Hypotension during spinal anaesthesia for caesarean section - a reply.

Abstract

We would like to thank Dr. Toyama for his interest in our article 1. Although we were unable to demonstrate the ability of perfusion index (PI) in predicting spinal anaesthesia-induced hypotension, Toyama et al. demonstrated that parturients with higher PIs were more likely to develop hypotension after spinal anaesthesia 2. However, Toyama et al.'s results contrast with those in a study by Sun et al., which showed that parturients who developed hypotension had lower pre-anaesthetic PIs than the normotensive group 3, although the differences were not significant. There may be several reasons for inconsistencies between the results of various studies. Firstly, there were several methodological differences, such as the definition of hypotension or infusion strategy, some of which were discussed in our article 1. Secondly, the characteristics of the participants may have varied between studies. For example, the range of PI values is wider in our study (1.3–17.2 in our study versus 0.7–8.6 in the study conducted by Toyama et al.), and the proportion of parturients with baseline PI values ≤3.5 was significantly lower in our study (17%) compared to that in Toyama's study (46%), PI 3.5 being the threshold value for prediction of hypotension in Toyama's study. Toyama et al. speculate that a higher PI indicates greater endothelial vasodilation, which is compensated for by higher sympathetic activity but is not blocked by spinal anaesthesia, resulting in venous pooling of the blood, and so hypotension. Although speculative, these results are interesting. In fact, re-analysis of our study failed to demonstrate any correlation between PI and low frequency to high frequency ratio (LF/HF) of heart rate variability, which is a relatively specific marker of sympathetic activity. The PI value is dependent on various factors including, but not limited to, vascular tone (endothelial, humoral, and neural) and sympathetic activity. It is conceivable that parturients with the same PI value have varied vascular tone and sympathetic activity balance. Therefore, the results of the studies conducted by Toyama as well as ours on PI appear to be insufficient to discuss the role of endothelial-dependent vascular tone in spinal anaesthesia-induced hypotension du-ring a caesarean section.