No abstract

The dynamic response of cerebral autoregulation to spontaneous changes in arterial blood pressure (ABP) is described by the relationship between cerebral blood flow velocity (CBFV) and resistance-area product (RAP). CBFV was measured with Doppler ultrasound in the middle cerebral artery and ABP with an intra-arterial catheter in 66 neonates. Spontaneous changes in mean ABP were automatically detected and the maximum derivative was used to synchronize the coherent averaging of corresponding CBFV and RAP transients. These were classified into two groups corresponding to intact (group A) or impaired (group B) autoregulation. The cross correlation between RAP and CBFV indicates a significant relationship with a time delay of 5 s for group A. The frequency response of RAP was estimated by the cross spectra with CBFV. Groups A and B present a similar amplitude spectra but the phase spectra of group A lags that of group B. The impulse responses of the two groups are also markedly different and were used to simulate the velocity response to a 5% step change in ABP. Impulse responses were also obtained for four different levels of pCO2 showing that hypercapnia leads to an impulse response similar to that of group B (impaired autoregulation). This method can be used to extend the usual dichotomic classification adopted in clinical studies of autoregulation.

The objective of the study was to test the hypothesis that dynamic cerebral pressure-autoregulation is associated with the outcome of patients with severe head injury and to derive optimal criteria for future studies on the predictive value of autoregulation indices. Repeated measurements were performed on 32 patients with severe head injury. Arterial blood pressure (ABP) was measured continuously with an intravascular catheter, intracranial pressure (ICP) was recorded with a subdural semiconductor transducer and cerebral blood flow velocity (CBFV) was measured with Doppler ultrasound in the middle cerebral artery. Transfer function analysis was performed on mean beat-to-beat values, using ABP or CBFV as input variables and CBFV or ICP as the output variables. A dynamic index of autoregulation (ARI) ranging between 0 and 9 was extracted from the CBFV step response for a change in ABP. No significant differences between survivors and non-survivors were found due to mean values of ICP, ABP, CPP, CBFV, pCO2, GCS, age or heart rate. The transfer functions between ABP-ICP and CBFV-ICP did not show any significant differences either. The median [lower, upper quartiles] ARI was significantly lower for non-survivors compared with survivors [4.8 (0.0, 5.9) v. 6.9 (5.9, 7.4), p= 0.004]. The correlation between ARI and GOS was also significant (r=0.464, p=0.011). Cohen's coefficient was optimal for a threshold of ARI= 5.86 (kappa 0.51, p=0.0036), leading to a sensitivity for death of 75%, specificity=76.5%, odds ratio =9.75 and overall precision = 75.8%. The difference in ARI values between survivors and non-survivors persisted when results were adjusted for GCS (p = 0.028). A similar analysis for the Marshall CT scale did not reach significance (p = 0.072). A logistic regression analysis confirmed that apart from the ARI, no other variables had a significant contribution to predict outcome. In this group of patients, death following severe head injury could not be explained by traditional indices of risk, but was strongly correlated to indices of dynamic cerebral pressure-autoregulation extracted by means of transfer function analysis. Future studies using a prospective design are needed to validate the predictive value of the ARI index, as estimated by transfer function analysis, in relation to death and other unfavourable outcomes.

Assessments of dynamic cerebral autoregulation usually measure the cerebral blood flow velocity (CBFV) response to changes in arterial blood pressure (ABP). We studied the effect of substituting ABP by cerebral perfusion pressure (CPP), expressed as the difference between ABP and intracranial pressure (ICP), in estimates of dynamic autoregulation obtained by transfer function analysis. CBFV, ABP and ICP were recorded during periods of physiological stability in 30 patients with severe head injury. Transfer function analysis was performed using the following combinations of input-output variables: ABP-CBFV, CPP-CBFV and CBFV-ICP. Frequency and time-domain (step response) functions were averaged for recordings with mean ICP < 20 mmHg (group A) and mean ICP > or = 20 mmHg (group B). The ABP-CBFV transfer function parameters and step response for group A were similar to previous studies in normal subjects, but group B showed deterioration of dynamic autoregulation. Radically different step responses were obtained from both groups for the CPP-CBFV transfer function and the coherence was not significantly improved. The CBFV-ICP transfer function had the highest values of coherence and indicates that changes in CBFV are the cause of spontaneous fluctuations in ICP. Furthermore, the ICP step response plateau was significantly higher for group B than for group A. An alternative calculation of the CBFV step response to changes in CPP resembled the corresponding responses for the ABP input. For spontaneous fluctuations in ABP, ICP and CBFV, it is not possible to calculate the CPP-CBFV transfer function directly due to the high positive correlation between ICP and CBFV, but an alternative estimate can be obtained by using the CBFV-ICP transfer function. The latter could also be useful as a method to assess intracranial compliance in head injury patients.