LETTERS TO THE EDITOR: Letters & Announcements
To the Editor:
We have read with interest the publication by Endoh et al. (1). We would like to comment on two methodological issues concerning the assessment of autoregulation based on the index of autoregulation that has been used by the authors and that is also known as the static rate of autoregulation (SRoR) (2).
SRoR is a percentage change in cerebrovascular resistance divided by percentage change in cerebral perfusion pressure. When patients are supposed to have normal intracranial pressure, arterial pressure may be used instead. However, this is an uncertain assumption. With a phenylephrine-induced increase in arterial pressure of substantial magnitude (around 20 mmHg), intracranial pressure may rise even in patients without obvious intracranial volume expansion (3).
Secondly, SRoR is a linear index, which is not ideal to describe the non-linear behavior of cerebral blood flow to changes in cerebral perfusion pressure. This index has been introduced to describe only a fragment of the autoregulatory curve—namely the autoregulatory plateau. To make a statement on the extent to which a drug affects autoregulation, ideally the autoregulatory curve (including upper and lower thresholds of autoregulation and the slope of the autoregulatory plateau before and after administration of the study drug) need to be known. With drug-induced hypotension, two possible phenomena may occur: the slope of the plateau may change or the lower limit of autoregulation can be exceeded. A change in SRoR does not allow distinguishing between these two scenarios. From this point of view, judging drug effects by comparing SRoR measured in two different ranges of arterial pressure (from 60 to 80 and from 80 to 100) is problematic.
Luzius A. Steiner, MD
Marek Czosnyka, PhD
1. Endoh H, Honda T, Ohashi S, et al. The influence of nicardipine-, nitroglycerin-, and prostaglandin E1
-induced hypotension on cerebral pressure autoregulation in adult patients during propofol-fentanyl anesthesia. Anesth Analg 2002; 94: 169–73.
2. Strebel S, Lam AM, Matta B, et al. Dynamic and static cerebral autoregulation during isoflurane, desflurane, and propofol anesthesia. Anesthesiology 1995; 83: 66–76.
3. Marmarou A, Takagi H, Shulman K. Biomechanics of brain edema and effects on local cerebral blood flow. Adv Neurol 1980; 28: 345–58.