Abstract
Objectives: To examine the brain’s microcirculatory response over the course of a continuous 5-min elbow movement task in order to estimate its potential role in grading vaso-neural coupling compared to the macrocirculatory response.Methods: We simultaneously recorded cerebral blood flow velocity (CBFV), changes in oxygenated/deoxygenated hemoglobin concentrations ([oxHb], [deoxHb]), blood pressure (BP), and end-tidal CO2 over 5-min periods of rest and left elbow movements in 24 healthy persons (13 women and 11 men of mean age ± SD, 38 ± 11 years). A low frequency range (0.07–0.15 Hz) was used for analysis by transfer function estimates of phase and gain.Results: Elbow movement led to a small BP increase (mean BP at rest 83 mm Hg, at movement 87; p < 0.01) and a small ETCO2 decrease (at rest 44.6 mm Hg, at movement 41.7 mm Hg; p < 0.01). Further, it increased BP-[oxHb] phase from 55° (both sides) to 74° (right; p < 0.05)/69° (left; p < 0.05), and BP-[deoxHb] phase from 264° (right)/270° (left) to 288° (right; p < 0.05)/297° (left; p = 0.09). The cerebral mean transit time at 0.1 Hz of 5.6 s of rest remained unchanged by movement. Elbow movement significantly decreased BP-CBFV gain on both sides, and BP-CBFV phase only on the right side (p = 0.05).Conclusion: Elbow movement leads to an increased time delay between BP and [oxHb]/[deoxHb] while leaving the cerebral mean transit time unchanged. Phase shifting is usually the more robust parameter when using a transfer function to estimate dynamic cerebral autoregulation; phase shifting at the microcirculatory level seems to be a better marker of VNC-induced changes than phase shifting between BP and CBFV.
Highlights
We mainly focused on Near-infrared spectroscopy (NIRS), because the blood pressure (BP)-cerebral blood flow velocity (CBFV) system is a more well-established method of assessing cerebrovascular dynamics, and because we wanted to interpret the BP-[oxHb] and BP-[deoxHb] findings in light of the best established method, we recorded CBFV
To assess the brain’s circulatory system by transfer function analysis (TFA), TFA is applied to the BP-CBFV relationship
If the TFA approach is applied to the microcirculation parameters [oxHb] and [deoxHb] and related to BP or CBFV as the driving parameter, our knowledge about the human microcirculation’s dynamic remains limited
Summary
Considering its pathophysiologic relevance to high blood pressure (BP), microangiopathic diseases, mild cognitive impairment, and dementia, the examination of cerebral microcirculation has been gaining widespread research interest (Birns et al, 2009; Pantoni, 2010; Marmarelis et al, 2014, 2017; Duncombe et al, 2017; de Heus et al, 2018; Müller et al, 2019). Near-infrared spectroscopy (NIRS) offers the opportunity to observe oxygenated and deoxygenated hemoglobin concentration changes ([oxHb], [deoxHb]) in the microcirculation of the brain’s upper cortical layer with a relatively high temporal resolution (Murkin and Arango, 2009). Their dynamic interplay with BP or cerebral blood flow velocity (CBFV) may potentially represent an early disease marker. In addition to the TFA of the BP-CBFV macrocirculatory relationship, the TF analyses of the dynamics of the interaction between BP and [oxHb], as well as of [deoxHb] (microcirculatory response), have shown that it is possible to grade CA failure (Reinhard et al, 2006; Cheng et al, 2012; Elting et al, 2018). Since the final aim of our study is to assess whether alterations in this VNC response interfere with patient’s daily life activities or rehabilitation capabilities, further studies on patients using this setup could not be justified if this hypothesis were to turn out not to be true
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