How to buy Dopplers
The purpose of this guide is to provide background information which will allow you to make an informed choice when selecting your doppler. We also wish to eliminate some of the “jargon” relating to Doppler instruments.
How dopplers work
During Doppler use, a handheld instrument (the probe: see below) is passed lightly overlying the skin above a blood vessel or over the tummy for foetal heart monitoring in pregnancy. A special ultrasound gel is used to improve transmission from the skin to the probe. The probe sends and receives sound waves which bounce off moving blood cells and are then processed through a microphone (see below) so that they are audible in the instrument's speaker or earphone. The movement of blood cells causes a change in pitch of the reflected sound waves (called the “Doppler effect”). If there is no blood flow there is no audible signal.
Figure 1 and 2. Doppler transducer or probe overlying a blood vessel with moving blood cells. Signals are sent and received by this probe and amplified within the device which also displays the heart rate.
Information from the reflected sound waves can also be processed by a computer to provide graphs that represent the flow of blood through the blood vessels. These graphs can be saved for future review or evaluation. Doppler can be combined with ordinary ultrasound (sonography) to assess whether blood is moving towards or away from the probe, and its relative velocity. This is called Doppler ultrasound or sonography.
Dopplers are used in many medical fields, including obstretrics and in vascular surgery where they are used as a cheaper non-invasive method of diagnosing and assessing the severity of peripheral vascular disease and assessing reflux in venous disease.
Dopplers in foetal heart monitoring: Obstretric Doppler auscultation
Advantages of the Doppler foetal monitor over a foetal stethoscope is the audio output, which allows people other than the user to listen to the heartbeat. Originally intended for use by health care professionals, dopplers are becoming popular for personal use.
The use of Dopplers in peripheral arterial disease (PAD)
Diagnosing patients with PAD allows them to be put on the correct treatment that would reduce deaths from heart attacks etc.
Measuring ABPI involves measuring the blood pressure at the ankle which is compared to the arm using a sphygmomanometer cuff and a Doppler probe whilst the patient is lying down (using an 8 MHz probe). Please refer to our Doppler resource page for how to measure ABPI.
The ABPI is simply the ratio of the ankle pressure divided by the arm blood pressure. A ratio of < 0.9 is diagnostic of PAD whilst a ratio of less than 0.5 indicates severe PAD. Those who might use Dopplers include vascular technologists, vascular surgeons, vascular lab managers, nurses as well as other allied medical ultrasound professionals. Remember it is not only the absolute ABPI that matters, the waveform is also important. A damped or monophasic waveform indicates severe PVD although the ABPI may be artificially high as in diabetics.
In addition to diagnosing PAD, Dopplers are commonly used by vascular surgeons to assess reflux in venous disease, particularly varicose veins.
Dopplers are available in different formats including the popular Hand Held and Table Top varieties; alternatively, they can be incorporated into a comprehensive vascular system.
Hand Held Dopplers
Table Top Dopplers
Additional features include automated cuff inflation/deflation, built-in printer, and graphic waveform display with step-by-step ABI instructions.
It is vital to use the correct probe for the task.
Probes come in different shapes and sizes depending on the application. For superficial vessels, a pencil type probe is ideal for accurate localisation of blood vessels beneath the skin. For obstetric applications a broader probe is utilized. Some probes are designed to be inserted into body cavities such as trans-vaginal probes (inserted through the vagina). These are useful in obese women in whom it is difficult to localise the baby’s heart through the tummy. Another consideration is whether to choose a waterproof probes, this is especially useful for obstetric probes.
In vascular applications an important consideration is whether to choose a mono directional or bi-directional system. In most situations where the Doppler will be used to measure ABPI (see above), mono directional systems are more than sufficient. However bi-directional systems are useful in situations where direction flow is important for example to detect reflux in venous disease.
Probe frequency. The 2 and 3 MHz probes are ideal for obstetric use although in some situations a transvaginal 5 Mhz probe is useful. In vascular surgery the 5 and 8 Mhz probes are ideal. Here is a list of probes and their application in more detail.
When purchasing a suitable Doppler system, you need a high quality, high performance system that offers value for money. We supply superior diagnostic products from Summit Doppler - a leading brand Doppler systems manufacturer, offering you the most advanced obstetrical and vascular Doppler systems available today. Most important is the high level of backup service which comes with these products.
These are just a few of the facts and product features of Doppler systems. We retail a full range, plus accessories, online. Please go to SJT Doppler pages for full details of our product range and to purchase products. Alternatively, if you would like to speak to someone about our Doppler systems, please contact us directly on Tel : 0844 272 1918.