Dynamic cerebral autoregulation is preserved during isometric handgrip and head-down tilt in healthy volunteers

Abstract

In healthy humans, cerebral blood flow (CBF) is autoregulated against changes in arterial blood pressure. Spontaneous fluctuations in mean arterial pressure (MAP) and CBF can be used to assess cerebral autoregulation. We hypothesized that dynamic cerebral autoregulation is affected by changes in autonomic activity, MAP, and cardiac output (CO) induced by handgrip (HG), head-down tilt (HDT), and their combination. In thirteen healthy volunteers, we recorded blood velocity by ultrasound in the internal carotid artery (ICA), HR, MAP and CO-estimates from continuous finger blood pressure, and end-tidal CO₂ . Instantaneous ICA beat volume (ICABV, mL) and ICA blood flow (ICABF, mL/min) were calculated. Wavelet synchronization index γ (0-1) was calculated for the pairs: MAP-ICABF, CO-ICABF and HR-ICABV in the low (0.05-0.15 Hz; LF) and high (0.15-0.4 Hz; HF) frequency bands. ICABF did not change between experimental states. MAP and CO were increased during HG (+16% and +15%, respectively, P < 0.001) and during HDT + HG (+12% and +23%, respectively, P < 0.001). In the LF interval, median γ for the MAP-ICABF pair (baseline: 0.23 [0.12-0.28]) and the CO-ICABF pair (baseline: 0.22 [0.15-0.28]) did not change with HG, HDT, or their combination. High γ was observed for the HR-ICABV pair at the respiratory frequency, the oscillations in these variables being in inverse phase. The unaltered ICABF and the low synchronization between MAP and ICABF in the LF interval suggest intact dynamic cerebral autoregulation during HG, HDT, and their combination.

Keywords: Dynamic cerebral autoregulation; head-down tilt; isometric handgrip; wavelet analysis.

Figure 1

Raw recordings of heart rate (HR), mean arterial pressure (MAP), cardiac output estimates (CO _bpc), end‐tidal CO ₂ (ETCO ₂) and internal carotid artery blood flow (ICABF) from one representative subject. During isometric handgrip (HG), HR, MAP and CO _bpc increased where as ETCO ₂ and ICABF did not change both in horizontal position and during head‐down tilt.

Figure 2

Time averaged wavelet power for mean arterial pressure (MAP) and internal carotid artery blood flow (ICABF), contour plot of coherence and plot of γ index against frequency during rest in one subject. Two peaks are identified in the power spectrums of both variables and the γ index: the Mayer wave peak at around 0.1 Hz and the respiratory peak (at ~0.2 Hz in this subject).

Figure 3

Time averaged wavelet power for cardiac output (CO _bpc) and internal carotid artery blood flow (ICABF), contour plot of coherence and plot of γ index against frequency during rest in one subject. Two peaks are identified in the power spectrums of both variables and the γ index: the Mayer wave peak at around 0.1 Hz and the respiratory peak (at ~0.2 Hz in this subject).

Figure 4

Time averaged wavelet power for mean arterial pressure (MAP) and internal carotid artery blood flow (ICABF), contour plot of coherence and plot of γ index against frequency during handgrip in the horizontal position in one subject (same as in Fig. 2). Two peaks are identified in the power spectrums of both variables and the γ index: the Mayer wave peak at around 0.1 Hz and the respiratory peak (at ~0.25 Hz). Compared to rest, a higher Mayer wave peak for MAP and a higher γ index at 0.1 Hz are observed.

Figure 5

Group mean (black line) and 95% CI (grey dashed lines) of frequency‐averaged wavelet coherence between mean arterial blood pressure (MAP) and internal carotid artery blood flow (ICABF) plotted over time, during handgrip (HG) and head‐down tilt combined with HG (HDT + HDT), for the low frequency (LF) and the high frequency (HF) interval. N = 12. The frequency‐averaged coherence between MAP and ICABF did not change over time during the HG maneuver in either body position or frequency interval.