- Email: [email protected]
Ultrasound in obstetric anaesthesia: a review of current applications
International Journal of Obstetric Anesthesia (2010) 19, 320–326 0959-289X/$ - see front matter c 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijoa.2010.03.006
REVIEW ARTICLE
www.obstetanesthesia.com
Ultrasound in obstetric anaesthesia: a review of current applications P. Ecimovic,a J.P.R. Loughreyb a b
Mater Misercordiae Hospital, Dublin, Ireland Rotunda Hospital, Dublin, Ireland
ABSTRACT Ultrasound equipment is increasingly used by non-radiologists to perform interventional techniques and for diagnostic evaluation. Equipment is becoming more portable and durable, with easier user-interface and software enhancement to improve image quality. While obstetric utilisation of ultrasound for fetal assessment has developed over more than 40 years, the same technology has not found a widespread role in obstetric anaesthesia. Within the broader specialty of anaesthesia; vascular access, cardiac imaging and regional anaesthesia are the areas in which ultrasound is becoming increasingly established. In addition to ultrasound for neuraxial blocks, these other clinical applications may be of value in obstetric anaesthesia practice. c 2010 Elsevier Ltd. All rights reserved.
Introduction
Ultrasound technology
The use of ultrasound in medical practice was first reported in the 1950s with one of the first descriptions of its use in clinical anaesthesia practice dating back 30 years.1 A number of recent reviews have recommended its usefulness in anaesthesia.2–4 One of the main limitations to the adoption of ultrasound in routine obstetric anaesthetic practice, apart from cost, is lack of opportunity to develop the necessary experience in image interpretation. To be successful at ultrasound guided techniques, there is a need to develop familiarity with relevant cross-sectional anatomy. Knowledge of anatomy, without an appreciation of what structures look like on ultrasound prevents appreciation of ‘sonoanatomy’. Obstetric neuraxial anaesthesia has been established for more than 50 years and has an acceptable safety record when performed without ultrasound. Experienced clinicians could contend that ultrasound adds little to patient experience. Initial reports of ultrasound application for pre-neuraxial block assessment in obstetric populations were first published over 25 years ago.5,6 However, there remains a shortage of good data charting the developing utility of ultrasound in obstetric anaesthesia practice and many published studies are from a relatively small number of enthusiasts (Table 1).
Piezo-electric crystals enable the transformation of electrical energy into mechanical sound vibration. Medical ultrasound has a frequency of 2–15 MHz, which is high above the audible range of 20 Hz–20 kHz in humans. The ultrasound transducer oscillates between the two modes, generating a pulse of ultrasound followed by a pause during which sound echos of the pulse are detected. The transducer detects both the intensity of the echo and the time required to travel back, enabling calculation of the distance of the reflecting interface. Most tissues are not homogenous and the sound wave stikes a series of interfaces. At each interface some of the wave is reflected back and detected by the transducer. The proportion of reflected and transmitted wave depends on acoustic impedance of tissues forming the interface. Tissues with high water content allow better passage of the waves. The ultrasound beam must be perpendicular to a structure to display its true echogenicity, so minor variations in probe orientation can produce different images. Bone reflects most of the waves back so structures beyond bone are not seen with ultrasound and a black/grey area is observed behind a thick white line of the bone surface.
Accepted March 2010 Correspondence to: Dr. J.P.R. Loughrey, Department of Anaesthesia, Rotunda Hospital, Parnell Sq, Dublin 1, Ireland. E-mail addresses: [email protected], [email protected]
Neuraxial blockade From the technical perspective, spinal ultrasound imaging is difficult as the area of interest is protected by bone. As the ultrasound beam does not pass through
P. Ecimovic, J.P.R. Loughrey Table 1
321
Clinical studies assessing the utility of ultrasound relevant to the obstetric population Study design
Methods
Findings
Observational
Labouring women n = 61 Pre-puncture US
Balki18
Observational
Currie6
Observational
Labouring women with BMI > 30; n = 46 Pre-puncture US depth measured; anaesthetist performing block blinded to depth Labouring women n = 75
Grau23
RCT
Successful identification of epidural space at premarked insertion point in 91.8% Mean BMI 29.7 (±4.8) Mean needle depth to epidural space 4.65 cm (±0.72) Agreement between US and needle depth statistically significant. 95% limits of agreement 0.67 to 0.69 cm 76% epidural placement at first attempt. 95% interval correlation between US and needle depth 1.3 to 0.7 cm. Mean needle depth to epidural space was 6.6 cm (+1.0) High degree of correlation between US measurements and subsequent depth of insertion of needle Number of puncture attempts: US 1.3 ± 0.6 vs control 2.2 ± 1.1 (P < 0.013) Effective analgesia: US 98% vs control 92%. (P < 0.03)
Grau11
RCT
Grau19
RCT
Grau12
Observational
Grau14
Observational
Grau22
RCT
Lee25
RCT
Schlotterbeck9
Observational
Wallace16
Observational
Arzola
15
Labouring women n = 300 Pre-puncture US vs control. All epidurals performed by single anaesthetist Labouring women with poorly palpable bony landmarks n = 72 Women scheduled for caesarean. n = 30 Pre-puncture US vs. real time US vs. control Labouring women cohort with 9 month follow up assessment n = 53 Labouring women n = 100 Labouring women n = 120 Assessment of US as teaching aid for residents; pre-puncture US vs control Postpartum women with previous dural puncture n = 36 Postpartum women n = 99 Accuracy of documented interspace assessment using ultrasound Obese women for caesarean section n = 36 Pre-puncture assessment
Number of puncture attempts: ultrasound 1.5 ± 0.9 vs control 2.6 ± 1.4 (p < 0.001) Epidural failure: ultrasound 0% vs control 5.5% (P > 0.05) Significant reduction in the number of attempts in both ultrasound groups (p = 0.036)
Soft-tissue channel between the spinal processes was narrower and skin-epidural space distance was greater in labour Strong correlation between distance measured by US and needle puncture US success 86% (±15) increasing to 94% after 50 epidurals; Control group success 60% (±16) increasing to 84% after 50 epidurals (P < 0.001) Higher incidence of abnormal ligamentum flavum anatomy in dural puncture group. (P < 0.0001) Sonographers not blinded Correct identification of puncture level in only 36.4%
Successful anaesthesia in all patients and correlation between US assessment and needle depth. No control group
RCT: randomised controlled trial, US: ultrasound.
bone well, the acoustic window is relatively narrow. High frequency transducers produce high resolution images with poor penetration as more sound is absorbed. Low frequency transducers have greater penetration but resolution is poorer. Most anaesthetists use a linear high frequency ultrasound probe for superficial vascular access and brachial plexus anaesthesia. How-
ever, these probes are generally insufficient for neuraxial imaging. A curved or curvilinear array low frequency (2–5 mHz) probe provides depth penetration required for imaging the ligamentum flavum-epidural complex with some compromise on image quality. Linear probes can be programmed to utilise lower frequency ultrasound, but usually down to a limit of 5–7 mHz.
322 With relatively little training, ultrasound can be used to determine an appropriate interspace for epidural or spinal anaesthesia. This is usually done with a longitudinal median or paramedian scan ascertaining sacral bone and imaging superiorly to directly visualise vertebral level (Fig. 1).7 The inaccuracy of clinical palpation to confirm an intervertebral level is well documented, as have the risks of an inadvertently high intervertebral space selection for spinal anaesthesia.8 Ultrasound imaging has been shown to be superior to palpation in correctly identifying lumbar intervertebral level, with traditional landmark palpation techniques erring to a higher level than was attempted or recorded in up to 50% of cases.9,10 As ultrasound equipment becomes more widely available, documentation of its use may become the accepted standard of care. Central neuraxial sonoanatomy has been defined in a number of papers.7,11,12 For neuraxial blocks, unlike most other ultrasound-guided regional anaesthesia blocks, ultrasound is usually used to provide a pre-procedure assessment and skin mark. A pre-puncture image identifies a needle insertion point in the midline-transverse (Figs. 2–4) or paramedian-longitudinal plane,13 and can also provide an estimate of depth to the epidural space from the skin. A correlation between ultrasound-measured depth of lumbar epidural space and direct measurement at the time of lumbar puncture has been demonstrated in patients with normal body mass index (BMI).14,15 In subjects with a higher BMI, due to increased potential for variation in depth assessment because of variable ultrasound probe skin pressure and poorer quality images, correlation may not be as
Ultrasound in Obstetric Anaesthesia accurate.16,17 However, Balki et al. have demonstrated a useful concordance with ultrasound-measured compared with needle depth of the epidural space in obese patients.18 Pre-procedure selection of a skin point does not guarantee successful placement if needle angulation does not replicate the ultrasound probe orientation. Inplane direct visualisation of needle progress through the tissues has been described.19,20 This technique is difficult for a single operator performing an epidural technique, as the loss of resistance technique, probe positioning and needle advancement require a third hand or skilled assistant. However, the use of epidural syringes with an autodetect loss of resistance function may allow a single skilled operator to perform ultrasound-guided epidural placement.21 It has been shown that ultrasound imaging improves the learning curve for successful insertion of epidural catheters for labour analgesia22 and decreases the incidence of side effects. Grau demonstrated reduced number of skin puncture attempts and needle redirections using ultrasound compared to control.11,19,23 In one study of 300 subjects, the ultrasound group had 1.3 ( ± 0.6) vs 2.2 ( ± 1.1) epidural needle puncture attempts with ventral needle advancement counted as one attempt.23 This may seem a relatively soft benefit for patients. However, it is the applicability of this technique to patients with more difficult landmarks that generates the greatest benefits. Ultrasound guidance has also been used for placement of epidural blood patch,24 and Lee et al. have suggested that pre-puncture imaging may identify defects in the ligamentum flavum predisposing some parturients to unintended dural puncture at that
Fig. 1 Longitudinal paramedian view of sacrum and L5 lamina. A longitudinal scan with the probe along the cranio-caudal axis of the spine can indicate middle of the probe location over a particular intervertebral space.
P. Ecimovic, J.P.R. Loughrey
323
Fig. 2 Transverse view of spinous process. For neuraxial blockade, a pre-puncture transverse image is more commonly used to identify a midline needle insertion point rather than ‘in-plane’ needle visualisation. The probe overlies a lumbar spinous process.
Fig. 3 Transverse view of interspinous window. The probe is positioned over a lumbar interspace revealing the ligamentum flavum-epidural area.
interspace.25 In addition to reduced skin punctures and needle redirections, enhanced analgesic success of ultrasound-guided epidural placement has also been reported.11,23 Thecal sac antero-posterior diameter can be measured using ultrasound although a correlation between this and predicted height of spinal anaesthesia has not been demonstrated.26
On the basis of these and other studies, some paediatric, in 2008 the National Institute for Clinical Excellence (NICE) took the view that ultrasound may be effective in achieving correct placement of epidural catheters.27 Those who may view ultrasound only as having a role when neuraxial blockade is technically difficult must accept that much experience is first required with routine
324
Ultrasound in Obstetric Anaesthesia
Fig. 4 Pre-puncture imaging of midline and depth of epidural space. The skin can be marked and epidural placement performed in the standard ‘blind’ fashion. The skin mark relates to the midpoint of the ultrasound probe and angulation of the needle should replicate the angle at which the probe achieved best image.
cases where palpable landmarks are easily identified. In parturients with scoliosis, a minimally rotated intervertebral space may be detected with ultrasound equipment.28 Initial bedside assessment using ultrasound of parturients with rarer conditions such as spina bifida occulta can help to define bony and ligamentous anatomic definition. Concerns for longer epidural siting times when ultrasound is used are overstated as it leads to fewer attempts at epidural space localisation and is likely to reduce overall procedure time in patients with difficult anatomic landmarks.
Vascular access Ultrasound-guided central venous cannulation gained momentum following a meta-analysis by Randolph et al. in 1996,29 which showed a reduction in placement failure, decreased incidence of multiple attempts and decreased rate of complications compared to standard landmark techniques. These results were confirmed by another meta-analysis in 2003 comparing the use of two-dimensional ultrasound guidance with an anatomic landmark technique.30 Based on these and other studies, NICE recommended that two-dimensional ultrasound guidance should be considered in most clinical circumstances where central venous cannulation is indicated.31 It is unclear if central venous cannulation in the third trimester is associated with a higher risk of pneumothorax due to anatomical changes in pregnancy, but headdown tilt is not comfortably tolerated in late pregnancy. Ultrasound-guided placement facilitates accurate central
vascular access with better patient positioning for comfort. It has proven to be most valuable in complicated cases, with coagulopathic, obese or hypovolaemic patients, those who cannot tolerate recumbent position, or those who have had multiple previous cannulations.32 For central venous catheterisation, it has been shown that real-time ultrasound guidance is better than the pre-puncture imaging of anatomy.33 The common technique for ultrasound-assisted central vein cannulation is the short-axis out-of-plane view. In addition to central venous catheters, ultrasound guidance is increasingly used for peripheral venous cannulation in patients with difficult venous access,34 and for insertion of arterial catheters for invasive blood pressure monitoring. This has obvious potential in obese parturients.
General anaesthesia Kubli et al. have published on effects of labour and opioids on gastric emptying and residual gastric volumes in parturients. Ultrasound-based technology in many of these studies was used to assess gastric volumes.35 As skills among obstetric anaesthetists improve, the idea of using ultrasound technology to assess gastric volumes in non-elective cases outside a research setting is thought-provoking and could influence clinical decision making with respect to anaesthesia technique. Location of the cricoid cartilage for correct application of cricoid pressure during rapid sequence induction is traditionally performed by the landmark technique, but may also be identified using ultrasound. The technology is
P. Ecimovic, J.P.R. Loughrey
325
useful to identify the cricothyroid membrane, which may lead to improved safety when considering percutaneous tracheal cannulation.36
Cardiac evaluation Transthoracic cardiac imaging is now feasible with the higher resolution equipment used in most centres for regional anaesthesia. Bedside ultrasound evaluation of the hypotensive parturient may help to differentiate noninvasively between hypovolaemia and other conditions such as cardiomyopathy and pulmonary embolism.37
Acute and chronic pain control There has been recent interest in abdominal wall infiltration techniques for post-caesarean analgesia.38 Field blocks such as the transversus abdominis plane (TAP) block, may be performed with enhanced accuracy using ultrasound. Although the block was originally described with a landmark technique, complications such as peritoneal injection or haematoma are possible. The sonoanatomy of the muscle planes is more clearly defined in the area between the level of the anterior superior iliac crest, the subcostal margin and the umbilicus.39 The TAP block as originally described uses clinical landmarks at the mid-axillary line, above the iliac crest, at the lumbar Triangle of Petit.40 However, the sonoanatomy, in terms of identifying muscle and fascial planes at this point are less clear. Performance of the block in a more anterior site with ultrasound may yield comparable efficacy and studies are awaited. Furthermore, it is likely that clinically useful catheter-based techniques may demonstrate the most benefit in postoperative analgesia, rather than single-shot techniques. Nevertheless, ultrasound will undoubtedly have a role in the evolution and development of abdominal field anaesthesia for post-caesarean analgesia as part of a multi-modal balanced analgesic plan. In addition, use of ultrasound for relatively safe and common indications such as abdominal field blockade will provide an appropriate learning zone for obstetric anaesthetists who do not have regular access to ultrasound in other settings. Pelvic girdle pain syndromes, which involve pubic symphysis, sacroiliac and coccygeal-related pain are a
Table 2
relatively rare but disabling constellation of syndromes which have been treated with local application of anaesthetic and steroid agents. Ultrasound may be used to avoid fluoroscopy and the risks of radiation to the fetus. Obturator nerve blockade, which has been described for chronic perineal pain syndromes has been performed using ultrasound.41
Cost The cost of purchasing high-resolution equipment with at least two different probes and accessories can be substantial and in the region of Eur 50,000 (Table 2). More basic ultrasound technology sufficient for central venous catheterisation is less expensive and can be justified on safety grounds. The cost justification for higher resolution equipment needed for neuraxial, abdominal and musculoskeletal use is harder to make, as patient benefits are more moderate. Devices commonly used by obstetricians may be readily available in patients areas and with appropriate specification, may be sufficient to explore initial neuraxial assessment. Ultrasound has been suggested as one area with improvement potential on future reductions in anaesthesia-related maternal mortality.42 A reduction in neuraxial injury from inappropriately high intervertebral space selection when spinal anaesthesia is planned may justify more widespread use. Until more data are available of the benefits to high-risk parturients, in addition to limited technical expertise, the cost of the equipment will be the main barrier to wider implementation.
Conclusion Placement of epidural catheters or spinal needles is an area where it is likely that more widespread use of ultrasound will occur. Ultrasound may become increasingly useful in other invasive procedures such as vascular access and field blocks. The pace of development in portable ultrasound technology will bring new refinements to image quality and we can expect wider availability of ultrasound technology on our delivery suites and operating rooms and further innovations in clinical application.
Cost of ultrasound equipment
Manufacturer
Model
Price
SonoSite
Mturbo with 2 probes (low and high frequency) S series with 2 probes LOGIQ e with low frequency probe and trolley LOGIQ e with 2 probes (low and high frequency) and trolley Mini Cart zone with one probe Zone ultra with one probe
Eur Eur Eur Eur Eur Eur
General electrics Zonare
Prices based on 2008 value excluding value added tax.;
48,000 40,000 43,000 50,000 45,000 35,000
326
Ultrasound in Obstetric Anaesthesia
References 1. Grange P, Foster P, Pretorius LK. Application of the Doppler ultrasound bloodflow detector in supraclavicular brachial plexus block. Br J Anaesth 1978;50:965–7. 2. Horlocker TT, Wedel DJ. Ultrasound-guided regional anesthesia: in search of the holy grail. Anesth Analg 2007;104:1009–11. 3. Liu SS, Ngeow JE, Yadeau JT. Ultrasound-guided regional anesthesia and analgesia: a qualitative systematic review. Reg Anesth Pain Med 2009;34:47–59. 4. Marhofer P, Chan VW. Ultrasound-guided regional anesthesia: current concepts and future trends. Anesth Analg 2007;104:1265–9. 5. Cork RC, Kryc JJ, Vaughan RW. Ultrasonic localization of the lumbar epidural space. Anesthesiology 1980;52:513–6. 6. Currie JM. Measurement of the depth to the extradural space using ultrasound. Br J Anaesth 1984;56:345–7. 7. Carvalho JC. Ultrasound-facilitated epidurals and spinals in obstetrics. Anesthesiol Clin 2008;26:145–58. 8. Reynolds F. Damage to the conus medullaris following spinal anaesthesia. Anaesthesia 2001;56:238–47. 9. Schlotterbeck H, Schaeffer R, Dow WA, Touret Y, Bailey S, Diemunsch P. Ultrasonographic control of the puncture level for lumbar neuraxial block in obstetric anaesthesia. Br J Anaesth 2008;100:230–4. 10. Whitty R, Moore M, Macarthur A. Identification of the lumbar interspinous spaces: palpation versus ultrasound. Anesth Analg 2008;106:538–40. 11. Grau T, Leipold RW, Conradi R, Martin E. Ultrasound control for presumed difficult epidural puncture. Acta Anaesthesiol Scand 2001;45:766–71. 12. Grau T, Leipold RW, Horter J, Conradi R, Martin E, Motsch J. The lumbar epidural space in pregnancy: visualization by ultrasonography. Br J Anaesth 2001;86:798–804. 13. Grau T, Leipold RW, Horter J, Conradi R, Martin EO, Motsch J. Paramedian access to the epidural space. the optimum window for ultrasound imaging. J Clin Anesth 2001;13:213–7. 14. Grau T, Leipold R, Conradi R, Martin E, Motsch J. Ultrasonography and peridural anesthesia. Technical possibilities and limitations of ultrasonic examination of the epidural space. Anaesthesist 2001;50:94–101. 15. Arzola C, Davies S, Rofaeel A, Carvalho JC. Ultrasound using the transverse approach to the lumbar spine provides reliable landmarks for labor epidurals. Anesth Analg 2007;104:1188–92. 16. Wallace DH, Currie JM, Gilstrap LC, Santos R. Indirect sonographic guidance for epidural anesthesia in obese pregnant patients. Reg Anesth 1992;17:233–6. 17. Stiffler KA, Jwayyed S, Wilber ST, Robinson A. The use of ultrasound to identify pertinent landmarks for lumbar puncture. Am J Emerg Med 2007;25:331–4. 18. Balki M, Lee Y, Halpern S, Carvalho JC. Ultrasound imaging of the lumbar spine in the transverse plane: the correlation between estimated and actual depth to the epidural space in obese parturients. Anesth Analg 2009;108:1876–81. 19. Grau T, Leipold RW, Fatehi S, Martin E, Motsch J. Real-time ultrasonic observation of combined spinal-epidural anaesthesia. Eur J Anaesthesiol 2004;21:25–31. 20. Karmakar MK, Li X, Ho AM, Kwok WH, Chui PT. Real-time ultrasound-guided paramedian epidural access: evaluation of a novel in-plane technique. Br J Anaesth 2009;102:845–54. 21. Riley ET, Carvalho B. The Episure syringe: a novel loss of resistance syringe for locating the epidural space. Anesth Analg 2007;105:1164–6. 22. Grau T, Bartusseck E, Conradi R, Martin E, Motsch J. Ultrasound imaging improves learning curves in obstetric epidu-
23.
24. 25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35. 36.
37. 38.
39.
40.
41.
42.
ral anesthesia: a preliminary study. Can J Anesth 2003;50: 1047–50. Grau T, Leipold RW, Conradi R, Martin E, Motsch J. Efficacy of ultrasound imaging in obstetric epidural anesthesia. J Clin Anesth 2002;14:169–75. Prasad GA, Simon G, Wijayatilake S. Ultrasound-assisted epidural blood patching. Int J Obstet Anesth 2008;17:376–7. Lee Y, Tanaka M, Carvalho JC. Sonoanatomy of the lumbar spine in patients with previous unintentional dural punctures during labor epidurals. Reg Anesth Pain Med 2008;33:266–70. Arzola C, Balki M, Carvalho JC. The antero-posterior diameter of the lumbar dural sac does not predict sensory levels of spinal anesthesia for Cesarean delivery. Can J Anesth 2007;54:620–5. NICE. Ultrasound guided catheterisation of the epidural space. National Institute for Health and Clinical Excellence:2008 (). McLeod A, Roche A, Fennelly M. Case series: ultrasonography may assist epidural insertion in scoliosis patients. Can J Anesth 2005;52:717–20. Randolph AG, Cook DJ, Gonzales CA, Pribble CG. Ultrasound guidance for placement of central venous catheters: a metaanalysis of the literature. Crit Care Med 1996;24:2053–8. Hind D, Calvert N, McWilliams R et al. Ultrasonic locating devices for central venous cannulation: meta-analysis. BMJ 2003;327:361. NICE. Guidance on the use of ultrasound locating devices for placing central venous catheters: National Institute for Health and Clinical Excellence:2002. (). Karakitsos D, Labropoulos N, De Groot E et al. Real-time ultrasound-guided catheterisation of the internal jugular vein: a prospective comparison with the landmark technique in critical care patients. Crit Care 2006;10:R162. Hosokawa K, Shime N, Kato Y, Hashimoto S. A randomized trial of image-basd skin surface marking versus real-time ultrasoundguided internal jugular vein catheterization in infants. Anesthesiology 2007;107:720–4. Bauman M, Braude D, Crandall C. Ultrasound-guidance vs Standard technique in difficult vascular access patients by ED technicians. Am J Emerg Med 2009;27:135–40. Kubli M, Scrutton MJ, Seed PT, O’Sullivan G. An evaluation of isotonic ‘‘sport drinks’’ during labor. Anesth Analg 2002;94:404–8. Nicholls SE, Sweeney TW, Ferre RM, Strout TD. Bedside sonography by emergency physicians for the rapid identification of landmarks relevent to cricothyroidotomy. Am J Emerg Med 2008;26:852–6. Beaulieu Y. Bedside echocardiography in the assessment of the critically ill. Crit Care Med 2007;35:S235–49. McDonnell JG, Curley G, Carney J et al. The analgesic efficacy of transversus abdominis plane block after cesarean delivery: a randomized controlled trial. Anesth Analg 2008;106:186–91. Tran TM, Ivanusic JJ, Hebbard P, Barrington MJ. Determination of spread of injectate after ultrasound-guided transversus abdominis plane block: a cadaveric study. Br J Anaesth 2009; 102:123–7. McDonnell JG, O’Donnell BD, Farrell T et al. Transversus abdominis plane block: a cadaveric and radiological evaluation. Reg Anesth Pain Med 2007;32:399–404. Fujiwara Y, Sato Y, Kitayama M, Shibata Y, Komatsu T, Hirota K. Obturator nerve block using ultrasound guidance. Anesth Analg 2007;105:888–9. Arendt KW, Segal S. Present and emerging strategies for reducing anesthesia-related maternal morbidity and mortality. Curr Opin Anaesthesiol 2009;22:330–5.
REVIEW ARTICLE
www.obstetanesthesia.com
Ultrasound in obstetric anaesthesia: a review of current applications P. Ecimovic,a J.P.R. Loughreyb a b
Mater Misercordiae Hospital, Dublin, Ireland Rotunda Hospital, Dublin, Ireland
ABSTRACT Ultrasound equipment is increasingly used by non-radiologists to perform interventional techniques and for diagnostic evaluation. Equipment is becoming more portable and durable, with easier user-interface and software enhancement to improve image quality. While obstetric utilisation of ultrasound for fetal assessment has developed over more than 40 years, the same technology has not found a widespread role in obstetric anaesthesia. Within the broader specialty of anaesthesia; vascular access, cardiac imaging and regional anaesthesia are the areas in which ultrasound is becoming increasingly established. In addition to ultrasound for neuraxial blocks, these other clinical applications may be of value in obstetric anaesthesia practice. c 2010 Elsevier Ltd. All rights reserved.
Introduction
Ultrasound technology
The use of ultrasound in medical practice was first reported in the 1950s with one of the first descriptions of its use in clinical anaesthesia practice dating back 30 years.1 A number of recent reviews have recommended its usefulness in anaesthesia.2–4 One of the main limitations to the adoption of ultrasound in routine obstetric anaesthetic practice, apart from cost, is lack of opportunity to develop the necessary experience in image interpretation. To be successful at ultrasound guided techniques, there is a need to develop familiarity with relevant cross-sectional anatomy. Knowledge of anatomy, without an appreciation of what structures look like on ultrasound prevents appreciation of ‘sonoanatomy’. Obstetric neuraxial anaesthesia has been established for more than 50 years and has an acceptable safety record when performed without ultrasound. Experienced clinicians could contend that ultrasound adds little to patient experience. Initial reports of ultrasound application for pre-neuraxial block assessment in obstetric populations were first published over 25 years ago.5,6 However, there remains a shortage of good data charting the developing utility of ultrasound in obstetric anaesthesia practice and many published studies are from a relatively small number of enthusiasts (Table 1).
Piezo-electric crystals enable the transformation of electrical energy into mechanical sound vibration. Medical ultrasound has a frequency of 2–15 MHz, which is high above the audible range of 20 Hz–20 kHz in humans. The ultrasound transducer oscillates between the two modes, generating a pulse of ultrasound followed by a pause during which sound echos of the pulse are detected. The transducer detects both the intensity of the echo and the time required to travel back, enabling calculation of the distance of the reflecting interface. Most tissues are not homogenous and the sound wave stikes a series of interfaces. At each interface some of the wave is reflected back and detected by the transducer. The proportion of reflected and transmitted wave depends on acoustic impedance of tissues forming the interface. Tissues with high water content allow better passage of the waves. The ultrasound beam must be perpendicular to a structure to display its true echogenicity, so minor variations in probe orientation can produce different images. Bone reflects most of the waves back so structures beyond bone are not seen with ultrasound and a black/grey area is observed behind a thick white line of the bone surface.
Accepted March 2010 Correspondence to: Dr. J.P.R. Loughrey, Department of Anaesthesia, Rotunda Hospital, Parnell Sq, Dublin 1, Ireland. E-mail addresses: [email protected], [email protected]
Neuraxial blockade From the technical perspective, spinal ultrasound imaging is difficult as the area of interest is protected by bone. As the ultrasound beam does not pass through
P. Ecimovic, J.P.R. Loughrey Table 1
321
Clinical studies assessing the utility of ultrasound relevant to the obstetric population Study design
Methods
Findings
Observational
Labouring women n = 61 Pre-puncture US
Balki18
Observational
Currie6
Observational
Labouring women with BMI > 30; n = 46 Pre-puncture US depth measured; anaesthetist performing block blinded to depth Labouring women n = 75
Grau23
RCT
Successful identification of epidural space at premarked insertion point in 91.8% Mean BMI 29.7 (±4.8) Mean needle depth to epidural space 4.65 cm (±0.72) Agreement between US and needle depth statistically significant. 95% limits of agreement 0.67 to 0.69 cm 76% epidural placement at first attempt. 95% interval correlation between US and needle depth 1.3 to 0.7 cm. Mean needle depth to epidural space was 6.6 cm (+1.0) High degree of correlation between US measurements and subsequent depth of insertion of needle Number of puncture attempts: US 1.3 ± 0.6 vs control 2.2 ± 1.1 (P < 0.013) Effective analgesia: US 98% vs control 92%. (P < 0.03)
Grau11
RCT
Grau19
RCT
Grau12
Observational
Grau14
Observational
Grau22
RCT
Lee25
RCT
Schlotterbeck9
Observational
Wallace16
Observational
Arzola
15
Labouring women n = 300 Pre-puncture US vs control. All epidurals performed by single anaesthetist Labouring women with poorly palpable bony landmarks n = 72 Women scheduled for caesarean. n = 30 Pre-puncture US vs. real time US vs. control Labouring women cohort with 9 month follow up assessment n = 53 Labouring women n = 100 Labouring women n = 120 Assessment of US as teaching aid for residents; pre-puncture US vs control Postpartum women with previous dural puncture n = 36 Postpartum women n = 99 Accuracy of documented interspace assessment using ultrasound Obese women for caesarean section n = 36 Pre-puncture assessment
Number of puncture attempts: ultrasound 1.5 ± 0.9 vs control 2.6 ± 1.4 (p < 0.001) Epidural failure: ultrasound 0% vs control 5.5% (P > 0.05) Significant reduction in the number of attempts in both ultrasound groups (p = 0.036)
Soft-tissue channel between the spinal processes was narrower and skin-epidural space distance was greater in labour Strong correlation between distance measured by US and needle puncture US success 86% (±15) increasing to 94% after 50 epidurals; Control group success 60% (±16) increasing to 84% after 50 epidurals (P < 0.001) Higher incidence of abnormal ligamentum flavum anatomy in dural puncture group. (P < 0.0001) Sonographers not blinded Correct identification of puncture level in only 36.4%
Successful anaesthesia in all patients and correlation between US assessment and needle depth. No control group
RCT: randomised controlled trial, US: ultrasound.
bone well, the acoustic window is relatively narrow. High frequency transducers produce high resolution images with poor penetration as more sound is absorbed. Low frequency transducers have greater penetration but resolution is poorer. Most anaesthetists use a linear high frequency ultrasound probe for superficial vascular access and brachial plexus anaesthesia. How-
ever, these probes are generally insufficient for neuraxial imaging. A curved or curvilinear array low frequency (2–5 mHz) probe provides depth penetration required for imaging the ligamentum flavum-epidural complex with some compromise on image quality. Linear probes can be programmed to utilise lower frequency ultrasound, but usually down to a limit of 5–7 mHz.
322 With relatively little training, ultrasound can be used to determine an appropriate interspace for epidural or spinal anaesthesia. This is usually done with a longitudinal median or paramedian scan ascertaining sacral bone and imaging superiorly to directly visualise vertebral level (Fig. 1).7 The inaccuracy of clinical palpation to confirm an intervertebral level is well documented, as have the risks of an inadvertently high intervertebral space selection for spinal anaesthesia.8 Ultrasound imaging has been shown to be superior to palpation in correctly identifying lumbar intervertebral level, with traditional landmark palpation techniques erring to a higher level than was attempted or recorded in up to 50% of cases.9,10 As ultrasound equipment becomes more widely available, documentation of its use may become the accepted standard of care. Central neuraxial sonoanatomy has been defined in a number of papers.7,11,12 For neuraxial blocks, unlike most other ultrasound-guided regional anaesthesia blocks, ultrasound is usually used to provide a pre-procedure assessment and skin mark. A pre-puncture image identifies a needle insertion point in the midline-transverse (Figs. 2–4) or paramedian-longitudinal plane,13 and can also provide an estimate of depth to the epidural space from the skin. A correlation between ultrasound-measured depth of lumbar epidural space and direct measurement at the time of lumbar puncture has been demonstrated in patients with normal body mass index (BMI).14,15 In subjects with a higher BMI, due to increased potential for variation in depth assessment because of variable ultrasound probe skin pressure and poorer quality images, correlation may not be as
Ultrasound in Obstetric Anaesthesia accurate.16,17 However, Balki et al. have demonstrated a useful concordance with ultrasound-measured compared with needle depth of the epidural space in obese patients.18 Pre-procedure selection of a skin point does not guarantee successful placement if needle angulation does not replicate the ultrasound probe orientation. Inplane direct visualisation of needle progress through the tissues has been described.19,20 This technique is difficult for a single operator performing an epidural technique, as the loss of resistance technique, probe positioning and needle advancement require a third hand or skilled assistant. However, the use of epidural syringes with an autodetect loss of resistance function may allow a single skilled operator to perform ultrasound-guided epidural placement.21 It has been shown that ultrasound imaging improves the learning curve for successful insertion of epidural catheters for labour analgesia22 and decreases the incidence of side effects. Grau demonstrated reduced number of skin puncture attempts and needle redirections using ultrasound compared to control.11,19,23 In one study of 300 subjects, the ultrasound group had 1.3 ( ± 0.6) vs 2.2 ( ± 1.1) epidural needle puncture attempts with ventral needle advancement counted as one attempt.23 This may seem a relatively soft benefit for patients. However, it is the applicability of this technique to patients with more difficult landmarks that generates the greatest benefits. Ultrasound guidance has also been used for placement of epidural blood patch,24 and Lee et al. have suggested that pre-puncture imaging may identify defects in the ligamentum flavum predisposing some parturients to unintended dural puncture at that
Fig. 1 Longitudinal paramedian view of sacrum and L5 lamina. A longitudinal scan with the probe along the cranio-caudal axis of the spine can indicate middle of the probe location over a particular intervertebral space.
P. Ecimovic, J.P.R. Loughrey
323
Fig. 2 Transverse view of spinous process. For neuraxial blockade, a pre-puncture transverse image is more commonly used to identify a midline needle insertion point rather than ‘in-plane’ needle visualisation. The probe overlies a lumbar spinous process.
Fig. 3 Transverse view of interspinous window. The probe is positioned over a lumbar interspace revealing the ligamentum flavum-epidural area.
interspace.25 In addition to reduced skin punctures and needle redirections, enhanced analgesic success of ultrasound-guided epidural placement has also been reported.11,23 Thecal sac antero-posterior diameter can be measured using ultrasound although a correlation between this and predicted height of spinal anaesthesia has not been demonstrated.26
On the basis of these and other studies, some paediatric, in 2008 the National Institute for Clinical Excellence (NICE) took the view that ultrasound may be effective in achieving correct placement of epidural catheters.27 Those who may view ultrasound only as having a role when neuraxial blockade is technically difficult must accept that much experience is first required with routine
324
Ultrasound in Obstetric Anaesthesia
Fig. 4 Pre-puncture imaging of midline and depth of epidural space. The skin can be marked and epidural placement performed in the standard ‘blind’ fashion. The skin mark relates to the midpoint of the ultrasound probe and angulation of the needle should replicate the angle at which the probe achieved best image.
cases where palpable landmarks are easily identified. In parturients with scoliosis, a minimally rotated intervertebral space may be detected with ultrasound equipment.28 Initial bedside assessment using ultrasound of parturients with rarer conditions such as spina bifida occulta can help to define bony and ligamentous anatomic definition. Concerns for longer epidural siting times when ultrasound is used are overstated as it leads to fewer attempts at epidural space localisation and is likely to reduce overall procedure time in patients with difficult anatomic landmarks.
Vascular access Ultrasound-guided central venous cannulation gained momentum following a meta-analysis by Randolph et al. in 1996,29 which showed a reduction in placement failure, decreased incidence of multiple attempts and decreased rate of complications compared to standard landmark techniques. These results were confirmed by another meta-analysis in 2003 comparing the use of two-dimensional ultrasound guidance with an anatomic landmark technique.30 Based on these and other studies, NICE recommended that two-dimensional ultrasound guidance should be considered in most clinical circumstances where central venous cannulation is indicated.31 It is unclear if central venous cannulation in the third trimester is associated with a higher risk of pneumothorax due to anatomical changes in pregnancy, but headdown tilt is not comfortably tolerated in late pregnancy. Ultrasound-guided placement facilitates accurate central
vascular access with better patient positioning for comfort. It has proven to be most valuable in complicated cases, with coagulopathic, obese or hypovolaemic patients, those who cannot tolerate recumbent position, or those who have had multiple previous cannulations.32 For central venous catheterisation, it has been shown that real-time ultrasound guidance is better than the pre-puncture imaging of anatomy.33 The common technique for ultrasound-assisted central vein cannulation is the short-axis out-of-plane view. In addition to central venous catheters, ultrasound guidance is increasingly used for peripheral venous cannulation in patients with difficult venous access,34 and for insertion of arterial catheters for invasive blood pressure monitoring. This has obvious potential in obese parturients.
General anaesthesia Kubli et al. have published on effects of labour and opioids on gastric emptying and residual gastric volumes in parturients. Ultrasound-based technology in many of these studies was used to assess gastric volumes.35 As skills among obstetric anaesthetists improve, the idea of using ultrasound technology to assess gastric volumes in non-elective cases outside a research setting is thought-provoking and could influence clinical decision making with respect to anaesthesia technique. Location of the cricoid cartilage for correct application of cricoid pressure during rapid sequence induction is traditionally performed by the landmark technique, but may also be identified using ultrasound. The technology is
P. Ecimovic, J.P.R. Loughrey
325
useful to identify the cricothyroid membrane, which may lead to improved safety when considering percutaneous tracheal cannulation.36
Cardiac evaluation Transthoracic cardiac imaging is now feasible with the higher resolution equipment used in most centres for regional anaesthesia. Bedside ultrasound evaluation of the hypotensive parturient may help to differentiate noninvasively between hypovolaemia and other conditions such as cardiomyopathy and pulmonary embolism.37
Acute and chronic pain control There has been recent interest in abdominal wall infiltration techniques for post-caesarean analgesia.38 Field blocks such as the transversus abdominis plane (TAP) block, may be performed with enhanced accuracy using ultrasound. Although the block was originally described with a landmark technique, complications such as peritoneal injection or haematoma are possible. The sonoanatomy of the muscle planes is more clearly defined in the area between the level of the anterior superior iliac crest, the subcostal margin and the umbilicus.39 The TAP block as originally described uses clinical landmarks at the mid-axillary line, above the iliac crest, at the lumbar Triangle of Petit.40 However, the sonoanatomy, in terms of identifying muscle and fascial planes at this point are less clear. Performance of the block in a more anterior site with ultrasound may yield comparable efficacy and studies are awaited. Furthermore, it is likely that clinically useful catheter-based techniques may demonstrate the most benefit in postoperative analgesia, rather than single-shot techniques. Nevertheless, ultrasound will undoubtedly have a role in the evolution and development of abdominal field anaesthesia for post-caesarean analgesia as part of a multi-modal balanced analgesic plan. In addition, use of ultrasound for relatively safe and common indications such as abdominal field blockade will provide an appropriate learning zone for obstetric anaesthetists who do not have regular access to ultrasound in other settings. Pelvic girdle pain syndromes, which involve pubic symphysis, sacroiliac and coccygeal-related pain are a
Table 2
relatively rare but disabling constellation of syndromes which have been treated with local application of anaesthetic and steroid agents. Ultrasound may be used to avoid fluoroscopy and the risks of radiation to the fetus. Obturator nerve blockade, which has been described for chronic perineal pain syndromes has been performed using ultrasound.41
Cost The cost of purchasing high-resolution equipment with at least two different probes and accessories can be substantial and in the region of Eur 50,000 (Table 2). More basic ultrasound technology sufficient for central venous catheterisation is less expensive and can be justified on safety grounds. The cost justification for higher resolution equipment needed for neuraxial, abdominal and musculoskeletal use is harder to make, as patient benefits are more moderate. Devices commonly used by obstetricians may be readily available in patients areas and with appropriate specification, may be sufficient to explore initial neuraxial assessment. Ultrasound has been suggested as one area with improvement potential on future reductions in anaesthesia-related maternal mortality.42 A reduction in neuraxial injury from inappropriately high intervertebral space selection when spinal anaesthesia is planned may justify more widespread use. Until more data are available of the benefits to high-risk parturients, in addition to limited technical expertise, the cost of the equipment will be the main barrier to wider implementation.
Conclusion Placement of epidural catheters or spinal needles is an area where it is likely that more widespread use of ultrasound will occur. Ultrasound may become increasingly useful in other invasive procedures such as vascular access and field blocks. The pace of development in portable ultrasound technology will bring new refinements to image quality and we can expect wider availability of ultrasound technology on our delivery suites and operating rooms and further innovations in clinical application.
Cost of ultrasound equipment
Manufacturer
Model
Price
SonoSite
Mturbo with 2 probes (low and high frequency) S series with 2 probes LOGIQ e with low frequency probe and trolley LOGIQ e with 2 probes (low and high frequency) and trolley Mini Cart zone with one probe Zone ultra with one probe
Eur Eur Eur Eur Eur Eur
General electrics Zonare
Prices based on 2008 value excluding value added tax.;
48,000 40,000 43,000 50,000 45,000 35,000
326
Ultrasound in Obstetric Anaesthesia
References 1. Grange P, Foster P, Pretorius LK. Application of the Doppler ultrasound bloodflow detector in supraclavicular brachial plexus block. Br J Anaesth 1978;50:965–7. 2. Horlocker TT, Wedel DJ. Ultrasound-guided regional anesthesia: in search of the holy grail. Anesth Analg 2007;104:1009–11. 3. Liu SS, Ngeow JE, Yadeau JT. Ultrasound-guided regional anesthesia and analgesia: a qualitative systematic review. Reg Anesth Pain Med 2009;34:47–59. 4. Marhofer P, Chan VW. Ultrasound-guided regional anesthesia: current concepts and future trends. Anesth Analg 2007;104:1265–9. 5. Cork RC, Kryc JJ, Vaughan RW. Ultrasonic localization of the lumbar epidural space. Anesthesiology 1980;52:513–6. 6. Currie JM. Measurement of the depth to the extradural space using ultrasound. Br J Anaesth 1984;56:345–7. 7. Carvalho JC. Ultrasound-facilitated epidurals and spinals in obstetrics. Anesthesiol Clin 2008;26:145–58. 8. Reynolds F. Damage to the conus medullaris following spinal anaesthesia. Anaesthesia 2001;56:238–47. 9. Schlotterbeck H, Schaeffer R, Dow WA, Touret Y, Bailey S, Diemunsch P. Ultrasonographic control of the puncture level for lumbar neuraxial block in obstetric anaesthesia. Br J Anaesth 2008;100:230–4. 10. Whitty R, Moore M, Macarthur A. Identification of the lumbar interspinous spaces: palpation versus ultrasound. Anesth Analg 2008;106:538–40. 11. Grau T, Leipold RW, Conradi R, Martin E. Ultrasound control for presumed difficult epidural puncture. Acta Anaesthesiol Scand 2001;45:766–71. 12. Grau T, Leipold RW, Horter J, Conradi R, Martin E, Motsch J. The lumbar epidural space in pregnancy: visualization by ultrasonography. Br J Anaesth 2001;86:798–804. 13. Grau T, Leipold RW, Horter J, Conradi R, Martin EO, Motsch J. Paramedian access to the epidural space. the optimum window for ultrasound imaging. J Clin Anesth 2001;13:213–7. 14. Grau T, Leipold R, Conradi R, Martin E, Motsch J. Ultrasonography and peridural anesthesia. Technical possibilities and limitations of ultrasonic examination of the epidural space. Anaesthesist 2001;50:94–101. 15. Arzola C, Davies S, Rofaeel A, Carvalho JC. Ultrasound using the transverse approach to the lumbar spine provides reliable landmarks for labor epidurals. Anesth Analg 2007;104:1188–92. 16. Wallace DH, Currie JM, Gilstrap LC, Santos R. Indirect sonographic guidance for epidural anesthesia in obese pregnant patients. Reg Anesth 1992;17:233–6. 17. Stiffler KA, Jwayyed S, Wilber ST, Robinson A. The use of ultrasound to identify pertinent landmarks for lumbar puncture. Am J Emerg Med 2007;25:331–4. 18. Balki M, Lee Y, Halpern S, Carvalho JC. Ultrasound imaging of the lumbar spine in the transverse plane: the correlation between estimated and actual depth to the epidural space in obese parturients. Anesth Analg 2009;108:1876–81. 19. Grau T, Leipold RW, Fatehi S, Martin E, Motsch J. Real-time ultrasonic observation of combined spinal-epidural anaesthesia. Eur J Anaesthesiol 2004;21:25–31. 20. Karmakar MK, Li X, Ho AM, Kwok WH, Chui PT. Real-time ultrasound-guided paramedian epidural access: evaluation of a novel in-plane technique. Br J Anaesth 2009;102:845–54. 21. Riley ET, Carvalho B. The Episure syringe: a novel loss of resistance syringe for locating the epidural space. Anesth Analg 2007;105:1164–6. 22. Grau T, Bartusseck E, Conradi R, Martin E, Motsch J. Ultrasound imaging improves learning curves in obstetric epidu-
23.
24. 25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35. 36.
37. 38.
39.
40.
41.
42.
ral anesthesia: a preliminary study. Can J Anesth 2003;50: 1047–50. Grau T, Leipold RW, Conradi R, Martin E, Motsch J. Efficacy of ultrasound imaging in obstetric epidural anesthesia. J Clin Anesth 2002;14:169–75. Prasad GA, Simon G, Wijayatilake S. Ultrasound-assisted epidural blood patching. Int J Obstet Anesth 2008;17:376–7. Lee Y, Tanaka M, Carvalho JC. Sonoanatomy of the lumbar spine in patients with previous unintentional dural punctures during labor epidurals. Reg Anesth Pain Med 2008;33:266–70. Arzola C, Balki M, Carvalho JC. The antero-posterior diameter of the lumbar dural sac does not predict sensory levels of spinal anesthesia for Cesarean delivery. Can J Anesth 2007;54:620–5. NICE. Ultrasound guided catheterisation of the epidural space. National Institute for Health and Clinical Excellence:2008 (). McLeod A, Roche A, Fennelly M. Case series: ultrasonography may assist epidural insertion in scoliosis patients. Can J Anesth 2005;52:717–20. Randolph AG, Cook DJ, Gonzales CA, Pribble CG. Ultrasound guidance for placement of central venous catheters: a metaanalysis of the literature. Crit Care Med 1996;24:2053–8. Hind D, Calvert N, McWilliams R et al. Ultrasonic locating devices for central venous cannulation: meta-analysis. BMJ 2003;327:361. NICE. Guidance on the use of ultrasound locating devices for placing central venous catheters: National Institute for Health and Clinical Excellence:2002. (). Karakitsos D, Labropoulos N, De Groot E et al. Real-time ultrasound-guided catheterisation of the internal jugular vein: a prospective comparison with the landmark technique in critical care patients. Crit Care 2006;10:R162. Hosokawa K, Shime N, Kato Y, Hashimoto S. A randomized trial of image-basd skin surface marking versus real-time ultrasoundguided internal jugular vein catheterization in infants. Anesthesiology 2007;107:720–4. Bauman M, Braude D, Crandall C. Ultrasound-guidance vs Standard technique in difficult vascular access patients by ED technicians. Am J Emerg Med 2009;27:135–40. Kubli M, Scrutton MJ, Seed PT, O’Sullivan G. An evaluation of isotonic ‘‘sport drinks’’ during labor. Anesth Analg 2002;94:404–8. Nicholls SE, Sweeney TW, Ferre RM, Strout TD. Bedside sonography by emergency physicians for the rapid identification of landmarks relevent to cricothyroidotomy. Am J Emerg Med 2008;26:852–6. Beaulieu Y. Bedside echocardiography in the assessment of the critically ill. Crit Care Med 2007;35:S235–49. McDonnell JG, Curley G, Carney J et al. The analgesic efficacy of transversus abdominis plane block after cesarean delivery: a randomized controlled trial. Anesth Analg 2008;106:186–91. Tran TM, Ivanusic JJ, Hebbard P, Barrington MJ. Determination of spread of injectate after ultrasound-guided transversus abdominis plane block: a cadaveric study. Br J Anaesth 2009; 102:123–7. McDonnell JG, O’Donnell BD, Farrell T et al. Transversus abdominis plane block: a cadaveric and radiological evaluation. Reg Anesth Pain Med 2007;32:399–404. Fujiwara Y, Sato Y, Kitayama M, Shibata Y, Komatsu T, Hirota K. Obturator nerve block using ultrasound guidance. Anesth Analg 2007;105:888–9. Arendt KW, Segal S. Present and emerging strategies for reducing anesthesia-related maternal morbidity and mortality. Curr Opin Anaesthesiol 2009;22:330–5.