Introduction Noninvasive blood pressure measurement In the early 1970’s Peñaz 23 described a new approach to continuous noninva- sive recording of blood pressure at the finger level, based on a volume-clamp method. The device was improved in its technical aspects resulting in an instru- ment called Finapres 17, which stands for Finger arterial pressure. Comparison with invasive recording demonstrated that finger blood pressure recording by Finapres provides an accurate estimate of the average radial blood pressure and allows a reasonably good estimate of intra-arterial blood pressure 21. Recently, a comparative study of simultaneous Finapres recording from different fingers in the same subject showed that although differences might exist in systolic, mean and diastolic blood pressure there is excellent agreement between dynamic indices 10. This makes noninvasive blood pressure recording of finger arterial pressure suitable for use in dynamic cerebral autoregulation evaluation. Transcranial Doppler sonography Transcranial Doppler sonography follows the same principles and assumptions of other applications of the Doppler effect to measure blood flow velocity in extracranial vessels: the Doppler probe generates an ultrasound beam, typically of 2 MHz for adults which is reflected by red blood cells in a large vessel with a frequency shift that is directly proportional to the velocity of the scattering elements 2. The frequency shift between transmitted ultrasound and reflected ultrasound allows calculation of the blood flow velocity through: c( f / f ) v ? t r (1) 2 ft coss where v is blood flow velocity, c is the speed of sound in blood,ft and fr are transmitted and received ultrasound frequency respectively and is the insona- tion angle between the ultrasound beam and the blood vessel direction at the depth of insonation. In the presence of laminar flow, the velocity distribution across the vessel diame- ter will be approximately parabolic. Consequently, the reflected Doppler fre- quency shift will comprise a distribution of frequencies, rather than a single value. Different alternatives exist to extract meaningful velocity information. The most common approach is to apply the fast Fourier transform (FFT) algorithm to short segments (typically!t = 5 ms) of the raw Doppler shifted signal, to obtain the spectral distribution of power at each frequency 12. From this distribution, either the maximum frequency (i.e. maximum velocity), or its intensity-weighted mean are extracted to represent the mean velocity for the time interval!t. The velocity distribution, and the maximum velocity envelope are normally displayed by most TCD devices as a colour coded sonogram, as represented in figure 3. The 13
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