Continuous Wave Doppler

What are the capabilities of CW doppler?

Evaluate the deep venous system for obstruction and venous incompetence.

What are the limitations of CW doppler?

It has a fixed sample size, no range resolution or ability to place the sample volume at a specific depth, no anatomic image, and it may be difficult to differentiate between patterns that are the result of deep venous obstruction and those resulting for extrinsic compression.

What makes the diagnosis of isolated calf vein thrombosis extremely difficult?

The paired deep veins in the calf.

What can cause a false positive study?

Extrinsic compression, peripheral arterial disease which can cause decreased venous filling, COPD which can result in elevated central venous pressure that alters the pressure gradients and reduces venous flow patterns, and improper doppler angle or probe pressure which can obliterate venous flow and/or result in accelerated flow that may be misinterpreted as flow in a diseased vessel.

What can cause a false negative study?

Partial thrombosis, collateral development, and duplicate deep veins.

How many PZT crystals does CW doppler use?

2 crystals, one continuously sending sound and the other continuously receiving the reflected waves.

What frequency probe does CW doppler use?

5MHz.

What angle to the skin surface should you use with CW doppler?

45-60 degrees.

What does correct vessel identification depend on?

Hearing the accompanying arterial signal because the deep veins are found adjacent to the corresponding artery.

How should you position the patient?

Just as you would for a lower venous duplex study, with the extremities lower than the level of the heart to facilitate venous filling.

What should you do if abnormal venous signals are obtained?

Reposition and reevaluate before coming to a reliable conclusion.

How do you begin the study?

Start on the asymptomatic side, place the probe at the inguinal ligament and identify the CFA, then move the probe medially to insonate the CFV.

What is the next step in the study?

Evaluate the CFV on the symptomatic side for the same flow patterns and responses to compression maneuvers.

What other veins are evaluated?

The femoral, popliteal, and posterior tibial veins.

Are the flow characteristics evaluated with CW doppler also evaluated with PW doppler with duplex scanning?

Yes.

What is spontaneity?

Spontaneous flow without augmentation.

Venous signal should be clearly at all sites with the exception of which veins?

Tibial, GSV, and radial/ulnar veins.

How will a patient who is cold, nervous, or in pain affect venous flow?

The veins will be vasoconstricted, reducing venous flow.

What venous signals are most likely abnormal?

Signals that are only evident following distal compression.

What provides documentation that the tibial, radial, or ulnar veins are patent?

Spontaneous flow or flow observed following distal augmentation.

What is the normal venous signal in the lower extremity?

Phasic with respiration.

What are continuous flow patterns in veins of the upper or lower extremity consistent with?

Proximal venous obstruction, but it can also be normally evident in patients with shallow respirations.

How is distal augmentation accomplished?

Manuel compression is applied distal to the transducer.

What should distal compression result in?

Augmentation, increasing the venous signal.

What is the absent of augmentation during the distal compression consistent with?

Obstruction.

Upon release of distal compression, what should happen?

There should be forward venous flow.

What would reversed doppler signals heard upon the release of distal compression suggest?

Incompetent valves (reflux).

What should happen with maximal proximal compression?

Venous flow should be halted.

What can be used as a substitute for the Valsalva maneuver?

Proximal compression.

What is augmentation during proximal compression indicative of?

Valvular incompetence, signifying venous reflux.

What should happen upon release of proximal compression?

There should be augmentation of the venous doppler signal.

What are no doppler signals being heard upon the release proximal compression consistent with?

An obstruction.

What can alter normal flow patterns?

Extrinsic compression from surrounding tissues and/or structures (tumors, pregnancy, ascites).

What kind of flow is normally heard in the subclavian vein?

Pulsatile, because of its close proximity to the heart.

When is pulsatile venous flow in the lower extremities evident?

In cases of fluid overload, chronic venous insufficiency, or increased pressure (congestive heart failure).

What is venous flow also related to?

Arterial peripheral resistance.

What will vasodilation result in?

More continuous flow with less respiratory variation.

What will vasoconstriction result in?

Trauma, pain, anxiety, or the need to conserve body heat may result in markedly decreased venous flow signals.