The modern practice of anaesthesia and critical care includes performing a number of invasive procedures e.g. vascular puncture (peripheral and central venous), nerve blocks and pleural aspiration. These historically have been performed using surface anatomical landmarks to guide needle placement. As a result of this both success and complication rates have varied considerably. Skill of the operator and individual patient variables, including size, anatomical variation and disease profile may affect outcomes. Although radiologists have for many years used 2D ultrasound for diagnostic purposes and to guide needle insertion, anaesthetists have not until recently had the resources or training to use such equipment. The last 3-5years has seen the development of affordable and portable ‘point of care’ ultrasound machines with enhanced capabilities, this along with the guidelines produced by National Institute of Clinical Excellence (NICE) has led to widespread adoption of the daily use ultrasound in anaesthesia.
Ultrasound is currenlt widely used to assist with Vascular access, Regional anaesthesia, Cardiac assessment, Lung assessment and central neuro-axial blockade. New developments include using ultrasound for abdominal blocks (Transverse Abdominal Plain block & Rectus sheath blocks) Paravertebral blocks and Caudals.
Ultrasound uses sound waves, above a frequency that the human ear can hear, to generate an image of body structures.
Audible sounds sensed by the human ear are in the frequency range of 20Hz to 20 kHz. Below 20Hz the sound waves are called infrasound and above 20 000Hz (20 kHz) they are called ultrasound. Sound waves used for medical imaging typically have a frequency in the range 3 - 15MHz.
Sound waves are produced by pressure disturbances that travel through a medium (e.g. air, water) as vibrations of the molecules. Without a medium (e.g. in a vacuum) there can be no sound.Sound waves are examples of longitudinal waves, which are waves that have vibrations along or parallel to their direction of travel.The propagation occurs when the molecules of the medium through which the sounds travels oscillate back and forth from their original rest positions, in the same line as the wave.
The membrane of a loudspeaker can generate sound waves by pushing molecules in front of it like a piston moving forwards and backwards. These molecules will then push the molecules in front of them, and so the disturbance travels away from a source at the speed of sound (c).
The sound waves are transmitted as an alternation series of compressions (zones of high pressure) and rarefactions (zones of low pressure).The motion of the these particles is caused by two factors; the pressure of the wave and the restoring forces of the molecules, or the elasticity, of the medium
There are some properties of the waves that are useful to define
Wavelength (?) is the distance between 2 consecutive, identical positions (such as peaks) in the pressure wave.
Frequency (f) is the number of peaks passing a point in one second or the number of oscillations per second. 1 Hertz (Hz) is 1 cycle per second.
Speed (c) is the distance the wave travels in a given time. This is determined by the elasticity and density of the medium and is a constant for a given medium.
Amplitude is a measure of the degree of change within a medium, caused by the passage of a sound wave and relates to the severity of the disturbance. Three of these quantities are related by the simple relationship that:
Since the speed of sound is a constant for a given medium the wavelength is inversely proportional to the frequency.The speeds of sound in various media are listed in the image. The speed of sound in soft tissues lies very close to an average value of 1540 ms-1. For a frequency range of 3 - 10 MHz, this gives the wavelength of 0.5 - 0.1 mm.
