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Flowmeters designed for the measurement of blood flow during surgical procedures.These flowmeters typically use transit time flow measurement (TTFM) to assess blood flow in vessels. TTFM employs small probes that are positioned in close proximity to the blood vessel to be assessed. Typically, the probes contain two piezoelectric crystals that function as both ultrasound transmitters and receivers and a reflecting plate. During the procedure, the probe is positioned such that the crystals are aligned with the direction of blood flow in the vessel (i.e., one upstream of the other) and the reflecting plate is facing the two crystals on the opposite side of the vessel. A series of ultrasound pulses is generated by each crystal, reflected by the reflecting plate, and received by the other crystal. The transit time of the pulses between the crystals is influenced by flow within the vessel, and TTFM flowmeters calculate the flow rate from the difference in upstream versus downstream transit times. Some of these flowmeters incorporate additional modalities such as ultrasound imaging and/or Doppler ultrasound-based flow velocity detection. Intraoperative ultrasonic blood flowmeters are typically used to assess vessel and/or graft patency during cardiac procedures (e.g., coronary artery bypass grafting), peripheral vascular procedures (e.g., carotid endarterectomy), or organ transplant procedures.
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