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Anesth Analg 2004;99:245-250 © 2004 International Anesthesia Research Society doi: 10.1213/01.ANE.0000120084.56136.15
Denise J. Wedel Section Editor |
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Abstract |
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The ideal technique for identifying the epidural space remains unclear. Five-hundred-forty-seven women in labor who requested epidural analgesia were randomly allocated to three groups according to the technique by which the epidural space was identified: 1) loss-of-resistance with air (air; n = 180), 2) loss-of-resistance with lidocaine (lidocaine; n = 185), and 3) loss-of-resistance with both air and lidocaine (air-plus-lidocaine; n = 182). We assessed ease of epidural catheter insertion, characteristics of the blockade, quality of analgesia, and complications. The inability to thread the epidural catheter occurred in 16% of the air, 4% of the lidocaine, and 3% of the air-plus-lidocaine patients (P < 0.001). More patients from the air group had unblocked segments (6.6% versus 3.2% and 2.2%, respectively; P < 0.02). The incidence of accidental dural puncture was greater in the air group (1.7% versus 0% in the other two groups; P < 0.02). Pain scores, time to onset of analgesia, upper sensory level, motor blockade, and the incidence of hypotension, transient neurological deficits, postpartum urinary retention, and postdural puncture headache were comparable. Identification of the epidural space with air was more difficult and caused more dural punctures than with lidocaine or air plus lidocaine. Additionally, sequential use of air and lidocaine had no advantage over lidocaine alone.
IMPLICATIONS: In laboring women receiving epidural analgesia, identifying the epidural space by loss-of-resistance with lidocaine was more effective and caused fewer complications than identifying the epidural space by loss of resistance with air.
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Introduction |
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Loss of resistance is widely used to identify the epidural space. Either air or a liquid such as saline or a local anesthetic can be used. An advantage of using air is easy identification of a sticky syringe plunger, whereas a liquid usually provides better proprioception. Both techniques also have drawbacks (1,2): for example, epidural injection of air carries several risks (3–10),1 and air bubbles within the epidural space can lead to incomplete (patchy) analgesia (11).
In contrast, the use of saline is reported to slow the onset and reduce the quality of epidural analgesia (12,13). Sarna et al. (14) reported that results were similar when air or liquid was used to identify the epidural space, but others found the use of liquid to be superior (15,16).
There is thus no consensus as to whether air or a liquid should be used for identifying the epidural space when using a loss-of-resistance technique (1,2). It is also possible that sequential use of air and a liquid will provide the advantages of both methods. We therefore compared the ease of performance, quality of analgesia, and complications associated with loss of resistance to air and lidocaine or with their sequential use.
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Methods |
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The local institutional human investigation committee at Wolfson Medical Center approved the study, and all patients gave written, informed consent. Between March and December 2002, 569 nulliparous laboring ASA status I and II women with singleton cephalic presentation at term and who requested epidural analgesia were prospectively enrolled in the study. Exclusion criteria were ASA physical status III or greater, preeclampsia, morbid obesity (body mass index >35 kg/m2), a history of drug or alcohol abuse, heavy smoking, or abnormal liver, hepatic, or hematological test results.
The parturients were hydrated with lactated Ringer’s solution (10 mL/kg). Epidural analgesia was administered when a patient’s uterine cervical dilation measured 2.5 to 5.5 cm. Epidural catheters were inserted by anesthesiologists with at least 5 yr of experience in obstetric anesthesia and who were familiar with the loss-of-resistance technique with both air and liquid. With patients in the sitting position, an 18-gauge Tuohy needle (Portex Inc., Keene, NH) was inserted at the L2-3 or L3-4 intervertebral space.
The loss-of-resistance method with a plastic syringe was used to localize the epidural space. Patients were randomly allocated to one of the three methods: 1) 3 mL of air (n = 180), 2) 3 mL of lidocaine 2% (n = 185), or 3) sequential use of air and lidocaine (n = 182). In the last group (air plus lidocaine), we first identified the epidural space with 3 mL of air and then reconfirmed correct position of the needle by injecting 3 mL of 2% lidocaine. In each case, all of the air or lidocaine was injected.
Once the needle was appropriately positioned, a multiorifice epidural catheter was threaded 2.5 cm into the epidural space. If after 5 min there was no blood or cerebrospinal fluid (CSF) aspiration, 3 mL of 2% lidocaine was administered through the catheter in all groups. After waiting an additional 5 min with no signs of subarachnoid block or intravascular injection, patients were turned into the left lateral position, and 10 mL of 0.2% ropivacaine was injected epidurally in 5-mL increments over 10 min. All subsequent analgesic management and study measurement (below) were performed by an anesthesiologist blinded to the method used to detect the epidural space.
Analgesia was maintained throughout the labor and delivery with a patient-controlled epidural analgesia device with a PCAM syringe pump (Model P500; IVAC Medical System, NH) with 0.2% ropivacaine administered as a 5 mL/h basal infusion and a patient-controlled bolus of 5 mL with a 20-min lockout interval and a 20 mL/h limit. Patients who requested additional analgesia were given up to an additional 10 mL of the same solution in 5-mL increments with the patient-controlled epidural analgesia device. This bolus was included in the 20 mL/h limit.
Monitoring consisted of noninvasive arterial blood pressure, maternal heart rate, tocodynamometry, and continuous external fetal heart rate measurement. The anesthesiologist performing the epidural block recorded the rate of catheter insertion failure, the occurrence of unintentional subarachnoid puncture, and accidental intravascular catheter insertion. Other variables recorded included cervical dilation at epidural insertion, use of oxytocin, duration of labor, and mode of delivery.
A blinded anesthesiologist assessed the following variables: the onset of sensory block (assessed by pinprick); the existence of unblocked segments; the extent of sensory and motor block (assessed by the modified Bromage score); the ability of the parturient to cooperate (push on demand) during delivery (graded as "yes" or "no"); and side effects or complications caused by the epidural analgesia, including hypotension (systolic blood pressure <100 mm Hg or a decrease of >20% from baseline), postpartum urinary retention, postdural puncture headache (PDPH), and transient neurological deficits.
The patients graded pain on a 100-mm-long visual analog scale (with 0 mm as no pain and 100 mm as the most pain imaginable). The visual analog scale scores, sensory upper limit, and motor blockade were evaluated 1 h after epidural catheter insertion and at the second stage of labor. If unblocked segments were present, the epidural catheter was repositioned, additional local anesthetic was administered, and the patient’s position was changed. Patients were excluded from the study if clinicians suspected accidental dural puncture or catheterization of a blood vessel.
The primary outcome was the inability to insert an epidural catheter. Although the incidence of difficult epidural catheter insertion is reportedly only 2% (16), we observed a 4.5% difficulty rate with fluid or the combination of air and fluid in a preliminary study. We therefore assumed a difficulty rate of 10% in the air group and a 4.5% difficulty rate in each of the other two groups. Using the sample size test for proportions, we calculated that 517 participants would provide an 80% power to detect a relative difference of 55% in the incidence of difficult catheter positioning. Secondary outcomes included accidental intravascular catheter placement, unblocked segments, and inadvertent dural puncture. Statistical data analysis was performed with SPSS 9.0 for Windows (SPSS Inc., Chicago, IL). Data were evaluated for normal distribution by using the Kolmogorov-Smirnov test. Continuous variables with distribution significantly differing from normal were compared by using the Mann-Whitney U- or median tests. Categorical data were described by using frequency counts and percentages and were compared by using 
2 (with the Yates correction) or Fisher’s exact tests, as appropriate. All tests were considered significant at P < 0.05.
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Results |
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Among the 569 parturients enrolled, 12 were excluded from the study because of obesity (n = 5), preeclampsia (n = 6), or prior administration of an anticoagulant (n = 1). Another 10 patients were excluded because the research team was unavailable (n = 6) or because there was precipitated labor (n = 4). Among the 547 patients who completed the study, 180 were in the air group, 185 were in the lidocaine group, and 182 were in the air-plus-lidocaine group. Demographic, morphometric, and labor characteristics were comparable in the three groups (Table 1).
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The epidural catheter could not be threaded into the epidural space in 16% of the patients in the air group, compared with only 3.8% in the lidocaine group and 3.3% in the combined air-plus-lidocaine group (P < 0.001). Unblocked dermatomal segments (one or two segments in each patient) occurred more frequently in the air group (6.6%) than in the lidocaine (3.2%) or air-plus-lidocaine (2.2%) groups. The incidence of inadvertent intravascular insertion of the epidural catheter was more frequent in the air group (17%) than in the lidocaine (6%) or air-plus-lidocaine (8%) groups (P < 0.02). The incidence of accidental dural puncture was 1.7% in the air group, whereas there were no dural punctures in the other two groups (P = 0.02). The incidences of hypotension, postpartum urinary retention, PDPH, and transient neurological deficit did not differ significantly among the groups (Table 2).
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There was no significant delay in the onset of the sensory block in the air group. Pain scores, Bromage scores, and upper sensory levels of block were comparable among the groups (Table 3). Patients in the three study groups were equally able to cooperate during delivery.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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We demonstrated that loss of resistance with 3 mL of air was associated with more difficulties in epidural catheter insertion, more unblocked epidural segments, and increased frequency of intravascular catheter placement and dural puncture than when either 3 mL of lidocaine or the sequential combination of air and lidocaine was used to locate the epidural space.
Previous studies reported slow onset and reduced quality of epidural analgesia when saline was used instead of air for loss of resistance (12,13). Presumably, saline slows onset and reduces block quality by diluting the local anesthetic that is subsequently injected. We sought to avoid dilution by performing the loss-of-resistance maneuver with lidocaine rather than saline. Consistent with this theory, we observed comparable onset time and improved block quality with lidocaine compared with air. We note, though, that our study did not include a comparison between saline and lidocaine. It thus remains possible that results would have been similar with each liquid.
A drawback of using lidocaine instead of saline for loss of resistance is the possibility of converting an accidental dural puncture into an actual subarachnoid block. This did not occur in our study, presumably because the fluid pushed the needle away from the dura, thereby minimizing the risk of inadvertent puncture.
The idea behind the combined technique (air plus lidocaine) was based on our observation in a pilot study that in some patients, the epidural space was difficult to identify with air but was easier to identify when we switched to lidocaine. The opposite (crossover from fluid to air) may also help, although this hypothesis was not tested in this study. The purpose of this alternative technique was also to identify the epidural space and push the dura anteriorly, thereby widening the epidural space and reducing the risk of accidental puncture of the dura or epidural veins.
Our findings are consistent with those of Beilin et al. (16) and Valentine et al. (15). Both reported that the loss-of-resistance technique with small amounts of saline (2 or 4 mL) was superior to using the same amount of air. The differences in the quality of analgesia and incidence of complications reported in other previous studies (1–16)1 can be explained by the different volumes of air or saline.
Both the air and saline loss-of-resistance techniques have well known advantages and disadvantages. The loss-of-resistance technique with air is associated with several negative side effects. For example, there is a 2% dural puncture rate, compared with only 0.3%–0.4% when saline is used (5–7,17). This may be caused by the difference in compressibility of lidocaine versus air, which may explain the more frequent incidence of epidural catheter insertion failure in the air group. Therefore, we believe that the loss-of-resistance technique with air is less reliable for locating the epidural space. Air in the epidural space appears to increase the occurrence of PDPH. The association between pneumoencephalos and PDPH is clear (6). As reported previously, we found that using air to locate the epidural space increased the incidence of unblocked segments (11). Other reported side effects of the epidural injection of air include emphysema of the neck (18,19), neck and shoulder pain (20), venous air embolism (9), and persistent neurological deficits (10). The epidural air bubbles may be enlarged by the concomitant use of nitrous oxide (21).
However, large volumes (i.e., 10 mL) of normal saline injected epidurally for loss of resistance may produce inadequate analgesia, presumably caused by the dilution of the local anesthetics (13). Large volumes of saline may also modulate the speed of onset of the epidural block (12,13). It was shown (13) that the number of segments with hypoesthesia for cold and pinprick after 8 mL of 1.5% mepivacaine administration was inversely related to the interval between the administration of saline solution during the loss-of-resistance method. When accidental dural puncture occurs with use of saline for locating the epidural space, the CSF can be differentiated from saline by its glucose and protein content (22).
Leaking of edema fluid may also occur (23). Addressing this issue, Russell (2) suggested that it is better to use a technique that reduces the incidence of complications rather than one that facilitates their diagnosis.
Previous studies (14,16) have reported a similar incidence of intravascular cannulation with air and saline loss-of-resistance techniques. In contrast, we found a 17% occurrence of intravascular cannulation in our air group, compared with only 6% in the lidocaine group—a statistically and clinically significant difference.
This difference could be explained by the different compressibility of air versus lidocaine. Therefore, injecting a liquid into the epidural space pushes the blood vessels away from the epidural catheter.
The position of the patient during the performance of epidural block was found to be related to the incidence of intravascular injection, with the head-down lateral recumbent position having a less frequent incidence (2%) than the sitting position (10.7%) (24). The incidence of postpartum urinary retention was comparable among the groups, despite the more frequent incidence of unblocked dermatomal segments in the air group.
The choice of the currently used loss-of-resistance techniques for locating the epidural space with air or saline generally depends on the anesthesiologist’s experience and personal preference. Among 404 surveyed obstetric anesthesiologists (25), 59% started their practice using air for loss of resistance, but only 37% are currently using air. Fifty-three percent use saline, and 6% use both techniques. Twenty-three percent had changed their technique from air to saline, but only 4% had changed from saline to air. A recent review concluded that the literature supports the use of a small volume of saline for loss of resistance not only because of better analgesia, but also for the decreased morbidity (26). Because liquids are incompressible, the transition from complete resistance to loss of resistance is more obvious; therefore, liquids are ideal for performance of needle insertion into the epidural space with the loss-of-resistance technique. The sequential use of air and lidocaine (air-plus-lidocaine group) did not change the quality of analgesia or the incidence of complications compared with the use of lidocaine alone. We believe, however, that the sequential combination of air and lidocaine technique (which was not reported in previous comparative studies) may strengthen the anesthesiologist’s belief that the needle is correctly placed.
In conclusion, this technique may be especially helpful in difficult epidural blocks. Our results demonstrate a reduced incidence of unblocked segments and complications with use of lidocaine rather than air for identifying the epidural space. Sequential use of both techniques in a given patient did not offer advantages over the use of lidocaine alone.
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Footnotes |
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1 Palmer SK, Tinnell CA. Complication rates for major regional blocks are different in general surgery compared to obstetrics. Anesthesiology 1994;81:A1210.
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References |
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- Scott DB. Identification of the epidural space: loss of resistance to air or saline? Reg Anesth 1997; 22: 1–2.
- Russell R. Loss of resistance to saline is better than air for obstetric epidurals. Int J Obstet Anesth 2001; 10: 302–6.[Web of Science]
- Stride PC, Cooper GM. Dural taps revisited: a 20-year survey from Birmingham Maternity Hospital. Anaesthesia 1993; 48: 247–55.[Web of Science][Medline]
- Gleeson CM, Reynolds F. Accidental dural puncture rates in UK obstetric practice. Int J Obstet Anesth 1998; 7: 242–6.
- Huffnagle SL, Norris MC, Arkoosh VA, et al. The influence of epidural needle bevel orientation on spread of sensory blockade in the laboring parturient. Anesth Analg 1998; 87: 326–30.
[Abstract/Free Full Text] - Laviola S, Kirvela M, Spoto MR, et al. Pneumocephalus with intense headache and unilateral pupillary dilatation after accidental dural puncture during epidural anesthesia for cesarean section. Anesth Analg 1999; 88: 582–3.
[Free Full Text] - Chadwick HS. An analysis of obstetric anesthesia cases from the American Society of Anesthesiologists closed claims project database. Int J Obstet Anesth 1996; 5: 258–63.
- Carter MI. Cervical surgical emphysema following extradural analgesia. Anaesthesia 1984; 39: 1115–6.[Web of Science][Medline]
- Naulty JS, Ostheimer GW, Datta S, et al. Incidence of venous air embolism during epidural catheter insertion. Anesthesiology 1982; 57: 410–2.[Web of Science][Medline]
- Nay PG, Milaszkiewicz R, Jothilingam S. Extradural air as a cause of paraplegia following lumbar analgesia. Anaesthesia 1993; 48: 402–4.[Web of Science][Medline]
- Dalens B, Bazin JE, Haberer JP. Epidural bubbles as a cause of incomplete analgesia during epidural anesthesia. Anesth Analg 1987; 66: 679–83.
[Free Full Text] - Iwama H. Injection volume of saline with loss of resistance method may affect the spread of epidural anesthesia. Anesthesiology 1997; 86: 507–8.[Web of Science][Medline]
- Okutomi T, Hoka S. Epidural saline solution prior to local anaesthetic produces differential nerve block. Can J Anaesth 1998; 45: 1091–3.[Web of Science][Medline]
- Sarna MC, Smith I, James JM. Paraesthesia with lumbar epidural catheters: a comparison of air and saline in a loss-of-resistance technique. Anaesthesia 1990; 45: 1077–9.[Web of Science][Medline]
- Valentine SJ, Jarvis AP, Shutt LE. Comparative study of the effects of air or saline to identify the extradural space. Br J Anaesth 1991; 66: 224–7.
[Abstract/Free Full Text] - Beilin Y, Arnold I, Telfeyan C, et al. Quality of analgesia when air versus saline is used for identification of the epidural space in the parturient. Reg Anesth Pain Med 2000; 25: 596–9.[Web of Science][Medline]
- MacArthur C, Lewis M, Knox EG. Accidental dural puncture in obstetric patients and long term symptoms. BMJ 1993; 306: 883–5.
- Aida S, Taga K, Yamakura T, et al. Headache after attempted epidural block: the role of intrathecal air. Anesthesiology 1998; 88: 76–81.[Web of Science][Medline]
- Thomas JE, Schachner S, Reynolds A. Subcutaneous emphysema as a result of loss of resistance identification of the epidural space. Reg Anesth 1982; 77: 44–5.
- Munro HM. Pain in the neck. Anaesthesia 1990; 45: 173.
- Petty R, Stevens R, Erickson S, et al. Inhalation of nitrous oxide expands epidural air bubbles. Reg Anesth 1996; 21: 144–8.
- el-Behesy BA, James D, Koh KF, et al. Distinguishing cerebrospinal fluid from saline used to identify the extradural space. Br J Anaesth 1996; 77: 784–5.
[Abstract/Free Full Text] - Zeidel A, Gingold A, Satunovsky E, et al. Bedside test for diagnosis of oedema fluid after extradural anaesthesia. Can J Anaesth 1998; 45: 664–6.[Web of Science][Medline]
- Bahar M, Chanimov M, Cohen ML, et al. Lateral recumbent head-down posture for epidural catheter insertion reduces intravascular injection. Can J Anaesth 2001; 48: 48–53.[Web of Science][Medline]
- Howell TK, Prosser DP, Harmer M. A change in resistance? A survey of epidural practice amongst obstetric anaesthetists. Anaesthesia 1998; 53: 238–43.[Web of Science][Medline]
- Shenouda PE, Cunningham BJ. Assessing the superiority of saline versus air for use in the epidural loss of resistance technique: a literature review. Reg Anesth Pain Med 2003; 28: 48–53.[Web of Science][Medline]
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