Prone Position, Cerebral Oximetry, and Delirium

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To JNA Readers:
The recent study by Soh et al1 examining the relationship between intraoperative cerebral oximetry changes and postoperative delirium in elderly patients undergoing major spinal surgery failed to demonstrate any significant relationship between the incidence or severity of regional cerebral desaturation and postoperative delirium. Although the data used to support the rationale for this and many similar trials are arguably somewhat weak with respect to the potential for such a relationship between desaturation and delirium, the negative findings for this present study nonetheless appear to indicate that cerebral desaturation may have no relationship to the subsequent development of postoperative delirium.
Several factors may have influenced these apparently negative findings. Indeed, what is absent from the study report, but clearly implicit in the type of operation being undertaken, is that these patients were in the prone position. This may have influenced the accuracy of the cerebral oximetry monitoring. Accordingly it would be interesting to know how the head was positioned, and specifically whether the forehead was being compressed in a horseshoe-type headrest restraint, or whether there was a Mayfield head restraint used (which would not compress the forehead tissues). In particular, if it was a horseshoe restraint, the pressure on the forehead may have impacted the accuracy of the INVOS Cerebral Oximeter 5100 device (Somanetics, Troy, MI), that they used in their study. Indeed, this device has repeatedly been shown to be prone to extracranial contamination from the forehead tissues2,3 and if the photon path length was altered by direct pressure on the forehead, it might have further compromised the quality of the signal and thus affected the algorithms that account for these extracranial tissues. Although it is uncertain whether this may have had an impact, considering this information at least assists one in optimally interpreting these data.
An additional potential limitation that could affect interpretation of these results relates to the nonblinded nature of the regional cerebral oxygen saturation (rSO2) data that was being collected. As the clinicians involved in the study would have been aware of the real-time rSO2 values, they may have been actively manipulating the physiological status of the patient to minimize any desaturation. Indeed, this may have accounted for the overall lower incidence of desaturation as compared with previous studies that have included blinded collection of rSO2 data.4
All that said, it is unlikely that the complex nature of perioperative delirium will likely be significantly impacted by manipulating 1 single parameter, be it with a physiological monitoring strategy, or a pharmacologic agent.5 However, before altogether dismissing the potential utility of cerebral saturation monitoring in this setting, larger, more adequately powered trials will likely be necessary.

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