A Doppler stethoscope sends ultrasound waves around the body to detect movement and record frequency shifts, making it more useful in noisy environments. It has applications in fetal ultrasound and emergency medicine.
A stethoscope is an instrument that allows people to hear sound inside an object, and a Doppler stethoscope is a particular variety. Regular stethoscopes, used by many medical professionals, amplify the sound the object makes. A Doppler stethoscope, on the other hand, can send sound waves around the body and record how often they bounce. The shift in frequency of a moving object, such as a heart inside a person’s chest or an ambulance traveling towards a traffic intersection, is called the Doppler effect. Compared to a regular stethoscope, a Doppler stethoscope is more useful in environments with high ambient noise, such as in a helicopter.
Sound travels in a wave and can have different frequencies. A frequency is the number of times a single wave repeats in a given period of time. Sound is measured in units of hertz (Hz), which individually represent one complete wave cycle per second. Humans can normally hear sounds in the range of 20 to 20,000 Hz, but noises produced by the body are usually in the range of 100 to 200 Hz. These noises include heartbeat and breathing, both caused basically by the movement of the organs involved. The heart contracts and relaxes to pump blood, while the lungs expand and contract to take in air.
Conventional stethoscopes are placed on the skin, and the doctor then listens to the amplified sounds the body makes. A Doppler stethoscope is also placed on the body, but it produces sounds in the ultrasound frequency range and sends these waves around the body. Ultrasound refers to any sound above the maximum hearing limit of 20,000 Hz for humans. Although the doctor can’t hear the ultrasound bouncing off the affected organs, the Doppler stethoscope turns the ultrasound into a sound signal that the doctor can hear.
Only moving body parts are suitable for the Doppler stethoscope, such as the lungs and circulatory system, as movement of the object in question is required for the Doppler effect to be present. When an object that produces sound approaches something that hears it, the frequency of the sound heard increases; as the object moves away, it becomes lower. This effect is used by emergency services, where the sirens sound different as they approach or move away.
As the ultrasound bounces off the moving part, it changes in frequency depending on how far away the part is at that moment. For example, a deflated lung is further from the front of the chest than an inflated lung, and so ultrasound detects the change in frequency. When it registers the change, it alters the audible sound that the doctor hears.
Compared with ordinary stethoscopes, Doppler stethoscope has a significant advantage. It hears the ultrasound it produces itself, and does not hear most of the noise already present in the environment. In noisy locations such as an emergency medical helicopter or in combat situations, the Doppler stethoscope provides a more accurate and clearer picture of an individual patient’s health. It also has applications in fetal ultrasound, where it can be used to detect a baby’s heartbeat in the womb.
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