The upper frequency limit of dynamic cerebral autoregulation

Key points

• Dynamic cerebral autoregulation (CA) is expressed by the temporal pattern of cerebral blood flow (CBF) recovery following a sudden change in arterial blood pressure (BP).
• Transfer function analysis of BP as input and CBF velocity (CBFV) as output can express dynamic CA through its amplitude (or gain) and phase frequency responses.
• The upper frequency limit (FupLim) at which dynamic CA can operate is of considerable physiological interest and can also provide additional information about worsening CA due to disease processes.
• In healthy subjects FupLim was strongly dependent on PaCO2 changes induced by four different breathing manoeuvres.
• The considerable inter‐subject variability in FupLim suggests that fixed frequency bands should not be adopted for averaging values of gain and phase in studies of dynamic CA.

Abstract

Dynamic cerebral autoregulation (CA) can be expressed in the frequency domain by the amplitude and phase frequency responses calculated by transfer function analysis (TFA) of arterial blood pressure (BP) and cerebral blood flow velocity (CBFV). We studied the effects of PaCO2 on the upper frequency limit (FupLim) of these responses and its inter‐subject 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 (EtCO2, capnography) were performed during five minutes 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 EtCO2 (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 EtCO2. These findings reinforce the key role of PaCO2 in CBF regulation. The considerable inter‐subject 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.