Flow, the volume of fluid passing a point during a certain period of time (mL/min or L/min), is life’s quintessential vital sign. By delivering oxygen and nutrients to cells, and then removing metabolic wastes, blood flow both gives and sustains life. When blood fails to flow, life ends. 

Precise flow measurement technology was not available until the mid-twentieth century, so it was preceded by a host of surrogate measures. Pressure, used by the body to control flow, was routinely measured to assess hemodynamics. Pulses are easily palpated, but represent only rough estimates of heart rate, not flow. Doppler technology measures only velocity of blood flow, not true volume flow. All are surrogates for knowing absolute flow, which today can be precisely measured by transit-time ultrasound technology, pioneered by Transonic. To measure is to know!