Measurement of cerebral autoregulation: clinical, physiological and technical concepts.

The human brain possesses little capacity to withstand interruptions in cerebral blood flow (CBF). As such, the regulation of blood flow at metabolically appropriate levels is of paramount importance in the maintenance of normal brain functioning. This blood flow control mechanism, referred to as cerebral autoregulation, is often disturbed or completely abolished after subrachnoid haemorrhage, tissue ischaemia and traumatic brain injury. Importantly, loss of autoregulation has important pathophysiological and clinical implications. It increases the likelihood of brain swelling, increases the risk of incidental hypoperfusion and therefore ischaemia, and is also predictive of a poor clinical outcome. Techniques for measuring autoregulation are numerous, employing a variety of tools, mathematical techniques and physiological principles. For the management of the neurointensive care patient, the ideal technique permits a robust, continuous measurement that describes in simple terms the adequacy of cerebral blood flow regulation, without unduly interrupting the critical care process or perturbing the patient. In practice, compromises must be made such that one or more of the aforementioned ideal properties of the method are absent. It is generally regarded that the ideal method should permit the measurement of autoregulation using intrinsic or “incidental” stimuli that do not disturb the patient unduly. Such intrinsic stimuli include the slow, spontaneous variations in blood pressure seen in healthy individuals and neurocritical care patients, variations in blood pressure occurring due to ectopic heart bearts or even respiratory variations either spontaneous or mechanically induced. There is little agreement yet on what constitutes the ideal method in the context of the neurocritical care patient, although there is growing support for a method based on the continuous correlation between slow fluctuations in arterial blood pressure and intracranial pressure. This method is not without its flaws however, and investigation into other techniques continues apace. Alternatives to analysing intracranial pressure include techniques using transcranial Doppler (TCD) ultrasound to record a surrogate measure of CBF, and examining the responses of TCD-based measures of CBF to changes in other parameters such as arterial blood pressure or cerebral perfusion pressure. Importantly, the success of these techniques is predicated on the availability and reliability of TCD signals. The TCD technique is difficult to learn, and obtaining the high level of skill required to reliably obtain signals using a blind scanning technique requires a lengthy period of supervised training. Importantly, the difficulty of performing TCD could theoretically be lessened with the use of hardware and/or software tools that simplify or speed up vessel identification and the locating of suitable acoustic windows. Easily determining the location of acoustic windows in the skull remains a key obstacle to performing TCD, for which few solutions exist. The aims of the research program leading to this thesis were to investigate techniques for obtaining high quality data to facilitate autoregulation measurement, to test the ability of novel algorithms and techniques to describe cerebral autoregulation, and to examine the clinical relevance of these methods in the setting of disorders affecting the cerebral vasculature. In doing so, this work aims to contribute to the advancement of autoregulation monitoring techniques toward the goals of being physiologically meaningful, reproducible and practically applicable in the neurointensive care setting.