1 Department of Anaesthetics, Glasgow Royal Infirmary, Glasgow G4 0SF, UK. 2 Department of Radiology, Western Infirmary, Glasgow, Glasgow G11 6NT, UK. 3 Department of Anaesthetics, Monklands Hospital, Airdrie ML6 0JS, UK 4 Present address: Department of Anaesthetics, Shelly Court, Gartnavel General Hospital,Glasgow G12 0WN, UK 5 Present address: Department of Radiology, Royal Alexandra Hospital, Paisley PA2 9PN, UK
Corresponding author. E-mail: mwatson@doctors.org.uk
Accepted for publication: November 11, 2002
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Abstract |
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Methods. Seventeen patients attending for elective magnetic resonance imaging (MRI) of the spine were studied. Ultrasonic identification of the L34 interspace was attempted by an anaesthetist and a marker was placed. A radiologist identified the anatomical location of the marker on the MRI scan.
Results. Thirteen out of 17 markers were at the L34 interspace; four were at the L23 interspace.
Conclusions. These results suggest that ultrasonography may be a useful adjunct to safe subarachnoid anaesthesia.
Br J Anaesth 2003; 90: 50911
Keywords: anaesthetic techniques, subarachnoid; measurement techniques, nuclear magnetic resonance; measurement techniques, ultrasound; risk
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Introduction |
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Methods and results |
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Comment |
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As in a previous study,2 in order to assess the value of ultrasonography as an adjunct, patients requiring MRI were used, and the interspace was identified with the patients spine flexed. The majority of the patients in our study had lumbar spine symptoms, so discomfort or difficulty with flexion might be expected. Our investigator had a low threshold for patient discomfort, resulting in discontinuation of three cases. In the clinical situation, many patients receiving spinal anaesthesia are either pregnant or elderly, so flexion may be difficult, as with our study population. The L34 interspace was selected for study as the highest that might be considered safe for spinal anaesthesia.1
The MRI results revealed four instances of inaccuracy. Narrowed interspaces at or below L34 were noted by the radiologist on MRI scans in five cases; in three of these five patients, the oil capsule had been placed one interspace too high. A high incidence of degenerative disc disease was reported on MRI. Apposition of adjacent lumbar spinous processes as a result of disc disease could partly explain our inaccuracy. In a younger, child-bearing population, disc degeneration may be less of a problem; in older age groups, progression of disc disease is reduced.6
As suspected by many clinicians, precise lumbar interspace identification by palpation is prone to error. Broadbent and colleagues2 confirmed this, showing that anaesthetists were 29% accurate, as determined by MRI. Ultrasonography was not investigated in this study. The inaccuracy was corroborated by Furness and colleagues,7 who showed that clinical identification by anaesthetists using palpation was 30% accurate, determined by lumbar spine x ray. In contrast, in the latter study, correct placement of markers using ultrasonography at the L34 interspace was 71%, which is comparable with 76% in our study. The important difference in the study by Furness and colleagues is that ultrasonography was performed by a consultant radiologist. Both previous studies also showed that clinical identification by anaesthetists was often inaccurate by two, three or four interspaces. Using ultrasound, markers were always within one interspace of the intended position.
The ultrasonic investigations in our study were performed by a trainee anaesthetist who had initially looked at ultrasonic images on a human volunteer; the feasibility of further study was then discussed with a radiologist. Before commencement of the study, 5 min of instruction on ultrasonic interpretation of lumbar spines and interspaces had been given by a radiologist. The technique was successfully cascaded to another trainee anaesthetist after two practice ultrasounds of less than 5 min duration. In comparison with many skills in anaesthesia, the interpretation of these ultrasonic images for anatomical purposes is relatively simple (Fig. 1), but our study was not designed to examine the learning curve. As anaesthetists are being taught by other anaesthetists to use ultrasound for the location of central veins, we see no reason why the same might not apply also to this ultrasonic technique.
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References |
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6 Wood GW. Lower back pain and disorders of intervertebral disc. In: Canale ST, ed. Campbells Operative Orthopaedics. St Louis: Mosby, 1998; 301492
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