Department of Anaesthesia, Urology & Nephrology Center, Mansoura, Egypt
*Corresponding author. E-mail: daliaat@hotmail.com
Accepted for publication: October 30, 2003
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Abstract |
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Methods. A total of 300 patients subjected to urological procedures and scheduled for spinal anaesthesia were independently assessed and stratified according to the categories of the difficulty predictors of spinal anaesthesia into one of nine grades (08) and randomized according to the experience of the anaesthetist into two groups (group A, staff with more than 15 yrs experience; group B, resident with more than 6 months but less than 1 yr in training). The number of attempts and levels, and success rate of the technique were the outcome variables. Data were analysed by multivariate analysis and receiver operating characteristic (ROC) curves.
Results. The bony landmarks of the back and the radiological characteristics of the lumbar vertebrae were two independent predictors of difficulty. Multivariate analysis indicated differences between junior and senior staff but ROC curves indicated no difference. Grade 4 was the difficulty score at or above which difficulty was expected whether or not radiological characteristics of the vertebrae were included.
Conclusions. Spinal bony landmarks and radiological characteristics of the lumbar vertebrae are independent predictors of difficulty during spinal anaesthesia. There is no difference between senior and junior anaesthetists. Grade 4 is the difficulty score at or above which difficulty is expected.
Br J Anaesth 2004; 92: 35460
Keywords: anaesthetic techniques, subarachnoid
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Introduction |
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Accurate preoperative prediction of potential difficulty can help reduce the incidence of multiple attempts, rendering the technique more acceptable and less risky to the patient. The quality of bony landmarks of the back is an independent difficulty predictor, while the significance of the anaesthetists level of experience is still unsettled.6 7 An objective scoring system might serve as a reproducible quantitative measure of the expected difficulty. This clinical study was designed to: (i) determine the predictive performance of the expected difficulty variables; (ii) compare senior and junior anaesthetists; (iii) develop a simple, accurate and easy applicable difficulty score.
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Methods |
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Development of preoperative difficulty score
The preoperative difficulty score for spinal anaesthesia was designed using patient characteristics familiar to the clinicians to make it easily applicable before anaesthesia. The rationale behind the definition of each difficulty category is based on general and accepted knowledge of spinal anaesthesia. For example, the subjective estimation of difficulty is increased in overweight adult patients. This will be greater in elderly obese patient, and even more when lumbar vertebral spinous processes are difficult to palpate. The difficulty categories are shown in Table 1. Age, BMI and spinal bony deformities are totally objective variables. Kyphosis, scoliosis and lordosis were considered as spinal bony deformities. The assessment of the quality of spinal bony landmarks is purely subjective. To avoid bias, this examination was performed separately before surgery by the senior investigator (MMA). The spinous processes of the lumbar vertebrae were assigned as clear and easily palpable, or unclear and difficult to palpate. The radiographic findings of the lumbar vertebrae, performed routinely in urological practice in our hospital, were categorized as in the difficulty score. The presence of osteophytes, ligament calcification or narrow intervertebral spaces were considered difficulty characteristics.
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Spinal anaesthesia
Patients were randomly allocated, through computer-generated numbers, into two groups. For group A, the anaesthetist had more than 15 yrs experience; for group B spinal anaesthesia was provided by a resident in training for more than 6 months and less than 1 yr. Spinal anaesthesia is frequently performed in our centre. A resident usually performs approximately five spinal punctures daily. In both groups, spinal puncture was performed with the patient in the sitting position, using a 22G spinal needle and after establishing the free flow of crystalloid solution in one arm vein. When spinal anaesthesia was complete, patients were laid supine and midazolam 13 mg was given.
Outcomes
The difficulty encountered in performing the spinal puncture was evaluated by three variables. First, the number of attempts required for successful needle placement at the initial spinal level. Each new skin puncture was considered another attempt. However, redirecting the needle without a new skin puncture was not considered an additional attempt. Second, the number of spinal levels before completing the puncture. Two levels only were allowed for the resident, after which the senior staff member had to take over and was allowed two more attempts. If they failed, the senior investigator had to complete the puncture. Third, the success or failure of spinal anaesthesia was recorded. Anaesthesia was considered complete and successful if the urological procedure was completed without any analgesic or anaesthetic supplementation.
Statistical analysis
The power of this clinical trial was retrospectively calculated using the GPower analysis program.8 Using post-hoc power analysis with accuracy mode calculations and assuming type-I error protection of 0.05 and medium effect size convention of 0.3, a total sample size of 300 patients produced a power of 0.99.
Patient characteristics are presented as mean (SD) and range, and were analysed by paired Students t-test. The association between patient predictive difficulty variables and the selected outcome variables was determined by univariate analysis using the Pearson 2 goodness-of-fit test. Predictive difficulty variables that were significant with univariate analysis were subjected to logistic multivariate stepwise regression analysis for the determination of preoperative difficulty variables that had an independent impact on the outcome variables.
Predictive accuracy was assessed for all predictive difficulty scores by building receiver operating characteristic (ROC) curves for the outcome variables using the statistical program Accu ROC for windows 95/98/NT version 2.4 (Accumetric corporation, Montreal, Canada). Accu ROC uses non-parametric methods,911 based on the MannWhitney U-test: to calculate the area under the ROC curve (AUC), its standard error (SE) and an estimate of its normal symmetric 95% confidence intervals; compare ROC curves from independent samples; and calculate other measures of test performance at all grades with 95% confidence intervals. AUC values of 0.50.7 suggest low accuracy and values greater than 0.7 confirm the usefulness of the difficulty classification as a difficulty predictor.12 In all calculations, P<0.05 was the significance level.
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Results |
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Univariate analysis of the difficulty predictors revealed that BMI, spinal bony landmarks and deformity, radiological characteristics of the lumbar vertebrae and the experience of the provider had significant impact on outcome variables. The results of the logistic multivariate stepwise regression analysis of these difficulty predictors are shown in Table 3. Two predictors only (spinal bony landmarks and radiological characteristics of the lumbar vertebrae) had a significant impact on attempts, levels and success of the spinal analgesia, while the experience of the anaesthetist had a significant impact only on the number of attempts and levels.
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Discussion |
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The experienced practitioner develops the tactile sense of needle advancement and completes spinal anaesthesia in a smooth, rapid and effective manner. However, patient characteristics and the anaesthetists level of experience may culminate in multiple attempts at more than one level, rendering the technique unpleasant to the patient and occasionally with dangerous complications. Neurological complications following neuraxial anaesthesia, leading to temporary or permanent disability have recently been reviewed.13 Spinal epidural haematoma,1416 although very rare, caused the US Food and Drug Administration to issue a warning in 1997.17 Epidural abscesses following neuraxial anaesthesia continue to be reported.18 19
Patient characteristics mostly determine the difficulty during spinal puncture. Patient characteristics have been described as difficulty predictors.6 7 Spinal bony landmarks were the only independent predictor of difficulty. In this study, spinal bony landmarks and radiological characteristics of the lumbar vertebrae were the only two independent predictors of difficulty during spinal puncture.
The anaesthetists level of experience has been evaluated as a predictor of difficulty during spinal puncture. In a study comparing staff/fellow anaesthetists, certified nurse anaesthetists and anaesthesia trainees,6 the providers level of experience had no effect. In another study comparing residents in training with anaesthetists of varying clinical experience,7 the providers level of experience was an independent predictor. In our study, multivariate analysis of difficulty predictors revealed a significant difference between senior and junior providers in the number of trials and levels but there was no significant difference in completing spinal anaesthesia. When the areas under the ROC curves were compared, there was no significant difference between senior and junior providers in either the number of trials or the number of levels. The explanation of this is complex. Multivariate analysis is a collection of techniques appropriate for the situation in which each individual provides observations simultaneously on several variables, and the random variation in these variables has to be studied simultaneously.20 The application of this method will only have produced a useful reduction in the dimensionality of the data if the components have an interpretation that appears to represent some meaningful characteristics. The ROC analysis uses non-parametric methods10 11 which are robust, and the hypotheses to be tested usually relate to the nature of the distribution as a whole rather than to the values assumed by some of its parameters.20 It is one of the most common measures used to describe the performance of the provider over all grades.9 Consequently, it can be safely concluded that there was no significant difference between senior and junior anaesthetists in this study.
Grade 4 was the difficulty score at or above which difficulty was predicted. This holds true whether or not the radiological characteristics of the lumbar vertebrae are included. Using this score, nearly two thirds of the patients were correctly stratified and classified. Furthermore, the likelihood ratio of a difficult score is greater than that of an easy score and, being relatively insensitive to changes in difficulty prevalence, it becomes an excellent descriptor of the score performance. The predictive values are sensitive to difficulty prevalence in the study population; this is why the predictive value of a difficulty score is low whereas the predictive value of an easy score is high. The thoughtful use of this difficulty score can stratify patients to the appropriate anaesthetist, reducing the number of trials and levels and improving the performance of spinal puncture.
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Acknowledgement |
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References |
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