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© 2001 International Anesthesia Research Society
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Introduction |
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 Top Introduction MethodsResultsDiscussionReferences |
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The ideal composition of epidural test doses is undetermined. Although lidocaine/epinephrine is well described, some clinicians use a small initial dose of the local anesthetic they have chosen for the block (1). Ropivacaine 0.75% has been used for epidural anesthesia in obstetrics (2), but initial doses of 2–3 mL failed to detect IV catheters (2,3), resulting in a seizure in one case (3). Addition of epinephrine to ropivacaine was suggested as a marker for IV injection (3), but this has not been investigated. Moreover, although ropivacaine has been used for spinal anesthesia (4–6), the clinical effects of intrathecal ropivacaine, and thus criteria for identifying its inadvertent intrathecal injection, have not been fully delineated. Therefore, we designed a randomized, double-blinded study to determine whether ropivacaine-epinephrine could be used as an effective test dose in parturients having elective cesarean delivery.
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Methods |
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TopIntroductionMethodsResultsDiscussionReferences |
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After ethics committee approval and written informed consent, we recruited 36 term parturients having elective cesarean delivery with a combined spinal-epidural technique. Patients received ranitidine premedication and IV preload of lactated Ringer’s solution 15 mL/kg. Monitoring included continuous electrocardiography, pulse oximetry and cardiotocography, and noninvasive arterial pressure. With the patient in the right lateral position and after skin infiltration with lidocaine 1%, the epidural space was located with a 16-gauge Tuohy needle at the L2-3 vertebral interspace, and a multiorifice epidural catheter was inserted 4 cm. The catheter was aspirated and checked for blood or cerebrospinal fluid. Lumbar puncture was then performed at the L3-4 vertebral interspace by using a 25-gauge Whitacre needle. Three injections were then given simultaneously over 5–10 s according to a randomized schedule:
- Group E (n = 12): 2 mL ropivacaine 0.75% and epinephrine 15 µg epidurally, 2 mL saline intrathecally, and 2 mL saline IV.
- Group IV (n = 12): 2 mL ropivacaine 0.75% and epinephrine 15 µg IV, 2 mL saline epidurally, and 2 mL saline intrathecally.
- Group S (n = 12): 2 mL ropivacaine 0.75% and epinephrine 15 µg intrathecally, 2 mL saline epidurally, and 2 mL saline IV.
- Group IV (n = 12): 2 mL ropivacaine 0.75% and epinephrine 15 µg IV, 2 mL saline epidurally, and 2 mL saline intrathecally.
Ropivacaine 0.75% was chosen as the concentration recommended by the manufacturer for epidural anesthesia for cesarean delivery. A volume of 2 mL was chosen on the basis of our previous investigations of intrathecal ropivacaine in parturients (unpublished data). Solutions were prepared by adding 0.15 mL epinephrine 1:1000 to 20 mL ropivacaine 0.75%. Randomization was performed by a research nurse according to a code contained in a sealed envelope. Immediately after injections, the epidural catheter was flushed with 1.5 mL saline. Patients were kept lateral for 1 min, then turned supine with left tilt.
Heart rate (HR) changes in the first minute were recorded electronically. Every 2 min for 10 min, one of two blinded investigators (EW or JL) recorded changes in sensation (pinprick) and motor function (modified Bromage scale [0 = able to lift extended leg at the hip, 1 = able to flex the knee but not lift extended leg, 2 = able to move the foot only, 3 = unable to move even the foot]). A positive HR change was defined by a >10 bpm increase, as recommended for obstetric patients (7). Subjective symptoms were not assessed. The prepared ropivacaine-epinephrine solution was then injected epidurally in increments to establish surgical anesthesia. Neonates were assessed by Apgar scores and umbilical cord blood gases. Data were compared with the Kruskal-Wallis test and Fisher’s exact test.
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Results |
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TopIntroductionMethodsResultsDiscussionReferences |
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Eight patients in Group S had adequate block for surgery after the test dose. For all other patients, epidural anesthesia was uneventfully established with the epidural catheter.
All patients in Group IV developed positive HR changes (Table 1). One patient in Group E had a false-positive HR change; a repeat test dose was negative, and the epidural catheter was used uneventfully for top-up. One patient in Group S had a false-positive HR change; in this patient, clear cerebrospinal fluid with no blood had been aspirated through the spinal needle, and there were clear signs of subarachnoid anesthesia after injection.
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One patient in Group S was unable to cooperate with sensory assessments. All other patients in this group developed sensory changes with upper level
T11 after 10 min (Fig. 1). Sensory changes also occurred in three patients in Group E and two in Group IV. Differentiation between Group S and the other groups was attained only after 10 min, and the lowest upper sensory level achieved in Group S differed from the highest upper sensory level achieved in Group E by only one dermatome.
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All patients in Group S developed Bromage scores
1, compared with no patients in the other groups (Table 1). However, this required waiting 8 min for all patients to develop signs. All infants were delivered in good condition. |
Discussion |
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TopIntroductionMethodsResultsDiscussionReferences |
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Although it is controversial, many clinicians continue to advocate the use of test doses to identify intravascular and intrathecal placement of intrathecal catheters (8). In our study, by using a double-blinded design, we attempted to simulate the clinical situation in which a test dose was injected without the patients or anesthesiologists being aware of whether the injection was epidural, subarachnoid, or IV. However, although ropivacaine-epinephrine identified IV injection in all patients, this test dose had serious limitations. These included the occurrence of false positives for HR changes, poor discriminatory ability with sensory criteria, slow onset of motor signs after intrathecal ropivacaine/epinephrine, and the occurrence of placebo sensory signs after intrathecal saline.
Although to identify IV injection our test dose had 100% sensitivity, it had only 92% specificity, implying that 8% of catheters might be incorrectly considered intravascular and removed. Although an upper sensory level T11 after 10 min identified subarachnoid injection, because the upper limit of the range of sensory levels in Group E (T12) approximated the lower limit in Group S (T11), sensory criteria are unlikely to be reliable. Unfortunately, we assessed only upper sensory limits; inclusion of lower limits may have increased sensitivity (9). We were surprised that some patients in Group IV developed sensory changes, which may have been a placebo effect. However, it was previously suggested that intrathecal saline may have some local anesthetic effect because it caused segmental hypesthesia to pinprick and cold and partial sympathetic block (10).
Although motor criteria were discriminatory, with all patients in Group S developing Bromage scores 1 compared with no patient in the other groups, because eight minutes were required for all patients to develop signs, these criteria are likely to be of limited usefulness in clinical practice.
In conclusion, 2 mL ropivacaine 0.75% with 15 µg epinephrine had serious limitations as an epidural test dose and should not be used to replace lidocaine-epinephrine.
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Acknowledgments |
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Supported,in part, by internal funding by the Chinese University of Hong Kong.
The authors wish to thank the midwives of the Labor Ward, Prince of Wales Hospital, Shatin, Hong Kong, China, for their help and cooperation.
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References |
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TopIntroductionMethodsResultsDiscussionReferences |
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- Abouleish E, Bourke D. Concerning the use and abuse of test doses for epidural anesthesia. Anesthesiology 1984; 61: 344–6.[Medline]
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Morton CPJ, Bloomfield S, Magnusson A, et al. Ropivacaine 0.75% for extradural anaesthesia in elective Caesarean section: an open clinical and pharmacokinetic study in mother and neonate. Br J Anaesth 1997; 79: 3–8.
[Abstract/Free Full Text] -
Abouleish EI, Elias M, Nelson C. Ropivacaine-induced seizure after extradural anaesthesia. Br J Anaesth 1998; 80: 843–4.
[Abstract/Free Full Text] -
van Kleef JW, Veering BTh, Burm AGL. Spinal anesthesia with ropivacaine: a double-blind study on the efficacy and safety of 0.5% and 0.75% solutions in patients undergoing minor lower limb surgery. Anesth Analg 1994; 78: 1125–30.
[Abstract/Free Full Text] - McDonald SB, Liu SS, Kopacz DJ, Stephenson CA. Hyperbaric spinal ropivacaine: a comparison to bupivacaine in volunteers. Anesthesiology 1999; 90: 971–7.[Web of Science][Medline]
- Gautier PhE, de Kock M, Van Steenberge A, et al. Intrathecal ropivacaine for ambulatory surgery. Anesthesiology 1999; 91: 1239–45.[Web of Science][Medline]
- Leighton BL, Norris MC, Sosis M, et al. Limitations of epinephrine as a marker of intravascular injection in laboring women. Anesthesiology 1987; 66: 688–91.[Web of Science][Medline]
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Birnbach DJ, Chestnut DH. The epidural test dose in obstetric patients: has it outlived its usefulness? Anesth Analg 1999; 88: 971–2.
[Free Full Text] - Morton CPJ, Armstrong PJ, McClure JH. Continuous subarachnoid infusion of local anaesthetic. Anaesthesia 1993; 48: 333–6.[Medline]
- Urban BJ, McKain CW. Local anesthetic effect of intrathecal normal saline. Pain 1978; 5: 43–52.[Medline]
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