Case Study
A primiparous 24-year-old parturient with 38 + 4 weeks of gestation requires labor analgesia at 11 p.m. The cervical dilation is minimal. An 18-gauge multiorifice epidural catheter was inserted at the L3–L4 interspace with a loss of resistance at 4.5 cm. Depth of insertion was 9.5 cm from the skin, which corresponds to 5 cm of catheter within the epidural space. After negative aspiration, an initial epidural loading dose of 15 ml of a mixture containing bupivacaine 0.1% (1 mg/ml) with 2 µg fentanyl per milliliter was administered. At 11.45 p.m., the pain score decreased from an initial 90/100 to 30/100. The parturient’s blood pressure remained stable, and she retained full motor strength of her lower extremities. Thereafter, patient-controlled epidural analgesia (PCEA) was started with the same epidural solution, allowing the parturient a 5-ml PCEA bolus with a lockout time of 20 minutes. At 1 a.m., the anesthesiologist was called to address breakthrough pain with a pain score of 60/100. Cervical dilation was 3 cm. Sensory level was at T11 bilaterally, and lower extremity motor function remained unaffected. Fifteen minutes following a manual bolus of 8 ml of the epidural solution, the parturient reported a pain score of 30/100. At 3 a.m. the anesthesiologist applied another manual bolus to address breakthrough pain. Thereafter, he decided to run an epidural background infusion of 7 ml/h bupivacaine 0.1% with fentanyl 2 µg/ml. No further intervention was requested to address breakthrough pain. At 11 a.m., a healthy 3.5-kg male baby was delivered via ventouse. At removal of the epidural catheter, the patient was very happy with her pain control and somewhat disappointed about her inability to bend both of her knees during the last 2 hours before delivery.
Key Points
Labor epidural analgesia was established with patient-controlled epidural analgesia (PCEA).
Following breakthrough pain, a background infusion was added to the analgesia regime.
The patient experienced improved analgesia but additional lower extremity motor block.
Discussion
Local Anesthetic Solutions for Maintenance of Labor Epidural Analgesia
The synergistic effect of local anesthetics and lipophilic opioid allows the use of solutions with low concentrations of local anesthetics. Typical mixtures use bupivacaine ≤ 0.1% (1 mg/ml) or ropivacaine ≤ 0.125% (1.25 mg/ml) combined with either fentanyl 2 µg/ml or sufentanil 0.5–1 µg/ml. This results in a reduction in total dose of local anesthetic, although the injected volume has been shown to be more important than the dose.1, 2 When compared with higher concentrations, low-dose epidural analgesia has the advantage of less motor block, more ambulation, less urinary retention, higher patient satisfaction, and fewer instrumental deliveries.2–5
Bupivacaine versus Ropivacaine
“Differential blockade” with substantially less motor block was believed to be a particular property of ropivacaine. Ropivacaine is an S-enantiomer that has approximately 75 percent of the potency of bupivacaine. If this reduced potency is taken into account, then there is little or no difference in the occurrence of motor block between these two local anesthetics.2, 4 Ropivacaine is associated with a lower risk of serious cardiotoxicity compared with bupivacaine.6 The IV dose causing CNS symptoms in healthy (nonpregnant) volunteers was 99 and 124 mg with bupivacaine and ropivacaine, respectively.6 However, with the current practice of using low concentrations of local anesthetics in labor analgesia, there is no clinically relevant difference in safety between bupivacaine and ropivacaine.
Patient-Controlled Epidural Analgesia
There are different options for maintaining epidural labor analgesia. The “low-tech” solution is a manual injection either by the midwife or by the anesthesiologist. This manual injection can be on a regular basis (e.g., every hour) or on maternal request. The downside is the potential time lag between request and delivery of the drug, as well as dependence on physical presence. Another potential limitation is the frequent manipulation at the epidural filter with the risks of infection and drug errors. These disadvantages can be overcome with an infusion pump or syringe pump for PCEA. A predefined volume is triggered by the parturient with a defined lockout period. Addition of a background infusion – usually between 3 and 8 ml/h of the epidural solution – reduces the number of interventions by the physician.2, 5
Breakthrough Pain
Breakthrough pain might be due to the epidural catheter or to obstetrical reasons, such as dystocia. Lack of adequate sensory blockade can be a sign of intravascular catheter migration, which is even possible after several hours of fully functional epidural labor analgesia.7 Unilateral analgesia can be due to anatomic barriers in the epidural space but is more likely to be due to lateral deviation of the catheter. Frequently, the catheter is withdrawn by 0.5–1 cm to resolve this issue, but this is not very likely to be successful8 unless the catheter is at more than 5 cm epidural depth.9
Inadequate analgesia is less frequent with multiorifice catheters.10 Frequently, a manual bolus of 5–10 ml of the solution used for PCEA can significantly improve analgesia. The substantially higher pressure generated by a manual bolus results in wider spread in the epidural space.11 In fact, the number of activated catheter orifices depends on the speed of injection.12 If a manual bolus does not improve analgesia and the extension of sensory block is adequate, a bolus of local anesthetic in higher concentrations, such as lignocaine 1% or bupivacaine 0.25%, might be helpful. The issue with the latter solution is that it might well result in motor block. Lower extremity motor block despite inadequate analgesia most likely represents epidural maldistribution. A manually injected bolus might improve analgesia.
Generally inadequate analgesia should be resolved in a timely manner. If simple measures are unsuccessful, then the best solution is probably to replace the epidural catheter.
Lower Extremity Motor Block
Lower extremity motor function should be quantified in order to compare the effect of different modes of epidural labor analgesia on motor function. The most frequently used scoring system is the modified Bromage score13 (Table 2.1).
Table 2.1 Modified Bromage Score.
| Score | Extent of motor block |
|---|---|
| 1 | Complete block (unable to move feet or knees) |
| 2 | Almost complete block (able to move feet only) |
| 3 | Partial block (just able to move knees) |
| 4 | Detectable weakness of hip flexion while supine (between scores 3 and 5) |
| 5 | No detectable weakness of hip flexion while supine (full flexion of knees) |
| 6 | Able to stand and to perform partial knee bend |
