Doppler Guide: Vascular vs Foetal

Vascular and foetal Dopplers work on the same principle and are not interchangeable. The difference is probe frequency, and it follows directly from physics.

Frequency and depth trade against each other

Higher frequency gives better resolution but penetrates less tissue. Lower frequency penetrates deeper with less resolution.

• 8 MHz — superficial peripheral vessels. The standard for ankle-brachial pressure index and peripheral pulse assessment.
• 5 MHz — deeper vessels, and larger or oedematous limbs where 8 MHz cannot reach.
• 4 MHz — deep vessels, obese patients.
• 2–3 MHz — foetal heart. Deep penetration through the abdominal wall and uterus.

This is why a foetal probe gives a poor peripheral signal, and a vascular probe will not reliably find a foetal heart.

Vascular Doppler use

The main application is ankle-brachial pressure index, measuring systolic pressure at the ankle against the arm to assess peripheral arterial disease. It is a routine part of diabetic foot assessment and leg ulcer work-up — both common in Singapore given diabetes prevalence.

Also used for locating pulses that cannot be palpated, and assessing arterial flow before procedures.

Foetal Doppler use

Detecting and counting the foetal heart rate in antenatal care, generally from around 10–12 weeks depending on the device and maternal habitus.

Worth stating plainly: these are clinical instruments. Consumer foetal Dopplers sold for home reassurance are a recognised safety concern, because a reassuring sound can delay presentation when something is wrong.

What to look for

• Interchangeable probes if you need more than one frequency.
• Audio quality and output — you are interpreting sound.
• Display — a rate readout helps but should not replace listening.
• Battery type and endurance.
• Gel is a consumable; factor it in.

Dopplers and probes are in diagnostic equipment.

Equipment guidance only; use of these devices requires clinical training.

This article is part of our guide to Building Your Diagnostic Kit: A Guide for GPs and Clinics.