The upper frequency limit of dynamic cerebral autoregulation.

Dynamic cerebral autoregulation (CA) can be expressed in the frequency domain by the amplitude and phase frequency responses calculated by transfer function analysis of arterial blood pressure (BP) and cerebral blood flow velocity (CBFV). We studied the effects of arterial P C O 2 ( P aC O 2 ) on the upper frequency limit (FupLim ) of these responses and its intersubject variability. Twenty-four healthy subjects (11 female, age 36.0 ± 13.4 years) were recruited. Recordings of CBFV (transcranial Doppler ultrasound), BP (Finometer) and end-tidal CO2 ( P ETC O 2 , capnography) were performed during 5 min at rest (normocapnia) and during four breathing manoeuvres: 5% and 8% CO2 in air and hyperventilation targeting reductions of 5 and 10 mmHg compared to normocapnia. FupLim was determined by the break point of the autoregulation index (ARI) curve as a function of frequency when the phase response was gradually set to zero. The five breathing conditions led to highly significant differences in P ETC O 2 (p < 0.0001), CBFV (P < 0.0001), ARI (p < 0.0001) and FupLim (p < 0.0001). FupLim ranged from 0.167 ± 0.036 Hz at the lowest values of hypocapnia (28.1 ± 1.9 mmHg) to 0.094 ± 0.040 Hz at the highest level of hypercapnia (41.7 ± 5.4 mmHg), showing a correlation of r = -0.53 (p < 0.001) with P ETC O 2 . These findings reinforce the key role of P aC O 2 in CBF regulation. The considerable intersubject variability of FupLim suggests that fixed frequency bands should not be adopted for averaging values of gain and phase in dynamic CA studies, and that the higher frequency band (0.20-0.40 Hz), in particular, does not contain relevant information about dynamic CA. Further investigations are needed to assess the information value of FupLim as a marker of dynamic CA efficiency in physiological and clinical studies.