Blood flow is highly complicated, because it is influenced by numerous factors including the state of the vascular bed, myocardial function, flow pulsatility and vascular geometry and compliance. To help us understand connections between blood flow and conditions of the heart and other blood vessels, there has always been the need for methods of obtaining flow information in vivo. These techniques became available during the last century, and although generally invasive and destructive, they have provided clinicians with information on blood supply, heart function, and the localized genesis and development of cardiovascular diseases. The earlier indirect methods relied on deduction of flow from differential pressure recordings. The introduction of the electromagnetic catheter-tip velocity probe, the electromagnetic flow meter and hot-film anemometry shifted the emphasis from pressure measurements toward flow measurements. Later, pulsed Doppler flow meters offered a noninvasive method for measurement of blood flow.
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