An analysis of transcranial doppler to interpret changes in cerebral circulation under +Gz

Introduction: The use of Transcranial Doppler (TCD) to measure the cerebral blood flow velocity (BFV) is one of the most elusive tasks under +Gz. The reason for this is the technical difficulty in keeping the TCD fixed during acceleration. There is no conclusive principle of the behavior of cerebral blood vessels under +Gz, despite earlier attempts in animal/human studies. In our study, we were able to overcome the technical difficulty and record the cerebral BFV of the middle cerebral artery under +Gz. Material and Methods: Twenty healthy adult males consented to participate in the study. High-performance human centrifuge was used to subject them to +Gz acceleration. The participants were instrumented with electrocardiography, thermistor bead, oxygen saturation probe, non-invasive blood pressure and TCD probe. Relaxed peripheral light loss (PLL) and straining PLL were recorded in a single gradual-onset rate profile. Results: The TCD data were retrieved and the data was plotted. The Doppler waveform varied with a change in +Gz. Pulsatility (Gosling) index was derived. The index increases as Gz level builds up, indicating an increase in arterial resistance. This increase was statistically significant. Conclusion: The understanding, so far, has been based on a presumption of vasoconstriction in the cerebral arteries. However, when monitoring TCD against increasing +Gz, it is not the presence or absence of the waveform that is of significance; however, it is the change in the pattern of the waveform that is noteworthy.