The Doppler effect is the apparent change in the frequency of emitted energy waves when the source is moving relative to the observer. The main medical application of this is the measurement of the shift in the frequency of emitted ultrasound to derive the velocity of blood flow, a procedure referred informally as a Doppler. The frequency shift is converted into an audible 'whoosh' sound. The higher the velocity, the higher pitch the sound. It can be used to estimate the extent of stenosis: a doubling of the flow velocity implies that blood is flowing twice as fast to compensate for an approximately 50% stenosis.
When combined with conventional ultrasound imaging to view vascular structures, the resulting investigation is called a duplex scan and is non-invasive method of assessing both the arterial and venous systems (see review ). Portable, battery-powered, hand-held devices are frequently used in vascular and cardiac surgery to measure venous and arterial flow including the flow of blood across heart valves as part of echocardiography.
Probes (transducers) of various dimensions and frequencies are available. The arterial signal is better sought with a physically smaller and higher frequency probe than the fetal heart.
A normal peripheral artery gives a triphasic signal:
- The first phase is the forward flow during systole.
- The second, a momentary reversal of flow during early diastole due to recoil of the blood vessels. There is reverse flow of blood into the left ventricule as the pressure falls in diastole, but prior to full closure of the aortic valve.
- Finally, a further lower velocity forward flow during the remainder of diastole, again due to recoil of blood vessels. This third phase may be absent in calcified or atherosclerotic vessels.
Conditions where it is useful
- Peripheral vascular disease
- Varicose veins
- Deep vein thrombosis
- Valvular heart disease
- Critical care monitoring (trans-oesophageal echocardiography to assess cardiac output)
- Monitoring of fetal heart sounds.