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1.8.1.4 What information one can obtained with spectral Doppler

Spectral Doppler provides information about the velocity of blood flow over time. Therefore, it permits measurement of maximal and mean velocity. Maximal velocity is equivalent to the highest point of the curve (below or above the zero line). Mean velocity, as the term denotes, is calculated as the mean of all velocities. This is done by simply tracing the curve and integrating the velocities below the curve (don't worry - the machine does it for you). Doppler curves also provide valuable information about the timing of events, especially when relating the Doppler curves to the ECG tracing. For instance, we are able to measure time intervals such as the ejection period, isovolumetric relaxation time, or diastolic filling time. By viewing the slope of the curves (i.e. deceleration and acceleration times), we can determine how quickly the velocity changes.

Velocities are also related to "pressures". To understand this we will have to first discuss a physical principle that was first described by the Swiss physicist Bernoulli in the 18th century. When a fluid passes through a narrow space, its flow velocity increases. This is similar to a river with rapids in those regions in which the river bed is narrow.

Pressure is increased before the point of obstruction and is low behind the obstruction. The difference between these pressures is known as the pressure gradient. The pressure gradient is actually the driving force behind the increase in velocity.

Bernoulli found that velocity increases exponentially with the pressure gradient. With the aid of Bernoulli's equation one can calculate the pressure gradient from the velocity. As pressure gradients are also related to the degree of obstruction, we can apply this principle to quantify lesions such as aortic stenosis or shunts. Bernoulli's equation is actually quite complex. It considers factors such as viscosity and flow coefficients. However, the simplified version of Bernoulli's equation is adequate for the purpose of medical ultrasound.

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Simplified Bernoulli's equation

 

The simplified version of Bernoulli's equation permits easy estimation of pressure gradients from velocities.

Bernoulli's equation is the most important formula in echocardiography. As you will see later, it will be used not only to derive maximum and mean pressure gradients but also for many other calculations.

Direct applications Indirect applications
Valvular stenosis AR Quantification
Defects (i.e. VSD, Coarctation, PDA) Diastolic function
TR signal (sPAP) dp/dt (contractility)

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