Detection of intravascular epidural catheter placement: a review.
Intravascular placement of an epidural catheter is recognised as a potentially fatal complication of epidural anaesthesia and analgesia. Up to 10% of epidural catheters may be inserted into an epidural vessel, the majority of which will be recognised; however, a proportion (I% of all epidural catheters inserted) may not be identified as lying intravascularly. Opinions differ on the optimal method for identifying intravascular catheters and no perfect method exists. Some debate the need for a test of correct location, as a lack of specificity may mean that a proportion of correctly located catheters are withdrawn and resited. This review outlines the incidence and risk factors associated with intravascularplacement and aims to evaluate the detection methods that have been described, in an attempt to answer the question: "What is the optimal way of detecting intravascular placement of an epidural catheter?"
Key Words: epidural, spinal, intravascular, anaesthesia, local anaesthesia
Intravascular placement of an epidural catheter is recognised as a potentially fatal complication of epidural anaesthesia. In this review we outline the incidence, risk factors and consequences of intravascular placement and then evaluate the methods of detection that have been described, in an attempt to answer the question: "What is the optimal way of detecting intravascular placement of an epidural catheter?"
The National Library of Medicine's PubMed, together with Ovid, were searched using the following keywords and combinations thereof. epidural, intravascular, intravenous, epidural complications, test dose. Articles obtained were reviewed for relevance and checked for other pertinent references, which were subsequently obtained. Once the relevant tests had been identified, further checks of the literature were undertaken using PubMed and Ovid in order to identify additional papers of relevance.
INCIDENCE, RISK FACTORS AND CONSEQUENCES
Up to 10% of epidural catheters may initially enter a blood vessel (1-4). The majority of misplaced catheters are identified and repositioned or resited before local anaesthetic is injected, but 1% may go undetected (1-3,5,6), or may migrate intravenously subsequently (5).
The most common predisposing factor for insertion of an epidural catheter into an epidural vessel is distension of epidural veins due to the effects of an intraabdominal mass. This especially applies during pregnancy and during labour; uterine contractions cause further distension of these vessels. Fluid status and vertebral level of insertion may also be relevant factors.
The technique of insertion can affect the probability of intravascular placement. The use of a combined spinal-epidural technique has been reported to decrease the incidence of intravascular placement', possibly by ensuring that the epidural catheter is inserted in the midline, avoiding the laterally located venous plexuses. The use of loss of resistance to saline, as opposed to air', and the midline approach, as opposed to the paravertebral approach (9), have no effect. However, the injection of saline into the epidural space prior to threading the catheter is reported to decrease the incidence of intravascular placement (1,10), although this has been debated (11,12). The type of catheter used is important, with softer, more pliable epidural catheters, such as the Flexitip Plus(TM] (Arrow), associated with a lower incidence of intravenous cannulation (13,14). However, the Flexitip Plus[TM] catheter has only a single outflow orifice and although this is not a universal finding, single-orifice catheters appear less likely to yield blood on aspiration if inserted intravascularly (4,15,16). Single orifice catheters may also be more prone to occlusion (17), resulting in unsatisfactory analgesia.
The potential consequences of intravascular placement include failed block, systemic local anaesthetic toxicity, seizures, cardiac arrest and death. Symptoms such as perioral numbness or dizziness may herald the onset of toxicity, but may be absent with bupivacaine, due to its low cardiovascular to cerebral side-effect ratio, and in anaesthetised or sedated patients. A failed block may result in further attempts at neuraxial anaesthesia, or conversion to general anaesthesia, with an increased risk exposure associated with each option.
METHODS FOR IDENTIFICATION OF INTRAVASCULAR PLACEMENT
The perfect method for identification of intravascular placement of an epidural catheter does not exist. Ideally, a test should allow the correct identification of those catheters placed within a vessel (100% sensitivity) and not allow correctly positioned catheters to be identified wrongly as being intravascular (100% specificity). The perfect test should also be quick and simple to perform, with reliable and easily identifiable end-points, be safe, tolerable and acceptable to the patient, and require no additional monitoring or equipment. A high sensitivity prevents false assurance that the catheter lies in the epidural space, and minimises the risk of complacency and decreased vigilance. A high specificity minimises the incidence of unnecessary removal of correctly positioned epidural catheters and the additional risks associated with repeat insertion, or a general anaesthetic. A study demonstrating 100% sensitivity and specificity would require a study population of greater than 118 patients in order to ensure that the lower end of the confidence range was greater than 95% for both (19).
All methods described in the literature have an substantial incidence of false positive and false negative results (4,19), making intravascular placement a diagnosis of exclusion. The possibility of a misplaced catheter must always be considered.
Direct physical methods
Direct methods involve identifying blood in the catheter lumen on placement by aspiration, capillary action or the effect of gravity.
Aspiration may fail to detect a proportion of intravascular catheters. Between 24 and 56% of single lumen epidural catheters situated intravascularly are not identified by aspiration (1,2,5). Proposed explanations for this include the low pressure within epidural veins and their tendency to collapse when a negative pressure is applied. The widely used multi-orifice catheters make detection by aspiration more likely than is the case for single terminal orifice designs (16,20). Norris et al reported that the incidence of false negatives with aspiration was comparable to or lower than the reported incidence of false negatives with Doppler and adrenaline 4. However, they failed to confirm the location of one catheter that yielded a positive result to adrenaline, despite negative aspiration. These authors concluded that aspiration was an effective means of diagnosing intravascular placement, but also noted that aspiration did not reliably identify intravascular placement when withdrawing the catheter from a blood vessel, and recommended that a test dose be carried out in this instance.
Aspiration is also more likely to give a false negative result when performed subsequent to the initial catheter placement. Epidural catheters demonstrate a significant incidence of blocked eyes when left in situ (17). Aspiration with the bacterial filter in situ is likely to fail if both air and liquid are present within the filter (21). However, removal of the bacterial filter for this purpose increases the risks of bacterial contamination. Therefore, once an epidural catheter has been inserted, it is difficult to reconfirm the position following subsequent doses, despite the possibility of catheters migrating intravascularly (2,22).
Meniscus (Shah) test
This test was originally described by Shah (23) and utilises a fluid meniscus within the epidural catheter. If the epidural catheter is correctly located in the epidural space, then the meniscus should fall when the distal end of the catheter is raised to a height of 30 cm above the insertion point and clear fluid (as opposed to blood) should be seen when the distal end is lowered by 30 cm. A further step has been described, in which 1 ml of air is injected prior to the 2 ml of saline to fill the catheter, so that the presence of air bubbles within the fluid in the latter step may differentiate it from cerebrospinal fluid (24). This test was reported to have sensitivity of 97.4% and specificity of 100%, but no comment was made on the power of the study. The study also failed to address the effect on the test of differing epidural pressures with changes in posture or of the influence of pregnancy and intrauterine contractions.
Other direct tests
The insertion of an absorbent wick into the hub of the Tuohy needle following injection of a low volume test dose has also been described (25). Blood staining of the wick may occur despite a negative aspiration test and implies intravascular placement of the needle. However, a catheter threaded through the Tuohy needle may be inserted into a vein even though the needle itself was located in the epidural space.
Various indirect methods have been proposed that may be used either alone or together with the direct methods. These methods can result in false positive results (4,26) and thus unnecessary reinsertion of the catheter.
An epidural test dose containing adrenaline is the best known pharmacological detection method, although its limitations, especially in labouring women, are well recognised (27,28). The usefulness of this test dose has been widely debated in the literature (4,26,29,30). Arguments against the routine use of test doses for intravascular placement have emphasised the lack of specificity, particularly in labouring women'" who arguably represent the largest population receiving epidural techniques. The routine use of a test dose may result in more unnecessary replacements of a correctly positioned catheter or unnecessary conversion to general anaesthesia.
A positive adrenaline test is generally considered to be an increase in heart rate of 20 beats per minute, or an increase in systolic blood pressure of 15 mmHg within two minutes of injection (32). However, this finding was based on a small sample of nine patients receiving repeated doses of adrenaline and monitored by non-invasive blood pressure measurement.
Various studies have suggested modifications of the diagnostic criteria for other patient populations. An increase in heart rate of 10 beats per minute may be optimal, especially in the pregnant population (33,34), but is likely to increase the number of false positive test responses, particularly during labour. An increase in heart rate of greater than 30 beats per minute in the 25 seconds following injection of a test dose without adrenaline or other vasopressor was reported in 12% of obstetric patients, in whom an epidural catheter was not located intravascularly (35). A significantly greater number demonstrated an increase in heart rate of 20 beats per minute. In addition to the fluctuations in heart rate that occur with labour pain, pregnancy attenuates the chronotropic response to adrenaline. The response to adrenaline may also be attenuated by volatile anaesthetic agents (36), [beta]-blockers (32) and aging (37,38). Regional anaesthesia can interfere with the cardiovascular response to adrenaline but appears to have little effect on the efficacy of the test (39), with increased effect demonstrated in high thoracic blocks. Another disadvantage of the adrenaline test dose is a decrease in patient mobility (40).
Changes in blood pressure or electrocardiograph T-wave amplitude may also be used as an end-point (41). A positive result is defined as a 25% or greater decrease in the T-wave amplitude of lead II, or a greater than 15% increase in systolic blood pressure (as measured using an arterial line). In elderly and anaesthetised patients these criteria are superior markers of a response to adrenaline than are changes in heart rate (42), but the routine use of such parameters is impractical, particularly outside the operating theatre environment.
The optimal test dose of adrenaline appears to be 15 [micro]g, this being sufficient for cardiovascular changes to be evident, while minimising adverse effects. Arguments against the use of adrenaline relate to its adverse effect on uterine blood flow (27,43,44) and its potential to cause myocardial ischaemia in elderly vascular surgical patients (19). Adrenaline is relatively contraindicated in pre-eclamptic women. The occurrence of a uterine contraction during injection of the test dose, or shortly thereafter, means that a further dose must be given, increasing the potential for compromised uterine blood flow. It is estimated that in clinical practice, 5% of patients may not be eligible for the use of adrenaline as a test dose, due to a contraindication to the use of adrenaline, or frequent uterine contractions precluding its interpretation (28). In addition, a second group of patients is likely to require a second dose of adrenaline because a uterine contraction occurs within 60 seconds of administration.
Other agents used to show a cardiovascular response to test placement include ephedrine and isoproterenol. The less cardiotoxic local anaesthetic agents, such as lignocaine or chloroprocaine, have been used to test for systemic local anaesthetic side-effects when initiating blocks (45,46), but this practice cannot be recommended.
A recent systematic review of epidural test doses failed to support the routine use of adrenaline as a test dose in pregnant women, due to its low positive predictive value, the lack of evidence supporting its benefit and concern regarding its effects on uteroplacental blood flow.
The only test with a sensitivity and positive predictive value >80 in pregnant women utilised the presence or absence of clinical signs and symptoms following epidural fentanyl in a dose of 100 [micro]g (34). However, this finding was based on a single clinical trial (47) and a case report (48). Fentanyl is ideally suited to a role in detecting intravascular placement, as it is used routinely as part of labour analgesia. However, in addition to a paucity of trials, other factors to consider are: 1) The optimal fentanyl test dose is unclear. Fentanyl 3 [micro]g/ml in conjunction with local anaesthetic may represent the optimal epidural bolus concentration for labour analgesia (49). For caesarean section, doses of epidural fentanyl in excess of 50 [micro]g as an adjunct to local anaesthesia have not been shown to convey additional benefit (50). 2) On the other hand, even at the doses described, fentanyl may have respiratory side-effects. There are rare reports of respiratory depression secondary to epidural fentanyl 100 [micro]g (51). Epidural fentanyl 80 [micro]g or in mean dose approaching 200 [micro]g by infusion has been shown to increase the incidence of maternal desaturation in the second stage of labour, but not to decrease neonatal well-being (52,53). 3) If fentanyl is used as a test dose for intravascular placement, it must follow a dose of local anaesthetic sufficient to exclude intrathecal placement. 4) When given through a multi-orifice catheter, only a proportion of the dose may enter the systemic circulation and thus prove ineffective. Nevertheless, fentanyl administration may prove a useful addition to vigilance, aspiration and other direct tests.
The injection of 1 ml of air as a marker, with Doppler auscultation at the precordium, has been described as safe (54) and is superior to an adrenaline test dose in sensitivity and positive predictive value (34). However, this method has also been shown to be less reliable when used with multi-orifice catheters, with a sensitivity of 82% and failure to identify at least one intravascular catheter (out of 11 proven intravascular catheters) (55). The lower sensitivity and specificity of the air test in multi-orifice catheters may be due to the preferential flow of injected air out of the more proximal ports, together with the possibility of multi-compartmental positioning.
A cold sensation experienced in the back upon initial injection of local anaesthetic into lumbar epidural catheters has also been shown to correlate with correct placement (56).
Tsui et al (57) recently described electrical stimulation via the epidural catheter to exclude both intrathecal and intravascular placement. This method uses electrical current to produce a motor response prior to, and following, injection of local anaesthetic to establish an epidural block. The current is transmitted via the epidural catheter to produce a positive motor response in a truncal or limb muscle, such that motor response to a very low current is consistent with intrathecal placement or proximity to a nerve root. Intravenous placement is indicated by stimulation persisting, or recurring, at pre-test levels. Published data assessing the sensitivity and specificity of this test are not available. Additional limitations are the requirement for a special epidural catheter with a metallic component to allow for conduction, a means of stimulating the catheter, the presence of air within the system interfering with conduction and inaccurate results with catheters positioned in more than one compartment. However, this test lends itself to repetition if intravenous placement or migration is suspected at any time that the catheter remains in situ.
Failure to establish a block
The importance of considering the location of the epidural catheter in cases when there has been failure to establish a satisfactory block cannot be overemphasised. This circumstance can be considered an indirect method of identifying possible intravascular catheters, although not one suitable for routine use.
Older texts and papers suggest the use of suxamethomium to identify an intravascular epidural catheter (58). A dose of 30 to 40 mg in an anaesthetised patient will cause apnoea within 60 seconds if the catheter is intravenous but if correctly located in the epidural space, up to twice the dosage has minimal effect on tidal volume and an onset of three to five minutes. A case in which suxamethonium was given inadvertently via the epidural resulted in muscle paralysis and apnoea, but of delayed onset (59).
The use of local anaesthetic as a test drug for intravascular placement, referred to previously, should now be viewed as historical (45,46).
LIMITATIONS OF DIRECT AND INDIRECT METHODS
None of the tests described above is reliable and reproducible. Given that aspiration demonstrates the majority of the intravascular multi-orifice catheters, the addition of a test utilising an indirect method may add little more than an increased likelihood of false positive results. Quantitatively, if one assumes that 10% of catheters are intravascular, and that aspiration will detect 97% of these 4 (3% false negative rate), then an additional test only seeks to identify the one in 300 that may be missed (60).
All epidural medications should be administered in an incremental fashion, so that systemic symptoms become evident before cerebral or cardiac toxicity occurs. However, incremental injections are unlikely to identify all intravascular catheters (61) because symptoms of local anaesthetic toxicity in obstetric patients show variable occurrence and because variable amounts of local anaesthetic will be injected through multi-orifice catheters located in more than one compartment.
VARIATION IN PRACTICE
The use of a test dose is more common in the United States of America (U.S.A.) than in Europe or Australia. In the U.S.A. the majority of practitioners use adrenaline test-dosing routinely (62), whereas in the United Kingdom, two surveys of obstetric anaesthetists demonstrated the incidence of routine use of adrenaline as part of the test dose for labour epidurals as 3% (63) and 5% (64). This is similar to findings relating to the insertion of thoracic epidurals (65). The frequency of use increased when utilising an epidural for caesarean section (63), probably due to the larger doses of local anaesthetic being used, although the purpose of adrenaline in this scenario could equally be because of its ability to expedite and prolong the block. Interestingly, one survey found that half of those surveyed would not employ any test for intravenous placement, including aspiration (64).
Our appraisal of the literature supports the view that aspiration and vigilance for the clinical effects of intravenously administered local anaesthetics are sufficient measures for the majority of epidural catheter insertions. If intravascular placement is strongly suspected, then an additional test such as mentioned above may be helpful. Of these, the adrenaline test dose is arguably the easiest and most familiar, although its limitations must be kept in mind. Other tests (such as the epidural stimulation test) may require equipment that is not readily available in the majority of clinical settings.
There is no single optimal way of testing for intravenously placed epidural catheters. Direct methods, such as aspiration, should be practised routinely. Indirect methods may also be considered, but can yield a false positive result, subjecting a patient to additional unnecessary risks. No single method is 100% sensitive and there is always the possibility that catheters may migrate from the epidural space. Therefore, there is no substitute for continued vigilance and administration of local anaesthetic in an incremental fashion.
Accepted for publication on December 6, 2006.
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D. N. BELL *, K. LESLIE ([dagger])
Department of Anaesthesia and Pain Management, Royal Melbourne Hospital, Melbourne, Victoria, Australia
* B.Med.Sci. (Hons), B.M., B.S. (Hons), Provisional Fellow.
([dagger]) M.B., B.S., M.D., M.Epi., F.A.N.Z.C.A., Staff Anaesthetist and Head of Research.
Address for correspondence: Dr D. N. Bell, Department of Anaesthesia and Pain Management, Royal Melbourne Hospital, Parkville, Vic. 3050. Reprints will not be available from the authors.
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|Author:||Bell, D.N.; Leslie, K.|
|Publication:||Anaesthesia and Intensive Care|
|Article Type:||Clinical report|
|Date:||Jun 1, 2007|
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