Background and Purpose--Mx is an index of cerebrovascular autoregulation. It is calculated as the correlation coefficient between slow spontaneous fluctuations of cerebral perfusion pressure (cerebral perfusion pressure=arterial blood pressure-intracranial pressure) and cerebral blood flow velocity. Mx can be estimated noninvasively (nMxa) with the use of a finger plethysmograph arterial blood pressure measurement instead of an invasive cerebral perfusion pressure measurement. We investigated the agreement between nMxa and the previously validated index Mx.

Methods--The study included 10 head-injured adults. Intracranial pressure was monitored with a parenchymal probe. Arterial blood pressure was monitored simultaneously with an arterial catheter and with the Finapres plethysmograph. Flow velocity in the middle cerebral artery was measured bilaterally with transcranial Doppler. Mx and nMxa were computed in both hemispheres, and asymmetry of autoregulation was calculated.

Results--Ninety-six measures of Mx and nMxa were obtained (48 for each side) in 10 patients. Mx correlated with nMxa (R=0.755, P<0.001; 95% agreement=±0.36; bias=0.01). Asymmetry in autoregulation assessed with Mx correlated significantly with asymmetry estimated with nMxa (R=0.857, P<0.0001; 95% agreement=±0.26; bias=-0.03).

Conclusions--The noninvasive index of autoregulation nMxa correlates with Mx and is sensitive enough to detect autoregulation asymmetry. nMxa is proposed as a practical tool to assess cerebral autoregulation in patients who do not require invasive monitoring.