Objective
Dynamic tests of cerebral autoregulation are becoming increasingly popular in clinical practice. The clinical usefulness of cerebral autoregulation is believed to depend on its close association with outcome; impaired autoregulation in the post-injury period implies a poorer outcome 1 . Various methods based on time-domain or frequency-domain analysis analysis of blood flow velocity fluctuation (measured non-invasively using Transcranial Doppler Ultrasonography), in response to spontaneous or provoked variations of arterial blood pressure (ABP) or cerebral perfusion pressure (CPP) have been described in the literature. The main advantage of such a methodology is that the test does not require any pharmacological alteration of ABP and the state of autoregulation can be monitored virtually continuously – as long as transcranial doppler probes may remain in place. Our objective was to investigate the clinical utility of two indices of dynamic autoregulation. One of them is based on changes in arterial pressure and the other, changes in cerebral perfusion pressure.
Methods
One hundred and eighty anaesthetised and ventilated head injured patients with intracranial pressure, arterial pressure and MCA blood flow velocity recorded intermittently (for periods of 10 minutes up to two hours) were studied. Indices of dynamic autoregulation were calculated as a moving correlation coefficient of 60 samples (total time 3 minutes) of 6 second mean values of blood flow velocity and arterial pressure (Mxa) or blood flow velocity and cerebral perfusion pressure (Mx), recomputed every 3 seconds. Values of Mx and Mxa were averaged over multiple recordings in each patient and correlated with outcome at 6 months post injury.
Results
Pearson's correlation coefficient between Mx and Mxa was significantly positive (R=0.85; p<0.000001). Mxa was significantly greater than Mx (0.22+/−0.22 versus 0.062+/−0.28; p<0.000001). The difference between Mx and Mxa significantly decreased with impaired autoregulation (R=−0.39; p < 0.000001) and with greater ICP (R=−.26; p<0.0005). Mx showed a significant correlation with outcome while the correlation between Mxa and outcome was much weaker and non-significant (R=0.25; p<0.0008 versus R= 0.14; p<0.07 (NS). Both indices correlated positively with ICP and negatively with CPP but this dependence was stronger for Mx than Mxa.
Conclusion
Although both indices are relatively well correlated with each other, differences between them may be considerable. The observation that the differences between indices decrease when ICP increases, contradicts the opinion that CPP rather than ABP should be taken into account only in cases when ICP is already elevated. When ICP is recorded, CPP rather than ABP should always be used in calculations. After head injury, the index based on cerebral perfusion pressure correlates more strongly with outcome than the index using arterial pressure alone, and therefore is clinically far more useful.