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* Correspondence: luke.fletcher@ austin.org.au Department of Anaesthesia, Austin Health, 145 Studley Road, Heidelberg, VIC 3084, Australia Full list of author information is available at the end of the article To the Editor, We read with interest the recent paper on “a novel non-invasive estimation of effective shunt fraction” (Chang et al. Intensive Care Medicine Experimental (2019) 7:49) [1]. The study retrospectively investigated the validity of various oxygenation indices in predicting the change in PaO2 in response to a change in FIO2 in critically ill patients. In particular, they investigated a method to estimate shunt fraction which substitutes in the denominator of the classical shunt equation of Berggren the term for mixed venous oxygen content (CvO2) with a term derived from transposition of the direct oxygen Fick equation which incorporates the ratio of oxygen uptake rate to pulmonary blood flow (VO2/Q). Their method then assumes that VO2 and Q may be treated as constants. We firstly observe that the equation they present (Eq. 2) is wrong. However, this error was not reproduced in the subsequent derivations in their Appendix 1 (Eqs. 3 and 4) and is unlikely to have invalidated their findings. We further point out that the correct derivation of the non-invasive equation was originally presented by our group in 2004–5 in a prospective series of studies in patients undergoing cardiac surgery [2, 3], and is as follows: Qs/Qt = (Cc’O2 − CaO2)/(Cc’O2 − CaO2 + VO2/Qt) We were able to demonstrate good agreement between this non-invasive shunt fraction measurement and the classical invasive shunt equation of Berggren, and also demonstrated how a measurement of non-shunt pulmonary blood flow could be used to produce the same result. This concept is particularly attractive because of the availability nowadays of minimally invasive estimates of effective pulmonary blood flow or cardiac output from a variety of technologies, without mixed venous blood sampling or right heart catheterisation. However, a key purpose of our studies was to determine the robustness of the concept to inherent imprecision in measurement of either VO2 or Q under clinical conditions.

Letter to the Editor: regarding “Predictive validity of a novel non-invasive estimation of effective shunt fraction in critically ill patients”