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From the Department of Anesthesia, Perioperative and Pain Medicine, Harvard Medical School, Brigham and Women's Hospital, 75 Francis Street, Boston, MA 02115 USA.
Address correspondence to: Lawrence Tsen MD. Phone: 617-732-8216; Fax: 617-277-2192; E-mail: lctsen{at}bics.bwh.harvard.edu)
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Abstract |
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Methods: Forty parturients in active spontaneous labour, with a singleton, vertex, term fetus, requesting analgesia were enrolled in a randomized, double blind fashion to receive a standardized CSE with either an active or inactive TENS unit. Prior to CSE placement, TENS intensity thresholds were determined with electrodes placed on the paraspinus muscles at T10-L1, and S2-4; TENS settings for mode, cycle, and pulse width were standardized. Data were collected at timed intervals on pain (VAS), sensory level (pinprick), motor blockade (Bromage), cervical dilatation, and duration of analgesia, and at delivery on fetal and neonatal outcome.
Results: The duration of the spinal portion of the CSE did not differ between groups (TENS off 91.1 ± 33 [mean ± SD] vs TENS on 83.1 ± 28 min, P=.42). Kaplan-Meier survival analysis and Mantel-Cox log rank analysis showed no difference between the two treatments (P=.28). Analgesia was comparable throughout the first hour of spinal analgesia.
Conclusion: In healthy labouring parturients, the application of a TENS unit did not alter the quality or duration of labour analgesia provided by the spinal portion of CSE analgesia.
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Introduction |
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Transcutaneous electrical nerve stimulation (TENS) has been used with some success for labour analgesia9 and has been noted to modulate nociceptive and deafferentation pain,1 with its effectiveness dictated, in part, by the frequency of stimulation chosen.2 Because local anesthetics, specifically bupivacaine, demonstrate a frequency-dependent blockade of nerve action potentials,3 we speculated that a TENS unit could directly or indirectly increase the quality and duration of the spinal portion of a CSE.
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Methods |
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With the request for analgesia, and after a fluid bolus of 1000 mL lactated Ringers solution, parturients were placed in the left lateral recumbent position. After sterile preparation with providone-iodine solution, and local infiltration with lidocaine 1%, the epidural space at the L 3-4 or L4-5 interspace was located with a 17-gauge Weiss-Touhy epidural needle by the loss-of-resistance-to-air technique. A 25-gauge Whitacre needle was placed via the shaft of the epidural needle as described previously,10 the dura was punctured, and when flow of cerebral spinal fluid was confirmed, bupivacaine 0.25% (1 mL) with sufentanil (10 µg in 0.2 mL) was given; the sufentanil was measured in a tuberculin syringe for accuracy. This was followed by placement of a 20G multihole epidural catheter, 3 cm into the epidural space.
Following placement of the CSE technique, the TENS unit either remained turned off, or was turned on, based on the randomization scheme described above. The unit was placed in a pouch to hide the controls from all participants. No epidural drugs were given until discomfort returned. The patient, the labour nurse, the obstetrician and the anesthesiologist recording the data were not informed of the status of the TENS unit.
Visual analogue scale pain scores, sensory level to pin prick at the midclavicular line, and motor blockade were assessed by a blinded observer at placement of CSE and at 3, 5, 10, 15, 30, 45, 60 min and at regular 30 min intervals thereafter; the assessments were also made prior to dosing of the epidural catheter. Visual analogue scores were done with an unmarked 100 mm plastic sliding scale indicator (Astra Laboratories, Worcester, MA, USA). Motor strength determinations utilized a modified Bromage score (0 = full flexion of knees and ankles, 1 = partial flexion of knees, full flexion of ankles, 2 = inability to flex knees, partial flexion of ankles; 3 = inability to flex knees and ankles). Cervical dilatation, use and amount of oxytocin and nalbuphine, fetal heart rate, neonatal sex, birthweight, and Apgar scores, and maternal side effects were recorded. Standardized protocols for pruritus, nausea, and hypotension were established.
Differences between treatment groups were tested by analysis of variance (ANOVA) for continuous data, or contingency table analysis for discrete data (chi-square with continuity correction or Fisher's exact test, as appropriate). The VAS scores were analyzed by repeated measures ANOVA. Nonparametric tests of VAS scores at individual time points yielded qualitiatively identical results. Duration of CSE analgesia during the spinal phase was tested initially by ANOVA. Inspection of the data suggested a non-normal distribution and one censored value (i.e., one patient delivered before request for additional analgesia, so the duration of her spinal analgesia could not be assessed). Kaplan-Meier survival curves were constructed and tested for a difference between group assignment with the Mantel-Cox logrank statistic. Statistical significance was assumed when P was less than 0.05.
Sample size for this trial was selected to detect a 30 min difference in CSE duration between groups with 90% power (ß=0.1) at the 0.05 significance level, based on published duration and standard deviation data from our institution.9 This analysis suggested 17 patients per group were necessary.
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Results |
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). Nulliparity itself was not associated with a difference in CSE duration or VAS scores (P=.25, P=.87, respectively, by ANOVA or repeated measures ANOVA).
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. Mantel-Cox log rank analysis showed no difference between the two treatments (P=.28). Analgesia was comparable throughout the first hour of spinal analgesia (Figure 2; repeated measures ANOVA for difference between TENS groups, P=0.25). Too few patients had adequate analgesia at time points beyond 60 min to be included in the analysis of analgesic quality.
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Discussion |
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We based our hypothesis on two findings. First, local anesthetics produce a greater degree of conduction blockade when the frequency of action potential impulses increases. This phenomenon, termed frequency dependent blockade, has been specifically demonstrated with bupivacaine to exist at higher than lower frequencies, i.e. 40 Hz12 and 15 Hz,3 but not at 9 Hz.3 As TENS utilizes a frequency-dependent mode of stimulation,2 in our study a frequency of 6-100 Hz, we speculated that a direct enhancement in the blocking potential of bupivacaine could occur. Second, in accordance to Melzack and Wall's electrophysiologically confirmed13,14,15 "gate control theory",16 the stimulation of large myelinated primary afferent A fibres act, via inhibitory circuits in the superficial laminae of the dorsal horn, to inhibit the transmission of small unmyelinated primary afferent A-delta and C fibres which are responsible for the transmission of pain. A frequency of 80 Hz has been reported to be the most effective in inhibiting nociception in the animal model,2 confirmed clinically (50-100 Hz),17 and thus utilized in our study (66-100 Hz). TENS has been demonstrated to provide labour analgesia in up to 44% of parturients11 and we speculated that this could independently lead to analgesia, thereby enhancing the analgesia provided by intrathecal medication.
Our results demonstrated that TENS did not make a difference in terms of quality (Figure 2
) or duration of analgesia (Figure 1). There may be several explanations for the lack of difference in quality of analgesia. First, the profound degree of analgesia provided by the CSE combination utilized in this study may have masked any benefit of the TENS unit. Bupivacaine 2.5 mg with sufentanil 10 µg has been demonstrated to provide excellent labour analgesia for first and second stages with extremely low VAS pain scores.6 Even with the addition of epinephrine, no improvement in the quality of the analgesia was noted for the first 90 min.18 Second, the small amount of bupivacaine used for our CSE technique may not have been sufficient to observe the frequency dependent effect described previously. Further studies are necessary to elucidate the importance of the amount of local anesthestic needed to observe the effect. Finally, a TENS unit may not add a considerable amount of analgesia; a review of randomized controlled trials of TENS during labour involving a total of 712 women concluded that, although additional analgesic interventions may be less likely with TENS during labour, 81% still requested adjuvant therapy.19
The absence of differences in duration may be due to the TENS unit being less effective as labour progresses.11 When used before an epidural technique, TENS was noted to delay request for alternative analgesia for only 10-20 min.20 In a more recent study, when TENS was used alone in nulliparous and multiparous parturients, adjuvant forms of analgesia were requested between 5 and 7 cm cervical dilatation.9 As the spinal portion of the CSE technique has been demonstrated to last into the second stage of labour in rapidly progressing labours, it may have already eclipsed the duration of a reliable TENS effect.
We recognize that our results could be due to inadequate TENS settings, as mode, intensity, pulse rate and width must be programmed. However, as guidance for our settings, we used the two TENS studies which demonstrated the greatest labour analgesia effects.21,11 In addition, to avoid a recent criticism of the TENS technique, we individually titrated the intensity (amplitude) settings to overcome the heterogeneous impedance of the skin and underlying tissues between the electrodes and the peripheral nerves.22 Moreover, although other TENS pad locations, behind the mastoid processes and between the eyebrows, have been used successfully,20,23 the location we used has been validated in positive outcome studies using TENS alone during labour.11,21
Some have raised the concern that TENS could interfere with fetal heart rate tracings,11 however, this was not witnessed in our review of fetal tracings, nor did we observe any incidents of non-reassurring fetal tracings24 subsequent to the CSE placement in either group. Moreover, neonatal Apgar scores were not different between the two groups.
In conclusion, the quality and duration of the spinal portion of the CSE technique, utilizing 2.5 mg intrathecal bupivacaine and 10 µg sufentanil, is not enhanced by the addition of a lumbar TENS unit.
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Footnotes |
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Accepted for publication October 3, 1999.
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Stewart A, Lambert DH, Concepcion MA, et al. Decreased incidence of tourniquet pain during spinal anesthesia with bupivacaine. A possible explanation. Anesth Analg 1988; 67: 833–7.
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Norris MC, Grieco WM, Borkowski M, et al. Complications of labor analgesia: epidural versus combined spinal-epidural techniques. Anesth Analg 1994; 79: 529–37.
5 Tsen LC, Thue B, Datta S, Segal S. Is combined spinal epidural analgesia associated with more rapid cervical dilation in nulliparous patients when compared to conventional epidural analgesia? Anesthesiology 1999; (In Press).
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— TENS Off —O— TENS On