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Abstract
The vertebral and carotid systems supply distinct areas of the brain and brainstem. The two systems are structurally joined by means of a multi-sided system of interconnected vessels (the circle of Willis) located at the base of the brain where they surround the stalk of the pituitary gland, th]e optic chiasm and optic tracts, and the hypothalamus. The term Cerebral Autoregulation (CA), describes the tendency of CBF to remain approximately constant when Mean Arterial blood Pressure (MAP) changes over a wide range, typically from 60 to 150 mmHg. from a functional perspective, the term cerebral autoregulation refers to the ability of cerebral arteries to maintain CBF (and therefore brain perfusion) at a relatively constant level despite fluctuations in CPP. from a physical perspective, autoregulation involves relatively rapid changes in the caliber of cerebral resistance vessels, principally the pre-capillary arterioles, in response to changes in transmural pressure as CPP varies. As a result of this phenomenon, CBF is relatively independent of CPP between the physiological limits of autoregulation, typically taken to be perfusion pressures of 50-60 mmHg for the lower limit and 150-160 mmHg for the upper. The Doppler effect is the phenomenon that occures when a sound wave with a certain frequency strikes a moving object, it is reflected with a different frequency. TCD uses a hand-held microprocessor-controlled transducer. It transmits a low-frequency (2 MHz) pulse-waved ultrasonic signal from skin surface across the cranial vault to the intracerebral arteries and receives the echoes along the same path. Acoustic windows are thin areas of bone or naturally occurring foramina or fissures that allow the best transmission of TCD signal. There are three naturally occurring acoustic windows: transtemporal, transorbital and transforaminal windows. TCD velocity measurement is affected by many factors like age, gender, hematocrit, fever, hypoglycemia, carbon dioxide, heart rate, cardiac output, blood pressure and brain activity. TCD applied in ICU on patient .