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Blood flowmeters designed to measure blood flow using ultrasound frequency (typically several megahertz) waves, either continuously or in pulses (using the Doppler effect). These flowmeters typically consist of a radiofrequency generator, cables, a probe with ultrasound transducers, controls, and a display and have computerized capabilities. In those instruments that use pulsed ultrasound waves, the wave reflected by the blood is sent back to the measuring instrument and the frequency shift is a measure of the blood flow velocity (Doppler effect); instruments that use continuous waves include two separate crystals: one to create and deliver ultrasound waves and the second to detect the reflected (i.e., echo) wave. Dedicated ultrasound blood flowmeters are available to perform noninvasive measurements (using a probe placed against the skin close to the target vessel) that may include vessels inside the cranium (i.e., transcranial measurements); invasive measurements (using a very tiny probe placed in direct contact with the vessel); or minimally invasive measurement (a ring-shaped probe placed at the tip of a catheter is introduced into the vessel). Instruments using a flow-sensor clamp to measure blood flow in external tubing during cardiopulmonary bypass and/or other procedures involving in vivo external processing of blood are also available. Clinical applications include diagnosis and/or treatment of air embolism, atherosclerosis, postphlebitic syndrome, varicose veins, or venous thrombosis. Some devices can also estimate left ventricular function at rest or during exercise by measuring flow through the aortic arch.
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