BJA Advance Access published online on January 19, 2007
British Journal of Anaesthesia, doi:10.1093/bja/ael361
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The coiling length of thoracic epidural catheters: the influence of epidural approach angle
Department of Anesthesiology, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Republic of Korea
* Corresponding author: Department of Anesthesiology, Seoul National University Hospital, 28 Yongon-dong Chongno-gu, Seoul, Republic of Korea 110-744. E-mail: bahkjh{at}snu.ac.kr
Accepted for publication November 24, 2006.
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Abstract |
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BACKGROUND: Thoracic epidural catheters are used for anaesthesia and postoperative analgesia. Usually, epidural catheters are placed without confirmation of their position despite frequent reports of complications as a result of malposition. In this study, we evaluated the threading length of thoracic epidural catheters without coiling and assessed the influence of two different epidural approach angles on the threading length without coiling.
METHODS: Eighty-three patients scheduled for thoracotomy were enrolled and randomly allocated into the acute angle group and the obtuse angle group. In both groups, skin insertion was performed at the T8–9 intervertebra level. Epidural access was performed under fluoroscopy using a paramedian approach at the T7–8 level in the acute angle group and at the T6–7 level in the obtuse angle group, and an end-hole 19-gauge epidural catheter was inserted. Coiling length, defined as the length of the catheter within the epidural space when any part of the catheter just begins to head caudally, was measured in both groups.
RESULTS: The coiling length was 7.4(4.4) cm (95% CI 6.0–8.7 cm) in the obtuse angle group compared with 4.9(3.3) cm (95% CI 3.8–6.0 cm) in the acute angle group (P = 0.005).
CONCLUSIONS: Approaching the thoracic epidural space with an obtuse approach angle provides longer coiling length. We recommend that an obtuse approach angle should be used to maximize the chance of the catheter reaching the intended level with minimum risk of coiling.
Keywords: anaesthetic techniques, epidural; approach angle; coiling length; equipment; catheters epidural
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Introduction |
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Thoracic epidural analgesia has become the standard method in postoperative pain control after thoracotomy.1–3 The epidural catheter tip position seems to be an important determinant of the extent of analgesia.4 However, it is well known that epidural catheters do not follow a straight and predictable course in the epidural space during catheter insertion.5–8
The risk of catheter coiling, knotting, or malposition may increase as the length of the catheter within the epidural space increases. On the other hand, insufficient length of the epidural catheter lying within the epidural space may increase the chance of accidental withdrawal. Therefore, it would be reasonable to search for a method that allows longer epidural threading length without coiling.
The coiling length,5 defined as the length of the catheter within the epidural space when any part of the catheter just begins to head caudally, is not fully established in thoracic epidural catheterization.8 We hypothesized that the approach angle of the epidural needle may affect the coiling length of the epidural catheter and consequently the position of the epidural catheter tip.
The purpose of this study was to determine the coiling length of the thoracic epidural catheter and to assess the influence of the approach angle on the coiling length.
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Methods |
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After obtaining approval from the hospital's review board and informed consent, 83 patients scheduled for lobectomies, bilobectomies, pneumonectomies, or oesophagectomies using a thoracotomy incision were randomly allocated into two groups, the acute angle group and the obtuse angle group, depending on the approach angle of the epidural needle. Patients with a history of spinal or epidural procedures or with contraindications to epidural catheterization such as coagulopathy or skin infection on the insertion site were excluded. Patients with compression fractures in the mid-thoracic region or severe degrees of vertebral deformities such as kyphosis, scoliosis, or hemivertebra were also excluded.
Epidural catheterization was performed the day before surgery using a paramedian approach with patients in the prone position, which allows patients to maintain their position comfortably and without movement during the procedure and thereby facilitates easier assessment of catheter position with less exposure to radiation. A large pillow was placed under the lower chest in order to augment the thoracic kyphosis to a degree used in the lateral decubitus position during epidural catheterization. After aseptic preparation, the T8–9 intervertebra level was identified by counting the vertebra and ribs from T1 and T12 under fluoroscopy. Before inserting the 17-gauge Tuohy epidural needle through the skin at the upper lateral corner of T9 vertebral body, 6–8 ml of lidocaine 1% was infiltrated throughout the expected pathway to the epidural space. The Tuohy needle was advanced towards the T7–8 intervertebra level in an attempt to access the epidural space at that level in the acute angle group (Fig. 1A), whereas epidural space access was attempted at the T6–7 intervertebra level in the obtuse angle group (Fig. 1B). The Tuohy needle (Arrow Inc., Reading, PA, USA) was guided by bony structures and anterior–posterior and lateral X-ray images. Once the epidural space was confirmed with a loss of resistance technique using 2–3 cc of air in a 10 cc syringe, a radio-opaque end-hole 19-gauge Flextip Plus® epidural catheter (Arrow Inc., Reading, PA, USA), made of polyvinyl chloride with stainless-steel wire reinforcement, was placed through the needle under fluoroscopy guidance. The epidural catheter was slowly advanced in 0.5-cm increments under fluoroscopy with the bevel of the needle facing cranially. The coiling length was measured in both groups by subtracting the length of the epidural needle (11.5 cm) from the threaded length read at the hub of the epidural needle when the catheter just began to coil caudally. If the epidural catheter would not coil, the length of the epidural catheter threaded into the epidural space was limited to 20 cm.
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If the catheter tip could not be placed at the intended level (T5–6 intervertebra level) owing to early coiling using an acute approach angle, catheterization was repeated using the obtuse angle and vice versa. In this case, the coiling lengths of the epidural catheter of both approach angles were obtained in the same fashion for analysis. A small amount (0.5 ml) of contrast medium (Iopamiro® 300, Bracco s.p.a., Milano, Italy) was used for final confirmation of catheter tip position.
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Statistical analysis |
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Sample size was calculated using values from a pilot study. Power calculation required 40 patients in each group to detect a 2.1-cm difference with 80% power and 5%
-error. Statistical analysis was performed using the SPSS Windows version 12.0 (SPSS, Chicago, IL, USA). Student's t-test was used to compare the data between the two groups and paired t-test was used to compare the data of patients who underwent epidural catheterization using both angles. Fisher's exact test was used to compare the frequency of paresthesia between the two groups. Data are expressed as mean(SD) unless specified otherwise. A P-value of <0.05 was considered to be statistically significant. |
Results |
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Initially, 83 patients were randomly allocated into the obtuse angle group (n = 41) and the acute angle group (n = 42). Two patients allocated into the acute angle group were excluded from the study because of epidural access failure. The two groups were similar in age, height, weight, and male to female ratio (Table 1). In 27 patients of the acute angle group, epidural catheterization was successful, but the catheter could not be placed at the intended level and thereby epidural access was repeated using the obtuse approach angle. In the obtuse angle group, the thoracic epidural catheter was successfully placed in all patients with no need for change of approach angle.
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The coiling length was 7.4(4.4) cm (95% CI 6.0–8.7 cm) in the obtuse angle group compared with 4.9(3.3) cm (95% CI 3.8–6.0 cm) in the acute angle group (P = 0.005) (Fig. 2). All epidural catheters began to coil within the epidural space before reaching the preset threading limit of 20 cm. There was no difference in the frequency of paresthesia during catheter insertion between the two groups (acute angle group vs obtuse angle group: 14/40 vs 17/41; P = 0.649). There was also a significant difference in the skin–epidural space length between the two groups [acute angle group vs obtuse angle group: 4.9(0.59) vs 6.9(0.78) cm; P < 0.001]. The estimated approach angle was 55–75° in the acute angle group and 35–45° in the obtuse angle group. The distance from midline to the skin insertion point was roughly 1.5–2 cm.
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In 27 patients whose epidural space was accessed using both approach angles, the coiling length using the obtuse approach angle was 2.3(3.0) cm longer than that when the acute approach angle was used (acute angle vs obtuse angle: 3.8(1.8) vs 6.1(2.7) cm; P < 0.001).
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Discussion |
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The main finding of our study is that when accessing the mid-thoracic epidural space using an obtuse epidural approach angle, the coiling length is longer compared with the use of an acute approach angle. Our results suggest that in order to minimize the risk of coiling, the insertion length of the thoracic epidural catheter should not exceed 3.8 cm when using an acute approach angle and 6.0 cm when using an obtuse approach angle.
The condition of the epidural space may vary from one patient to another and may have a significant effect on the coiling length. Analysis of the 27 patients, in whom both approach angles were used, also demonstrated a significantly longer coiling length with the obtuse angle by an average of 2.3 cm. We think that the coiling lengths obtained from these patients exclusively reflect the influence of the approach angle.
Concerning thoracic epidural catheters, data for coiling lengths are very limited in the current literature.8 For lumbar epidural catheters, the coiling length using single orifice epidural catheters was found to be 3 cm.5 In contrast to the traditionally recommended insertion length of 2–4 cm of insertion length for surgical patients,9 10 a prospective randomized trial in healthy parturients suggested that the epidural catheter should be inserted 6 cm for prolonged labour or Caesarean delivery.11 A study using multiorifice lumbar epidural catheters for labour analgesia recommended an insertion length of 5 cm to avoid intravenous insertion.12 There are few studies concerning the insertion technique and position of thoracic epidural catheters. A study comparing midline and paramedian thoracic epidural catheterization demonstrated that paramedian approaches were associated with higher straight advance rates.13
Despite controversy over the technical ease of the paramedian approach,14 15 previous studies on lumbar epidural catheterization have shown that the paramedian approach of the epidural space is associated with less paresthesia,16 faster and easier catheter insertion,16 and fewer technical problems17 compared with the midline approach. Approaching the thoracic epidural space with an obtuse angle in combination with the paramedian approach may provide additional potential advantages other than the longer coiling length. We speculate that, with the obtuse approach angle, the epidural needle approaching the spinal column less acutely potentially increases the margin of safety in regard to spinal cord injury. It also allows a longer travelling length from the skin to the epidural space, which might be associated with less bacterial colonization.18 Frequent failure of the thoracic ligamentum flavum to fuse may also render the paramedian approach more favourable.19
There are some limitations to this study that may be worth consideration. In our study, epidural catheters were placed with the patients in the prone position, which is not often used for placing thoracic epidural catheters. Additionally, there might be a possibility that the milieu of the epidural space in the prone position was different from that in the lateral decubitus or sitting position. However, we believe that the coiling length would not be different because the degree of the thoracic kyphosis in our study was similar to that used for thoracic epidural catheterization in the lateral decubitus or sitting position. In addition, although a different epidural level was accessed depending on the approach angle (T7–8 in the acute angle group vs T6–7 in the obtuse angle group), we think that the influence of one level difference on the coiling length is minimal in the mid-thoracic epidural space, not likely to alter the results of our study.
In conclusion, approaching the mid-thoracic epidural space in a paramedian approach with an obtuse angle provides longer coiling length. We suggest that when inserting a thoracic epidural catheter, an obtuse approach angle can maximize the chance of the catheter reaching the intended level with minimum risk of coiling.
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Acknowledgements |
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This study was supported solely by the Department of Anesthesiology. There are no financial relationships to disclose.
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Footnotes |
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This study was presented for poster discussion as an abstract at the 2006 European Society of Anesthesiologists meetings. 
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References |
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