Doppler Effect

No discussion of sound would be complete without revisiting the Doppler Effect.  First described by the Austrian Christian Doppler in the 1842, it explains how the frequency of a wave is perceived to change relative to movement.  The central premise of the Doppler Effect is a change in the frequency of a detected wave when the source or detector is moving.  A change in pitch of a moving ambulance is a familiar example of the Doppler Effect.  It is also seen every day in weather tracking and police radar detection systems.  In diagnostic ultrasound, Doppler is used to detect and measure blood flow, not to create an image.  This is accomplished in two ways; color-flow and spectral analysis.  When a transducer is placed over a blood vessel so that the flow of blood is directed toward the transducer, the reflected wave will have a higher frequency than the incident signal.  This is known as a positive Doppler shift.  Conversely, if the flow of blood is moving away in relation to the transducer, a lower frequency will be reflected.  This is called a negative Doppler shift.  The angle between the receiver and the transmitter plays an important role in determining the amount of shift.  Doppler shifts calculated by ultrasound systems use the cosine of the angle between the axis of the ultrasound beam and the direction of flow.  Maximum Doppler shift occurs at 0 degrees (cosine of 0 = 1) when flow is either directly towards or away from the transducer.  When flow is perpendicular (90 degrees) to the ultrasound beam (cosine of 90 = 0), no shift is detected.  Ultrasound systems have the ability to incorporate both color flow and spectral analysis to determine direction and velocity of flow.  When these two technologies are incorporated with B-mode imaging, virtually all complications of central venous cannulation can be eliminated.

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The image below shows a longitudinal view of the carotid artery demonstrating color flow Doppler superimposed over a B mode image.  Blood flow is moving caudal to cephalad, as indicated by the white arrow.  The area inside the box represents the axis of the Doppler ultrasound.  The yellow lines denote 90 degrees, or no flow, as shown in the scale at the upper left.  Blood flowing through the carotid toward the center point (90 degrees) results in a positive Doppler shift, represented by the red color.  As it passes the center point, blood is moving away in relation to the center point and a negative Doppler shift occurs.  Both the red and blue color represent arterial blood flow, the difference in color being the relation of flow to the ultrasound beam.

 

Spectral analysis has very little application in regional anesthesia.  However, it can be an invaluable tool when used for vascular access.  Below are two B mode images of the subclavian vein using both color-flow Doppler and spectral analysis.  There are good color-flow Doppler signals in each image, however spectral analysis on the image at the right reveals a proximal obstruction (in this case the proximal subclavian/superior vena cava just above the right atrium).  Attempting to place a central venous catheter in this patient would not only prove futile, but could also potentially harm the patient.


References

Zagzebski JA. Physics and instrumentation in Doppler and B-mode ultrasonography. In: Zweibel WJ. Introduction to Vascular Ultrasonography. 4th ed. Philadelphia, PA: W.B. Saunders Company; 2000:17-43.

Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

Kremkau F W. Doppler Ultrasound: Principles and Instruments. Philadelphia, PA: W.B. Saunders Company; 1990:5-51.

Taylor K J, Holland S. Doppler us. part i. basic principles, instrumentation, and pitfalls. Radiology. 1990; 174(2):297-307.

 

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