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Year : 2007  |  Volume : 51  |  Issue : 3  |  Page : 176-183  

Ultrasound in anaesthesia

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 Acceptance23-Apr-2007
Date of Web Publication20-Mar-2010

Correspondence Address:
P N Jain
902, Fun Tower GD, Ambedkar Road, Parel, Bhoiwada, Mumbai-400012
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Source of Support: None, Conflict of Interest: None

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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

How to cite this URL:
Jain P N, Ranganathan P. Ultrasound in anaesthesia. Indian J Anaesth [serial online] 2007 [cited 2021 Jul 28];51:176-83. Available from: https://www.ijaweb.org/text.asp?2007/51/3/176/61139

   Introduction Top

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 Top

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 Top

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 Top


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

1. Transducer

Converts electrical to mechanical energy and vice­versa. It serves two functions

  • It converts electrical energy provided by the transmitter into acoustic pulses directed into the patient
  • It receives the reflected echoes
2. Receiver and processor

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 [2][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 Top

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 Top

  1. Transducer selection: The correct frequency should be selected depending on the depth of penetration, and the resolution needed
  2. 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
  3. Focus: For optimum image, the area under examina­tion should be within the focal area of the ultrasound beam

   Doppler ultrasound Top

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 Top

The applications of ultrasound in anaesthesia include

  1. Ultrasound for vascular access
  2. Ultrasound guided regional anaesthesia
  3. Trans-esophageal echocardiography

   Ultrasound for vascular access Top

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

   Ultrasound guided central venous cannulation Top

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%. [3],[4]

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: 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

The needle puncture may be made in two ways:

  • 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
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[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. [4] 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-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. [5],[6] 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. [7] 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. [7]

   Ultrasound for arterial cannulation Top

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. [9],[10]

   Ultrasound guided regional anaesthesia Top

The features of any imaging technique used for re­gional anaesthesia should include: [11]

  • 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 Top

A successful regional block requires optimum distri­bution of local anaesthetic around nerve and plexus struc­tures. Ultrasound imaging has the following advantages: [12],[13],[14],[15]

  • 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 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. [13]

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. [15]

Nerve stimulation may be combined with ultrasound guidance to confirm nerve- needle contact. However, this has not been shown to confer any advantages. [16],[17] A number of clinical studies [14],[18] 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 Top

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. [11] Studies on the use of ultrasound for lum­bar epidurals [19] 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. [20],[21] In one case series [22] , 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. [23],[24]

   Ultrasound in paediatrics Top

Ultrasonographyis particularly useful for neuralblocks 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.[Figure 4]

Studies [19],[26],[27],[28] 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 Top

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. [29] 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. [7],[12],[13]

   Ultrasound for trans-oesophageal echocardiography Top

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 Top

The use of laryngeal ultrasound to detect patients at risk of post-extubation stridor, by evaluating peri-cuff air­flow has been described. [30] Ultrasound has also been shown to be as effective as MRI to assess subglottic diameter, to calculate appropriate endotracheal tube size. [31] 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. [18]

   References Top

1.Merritt CRB. Physics of Ultrasound. In: Rumack CM, Wilson SR, Charboneau JW eds. Diagnostic Ultrasound. St Louis, Mosby .1998: 3-33  Back to cited text no. 1
2.Taylor PM. Ultrasound for anaesthetists. Current Anaesthesia& Critical Care 2003; 14: 237-249.  Back to cited text no. 2
3.National Institute for Clinical Excellence. Guidance on the use of ultrasonic locating devices for placing central venous catheters [NICE technology appraisal no 49]: September 2002.  Back to cited text no. 3
4.Randolph AG, Cook DJ, Gonzales CA, Pribble CG. Ultrasound guidance for the placement of central venous catheters - a meta­analysis. Crit Care Med 1996; 24: 2053-2058.  Back to cited text no. 4
5.Hind D, Calvert N, McWilliams R, et al. Ultrasonic locating devices for central venous cannulation - a meta-analysis. BMJ 2003; 327: 361-4.  Back to cited text no. 5
6.Schummer W, Schummer C, Tuppatsch H, Fuchs J, Bloos F, Huttemann E. Ultrasound guided central venous cannulation: Is there a difference between Doppler and B-mode ultrasound? J Clin Anesth 2006; 18: 167-172.  Back to cited text no. 6
7.Hatfield A, Bodenham A. Ultrasound: an emerging role in anaes­thesia and intensive care. Br J Anaesth 1999; 83: 789-800.  Back to cited text no. 7
8.Davey A, Boyd C. Central venous access - anatomy and Ultra­sound. studentBMJ 2006; 14: 397- 440.  Back to cited text no. 8
9.Maecken T, Grau T. USG for vascular access. Crit Care Med 2007; 35: S178-185.  Back to cited text no. 9
10.Levin PD, Sheinin O, Gozal Y. Use of ultrasound in insertion of radial artery catheters. Crit Care Med 2003; 31: 481-484.  Back to cited text no. 10
11.Oldman MJ, Nicholls BJ. Imaging techniques and regional ana­esthesia. Current Anaesthesia and Critical Care 2004; 15: 255­-261.  Back to cited text no. 11
12.Dureja GP. Guidance in regional anaesthesia: Is it the gold stan­dard? Editorial. J Anaesth Clin Pharmacol 2007; 23: 119-120.  Back to cited text no. 12
13.Kumar PA, Brooks GW, Arora H. Ultrasound guidance in re­gional anaesthesia. J Anaesth Clin Pharmacol 2007; 23: 121-128.  Back to cited text no. 13
14.Andrew T. Gray. Ultrasound-guided regional anaesthesia: cur­rent state of the art. Anesthesiology 2006; 104: 368-373.  Back to cited text no. 14
15.Marhofer P, Greher M, Kapral S. Ultrasound guidance in re­gional anaesthesia. Br J Anaesth 2005; 94: 7-17.  Back to cited text no. 15
16.Chan V, Perlas A, McCartney C, Brull R, Xu D. Ultrasound guidance improves success rate of axillary brachial plexus block. Can J Anaesth 2007; 54: 176-182.  Back to cited text no. 16
17.Beach ML, Sites BD, Gallagher JD. Use of a nerve stimulator does not improve efficacy of ultrasound-guided supraclavicular nerve blocks. J Clin Anesth 2006; 18: 580-84.   Back to cited text no. 17
18.Grau T. Ultrasonography in the current practice of regional anaesthesia. Best Practice and Research Clinical Anaesthesiology 2005; 19: 175-200.  Back to cited text no. 18
19.Grau T. The evolution of ultrasound imaging for neuraxial anaes­thesia. Can J Anaesth 2003; 50: R1-R8.  Back to cited text no. 19
20.Furness G, Reilly MP, Kuchi S. An evaluation of ultrasound imaging for identification of lumbar vertebral level. Anaesthesia 2002; 57: 277-283.  Back to cited text no. 20
21.Watson MJ, Evans S, Thorp JM. Could sonography be used by an anaesthetist to identify a specified lumbar interspace before spinal anaesthesia? Br J Anaesth 2001; 90: 509-511.  Back to cited text no. 21
22.McLeod A, Roche A, Fennelly M. Ultrasonography may assist epidural insertion in scoliosis patients. Can J Anaesth 2005; 52: 717-720.  Back to cited text no. 22
23.Peterson M, Abele J. Bedside ultrasound for difficult lumbar puncture. The Journal of Emergency Medicine 2005; 28: 197­-200.  Back to cited text no. 23
24.Ferre RM, Sweeney TW. Emergency physicians can easily ob­tain ultrasound images of anatomical landmarks relevant to lum­bar puncture. American Journal of Emergency Medicine 2007; 25: 291-296.  Back to cited text no. 24
25.Rapp H, Grau T. Ultrasound imaging in paediatric regional ana­esthesia. Can J Anaesth 2004; 51: 277-278.  Back to cited text no. 25
26.Willshcke H, Bosenberg A, Marhofer P, et al. Epidural catheter placement in neonates: sonoanatomy and feasibility of ultrasonographic guidance. Regional Anesthesia and Pain Medi­cine 2007; 32: 34-40.  Back to cited text no. 26
27.Rapp H, Grau T. Ultrasound guided regional anaesthesia in pae­diatric patients. Techniques in Regional Anesthesia and Pain Management 2004; 8: 179-198.   Back to cited text no. 27
28.Kil HK, Cho JE, Kim WO, Koo BN, Han SW, Kim JY. Pre­puncture ultrasound measured distance: an accurate reflection of epidural depth in infants and small children. Regional Anesthesia and Pain Medicine 2007; 32: 102-106.  Back to cited text no. 28
29.Chen C, Tang S, Hsu T et al. Ultrasound guidance in caudal epidural needle placement. Anesthesiology 2004; 101: 181-184.   Back to cited text no. 29
30.Ding LW, Wang HC, Wu HD, Chang CJ, Yang PC. Laryngeal ultrasound: a useful method in predicting post-extubation stri­dor. Eur Respir J 2006; 27: 384-389.   Back to cited text no. 30
31.Lakhal K, Delplace X, Cottier J et al. The feasibility of ultra­ sound to assess subglottic diameter. Anesth Analg 2007; 104: 611-614.  Back to cited text no. 31


  [Figure 1], [Figure 2], [Figure 3], [Figure 4]


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