Abstract
Objectives: At present, there is no standard bedside method for assessing cerebral autoregulation (CA) with high temporal resolution. We combined the two methods most commonly used for this purpose, transcranial Doppler sonography (TCD, macro-circulation level), and near-infrared spectroscopy (NIRS, micro-circulation level), in an attempt to identify the most promising approach.Methods: In eight healthy subjects (5 women; mean age, 38 ± 10 years), CA disturbance was achieved by adding carbon dioxide (CO2) to the breathing air. We simultaneously recorded end-tidal CO2 (ETCO2), blood pressure (BP; non-invasively at the fingertip), and cerebral blood flow velocity (CBFV) in both middle cerebral arteries using TCD and determined oxygenated and deoxygenated hemoglobin levels using NIRS. For the analysis, we used transfer function calculations in the low-frequency band (0.07–0.15 Hz) to compare BP–CBFV, BP–oxygenated hemoglobin (OxHb), BP–tissue oxygenation index (TOI), CBFV–OxHb, and CBFV–TOI.Results: ETCO2 increased from 37 ± 2 to 44 ± 3 mmHg. The CO2-induced CBFV increase significantly correlated with the OxHb increase (R2 = 0.526, p < 0.001). Compared with baseline, the mean CO2 administration phase shift (in radians) significantly increased (p < 0.005) from –0.67 ± 0.20 to –0.51 ± 0.25 in the BP–CBFV system, and decreased from 1.21 ± 0.81 to −0.05 ± 0.91 in the CBFV–OxHb system, and from 0.94 ± 1.22 to −0.24 ± 1.0 in the CBFV–TOI system; no change was observed for BP–OxHb (0.38 ± 1.17 to 0.41 ± 1.42). Gain changed significantly only in the BP–CBFV system. The correlation between the ETCO2 change and phase change was higher in the CBFV–OxHb system [r = −0.60; 95% confidence interval (CI): −0.16, −0.84; p < 0.01] than in the BP–CBFV system (r = 0.52; 95% CI: 0.03, 0.08; p < 0.05).Conclusion: The transfer function characterizes the blood flow transition from macro- to micro-circulation by time delay only. The CBFV–OxHb system response with a broader phase shift distribution offers the prospect of a more detailed grading of CA responses. Whether this is of clinical relevance needs further studies in different patient populations.
Highlights
METHODSCerebral autoregulation (CA) describes the ability of the cerebrovascular system to provide a continuous steady-state blood supply to the brain over a wide range of blood pressure (BP) levels
CO2 induced CBF or its velocity (CBFV) increase correlated significantly with the CO2 induced oxygenated Hb (OxHb) increase [r = 0.72, p < 0.005)], as well as with the CO2 induced total oxygenation index (TOI) increase [r = 0.69; 95% CI: 0.30, 0.80; p < 0.005]
The use of TF to estimating CA via analysis of the BP–CBFV relationship clarified the dependence of results on several technical aspects (Meel-van den Abeelen et al, 2014), including how signals are averaged, the window selected for FFT, and decisions regarding signals smoothing or analysis of relative values
Summary
METHODSCerebral autoregulation (CA) describes the ability of the cerebrovascular system to provide a continuous steady-state blood supply to the brain over a wide range of blood pressure (BP) levels. Low BP leads to low cerebral perfusion and may result in ischemia (Ringelstein et al, 1988; Kleiser and Widder, 1992). TCD measures blood flow velocity in large cerebral arteries. The actual measured blood flow velocity depends on several factors, mainly on the BP gradient across the vessel bed and on the vessel diameter. Metabolic factors such as partial pressure of carbon dioxide (pCO2), mental activity, or [H+] concentration in the brain tissue may affect vessel diameter and velocity. The measured velocity represents the actual brain demands and corresponds closely to the cerebral blood flow (CBF) when vessel diameter does not change considerably. The mechanisms of CA transform macroangiopathic blood flow to microangiopathic capillary flow, linking the processes
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