We appreciate the interest that Dr. Critchley has taken in our study1and thank him for his letter. We agree that the real accuracy and precision of thermodilution are crucial to the discussion. Although thermodilution continues to be chosen as the clinical reference standard for validation studies, we would encourage further research to determine its precision, using high-precision tools, such as indwelling flow probes that can be adapted for use during major surgery and critical care. The evidence from recent studies using such invasive gold standards is that thermodilution has no better precision than the other methods surveyed during conditions of unstable hemodynamics.2,3Furthermore, we welcome continued work by developers to improve the performance of new devices, and their subsequent independent testing in a variety of clinical scenarios.

Dr. Critchley's comments confirm that the ± 20% criterion for agreement with the true cardiac output is essentially an arbitrary one. Our data suggest the likely limits of agreement of each generic method with the true cardiac output are closer to ± 30%. We leave judgment of the acceptability of this for clinical decision-making to the interested clinician. However, the studies quoted above suggest it is likely that this is the real precision of thermodilution that we have been routinely working with for many years, while managing patients during cardiac surgery.

*Austin Hospital and University of Melbourne, Melbourne, Australia. phil.peyton@austin.og.au

Peyton PJ, Chong SW: Minimally invasive measurement of cardiac output during surgery and critical care: A meta-analysis of accuracy and precision. Anesthesiology 2010; 113:1220–35
Botero M, Kirby D, Lobato EB, Staples ED, Gravenstein N: Measurement of cardiac output before and after cardiopulmonary bypass: Comparison among aortic transit-time ultrasound, thermodilution, and noninvasive partial CO2rebreathing. J Cardiothorac Vasc Anesth 2004; 18:563–72
Bajorat J, Hofmockel R, Vagts DA, Janda M, Pohl B, Beck C, Noeldge-Schomburg G: Comparison of invasive and less-invasive techniques of cardiac output measurement under different haemodynamic conditions in a pig model. Eur J Anaesthesiol 2006; 23:23–30