5. Ultasound

sound waves which exceed the threshold of human hearing (around 20,000 Hz) are described as ultrasonic Medical ultrasound uses frequencies of 2–15 MHz

These waves are generated by applying a high-frequency alternating voltage to the two sides of a piezo-electric crystal transducer (which deforms when a voltage is applied to it). This changes the thickness of the crystal, which then emits ultrasonic radiation of the same frequency as the applied potential difference. The crystal also transduces the reflected waves back into an electrical signal from which a computer-generated cross-sectional image can be displayed.

The signals are unable to penetrate bone or gas-filled structures, including the lung, and so ultrasound studies of these structures are not possible. Reflected signals are strongest from the interface between tissues of different density, such as air and blood, and when the structure being examined is perpendicular to the angle of the beam.

tissue that is highly reflective (hyperechoic) appears white. (Examples include bone and fascial planes.) Weakly reflected waves (hypoechoic) are darker. (Examples include muscle and fat.) Nerves can be either hyperechoic or hypoechoic. The cervical nerve roots in the neck, for example, are hypoechoic and appear dark, but by the time they have formed divisions at the lateral border of the first rib are hyperechoic and white. Blood does not reflect (anechoic) and so blood vessels appear black. Air–tissue interfaces reflect strongly.

the higher the frequency the better the resolution of the image, but this is at the expense of tissue penetration. Lower frequencies will produce images from deeper structures, but their definition is less good.

This can be expressed as the ‘half-power distance’, which is the depth at which the sound is halved. This depth is 3,800 mm for water and less than 1 mm for air and lung. Sound is attenuated by bone (2–7 mm) and also by muscle (6–10 mm).

ultrasound moves through tissue at 1,540 m/s. This rapid transmission and reception of pulses of sound allows the generation of dynamic images.

These are generated by probes which comprise an array of parallel piezoelectric elements that are activated in sequence, rather than simultaneously. This wavefront can, in practice, scan a 90’ sector of tissue, with the reflected echoes processed into a two-dimensional picture.

The Doppler effect describes the change in the frequency of sound and ultrasound if either the emitter or the receiver is moving. Colour flow Doppler is able to display blood flow in real time, using three basic colours. Blood flow towards the transducer is red, whereas that away from the transducer is blue. It is clearly important not to assume that these colours indicate arterial and venous blood. The colour green can be added when blood flow velocity exceeds a preset limit. In areas of turbulent flow, such as may occur across a diseased cardiac valve, all three colours may be displayed

Critical care: Ultrasound scans of the abdomen and thorax can identify fluid collections, which can then be drained under ultrasound guidance. Cranial scanning is routinely used in neonatal intensive care to detect intraventricular haemorrhage and midline shift Echocardiography in critical care: echocardiography is increasingly popular in critical care to assess cardiac function. Imaging can be structural, identifying, for example, pericardial effusion or abnormalities of ventricular wall and cavity size, and it can be haemodynamic, utilizing Doppler techniques to view blood flow through the valves and cardiac chambers Central venous cannulation: ultrasonic-guided cannulation is now routine, particularly for the internal jugular route.

• Air embolism: the interface between air and blood generates a strong reflected signal, and a Doppler probe over the praecordium is sensitive enough to produce ultrasound images from bubbles as small as 2 mm in diameter. • Ultrasonic devices: the principles of ultrasound can be used in gas flowmeters, in cleaning devices and in humidifiers. • TOE • Oesophageal Doppler monitoring (ODM): • Regional nerve blockade: • Gastric emptying. • Cricothyroid membrane

TOE: modern TOE probes allow 180’ views of the heart, and the absence of large tissue masses between the probe and the myocardium allows for well-defined ultrasound images. It has specialist cardiac uses such as the assessment of valvular heart disease, the diagnosis of bacterial endocarditis, the identification of atrial thrombus and the investigation of congenital heart disease. It can identify aortic atherosclerosis, aortic dissection and disease, can assess paracardiac masses. For the general anaesthetist, its main value lies in the intra-operative determination of left ventricular preload and function, the diagnosis of acute left ventricular dysfunction and myocardial ischaemia, and the detection of air embolism. (Complications are mainly mechanical, and relate to the passage and presence of a firm probe within the thin-walled oesophagus with the consequent risk of perforation. T he reported complication rate is very low; in one [early] series of 10,419 awake patients there were only two cases of bleeding.)

ultrasound-guided regional anaesthesia (UGRA) is now regarded by many as a technique that is faster, safer and more efficacious than either landmark or nerve-stimulator assisted methods. The evidence for these assumptions is absent, and the controlled trials to support this view may be a long time coming. As the complication rate of nerve blocks is already low, the numbers of patients required to demonstrate a difference are impractically large. Few would argue, however, with the intuitive proposition that if the needle tip is visible and if the local anaesthetic is seen spreading circumferentially around the nerve, then more successful and safer blocks seem likely. Nerve fascicles themselves are dark, whereas supporting connective tissue tends to be brighter and more hyperechoic. This is a generalization because the varying structure of the fascia which invests a particular nerve means that the same nerve may have a different ultrasound appearance along its course.

Superficial nerves and plexuses are more suitable for UGRA than those sited more deeply. The sciatic nerve in the buttock, for example, is a large structure that is nonetheless difficult to identify because of attenuation of the beam by surrounding gluteal muscles. The advancing needles are best displayed if they are parallel to the probe face; at angles greater than 45’ they become difficult to see. In central neuraxial techniques, ultrasound-assisted location may help confirm the depth to the epidural space, the midline or the spinal level, but real-time guidance is more difficult given that both spinals and epidurals are two-handed techniques, and so its use is not yet routine.

Ultrasound can also be used to assess gastric contents (which is of particular interest in obstetric and emergency anaesthesia) by scanning the gastric antrum. Its value is limited by a high false negative rate of up to 25%, and so it is not currently possible to state unequivocally that the stomach is empty, but the false positive rate is low and so it does allow a broad assessment of risk.

The high failure rate of emergency front-of-neck access has been attributed largely to the inability accurately to locate the cricothyroid membrane. Ultrasound of the neck prior to induction of anaesthesia provides the most reliable means of identification.