Ultrasound Blocks for the Anterior Abdominal Wall

24 | Ultrasound Blocks for the Anterior Abdominal Wall
2. Ultrasound and Regional Anesthesia
Gabriele Aletti
Sound Waves
The application of pressure to a medium for a given period of
time causes the compression of its molecules that will become
closer to the subsequent molecules. The pressure energy will be
propagated deeper between adjacent molecules in the direction
of the compression. The movement of the molecules will
propagate in the form of a pressure wave. A wave is a
disturbance in a medium traveling through it at a constant
speed. The periodic application of pressure will generate more
waves that will travel through the medium.
Sound waves are subsequent and periodic high pressure and
low pressure waves of molecular vibration. They travel
longitudinally through a physical medium determining high
pressure areas (compression) and low pressure areas (rarefaction)
along the direction of propagation. Each wave has a frequency
(f) of propagation measured as cycles per unit of time.
The wavelength ( λ) is defined as the geometric distance at an in-
stant between two successive high pressure pulses or two
successive low pressure pulses. The wavelength of sound
decreases as frequency increases. The speed of propagation of