Dynamic cerebral autoregulation during repeated squat-stand maneuvers

Abstract

Transfer function analysis of spontaneous oscillations in blood pressure (BP) and cerebral blood flow (CBF) can quantify the dynamic relationship between BP and CBF. However, such oscillation amplitudes are often small and of questionable clinical significance, vary substantially, and cannot be controlled. At the very low frequencies (<0.07 Hz), coherence between BP and CBF often is low (<0.50) and their causal relationship is debated. Eight healthy subjects performed repeated squat-stand maneuvers to induce large oscillations in BP at frequencies of 0.025 and 0.05 Hz (very low frequency) and 0.1 Hz (low frequency), respectively. BP (Finapres), CBF velocity (CBFV; transcranial Doppler), and end-tidal CO(2) (capnography) were monitored. Spectral analysis was used to quantify oscillations in BP and CBFV and to estimate transfer function phase, gain, and coherence. Compared with spontaneous oscillations, induced oscillations had higher coherence [mean 0.8 (SD 0.11); >0.5 in all subjects at all frequencies] and lower variability in phase estimates. However, gain estimates remained unchanged. Under both conditions, the "high-pass filter" characteristics of dynamic autoregulation were observed. In conclusion, using repeated squat-stand maneuvers, we were able to study dynamic cerebral autoregulation in the low frequencies under conditions of hemodynamically strong and causally related oscillations in BP and CBFV. This not only enhances the confidence of transfer function analysis as indicated by high coherence and improved phase estimation but also strengthens the clinical relevance of this method as induced oscillations in BP and CBFV mimic those associated with postural changes in daily life.

Fig. 1.

Effects of the repeated squat-stand maneuvers on mean arterial blood pressure (BP), cerebral blood flow velocity (CBFV), and respiration. Raw waveform data from a representative individual during repeated squat-stand maneuvers at 0.1 Hz (5-s squat, 5-s stand) are shown. CO₂, CO₂ waveform (capnography); CBFV, transcranial Doppler; BP, mean arterial BP (Finapres). The graph starts with 5 s squatting, leading to increases in BP and CBFV, followed by 5 s standing, causing reductions in BP and CBFV. A total of 30 s is displayed, showing 3 full cycles of the maneuvers. Note that despite strong hemodynamic effects, there is no distortion of waveforms.

Fig. 2.

Spectral analysis of hemodynamic changes under resting conditions and during repeated squat-stand maneuvers. Data from 1 subject, showing beat-to-beat variability in BP and CBFV (top), and results of spectral analysis of these data (bottom). PSD, power spectral density. A: resting condition (spontaneous oscillations). B: oscillations induced by squat-stand maneuvers at 0.025 Hz. C: oscillations induced at 0.05 Hz. D: oscillations induced at 0.1 Hz.

Fig. 3.

Spectral power of BP and CBFV for spontaneous and induced oscillations. A: spectral power of spontaneous oscillations in BP (in mmHg²; black bars) and CBFV [in (cm/s)²; gray bars]. Bars represent group mean and SE. VLF, very low frequency band (0.02–0.07 Hz); LF, low-frequency band (0.07–0.2); HF, high-frequency band (0.2–0.35 Hz). Statistical comparisons were made using Friedman test with Dunn posttest. *P < 0.001 (BP); **P = 0.005 (CBFV). B: spectral power of BP (black bars) and CBFV (gray bars) during oscillations induced by periodic squatting at 3 frequencies: 0.025 and 0.05 Hz (VLF range) and 0.1 Hz (LF range). Note the different y-axis scale (factor 30) for spectral power compared with A. *P = 0.001 (BP). **P = 0.014 (CBFV). Differences in spectral power between spontaneous and induced oscillations were highly significant (P < 0.0001).

Fig. 4.

Transfer function analysis of spontaneous and induced oscillations. Group results of transfer function analysis of spontaneous oscillations (resting conditions) and oscillations induced by repeated squat-stand maneuvers at 3 different frequencies. The background plots are frequency distribution graphs, obtained from spontaneous oscillations, of mean (black line) and SE (gray area) phase, gain, and coherence plotted against oscillation frequency. Squared boxes represent mean (SE) phase, gain, and coherence within the 3 frequency bands VLF, LF, and HF (see Fig 2). For comparison, results obtained from repeated squat-stand maneuvers are plotted as black dots (mean and SE) at their respective frequencies (0.025, 0.05, and 0.1 Hz). Statistical comparisons were made using Friedman test with Dunn posttest. *P < 0.05 for differences between frequency bands within each group. **P < 0.05 for differences between the 2 groups (spontaneous oscillations vs. induced oscillations).