Reply

Sir, In the February 2002 issue ofActa Anesthesiologica Scandinavica, Stenqvist and colleagues published ‘Clinical evaluation of a partial CO2 rebreathing technique for cardiac output monitoring in critically ill patients’ (pp. 152–9). The paper presents the results of a methods comparison study where the authors evaluated the agreement (bias and limits of agreement)(1–3) between cardiac output determined with a new, convenient, non-invasive CO2 rebreathing technique and thermodilution. The discussion of the results of the study starts with an important point: ‘The problem when comparing alternative methods for cardiac output measurement is that the usual reference method, thermodilution, is not a true ‘gold standard’ and has substantial variability of its own. Consequently, an important part of the observed difference between the two techniques is due to variability in the thermodilution technique itself’. The question is then, what prompted the authors not to attempt to quantify the substantial variability, both in the reference and experimental method, by assessing the repeatability of measurements from replicate experiments (1,2) during a period of stable hemodynamics? In my opinion, the authors, by this omission, are left with a problem that is usually considered insoluble: one equation with two unknowns (4). The authors chose, as the reference method, bolus thermodilution with three 10 ml injections of iced saline randomly throughout the breathing cycle ‘accepting 10% spread of the values’ in 11 patients and continuous thermodilution cardiac output in four patients. Ordinarily, one does not use two different reference techniques in correlation or Altman-Bland analyses. Why was it permissible in this investigation? I also wonder what the expression ‘accepting 10% spread of the values’ means? If for example the first thermodilution cardiac output is 5.0 l/min and the next 5.7 l/min, is this determination then discarded? and what if the next value again is 5.7 l/ min? Is it also discarded or is the 5.0 l/min? Stenqvist and collaborators included 7–10 data sets per patient in their correlation and Altman-Bland analyses. This may spuriously inflate the value of the correlation coefficient and narrowed down the limits of agreement.What measures were adopted to correct this problem? The main result of the report is that cardiac output determinations with thermodilution and CO2 rebreathing agreed within 1.7 l/min in 95% of the patients. If one is prepared to accept that differences in cardiac output of 1.7–2.0 l/min are clinically irrelevant, the authors’ contention that ‘the partial CO2 rebreathing technique provides a useful . non-invasive estimation of cardiac output’ is justified. Alternatively, it could also be argued that, with discrepancies of this magnitude, one might as well trust ones clinical judgement. Finally, the authors state that partial CO2 rebreathing cardiac output ‘may be used to monitor central hemodynamics in a large number of critically ill patients’. I assume that the phrase ‘large number’ means that Stenqvist and colleagues feel that CO2 rebreathing cardiac output can replace determinations with thermodilution in many patients; but then again, apparently, not in all. It would be interesting to learn how one distinguishes