 While it has long been possible to measure pressure in real time from the left ventricle, measuring the volume was technically more difficult.The use of ultrasonic sonomicrometry and the development of the conductance catheter triggered renewed interest in PV loops studies.In sonomicrometry, small ultrasonic transducers (usually referred to as "crystals") transmit signals to each other, and the distance between them is accurately determined based on the transit-time of the signals.