Cerebral autoregulation (CA) describes the capability of the brain to maintain its flow as relatively stable over a wide rage of mean arterial pressures (MAPs), for example, from 60 to 150 mm Hg. Because the brain is encompassed in the skull, CA is vital for preventing cerebral edema and hemorrhage, as illustrated in some patients experiencing acute liver failure. Also, CA counteracts the effect on cerebral blood flow (CBF) of a reduction in MAP in response to, for example, hemorrhage or a change in body position. Yet, when rising from a supine or seated position, one sometimes experiences symptoms indicative of a reduced CBF, like blurred vision. Thus, CA is of general clinical interest. For example, diabetic patients have increased risk of developing ischemic or hemorrhagic stroke, and in some diabetic patients CA is affected.1
In the BTF guidelines, autoregulation has been mentioned for the first time and the caveat has been raised that in individual cases the traditional CPP limits may not be appropriate. Some evidence as to whether ICP-guided or CPP-guided therapy is better may depend on the state of autoregulation: if the autoregulation is intact a CPP directed therapy could be used, with a higher probability of favourable outcome, whereas if autoregulation is apsent ICP-guided therapy may yield better results.
Because of these findings, a continuous assessment of both static and dynamic cerebral autoregulation has become an interesting adjunct to current neuromonitoring strategies. Cerebrovascular autoregulation may be determined by various approaches. One of the two most used bedside techniques is the online correlation between ICP and MAP (pressure reactivity index, PRx) and between middle cerebral artery (MCA) blood flow velocities and MABP (Mx). Both strategies provide immediate and dynamic information on autoregulation at the bedside.
Using these techniques cerebral autoregulation can be monitored continuously and the decision between ICP-directed and CPP-directed therapies could be based on the autoregulatory state. It is noteworthy that the autoregulatory state is not a static but rather a dynamic variable, suggesting that the individualised treatment strategy should be re-evaluated regularly over the time course of the acute phase following brain injury.
NICEM Consensus on neurological monitoring in acute neurological disease statement was:
• In general, ICP and CPP target values should be ≤ 20 mmHg and≥ 50 to ≤ 70 mmHg, respectively.
• CPP values> 70 mmHg should only be targeted if cerebrovascular autoregulation is intact.
• If cerebrovascular autoregulation is lost, ICP-directed therapeutic strategies should be considered. 2
1 Dynamic Cerebral Autoregulation and Monitoring Cerebral Perfusion. Niels H. Secher, Johannes J. van Lieshout. Hypertension.2010; 56: 189-190. Published online before print June 14, 2010, doi: 10.1161/HYPERTENSIONAHA.110.154971
2 NICEM consensus on neurological monitoring in acute neurological disease. Peter J.D. Andrews, Giuseppe Citerio, Luca Longhi, Kees Polderman, Juan Sahuquillo, Peter Vajkoczy. Intensive Care Med (2008) 34:1362–1370. DOI 10.1007/s00134-008-1103-y
3 Howells et al, J Neurosurg 2005 102: 311-317