Field block: an additional technique of potential value for breast surgery under general anaesthesia.

artefacts, such as arterial blood sampling, were removed manually. The respiratory variability of the arterial pressure was compared with the variability of the plethysmograph trace in several ways. Twenty-second epochs (enclosing at least three respiratory cycles) were used to determine the periodic variability. An adjustment to the raw waveform data was made to compensate for any general upward or downward trend in the analysis period. Systolic pressure variation and pulse pressure variation were both investigated. The plethysmogram, not having a zero baseline, was analysed by determining the envelopes that characterized the peaks and troughs, and in another analysis only the peak values were used. No matter which technique was used, the correlations between pressure variation and plethysmographic variation were very poor and the Bland–Altman plots indicating that the substitution of respiratory related plethysmographic variation for pressure variation are inappropriate. The graph (see Fig. 1) represents 90 min of one patient’s data (nine 10-min periods during a 5-h procedure). Each measurement is from a 20-s epoch. The results above are typical and in my view it made the use of the plethysmographic waveform variability not an appropriate substitute for respiratory-associated arterial pressure variation, I believe their comments about the drawbacks of arterial pressure monitoring are also overstated. This study was not as well-controlled as that by Cannesson and colleagues, but was carried out in realistic clinical situations. I think the clinical usefulness of respiratory-associated changes in the optical plethysmograph is still to be proven.