Abstract
The time constant (τ[s]) estimates how fast the arterial part of the cerebrovascular bed fills with blood volume during the cardiac cycle, whereas a product of τ and heart rate (HR) (τ*HR[%]) assesses how this period of arterial filling is related to an entire heart cycle. In this study we aimed to investigate cerebral hemodynamics using τ and τ*HR during a progressive lower body negative pressure (LBNP) test.
Transcranial Doppler cerebral blood flow velocity (CBFV), Finapres arterial blood pressure (ABP), and HR, along with end-tidal CO2, were simultaneously recorded in 38 healthy volunteers during an LBNP test. The τ was estimated using mathematical transformation of ABP and CBFV pulse waveforms. After a gradual shortening of τ from baseline (0.20 ± 0.06 s) to maximal LBNP before the onset of presyncope (0.15 ± 0.05 s), we observed a significant increase in τ at presyncope (0.24 ± 0.15 s; p = 0.0001). In the course of LBNP, the τ*HR did not significantly change from baseline (25.6 ± 5.7 % vs 26.6 ± 8.9 %, p = n.s.) except for presyncope, when it increased to 40.4 ± 21.1 % (p < 0.000001). Because the time needed to fill the arterial part of the cerebrovascular bed with blood is prolonged during presyncope, an increased part of the heart cycle seems to be spent on the cerebral blood supply.
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Grants
MK was supported by the Ministry of Polish Science and Higher Education. CH was supported through a Feodor-Lynen Scholarship of the Alexander-von-Humboldt Research Foundation, Germany.
Conflict of Interest
ICM+ is a software program for brain monitoring in clinical/experimental neurosciences, licensed by Cambridge Enterprise Ltd (www.neurosurg.cam.ac.uk/icmplus). MC has an interest in part of the licensing fee.
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Magdalena, K., Czosnyka, M., Diehl, R.R., Haubrich, C. (2016). The Interaction Between Heart Systole and Cerebral Circulation During Lower Body Negative Pressure Test. In: Ang, BT. (eds) Intracranial Pressure and Brain Monitoring XV. Acta Neurochirurgica Supplement, vol 122. Springer, Cham. https://doi.org/10.1007/978-3-319-22533-3_28
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DOI: https://doi.org/10.1007/978-3-319-22533-3_28
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