Applications of ultrasound in anaesthesia

Applications of ultrasound in anaesthesia

The applications of ultrasound in anaesthesia include

1. Ultrasound for vascular access

2. Ultrasound guided regional anaesthesia

3. Trans-esophageal echocardiography

ULTRASOUNDFORVASCULARACCESS

Ultrasound can be used to reduce complications associated with the cannulation of veins and arteries.

ULTRASOUND GUIDED CENTRAL VENOUS CANNULATION

Indications for central venous catheter insertion

include:

 Haemodynamic monitoring

 Intravenous delivery of blood products and drugs

 Haemodialysis

 Total parenteral nutrition

 Cardiac pacemaker placement

 Difficult peripheral access

Commonly used sites for central venous cannulation

are

 The internal jugular vein (IJV)

 The subclavian vein (SV)

 The femoral vein (FV).

The traditional “landmark” method of central venous cannulation relies on surface anatomical landmarks. The literature failure rates for initial CVC insertion with this method have been reported to range between 10% and 35%.

The most common complications associated with CVC placement are:

 Arterial puncture

 Pneumothorax

 Nerve injury

 Multiple unsuccessful attempts

 Malposition of catheter

 Arteriovenous fistula formation

The risk of complications increases, depending upon:

 Difficult anatomy: obesity, short neck, scarring due to surgery or radiation

 Repeated catheterization: increased risk of

thrombus formation

 Coagulopathies

 Patients on mechanical ventilation

The advantages of ultrasound-guided central

venous catheterization include:

 Identification of the vein

 Detection of variable anatomy

 Detection of intravascular thrombi

 Avoidance of inadvertent arterial puncture.

Two types of ultrasound guidance are described:  two dimensional (2-D) imaging ultrasound guidance and audio guided Doppler ultrasound guidance.

Two-dimensional ultrasound provides a real-time image of the anatomy. Audio-guided Doppler ultrasound helps to localize the vein and differentiate it from its companion artery. However it does not give an idea about the depth of the vessel

The needle puncture may be made in two ways:

 Indirectly: after pre-procedure identification of the vessel by ultrasound. This technique may not have any advantage over conventional ‘landmark’ identification of vascular structures.

 Directly: under real-time visualization

Machines designed for vascular access (e.g.Siterite) usually provide B-mode 2-D real-time images; generally using 2.5 to 10 MHz probes. Needles are seen more easily in longitudinal section; however relationship of the needle to surrounding structures is better appreciated in transverse section. In the absence of direct view, tissue distortion produced by needle movement can indicate the direction.

A guide may be present on the ultrasound probe to

facilitate needle insertion. Sterile gel is used between the probe and the skin surface, and sterility of the probe is maintained by covering it with a transparent plastic sheath.

Arteries appear round in cross-section, are pulsatile, and not easily compressible with pressure applied by the probe.

Veins are more irregular, vary in size with respiration and are easily compressible. A meta-analysis of 12 randomized controlled trials evaluating the effect of realtime ultrasound guidance using regular or Doppler ultrasound for central venous catheter placementwas conducted and they found a reduction in placement failure, decreased need for multiple attempts, and decreased complications, as compared to the standard landmark technique.Another meta-analysis of 7 trials was carried out comparing the use of 2-D ultrasound versus landmark method for central venous cannulation in adults.5 It showed that for IJV cannulation, 2-D ultrasound guidance was associated with reduced risks of failed catheter placements, catheter placement complications, failure on the first catheter placement attempt, and fewer attempts to achieve successful catheterization.

The difference between the 2-D ultrasound method and the landmark method in the time taken to insert a catheter successfully was small and not statistically significant. For subclavian vein cannulation, 2-Dultrasound guidance was associated with reduced risks of catheter

placement failure and catheter placement complications.

In the cannulation of the IJV in infants, 2-D ultrasound guidance was significantly better than the landmark method in terms of reductions in the risk of failed catheter placements, the risk of catheter placement complications, and the number of attempts required before catheterization was successful. Using 2-D ultrasound guidance, successful cannulation was achieved more quickly than with the landmark method, although this result was not statistically significant.

2D ultrasound was also found to be superior to

Doppler ultrasound for IJV and subclavian vein procedures.

Based on this meta-analysis, the NICE (National

Institute for Clinical Excellence – NHS) has recommended that the use of two-dimensional (2-D) imaging ultrasound guidance should be considered in most clinical circumstances where CVC insertion is necessary.3, The use of ultrasound for vascular access may be particularly helpful in haemodialysis patients who need wide bore access, present for repeated cannulation, may not be able to lie supine, and may have underlying coagulopathy or platelet dysfunction.7Ultrasound can also be used as an alternative to X-ray to check for malposition of central venous catheters and peripherally inserted central catheters. Routine ultrasound examination of recently cannulated veins can also be done to rule out presence of thrombi, prior to re-cannulation.

Ultrasound for arterial cannulation

Arterial cannulae are inserted for blood pressure

monitoring and blood gas sampling. Studies comparing the use of ultrasound versus blind technique for radial artery cannulation have found that ultrasound guidance decreases the number of attempts, and improves the overall success

rate of cannulation.

Ultrasound guided regional anaesthesia

The features of any imaging technique used for regional anaesthesia should include:

 Good resolution

 Safety - for both patient and operator – minimal

exposure to radiation

 Offer real time guidance

 Portability

 Should not require additional personnel to operate

Among currently available imaging techniques, ultrasound is the most compatible with these criteria In routine anaesthetic practice, ultrasound can be used for

1. Peripheral nerve plexus blocks

2. Central neuraxial blocks in children and in difficult anatomical situations in adults

3. In procedures for chronic pain

Peripheral nerve blocks

A successful regional block requires optimum distribution of local anaesthetic around nerve and plexus structures.

Ultrasound imaging has the following advantages:

 Direct visualization of neural structures

 Direct visualization of related structures like blood vessels and tendons, which helps to identify nerves

 Guidance of the needle under real-time visualization

 Avoid complications like intravascular and

intraneuronal injection

 Monitor the spread of local anaesthetic

 Allows repositioning of the needle after an initial injection to allow better delivery of local anaesthetic to areas that may not be completely blocked with a single dose

 Can be used in patients with poor twitch response to nerve stimulation On high-resolution ultrasonography, nerves appear as honey comb structures with hypoechoic fascicles surrounded by hyperechoic tissue. 10-15MHz probes are used for the brachial plexus at the interscalene or supraclavicular level. Deeper nerves like the sciatic, infraclavicular and popliteal require the use of lower frequency – 4-8 MHz – probes.

For ultrasound-guided nerve block, all the anatomical structures in the target area have to be visualized.

The penetration depth, the frequencies, and the position of the focal zones are optimized. The visibility of the needle on ultrasound is affected by the angle of insertion – reduced at steep angles - and the gauge of the needle – large bore needles are easy to visualize. The out-of-plane needle approach involves inserting the needle so that it crosses the plane of imaging near the target. The needle is not visible during insertion. The in-plane needle approach the needle is inserted within the plane of imaging to visualize the entire shaft and tip.

Once the needle is optimally in place, the local anaesthetic is administered under direct sonographic visualization until the nerve structures are surrounded by local anaesthetic. If the local anaesthetic does not spread in the right direction, the needle can be repositioned accordingly.

Air bubbles can cause shadowing and have to be removed prior to injection. Bicarbonate containing solutions are avoided because of CO2 production, which can interfere with imaging.

Nerve stimulation may be combined with ultrasound guidance to confirm nerve- needle contact. However, this has not been shown to confer any advantages A number of clinical studies have examined block characteristics

with ultrasound guidance at different anatomical locations.

All studies found improved block characteristics,

including reduced onset time and improved quality of block.

The dose of local anaesthetic required was reduced. The incidence of paraesthesia was also decreased, which could minimize post-procedure neuropraxia. The block performance time was not significantly increased. Complications like neurological damage and vessel puncture were avoided.

Central neuraxial blockade

Ultrasound guidance for neuraxial anaesthesia is limited by the presence of bony structures like laminae, spinous processes and transverse processes, which do not allow the ultrasonic be amto pass through. Also, the depth of the epidural space in adults needs imaging with low frequency

probes, which gives poor resolution. Present studies indicate that ultrasonography should be used along loss of resistance techniques, to guide needle orientation, and to give an idea of the depth atwhich the ligamentum flavum should be encountered.11Studies on the use of ultrasound for lumbar epidurals have shown good correlation between ultrasonographically measured data on the depth of the lumbar epidural space and direct measurement at the time of lumbar puncture. Ultrasound guidance is associated with significant reduction of the puncture attempts, reduction in

the number of puncture levels, more precise application of the catheter, and improvement of analgesia quality and patient satisfaction. Ultrasound visibility has been shown to be higher in the paramedian as compared to the median

plane. Ultrasound imaging has been shown to be superior to clinical palpation as a method of identifying lumbar intervertebral level In one case series, ultrasound guidance was used to determine the least rotated vertebral body for epidural catheter insertion in patients undergoing scoliosis surgery. Ultrasound has also been used to identify landmarks prior to difficult lumbar subarachnoid puncture.

Ultrasound in paediatrics

Ultrasonography is particularly useful for neural blocks in children for the following reasons:

 Variability in anatomy according to age and

constitution of the patient.

 Regional blocks are usually performed under

anaesthesia or sedation - adverse effects may not

be detected.

 Because of the superficial location of most neural structures in children, one can use higher frequency ultrasonic probes, with better resolution.

Spinous interspaces and intervertebral foramina allow the ultrasonic beam to penetrate through, to visualize deeper structures.

Studies have shown that ultrasound provides

information on the distance of skin-to-ligament flavum in neonates, infants and children. Hence, the risk of dural puncture is reduced and the spread of local anaesthetic can also be visualized.

Pain interventions

The use of ultrasound has been shown to have 100% accuracy in locating the caudal space and guiding epidural needles for caudal injections for low back pain. Use of ultrasound for facet joint injections, lumbar sympathetic blocks, celiac plexus blocks, stellate ganglion blocks and

identification of myofascial trigger points has also been described.

Ultrasound for trans-oesophageal echocardiography

 Currently available TEE probes combine multiplanar ultrasound for cardiac imaging, with Doppler to view blood flows.

TEE is used in anaesthesia to:

 Assess adequacy of repair and detect residual

pathology or prosthetic valve dysfunction in patients undergoing surgery for valvular and congenital heart disease

 Diagnose ongoing ischemia by detecting fresh regional wall motion abnormalities in patients with ischemic heart disease

 Assess left and right ventricular function, and volume status in patients with severe haemodynamic instability

 As a sensitive tool for early detection of pulmonary embolism, especially in patients undergoing neurosurgery in the sitting position

 Transesophageal stress echocardiography to detect coronary artery disease and viability.

Newer applications

The use of laryngeal ultrasound to detect patients at risk of post-extubation stridor, by evaluating peri-cuff airflow has been described. Ultrasound has also been shown to be as effective as MRI to assess subglottic diameter, to calculate appropriate endotracheal tube size. Ultrasound has been used to visualize CSF leak in cases of post-dural puncture headache, and for the application of epidural blood patch under real-time depiction.