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Original Article

Influence of posture on the incidence of vein cannulation during epidural catheter placement

Harney, D.*,1; Moran, C. A.*,1; Whitty, R.*; Harte, S.*; Geary, M.; Gardiner, J.*

Author Information
European Journal of Anaesthesiology: February 2005 - Volume 22 - Issue 2 - p 103-106
doi: 10.1017/S0265021505000190

Abstract

Complications following epidural placement for women in labour are uncommon and rarely lead to serious sequelae once patient selection criteria are adhered to. Epidural vein cannulation, however, continues to be a particular problem in parturients due to distension of epidural veins [1]. Epidural vein engorgement is maximal when the pregnant woman is in the supine position and minimal in the lateral position [2]. The incidence of epidural vein cannulation varies between 5% and 12% depending on the size of the Tuohy needle and the type of epidural catheter used [3]. Epidural insertion technique also appears to reduce inadvertent vein cannulation [4,5].

Epidural vein cannulation can lead to a number of problems for the labouring mother, ranging from inadvertent intravascular injection of local anaesthetics to extradural haematomas leading to spinal cord compression and epidural abscess formation. The possibility of expansion of an epidural haematoma following local anaesthetic injection into the epidural space also exists. The latter is based on the observation that lignocaine has been shown to have an inhibitory effect upon blood coagulation [6].

Current anaesthetic department protocols in our hospital allow for epidural placement in the left lateral or sitting positions. In general, the sitting position appears to be favoured by junior staff as it provides easier identification of the patient's midline thus facilitating easier epidural placement. Following an initial observation of an apparently high incidence of epidural vein cannulation in patients undergoing epidural catheter placement in the sitting position a randomized controlled trial was conducted to document whether such an association existed.

Patients and methods

Following institutional Ethics Committee approval and written informed consent, 209 patients in established labour requesting epidural analgesia were randomly allocated using computer generated random numbers to receive epidural catheter placement in the sitting or lateral position. Parturients with maternal hypertension, morbid obesity (body mass index (>35), previous spinal or disc surgery or any contraindication to epidural analgesia including anticoagulation, refusal or failure to site on the second attempt, were excluded from the study. The study commenced once all the departmental trainee anaesthetists were proficient at epidural insertion using both patient positions.

All participating anaesthetists had a minimum of 3 months experience inserting epidural catheters into obstetric patients and had their technical abilities assessed by a senior member of staff prior to the study commencing. In all subjects participating in this study a Portex mini pack epidural system was used. This contained an 18-G Tuohy epidural needle, a radio-opaque epidural catheter with a blunt end and three helical placed side holes near the catheter tip, and a disposable plastic syringe assembly (SIMS PORTEX Ltd., Hythe, Kent CT21 6JL, UK). The epidural space was identified with an 18-G Tuohy needle with the tip in a cephalad direction at the L3/L4 level using a loss of resistance to air technique. The epidural catheter was then advanced into the space. Distance of catheter insertion was calculated as the difference between the length of the catheter at the skin and the depth when loss of resistance was achieved. When blood appeared in the catheter, this was documented as an epidural vein cannulation, and the catheter was withdrawn until <3 cm remained in the epidural space. If blood continued to flow freely, the catheter was removed, and a second attempt was used to place the catheter at the next higher interspace level (L2/L3). Two attempts to place the catheter were allowed otherwise the patient was excluded from the study.

No test dose was used. Following epidural catheter placement an initial dose of isobaric bupivacaine 0.25% 10 mL and 100 μg fentanyl was administered in incremental doses over 10 min. The anaesthetist completed a data sheet which included information on patient position (left lateral or sitting), years of experience of the anaesthetist in inserting epidural catheters, patient height and weight, previous epidurals (yes or no), cervical dilatation in centimetres, epidural vein cannulation, initial catheter distance and final catheter insertion distance. Efficacy of epidural analgesia was estimated on a visual analogue scale (VAS) ranging from 0, no pain to 100, worst pain possible; we arbitrarily defined adequate analgesia as a VAS ≤40 mm and inadequate analgesia as a VAS >40 mm.

A continuous epidural infusion (bupivacaine 0.1% with fentanyl 2 μg mL−1) was employed at a rate of 0-15 mL hourly to maintain analgesia. Automated non-invasive haemodynamic monitoring (Colin Press-Mate monitors) recorded patient blood pressures and heart rates. Block height was assessed by the operating anaesthetist initially with a dermatomal level of T10-L1 for analgesia and then monitored and recorded by a midwife during labour. Analgesia was assessed at 15-min intervals throughout the period of labour.

Statistical analysis was performed using commercial software (SPSS for Windows, version 6.1; SPSS, Chicago, IL, USA). Data for categorical variables (previous epidural, poor epidural function, patients with epidural vein cannulation) are presented as numbers and percent. Data for continuous variables (height, weight, cervical dilatation, doctor grade, initial catheter distance inserted) are presented as means and standard deviations (SD) if they were normally distributed, and as medians and ranges when the data were not normally distributed. Linear regression analysis was used to determine if variables were significantly correlated. To further determine which variables were independently associated, multiple regression analysis was subsequently applied. For continuous variables where data were not distributed normally, logarithmic transformation of the data was first performed before using linear and multiple regression analyses. Analysis of variance (ANOVA) was used for categorical variables. Comparisons of means were performed using the t-test whereas comparisons of medians were performed using the U-test. P < 0.05 was considered as significant.

Results

There was no significant difference between the groups with regard to age, height, weight or cervical dilatation at the time of epidural catheter insertion. There was no difference in the distance of epidural catheter insertion (Table 1). The overall incidence of epidural vein cannulation was 9.6%. In the sitting position group, 16 of 102 (15.7%) epidurals were diagnosed as vein cannulations, compared with 4 of 107 (3.7%) in the lateral position group (P = 0.011).

Table 1
Table 1:
Results expressed as mean (range) unless otherwise sated.

Operator experience (range, 3 months to 4 yr) was similar in both groups and was not associated with vein cannulation. In absence of epidural vein trauma all epidurals functioned satisfactorily. A significant correlation between epidural vein trauma and poor labour analgesia existed, however (P = 0.006). Data for both patient posture and analgesia remained independently significant on multiple regression analysis (posture, P = 0.009; analgesia, P = 0.006) (Table 2).

Table 2
Table 2:
Correlation coefficient between epidural vein cannulation and risk factors.

Discussion

Earlier studies have reported on an incidence of epidural vein cannulation of between 1% and 12% [7,8]. In this study we found an overall incidence of 9.6%. We further noted that patient posture at the time of epidural catheter insertion played a role in determining the incidence of epidural vein trauma. A significant difference existed between the two groups of patients with an incidence of epidural vein cannulation of 15.7% in the sitting group and 0.4% in the lateral position group. Although our numbers were small, we also noted a significant association between poor epidural function and epidural vein trauma. The literature supports an overall incidence of inadequate analgesia following epidural placement of up to 17% citing various factors such as outward catheter migration, poor anaesthetic technique and air bubbles in the epidural space as being causative [9,10]. In our study, however, poor or inadequate epidural function was limited to those patients who had a bloody tap. We postulate that the space occupying effect of a haematoma resulting from trauma to an epidural vein may account for this observation. Apart from these observations, no other adverse sequelae were noted in either group. The use of an epinephrine-containing test dose has been recommended as a reliable marker of inadvertent epidural vein cannulation, and hence to reduce the incidence of systemic injection of local anaesthetics [11]. Epidural vein cannulation, however, can lead to vein trauma and haematoma formation.

Epidural vein trauma has the potential to result in significant harm to the patient. At the simplest level, initial epidural vein cannulation or trauma frequently necessitates replacement of the epidural catheter with all the attendant risks that a second attempt at instrumenting the epidural space carries. Epidural haematoma leading to adverse neurological sequelae can be one of the most serious complications following epidural catheter placement. This type of haematoma tends to arise largely in patients with a pre-existing coagulopathy. In patients with normal haemostatic profiles, trauma to the network of epidural veins results in a small amount of bleeding which usually clots without any untoward events. Reports of significant epidural haematoma in patients given anticoagulants following epidural placement exist, however [12], particularly where epidural vein trauma has been noted at the time of epidural catheter insertion. This observation has led to the recommendation that epidural catheters should be left in situ until the haemostatic mechanisms of the patient have returned to normal. The introduction of routine postoperative thromboprophylaxis, especially with low-molecular-weight heparins, has led to an increase in the incidence of epidural haematoma being reported. Enoxaparin, for example, has been implicated in epidural haematoma in patients following epidural catheter removal, particularly when a bloody tap has been noted at the time of insertion [13,14].

Engorgement of the plexus of epidural veins secondary to the effects of gravity and caval obstruction by the gravid uterus are the most likely explanation for our high rate of blood tap in the sitting position. This observation correlates with the previously documented evidence of raised epidural space pressures with resultant vein engorgement that vary with patient position, being maximal in the supine position and minimal in the lateral position [15]. Unexpectedly, other potential risk variables such as stage in labour, operator experience, the distance the catheter was advanced into the epidural space and patient height or weight were not predictive for epidural vein cannulation.

Our results suggest that simply using the lateral position will significantly reduce the incidence of epidural vein cannulation during catheter insertion. This may reduce the potential for harm to the patient, particularly in the current climate of widespread thromboprophylaxis with low-molecular-weight heparins. We recommend that epidurals be inserted in left lateral position to reduce the incidence of bloody tap, and, in doing so, to improve the quality of analgesia.

Acknowledgements

Drs John Loughrey and Noreen Dowd for their helpful comments.

References

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Keywords:

ANALGESIA EPIDURAL; complications; posture; ANALGESIA OBSTETRICAL; complications; patient position

© 2005 European Society of Anaesthesiology