concentration and tissue metabolism of a substance are constant, and when the blood flow is stable. If these criteria are not satisfied, care is needed in the interpretation of the results obtained. It should be recognized that the arterio-venous difference technique only measures the net exchange of a substance with the tissue, and that tracers are needed if unidirectional flux needs to be estimated. The other factors which must be borne in mind when intending to use this technique are the transit times of blood and the substance of interest through a tissue, the volume of distribution of the substance in the tissue, and the possibility that the venous samples obtained are derived from a mixture of different tissues.

Summary

Background and objective: Despite the introduction of various less-invasive concepts of cardiac output measurement, pulmonary arterial thermodilution is still the most common measurement technique. Methods: This prospective controlled study was designed to compare different methods of cardiac output measurement simultaneously. Pulmonary arterial thermodilution, transpulmonary thermodilution (PiCCO™), trans-oesophageal echo-Doppler probe (HemoSonic™) and partial carbon dioxide rebreathing technique (NICO™ monitor) were evaluated against a peri-aortic transit-time flow-probe as reference method in a clinically relevant animal model. After approval from the Local Ethics Committee on Animal Research, the investigations were conducted in nine anesthetized domestic pigs. Systemic haemodynamics were modulated systematically by the application of catecholamines, caval occlusion and exsanguination. Statistical analysis was performed with Bland–Altman and linear regression. Results: A total of 366 paired cardiac output measurements were carried out at a reference cardiac output between 0.5 and 7 L min−1. The correlation coefficients for pulmonary arterial and transpulmonary thermodilution against reference were 0.93 and 0.95, for trans-oesophageal Doppler and partial rebreathing technique 0.84 and 0.77. Pulmonary arterial thermodilution and transpulmonary thermodilution showed comparable bias and limits of agreement. Where HemoSonic™ showed an overestimation of cardiac output at a higher precision, NICO™ overestimated low and underestimated higher cardiac output values. Conclusions: Our data suggest that pulmonary arterial thermodilution and PiCCO™ may be interchangeably used for cardiac output measurement even under acute haemodynamic changes. The method described by Bland and Altman demonstrated an overestimation of cardiac output for both thermodilution methods. HemoSonic™ and NICO™ offer non-invasive alternatives and complementary monitoring tools in numerous clinical situations. Trend monitoring and haemodynamic optimizing can be applied sufficiently, when absolute measures are judged critically in a clinical context. The use of the NICO™ system seems to be limited during acute circulatory changes.