Variability of time-domain indices of dynamic cerebral autoregulation.

The intra- and inter-subject variabilities of the cerebral dynamic autoregulatory index (ARI) were studied in a group of 14 healthy subjects aged 23-51 years. An alternative index, derived from autoregressive-moving average (ARMA) modelling of the arterial blood pressure (ABP)-cerebral blood flow velocity (CBFV) dynamic relationship, named ARMA-ARI, is also proposed. The susceptibility of both indices to physiological sources of variability was studied by performing measurements during spontaneous respiration (SR), and controlled breathing at 6, 10 and 15 breaths min(-1). ABP was measured non-invasively (Finapres), CBFV was recorded with Doppler ultrasound in both middle cerebral arteries and end-tidal CO2 (EtCO2) was estimated with an infrared capnograph. ARI and ARMA-ARI were calculated as a summary measure for the whole of each recording period, and also continuously, using a 60 s moving data window. Respiration did not have an effect on either of these indices, despite significant, but relatively small, reductions in EtCO2 at 10 and 15 bpm, compared to SR. Very significant differences were observed between ARI and ARMA-ARI in relation to their stability, variability and sensitivity to discriminate between subjects. For continuous estimates the coefficient of variation of ARI was 30 +/- 21% compared to 15 +/- 8% for ARMA-ARI (p < 0.000). The cumulative probability distributions were also significantly different for the two indices for each of the respiratory manoeuvres. The greater stability and reduced variability of ARMA-ARI, in relation to the classic ARI, suggest that the former should be used in future studies of dynamic autoregulation, mainly in situations where an improved temporal resolution might be required, such as the investigation of vaso-vagal syncope or the physiology of exercise.