Nuffield Department of Anaesthetics, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, UK E-mail: caroline.rob@virgin.net
Accepted for publication: August 19, 2002
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Abstract |
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Methods. We studied 113 children undergoing elective surgery. During insertion of the caudal block, a stethoscope was placed over the lower lumbar spine and the presence or absence of an audible swoosh noted. The operators clinical impression of successful insertion was also recorded.
Results. The overall success rate of caudal anaesthesia was 95.6%. Of the 108 patients with a successful block, 98 had a positive swoosh test. There were no false positive results. Calculations show the swoosh test to have a sensitivity of 91%, a specificity of 100% and a positive predictive value of 100%.
Conclusions. The swoosh test is a simple and accurate test to confirm successful caudal insertion in children, and is especially useful as a teaching aid for anaesthetists new to the technique.
Br J Anaesth 2003; 90: 625
Keywords: anaesthetic techniques, epidural
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Introduction |
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The whoosh test has not been extensively studied in children. Based on their experience in only two patients, Bollinger and Mayne7 suggested that it is not reliable because of false positive tests. There has also been a case report of probable venous air embolism in a child following a whoosh test.8 In addition, the high incidence of neurological complications associated with the use of air to identify the epidural space in children9 means that many anaesthetists would consider the original whoosh test to be contraindicated in children.
Observation of a distinct swoosh during injection of local anaesthetic into the caudal space in a child led us to investigate what we have christened the swoosh test. This modification of the whoosh test avoids the use of air, thereby permitting its safe use in children. In this study we have investigated the accuracy of our swoosh test in children and also compared it with the more traditional end-points of successful caudal insertion.
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Methods |
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Anaesthesia was induced either via i.v. administration of propofol 35 mg kg1, or inhalation of sevoflurane 8% in oxygen 100%. Patients were positioned in the left lateral position and the sacral hiatus was identified using the bony landmarks of the sacrum. Blocks were performed under aseptic precautions according to the method described by Dalens10 with a 22G Venflon® (Becton Dickinson, Helsingborg, Sweden) in all but two patients in whom 24G Neoflona® (Becton Dickinson) was used. In this technique, the stylet of the cannula is withdrawn slightly after penetration of the sacrococcygeal membrane and the cannula is advanced into the sacral canal to a position corresponding approximately to the level of S3. Compared with a needle technique this is felt to reduce the risk of inadvertent placement, either in the subarachnoid space or a blood vessel.
Injection of 0.51 ml kg1 of bupivacaine 0.25% was then performed depending upon the height of the block required, according to the Armitage regimen.11 Inadvertent dural puncture or intravascular placement was excluded by negative aspiration, both initially and after 2 ml of injection to exclude placement in a small vessel. In addition, the volume of local anaesthetic was injected slowly with continuous electrocardiogram monitoring. During injection, a stethoscope was placed over the lower lumbar spine, corresponding to an area immediately above the end of the cannula. If the injection was heard, this was recorded as a positive result. If the result was inaudible or equivocal, it was recorded as a negative test. Subsequently, the operators clinical impression of successful placement was recorded based on the following five predictors: a loss of resistance on piercing the sacrococcygeal membrane, ease of threading the cannula, an approximate angle of 45° between the cannula and patients skin, ease of injection of the local anaesthetic and lack of subcutaneous swelling at the injection site. If three or more were positive then the clinical impression was recorded as successful. At the postoperative visit, the success of the caudal block in terms of providing analgesia was noted. An internet-based program (http://www.hutchon.freeserve.co.uk/Diagnostic-test.htm) was used to calculate the sensitivity, specificity, positive predictive values and confidence intervals of the swoosh test and clinical observation of successful caudal placement.
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Results |
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Discussion |
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Normally, the operator relies upon a number of end-points to confirm successful placement of the needle or cannula. These are typically a give on piercing the sacrococcygeal membrane, the ease of injection of the local anaesthetic solution and the lack of subcutaneous swelling after injection. The whoosh test was first described in adults undergoing caudal epidural steroid injections for back pain.4 Approximately 2 ml of air are injected through the caudal needle following placement, with a resulting whoosh heard through a stethoscope positioned over the thoracolumbar spine if needle placement is correct. In its original description, only 26 patients were studied, although the test was positive in all patients with a correctly placed needle as identified by epidurography. Sensitivity and specificity were not described. Eastwood and colleagues5 reported their findings in 131 adults receiving a caudal steroid injection and found the whoosh test to have a sensitivity of 94% but a specificity of only 20% with a significant number of false positive results. More recently, the use of a nerve stimulator to confirm successful placement has been described. Singh and Khan13 described its use in 15 children, with only one child failing to receive good analgesia. All 15 had a positive whoosh test, but only 80% had documented motor stimulation with the nerve stimulator. In the other report, all 32 children studied had motor activity in the anal sphincter.14 Use of a nerve stimulator was not compared with the whoosh test in this study.
There are no reports on the accuracy of the whoosh test in children, although a brief report on its use in two children suggested it to be inaccurate.7 In addition, the use of air in the epidural space is probably best avoided in children as it may be associated with neurological damage.9 There has also been a report of probable venous air embolism in a child following the use of an air whoosh test.8 Puddy15 has described a modification of the whoosh test that involves the addition of a small quantity of air to the local anaesthetic solution (whoosh test 2), but this would still seem to be contraindicated. Our observation of a swoosh during injection of local anaesthetic into the caudal space in a child lead us to investigate the swoosh test as a predictor for successful caudal anaesthesia. The test could just as easily be performed with a volume of sodium chloride 0.9%, which would avoid the situation where local anaesthetic has been administered subcutaneously thereby causing the caudal to be abandoned because of potential toxicity. Whilst this particular modification has not been investigated, the results are likely to be similar.
Our observations show that a positive swoosh test is a highly sensitive predictor of a successful caudal anaesthetic, with a positive predictive value of 100%. We had a number of false negatives but nevertheless the overall sensitivity of the test was above 90%. In particular, we had no false positive results, a highly desirable characteristic for a diagnostic test, giving a specificity of 100%. We were unable to demonstrate a statistically significant difference between the predictive values of the swoosh test and clinical impression of successful cannula insertion. The results are unaffected by using a loss of resistance on piercing the sacrococcygeal membrane as the single end point of success. In their study in adults using an air whoosh test, Chan and colleagues6 found a positive predictive value of 78% for a give on insertion, compared with 97.7% for the whoosh test. The higher predictive value for clinical judgement in our study is almost certainly related to our selected patient group.
We conclude that the swoosh test is both simple and accurate. It compares favourably with previous descriptions of the whoosh test and to more complicated methods of localizing the caudal space. It is especially useful when caudal insertion has been difficult and clinical impression is equivocal and as a teaching aid for anaesthetists new to the technique of caudal anaesthesia in children.
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References |
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