Assessment of cerebral autoregulation using sinusoidal lower body negative pressure

The objective of this work is to assess sinusoidal lower body negative pressure as a stimulus to improve measurements of cerebral autoregulation. The stimulus produces periodic variations in blood pressure and cerebral blood flow velocity. The two periodic waveforms are related by their phase difference and the ratio of their amplitudes 9gain). Literature on the phase difference and gain, generated with a variety of stimuli, is discussed and used to justify the assumption that they reflect the competence of cerebral autoregulation. A theoretical model was developed that suggests that non-regulating flow dynamics introduces complications to this interpretation when intracranial pressure is abnormal. Many measurements were performed on healthy volunteers. Phase difference was shown to be a repeatable index but gain was shown to have doubtful accuracy and poor repeatability. The measurement of blood pressure by the Finapres was shown to be the primary source of variability in gain. Variability in phase difference measurements was attributed to instrumentation errors and differences between individuals by the physiological variation within an individual were relatively small. A new median analysis was shown to reduce variability. It was improved further when respiration was synchronized to the stimulus. Breathing of air with 5% CO2 was shown to reduce the phase difference, supporting the use of phase difference as an index of cerebral autoregulation. Sinusoidal lower body negative pressure is a valid stimulus for the assessment of cerebral autoregulation that has great value as a tool for researching the cerebral autoregulation physiology. Development for use in routine practice is possible provided its value is established in the clinical management of patients and engineering is undertaken to enable the measurement to be made more comfortable.