To the Editor:
Sigurdsson et al. conducted and reported a superb investigation examining measurement of cardiac output in small children by extracorporeal arteriovenous ultrasonography with reference to an aortic flow probe.1 I note that they measured five consecutive repeated cardiac output measurements simultaneously by both methods in subjects (n = 43) and appear to use all of the data points (215) to measure bias and limits of agreement using standard Bland–Altman analysis.2 Since their original paper in 1986, Bland and Altman have detailed special considerations for repeated measures from the same subjects and warned that “if each pair of X and Y measurements is treated as if from a different individual the structure of the data is ignored and incorrect estimates are likely; specifically, the interval between the limits of agreement may be too narrow.”3 Bland and Altman developed specific techniques for repeated measures in the same subject with the exact statistical method dependent on whether the physiologic variable (e.g., cardiac output) is changing or not. Myles and Cui expanded on this technique and offered additional methods to overcome violation of the assumption of independent sampling that occurs when all repeated measures are treated as if they are from different individuals.4 I wonder if it would be possible for the authors to use the more modern Bland–Altman analysis for their data and provide updated results that remain consistent with statistical assumptions of independent sampling.
The author declares no competing interests.