Ultrasound control for presumed difficult epidural puncture

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Acta Anaesthesiol Scand 2001; 45: 766–771 Printed in Denmark. All rights reserved

Copyright C Acta Anaesthesiol Scand 2001 ACTA ANAESTHESIOLOGICA SCANDINAVICA

ISSN 0001-5172

Ultrasound control for presumed difficult epidural puncture T. GRAU, R. W. LEIPOLD, R. CONRADI and E. MARTIN Department of Anaesthesiology, University of Heidelberg, Germany

Background: The efficacy of epidural anaesthesia depends on the accurate identification of the epidural space (ES). Abnormal anatomical conditions may make the procedure difficult or impossible. The aim of this study was to investigate whether prepuncture ultrasound examination of the spinal anatomy might be beneficial in expected cases of difficult epidural anaesthesia. Methods: We used digital ultrasound equipment with a 5-MHz transducer to assess the anatomy of the ES and the posterior parts of the spinal column. We examined 72 parturients with abnormal anatomical conditions who were scheduled for epidural anaesthesia. The women were randomised into two equal groups. In all patients, the standard loss of resistance technique was used. In the ultrasound group, an ultrasound examination of the appropriate spinal region was conducted prior to epidural puncture. ES depth seen on the ultrasound images was com-

pared to the ES depth measured by the needle. We compared the number of puncture attempts with the standard method (control group) to the number of attempts under ultrasound guidance. Results: Ultrasonography significantly improved operating conditions for epidural anaesthesia. The maximum VAS scores and patient acceptance were significantly better. Conclusions: With ultrasound measurement of the ES depth, the quality of epidural anaesthesia was enhanced.


dural catheter may cause asymmetrical diffusion of drugs in the ES (16). It has long been known that ultrasonography can provide information about the anatomy of the lumbar spine. The first studies in the field of ultrasound measurement of the ES date back 20 years, when Cork et al. (17) and Currie (18) observed a strong relationship between the ES depth seen on ultrasound images and the depth measured by the epidural needle. Wallace and colleagues (20) confirmed that puncture depth is predictable from an ultrasound depth measurement. In the most recent study, Bonazzi and de Gracia found a very high correlation (99%) between the two (19). They concluded that ultrasound measurement of the ES prior to ES puncture is a useful diagnostic method, particularly when anatomical landmarks are obscured. Nevertheless, no further studies have been carried out, nor has the method found clinical acceptance. In recent years, ultrasound imaging technology has become more versatile. Modern computer technology has led to considerable enhancements in imaging, differentiation between similar echo structures and suppression of white noise. In 1992, Wallace et al. (20) were the first to receive a well-defined echo from the

anaesthesia (EDA) via an indwelling catheter provides a means by which the level and duration of anaesthesia can be adjusted to individual needs. The utilisation of EDA has increased over the past few decades, especially for labour and delivery (1, 2). The epidural space (ES) has been described as a potential space rather than a true cavity (3–5, 11). Reynolds et al. (5), Hogan et al. (6) and Westbrook et al. (7) reported the ES to have a ‘‘sawtooth configuration’’ in the sagittal view. Therefore, ‘‘ES depth’’ depends on the needle trajectory. All attempts to relate this depth to patient variables such as height or weight have proved to be inadequate for clinical use (6–9). Bevacqua and colleagues (10) found that the variability in posterior ES anatomy affects the successful administration of drugs. In the presence of obesity, scoliosis or oedema, the ES should be punctured only by experienced practitioners. Because of the high variability in the skin-to-ES distance (11, 12) and the inhomogeneous structure of the ES itself (13–15), its accurate identification is a technically demanding procedure. Any abnormal anatomical conditions may make it difficult to place the Tuohy needle correctly, and inaccurate placement of the epiPIDURAL


Received 14 August, accepted for publication 14 December 2000

Key words: Epidural anesthesia, difficult puncture; ultrasonography; diagnostic use. c Acta Anaesthesiologica Scandinavica 45 (2001)

Ultrasound for difficult epidural puncture

flaval ligament. We used more advanced ultrasound equipment in our study and were able to identify the ligament as well as the dura mater and, subsequently, the ES. Ultrasound scanning gives an accurate reading of structures up to a depth of 12 cm under the skin surface. As the ES is located at a depth of 20–90 mm (3, 4), pre-puncture ultrasound examination is theoretically possible. The aim of this study was to investigate whether pre-puncture ultrasound examination of the spinal anatomy might be beneficial in expected cases of difficult epidural anaesthesia.

pant had a history of spinal surgery or trauma (except prior EDA), nor was there evidence of congenital spinal abnormalities in any of the patients. The participants were randomised into two equal groups with a closed-envelope technique. In 36 parturients the ES was punctured without previously conducting an ultrasound examination (control group). The remaining 36 parturients had an ultrasound examination of the appropriate spinal region prior to epidural puncture (US group). Epidural anaesthesia was always performed by the same operator (TG) using the same technique. With the parturient in a sitting position, the skin and underlying tissues were infiltrated with a local anaesthetic in the midline at the mid-point of the L3–L4 interspace. An epidural needle (Tuohy G 17, BraunTM Melsungen, Germany) was inserted in the sagittal plane using the standard ‘‘loss-of-resistance-to-saline’’ technique (LOR) to identify the ES. A closed-tip, multi-orifice catheter was advanced 4–5 cm into the ES. A test dose of 2 ml of bupivacaine 2.5 mg mlª1 was given through the catheter before the main dose (10– 13 ml of bupivacaine 0.25 mg mlª1 plus sufentanil 10 mg) was administered in increments. After administering the drug, the time to the first sign of sensory block and the time to complete analgesia were documented. The extent of sensory block was evaluated by cold and the degree of motor block according to Bromage. Data were aquired every hour or on patients’ request for analgesia. Unilateral or asymmetrical blockade was considered as an effect of lateral placement of the catheter (21, 22); symmetrical, but ‘‘patchy’’ anaesthesia was interpreted to be the

Methods Approval for this study was granted by the local ethics committee, and written informed consent was obtained from all study patients. For initially evaluating the variation in treatment we used this prospective (closed-envelope) randomised study protocol for 72 parturients with presumed difficult puncture. Inclusion criterion was the expectation of a difficult EDA: Roughly two out of three parturients had either a history of difficult epidural anaesthesia (36%) or substantial alterations of the lumbar spine such as scoliosis, kyphosis or hyperlordosis (26%). Thirtyeight percent had a Body Mass Index ⬎33 kg/m2). Parturients who did not require EDA or in whom EDA was contraindicated due to for example relevant clotting disorder, infectious skin disease or other acute infections, were excluded from the study. To avoid delay due to any additional diagnostic procedures, obstetric emergencies were excluded as well. No partici-

Table 1 Demographic and basic parameters (ESΩEpidural space; EDAΩEpidural anaesthesia). Control group

Age Weight Height ASA Study time Min. dist. skin-ES Max. dist. skin-ES Measured angle Punctured angle Punctured depth Catheter depth First effects of EDA Compl. effects of EDA VAS before EDA Max. VAS under EDA Patients’ satisfaction

years kg cm min mm mm degree degree mm mm min min scale scale scale 1–6

Ultrasound group





30.8 90.2 163.5 2.1 4.4 ª ª ª 16.3 60.1 107.9 5.5 18.2 4.2 1.8 2.1

4.8 23.5 16.7 0.6 1.4 ª ª ª 9.4 16.0 15.5 3.8 4.9 4.1 2.7 1.3

30.5 92.4 164.5 2.3 5.2 47.0 59.2 16.7 16.2 57.5 106.0 4.4 16.1 4.1 0.8 1.3

5.6 24.4 8.6 0.6 1.5 9.5 10.4 5.7 7.5 11.0 12.9 2.4 4.3 4.1 1.4 0.5


T. Grau et al.

Fig. 1. The visibility of structures in the lumbar epidural area on ultrasound are presented. The visibility of the epidural space, the ligamentum flavum and dura mater is divided into the catagories none, sufficient or good.

consequence of an inhomogeneous spread of anaesthetics. Parturients in the US group underwent ultrasonography of the lumbar spine prior to EDA. Longitudinal and transverse ultrasound scans were performed between contractions and taken for documentation. The visibility of the ES in the ultrasound scan was rated as ‘‘none’’, ‘‘sufficient’’ or ‘‘good’’. We compared the skin to ES depth seen on the ultrasound images to the depth measured at puncture and calculated the precision of the ultrasound examination. Other evaluations included ultrasound visibility of ES-adjacent structures. We considered every redirection of the needle, even without further skin puncture, as a ‘‘puncture attempt’’. This convention seemed necessary, as otherwise a successful puncture in only one attempt could mean a direct hit immediately after skin puncture as well as a lengthy procedure with several relocations of the needle. If the ES could not be cannulated in the first puncture site, a neighbouring intervertebral space was punctured. The number of puncture attempts and the number of puncture sites were recorded. Twenty-four hours post partum, parturient satisfaction with analgesia during labour was assessed using the Visual Analogue Scale (VAS) pain score plus a numerical, six-point verbal score (1Ωvery good, 2Ωgood, 3Ωaverage, 4Ωsufficient, 5Ωunsatisfactory, 6Ωinsufficient). All patients were interviewed regarding EDA side effects. We used SonoaceTM 6000 ultrasound equipment


(Kretz, Marl, Germany) with a 5-MHz transducer. Distances and angles were calculated using the built-in distance and angle program. Scanning was performed in the midline overlying the L3–L4 interspace in the transverse and longitudinal planes. This provided a three-dimensional image of the spinal ligaments and adjacent bony structures. The angle of the needle at the puncture was measured with a conventional set square with respect to a horizontal line in the sitting patient. We recorded all data on standardised forms using tick boxes. Discrete variables (puncture attempts and number of vertebral levels) were analysed using the Chi-squared test with Yates’ correction. Precision of ES detection (depth on ultrasound vs. LOR method) was calculated using the Bland–Altman Test (23). ‘‘Precision’’ was defined as the 95% confidence interval for the difference between US and LOR depth measurements. Excel 97TM MicrosoftTM, SPSS 7.5TM and Primer Biostatistik 4.04TM (S.A. Glantz) software were used for statistical analysis. Data are given as mean∫SD unless otherwise stated.

Fig. 2. Relation between skin-to-ES distance obtained by ultrasound (measured depth and during epidural puncture).

Fig. 3. Relation between ideal angle of needle trajectory obtained by ultrasonography (measured puncture angle) and the angle of the epidural needle, when entering the epidural space (punctured angle). Angles are given in degrees in relation to the transverse plane in a patient, who is sitting.

Ultrasound for difficult epidural puncture

Fig. 4. Native transverse and longitudinal ultrasound scans. The resolution of the printed pictures is effectively not as good as on the US screen.

Fig. 5. This picture gives descriptive information on the sonomorphology and localisation of the relevant structures: the position of the dura mater, the processi spinosi, the ligamentum flavum and the corpora vertebrae are defined.

Results The groups were similar regarding age, weight, body height and ASA risk factors (Table 1). In the US group

all relevant anatomical structures could be identified (Fig. 1; Fig. 4, 5). In the control group, 2.6∫1.4 puncture attempts were made as compared to 1.5∫0.9 attempts in the US group (P⬍0.001) (Table 2). In the


T. Grau et al. Table 2 The effects of ultrasound study on different parameters. The data are presented as frequencies and as mean and standard deviation, including P-values.

Number of puncture attempts Number of puncture sites Catheter advancement attempts Patients with asym. EDA* diffusion Patients with patchy EDA* diffusion Epidural failure VAS before EDA Maximum VAS Patients’ satisfaction Headache Backache

Control group

Ultrasound group


2.6∫1.4 1.5∫0.7 1.3∫0.6 5 3 2 4.2∫4.1 1.8∫2.7 2.1∫1.3 4 7

1.5∫0.9 1.27∫0.51 1.1∫0.6 2 1 0 4.1∫4.1 0.8∫1.4 1.3∫0.5 2 7

P⬍0.001 P⬍0.05 P⬍0.003 n.s. n.s. n.s. n.s. P⬍0.035 P⬍0.006 n.s. n.s.

* EDAΩEpidural anaesthesia.

control group, 1.5∫0.7 intervertebral spaces had to be punctured before the ES was successfully reached and in the US group 1.3∫0.5 (P⬍0.05) (Table 2). An average of 1.3∫0.6 catheter advancement attempts were made in the control group and 1.1∫0.4 in the US group (P⬍0.003) (Table 2). The VAS pain scores before EDA were similar in the two groups; 4.2∫4.1 in the control group and 4.1∫4.1 in the US group. During labour and EDA, the maximum VAS pain score was 1.8∫2.7 in the control group and 0.8∫1.4 in the US group (P⬍0.035). Patients’ satisfaction with epidural anaesthesia was rated in a verbal rating score as 2.1∫1.3 in the control group, and as 1.3∫0.5 in the US group (P⬍0.006) (Table 2). We found similar skin-to-ES distances in the two groups. The ultrasound-measured depth and angle correlated with the puncture depth and angle (Fig. 2, 3). For the prediction of the skin-to-ES distance we found a 95% precision (pr) (Bland–Altman) calculated as prΩ7.9 mm. Side differences in the distribution of asymmetrical spread of sensory blockade was found in 14.7% of the control group and in 5.5% of the US group (n.s.). Patchy anaesthesia was seen in 8.8% and 2.7% of the parturients, respectively (n.s.) (Table 2). Four parturients in the control group versus two in the US group complained of mild headache after delivery. In each group seven parturients suffered from a light backache postoperatively (Table 2).

Discussion We found ES puncture to be significantly facilitated by pre-puncture findings of spinal anatomy obtained by ultrasonography. The ultrasound image provides three important pieces of information that may ac-


count for the significant reduction of puncture attempts in the US group: the optimal skin puncture site, the ideal direction of needle advancement and, above all, the skin-to-ES distance. This information enables the anaesthesiologist to be more decisive in performing the puncture rather than having to ‘‘feel the way’’ to the epidural space. Sutton and Linter (11) examined the skin-to-ES distance range in a large obstetric population. They found only 76% of the 3011 ES examined within a ‘‘normal’’ range (4–6 cm). In 16% of parturients, the ES was located at a ‘‘shallow’’ depth (2–4 cm). This distribution of skin-to-ES depth agrees with other reports (3, 15, 25). As the skin-to-ES distance ranges from 2 to 9 cm, the flaval ligament can be anywhere in this 7-cm span. According to our data, pre-puncture ultrasound examination reduces this ‘‘probable depth to midline epidural entry’’ to a 7.9-mm span in 95% of all cases. However, the value of these findings is limited by the puncture technique itself, and strict interpretations of distances on pre-puncture ultrasound images are subject to systematic errors. A number of reasons account for this: A) The blunt Tuohy needle generates high pressure during passage through tissue (max. 180 mmHg), especially in the flaval ligament (⬎400 mmHg) (26). This can cause a tissue deformation which accounts for a difference in the skin-to-ES depths measured. B) The Tuohy needle must be inserted beyond the flaval ligament to allow the passage of the catheter. Hence the skin-to-flaval ligament distance measured by ultrasound is a priori shorter than the depth of needle insertion at entry of the ES. C) If the angle at which the epidural needle is in-

Ultrasound for difficult epidural puncture

serted differs from the angle used at scanning, the required depth of needle insertion will be greater than the distance measured by ultrasonography. This happens when the trajectory of an epidural needle strays away from the midline or is inserted via a more cranial or caudal angle. Such off-axis trajectories will cause the needle to enter the ES at a depth that is greater than the skin-to-ES depth measured by ultrasonography. Thus, ultrasound measurements of the skin-to-ES depth must be expected to differ slightly from the length of needle insertion actually required for epidural puncture. This degree of uncertainity can only be avoided by using real-time ultrasound to guide the puncture. As ultrasonography is being used in clinical diagnosis throughout the world in many medical fields, the equipment is readily available everywhere. Major advantages over computerised axial tomography and magnetic resonance imaging are that ultrasound is minimally invasive, mobile and less expensive. The investigation is fast and free of discomfort and does not present a risk for mother or child. Patients’ satisfaction with anaesthesia was very high in both groups. Given the significant difference in the number of needle manipulations and the improved VAS score, the higher satisfaction with the technique in the ultrasound group is easily explained. We found no disadvantages related to ultrasound scanning. When a difficult puncture of the ES is expected, ultrasound images facilitate the procedure. Further investigations seem warranted, especially in the field of real-time ultrasound examination. With increased information on the individual anatomy, a general quality enhancement of epidural analgesia could be expected.

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Address: Thomas Grau, MD University of Heidelberg Department of Anaesthesiology Im Neuenheimer Feld 110 D-69120 Heidelberg Germany e-mail: thomas_grau/med.uni-heidelberg.de website: www.peridural.com


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