The ‘swoosh’ test—an evaluation of a modified ‘whoosh’ test in children

R. M. L’E. Orme* and S. J. Berg

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


    Abstract
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Background. Caudal analgesia is widely used in paediatric anaesthetic practice. In adults, the ‘whoosh’ test has been recommended as a guide to successful needle insertion, but it has not been extensively studied in paediatric patients. We have investigated a modification of the ‘whoosh’ test, which we have christened the ‘swoosh’ test. It avoids the injection of air by performing auscultation during injection of the local anaesthetic solution. We have compared it with clinical judgement of correct placement.

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 operator’s 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: 62–5

Keywords: anaesthetic techniques, epidural


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Caudal analgesia is widely used in anaesthetic practice for providing perioperative analgesia for lower abdominal, penile, rectal and lower limb surgery. Whilst it is a relatively simple technique, it relies on the ability to locate the anatomical landmarks of the sacral cornua and the end-points of successful insertion, typically a give on piercing the sacrococcygeal membrane. In adults, the success rate may be as low as 75%.1 However, in children, it is in the order of 96%,2 as the anatomical landmarks tend to be more reliable.3 The ‘whoosh’ test was developed in the chronic pain clinic setting as a useful guide to successful needle placement.4 It involves the injection of approximately 2 ml of air through the caudal needle, with a ‘whoosh’ being heard via a stethoscope placed over the thoracolumbar spine if placement is correct. In the same setting, it was subsequently shown that the ‘whoosh’ test was superior to clinical judgement in detecting successful needle placement.5 It has also been advocated as a teaching aid for anaesthetists new to the technique of caudal anaesthesia.6

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.


    Methods
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
The study was approved by the local ethics committee. Children undergoing all types of general surgery were recruited to the study, which ran between October 2000 and December 2001. Basic patient characteristics were recorded: age, sex, weight and ASA grade. The type of surgery being performed and the grade of anaesthetist performing the caudal block were also noted. All blocks were performed on anaesthetized patients after securing i.v. access and following establishment of minimum recommended monitoring.

Anaesthesia was induced either via i.v. administration of propofol 3–5 mg kg–1, 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 0.5–1 ml kg–1 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 operator’s 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 patient’s 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.


    Results
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
In total, 113 children were recruited to the study (Table 1). The median age was 1.75, with a range from 1 month to 13 yr. The operative procedures included inguinal herniotomy (32), orchidopexy (27), ligation of patent processus vaginalis (17), circumcision (14), rectal procedures (11), hypospadias repair (seven) and intra-abdominal surgery (five). The majority of children were boys (99 vs 14). Sixty-three of the caudal blocks were performed by consultants and 50 by anaesthetic trainees.


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Table 1 Patient characteristics. Data are median (interquartile range) [range], mean (SD) or number (proportion)
 
The results are shown in Table 2. In total, 108 patients (95.6%) had a successful block as judged by the lack of requirement for supplemental perioperative analgesics and the presence of good postoperative analgesia. Cannula placement was felt to be successful in 100% of patients, whereas only 91% (98 patients) had a positive ‘swoosh’ test. In the five patients whose block failed, one cannula was felt to be correctly placed (false positive). The ‘swoosh’ test was negative in all patients with a failed block. Calculations show the ‘swoosh’ test to have a sensitivity of 90.7%, a positive predictive value of 100% (95% confidence intervals 96.2–100%), and a specificity of 100%. This compares with clinical judgement, which has a sensitivity of 100%, a positive predictive value of 99.1% (95% confidence limits 94.9–99.8%) and a specificity of 80%.


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Table 2 Comparison of the ‘swoosh’ test with clinical impression for predicting successful caudal placement. Data are number (proportion)
 

    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Caudal anaesthesia has become widely used in paediatric anaesthetic practice since its first description almost 70 yr ago.12 In adults, there is a significant failure rate, but in children success rates are reported to be higher. In their review of 750 consecutive caudal blocks in children, Dalens and Hasnaoui2 demonstrated a success rate of 96%. We have found similar results in this study, with an overall success rate of 95.6%. The higher failure rate in adults is almost certainly related to anatomical considerations, as the sacral hiatus becomes smaller with age due to increasing posterior fusion of the fourth and fifth sacral vertebrae, and the anatomical landmarks used to locate the hiatus become less distinct.3

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.


    References
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
1 White AH. Injection techniques for the diagnosis and treatment of low back pain. Orthop Clin North Am 1983; 14: 553–67[ISI][Medline]

2 Dalens B, Hasnaoui A. Caudal anesthesia in pediatric surgery: success rate and adverse effects in 750 consecutive patients. Anesth Analg 1989; 68: 83–9[Abstract]

3 Willis RJ. Caudal epidural blockade. In: Cousins MJ, Bridenbaugh PO, eds. Neural Blockade, 2nd Edn. Philadelphia: Lippincott, 1988; 361–83

4 Lewis MPN, Thomas P, Wilson LF, Mulholland RC. The ‘Whoosh’ test. A clinical test to confirm successful needle placement in caudal epidural injections. Anaesthesia 1992; 47: 57–8[ISI][Medline]

5 Eastwood D, Williams C, Buchan I. Caudal epidurals: the whoosh test. Anaesthesia 1998; 53: 305–7[CrossRef][ISI][Medline]

6 Chan SY, Tay HB, Thomas E. ‘Whoosh’ test as a teaching aid in caudal block. Anaesth Intens Care 1993; 21: 414–15[ISI][Medline]

7 Bollinger P, Mayne P. The ‘Whoosh’ test in children. Anaesthesia 1992; 47: 1002–3[ISI][Medline]

8 Guinard JP, Borboen M. Probable venous air embolism during caudal anesthesia in a child. Anesth Analg 1993; 76: 1134–5[ISI][Medline]

9 Flandin-Bléty C, Barrier G. Accidents following extradural analgesia in children. The results of a retrospective study. Paediatr Anaesth 1995; 5: 41–6[ISI][Medline]

10 Dalens B. Caudal anaesthesia. In: Dalens B, ed. Regional Anaesthesia in Infants, Children and Adolescents. Baltimore: Williams and Wilkins, 1995; 171–99

11 Armitage EN. Regional anaesthesia. In: Sumner E, Hatch DJ, eds. Textbook of Paediatric Anaesthetic Practice. London: Balliere Tindall, 1989; 213–33

12 Campbell MF. Caudal anesthesia in children. Am J Urol 1933; 30: 245–9

13 Singh M, Khan RM. Use of a peripheral nerve stimulator for predicting caudal epidural analgesia. Anaesthesia 2000; 55: 830–1

14 Tsui BCH, Tarkilla P, Gupta S, Kearney R. Confirmation of caudal needle placement using nerve stimulation. Anesth Analg 1999; 91: 374–8[Abstract/Free Full Text]

15 Puddy BR. The ‘whoosh’ test. Anaesthesia 1999; 54: 614





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