General Principles for Measuring Arterial Waves

Abstract

The transducer used to measure pressure, flow and diameter should obey the three criteria for the faithful reproduction of an event: amplitude linearity, adequate frequency response and phase linearity. The errors in phase introduced by a catheter–manometer system are not regarded as very important until the calculation of such complex quantities as arterial impedance and external ventricular power became of interest. The various methods used to measure blood flow, stroke volume and cardiac output are discussed in this chapter, along with the description and performance of single and multisensor catheters and guidewires for measuring pulsatile pressure and blood flow velocity. With the advancements in guidewire technology, new diagnostic parameters that combine both pressure and blood flow velocity can be obtained. Several devices are now available that measure beat-to-beat pulsatile blood pressure noninvasively. The Nexfin (ClearSight) is a new device using finger cuff technology. As opposed to the Finapres, the brachial arterial blood pressure wave is reconstructed from the measured finger arterial blood pressure wave. Pressure waveforms recorded with the Finapres compare favorably with those recorded directly in the radial artery. The BPro with A-Pulse is a wristband device that resembles a wristwatch. It uses a tonometer to measure the radial pressure wave and displays it in real time. This system has been validated against invasive measurements and can be used to measure central blood pressure continuously over a 24 hour period. Another wrist-worn tonometer was recently developed by Microsoft Research and evaluated. This instrument also permits accurate registration of arterial pressure waves using the principle of applanation. Measuring peripheral arterial pressure waves has been solved in the SphygmoCor Xcel and other devices by using the oscillometric method to measure the arterial wave. Measurement of the pulsatile blood flow velocity waveform and SV can be obtained in both the main pulmonary artery and the ascending aorta using MRI. The most widely used application of ultrasound to the measurement of blood flow velocity and diameter is brachial artery flow–mediated dilation (FMD) and is discussed in Chapter 6.