|Year : 2007 | Volume
| Issue : 3 | Page : 176-183
Ultrasound in anaesthesia
PN Jain1, Priya Ranganathan2
1 MD, MNAMS, Professor, Dept of Anaesthesiology, Critical Care & Pain, Tata Memorial Hospital, Mumbai, India
2 MD, DNB Asstt. Professor, Dept of Anaesthesiology, Critical Care & Pain, Tata Memorial Hospital, Mumbai, India
|Date of Acceptance||23-Apr-2007|
|Date of Web Publication||20-Mar-2010|
P N Jain
902, Fun Tower GD, Ambedkar Road, Parel, Bhoiwada, Mumbai-400012
Source of Support: None, Conflict of Interest: None
Ultrasound technology is a rapidly emerging science and the field of anaesthesia has not remained untouched by its widespread applications. It is playing an increasing role in vascular access, in regional anaesthesia for nerve blocks and as a transoesophageal echocardiography tool for cardiac imaging and viewing blood flows. It has special applications to assess the depth of epidural space in cases of difficult anatomy or in an otherwise high risk patient where interventional procedure is required. As the ultrasound guidance is becoming standard practice of future, anaesthesiologists need to develop a thorough understanding of this technology& practical skills by training themselves.
Keywords: Anaesthesia, Ultrasound
|How to cite this article:|
Jain P N, Ranganathan P. Ultrasound in anaesthesia. Indian J Anaesth 2007;51:176-83
| Introduction|| |
The technology of ultrasound in medicine has evolved leaps and bounds over the years. Modern ultrasound machines are more compact & portable, with better resolution and enhanced tissue penetration making it a handy tool for identification and desired intervention in various body structures. Anaesthesiologists have been performing diverse interventional procedures using anatomical landmarks over so many years with variable success rates, risks, and consequences of complications. The ultrasound imaging can play a major role in the field of anaesthesiology, critical care & pain to perform with precision and reduce complications. Ultrasound has been shown to offer excellent guidance for difficult venous access, epidural space identification, delineating nerve plexuses for chronic pain nerve blocks, in transoesophageal echocardiography or recently in vehement use for regional anaesthesia. However, the use of ultrasound in daily clinical practice will require not only high precision machines but also a high degree of training of anaesthesia users. The training in ultrasound techniques in near future will become part of the core training of every anaesthesiologists, just as laparoscopic work is for surgeons. Anaesthesiologists need to develop a thorough knowledge of sonoanatomy involved and acquire clear concepts for both ultrasound technology and skills to visualize various structures intended to be manipulated. A day is not far the ultrasound imaging may become an important component of anaesthesia machine. This review attempts to highlight the basics of ultrasound and its use in regional anaesthesia, venous access and as transoesophageal echocardiography tool.
| The historical perspective|| |
The association between ultrasound and living systems has been studied since 1920's. The discovery of piezoelectric effect and its utility in construction of high frequency mechanical vibrating sources coupled with high frequency electronic drives provided the basis of this great work. In 1960's physicians began to accept ultrasound and used this technique in the clinics. The 1970's witnessed the widespread use of ultrasound in clinical medicine.
| Basic physics|| |
Sound is produced when mechanical energy travels through matter as a wave, producing alternate compression and rarefaction. Ultrasound imaging is based on the scattering of sound energy by interfaces formed of materials of different properties. The amplitude of reflected energy is used to generate ultrasound images. Frequencies used for ultrasound are higher than those in the audible range, and typically vary from 2 to 15 MHz for diagnostic procedures
| Parts of ultrasound|| |
Generates precisely timed, high amplitude voltage to energize the transducer. It also controls the rate of pulses emitted (pulse repetition frequency) - the ultrasound pulses must be spaced with enough time between the pulses, to permit sound to travel to the depth of interest and return before the next pulse is sent
Converts electrical to mechanical energy and viceversa. It serves two functions
2. Receiver and processor
- It converts electrical energy provided by the transmitter into acoustic pulses directed into the patient
- It receives the reflected echoes
These detect and amplify the backscattered energy and manipulate the reflected signals for display
3. Image display
Earliest A-mode devices displayed the voltage produced across the transducer as a vertical deflection on the face of the oscilloscope. Only the position and strength of a reflecting structure could be recorded.
M-mode ultrasound displays echo amplitude and shows the position of moving reflectors. It is used in the evaluation of cardiac chambers, valves and vessel walls.
Real-time B-mode display uses multiple ultrasound pulses to generate a two-dimensional image.
Propagation of sound
Ultrasound transducers work on the principle of piezoelectricity. Within the transducer are arrays of piezoelectric crystals, which have the property of changing shape when an electrical voltage is applied. Application of a voltage, which oscillates at the resonant frequency of the crystal, enables electrical energy to be converted into sound energy [Figure 1]. Modern systems have arrays that are structured to allow the sound waves generated by one crystal to interact with those from other crystals. Consequently the sound waves can be amplified or diminished.
The sound wave is propagated through the body tissues and interactions occur between the wave and the tissues. If the sound is transmitted through a homogeneous structure the principal interaction is absorption of the sound. The rate of absorption is least in fluids and greatest in solid structures. The majority of body tissues are not homogeneous and the sound wave strikes a series of interfaces. At each interface the wave can be reflected or refracted. Refraction is usually insignificant. The waves that are reflected back to the transducer strike the piezo electric crystal. The crystal converts sound into electrical energy. The distance of the reflector can be calculated by calculating the time taken for the sound to travel from and to the transducer. The amplitude of the reflected sound can be used to calculate the reflectivity of the object.
The proportion of sound reflected or transmitted at an interface depends upon the difference in acoustic impedance between the tissues forming the interface. The acoustic impedance is measured in Rayls and is the product of the density of the tissue and the velocity with which it propagates sound. Air and bone have different impedance compared to other tissues; therefore, at such interfaces the majority of sound is reflected. Hence, ultrasound cannot be used to image deep to bone or air.
Acoustic impedance of different materials [Additional file 1]
High frequency transducers produce higher resolution images but the sound waves are absorbed more as they pass through the body. Low frequency transducers have greater penetration, but poor resolution
| Resolution|| |
This refers to the ability of the device to differentiate two closely situated objects as distinct structures. Axial resolution is measured along the axis of the ultrasound beam in its direction of propagation. It is directly proportional to the ultrasound frequency. Transverse resolution is measured at 90 degrees to axial resolution. It depends on the width of the pulse beam. Axial resolution is always superior to transverse resolution
| System set-up|| |
- Transducer selection: The correct frequency should be selected depending on the depth of penetration, and the resolution needed
- Depth: The operator can select the depth of tissue that is displayed on the monitor. With greater depth, structures appear smaller but a wider anatomical area can be covered
- Focus: For optimum image, the area under examination should be within the focal area of the ultrasound beam
| Doppler ultrasound|| |
The Doppler principle is the phenomenon in which sound transmitted from a moving object is perceived by a stationary observer to be of a different frequency depending upon the velocity and direction of travel. Thus, changes in frequency (frequency shift) can be used to calculate velocity of movement of blood.
| Applications of ultrasound in anaesthesia|| |
The applications of ultrasound in anaesthesia include
- Ultrasound for vascular access
- Ultrasound guided regional anaesthesia
- Trans-esophageal echocardiography
| Ultrasound for vascular access|| |
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:
Commonly used sites for central venous cannulation are
- Haemodynamic monitoring
- Intravenous delivery of blood products and drugs
- Total parenteral nutrition
- Cardiac pacemaker placement
- Difficult peripheral access
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 internal jugular vein (IJV)
- The subclavian vein (SV)
- The femoral vein (FV).
The most common complications associated with CVC placement are:
The risk of complications increases, depending upon:
- Arterial puncture
- Nerve injury
- Multiple unsuccessful attempts
- Malposition of catheter
- Arteriovenous fistula formation
The advantages of ultrasound-guided central venous catheterization include:
- Difficult anatomy: obesity, short neck, scarring due to surgery or radiation
- Repeated catheterization: increased risk of thrombus formation
- Patients on mechanical ventilation
Two types of ultrasound guidance are described: twodimensional (2-D) imaging ultrasound guidance and audio guided Doppler ultrasound guidance. Two-dimensional ultrasound provides a real-time image of the anatomy. Au-dio-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
- Identification of the vein
- Detection of variable anatomy
- Detection of intravascular thrombi
- Avoidance of inadvertent arterial puncture.
The needle puncture may be made in two ways:
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.
- Indirectly: after pre-procedure identification of th vessel by ultrasound. This technique may not have any advantage over conventional 'landmark' identification of vascular structures.
- Directly: under real-time visualization
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[Figure 2] AB. A meta-analysis of 12 randomized controlled trials evaluating the effect of realtime ultrasound guidance using regular or Doppler ultrasound for central venous catheter placement was 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.  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-D ultrasound 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.  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.  Ultrasound 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: 
Among currently available imaging techniques, ultrasound is the most compatible with these criteria
- Good resolution
- Safety - for both patient and operator - minimal exposure to radiation
- Offer real time guidance
- Should not require additional personnel to operate
In routine anaesthetic practice, ultrasound can be used for
- Peripheral nerve plexus blocks
- Central neuraxial blocks in children and in difficult anatomical situations in adults
- 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: ,,,
On high-resolution ultrasonography, nerves appear as honeycomb structures with hypoechoic fascicles surrounded by hyperechoic tissue. 10-15 MHz 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. 
- 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
For ultrasound-guided nerve block, all the anatomical structures in the target area have to be visualized[Figure 3] AB. 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 - largebore 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 character istics 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 perfor - mance 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 beam to 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 at which the ligamentum flavum should be encountered.  Studies 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 insertionin patients undergoingscoliosis surgery. Ultrasound has also been used to identify landmarks prior to difficult lumbar subarachnoid puncture. ,
| Ultrasound in paediatrics|| |
Ultrasonographyis particularly useful for neuralblocks in children for the following reasons:
Spinous interspaces and intervertebral foramina allow the ultrasonic beam to penetrate through, to visualize deeper structures.[Figure 4]
- 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.
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|| |
A detailed review of TEE is beyond the scope of this article. 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. 
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]