CME Module 14 : When Your Epidural Doesn't Work - Is It Always Our Fault? Next >

Epidemiology

Numerous authors have studied the incidence of failed epidurals for labor. For example, Pan 1 reviewed over nineteen thousand deliveries in North Carolina and found an overall failure rate of 12%. In 6.8% of patients, analgesia was initially adequate but subsequently became inadequate and required replacement, 1.5% of which required multiple replacements. For Cesarean Section, 7.1% of pre-existing epidural catheters required replacement. In 1998 Paech 2 prospectively reviewed approximately 11,000 cases in Australia and found a 4.7% reinsertion rate. Eappen 3 from Boston found an even higher replacement rate of 13.1%.  

Etiology

The causes of failed or dysfunctional epidurals can be divided into 6 major areas – performance factors, patient factors, surgical factors, anatomical considerations, methodology and equipment malfunction and defects.

 1). Performance factors

Although little research has been done, clinical experience and intuition would suggest that epidural failure varies inversely with the technical skills and experience of the operator. In an interesting study of anesthesia trainees, Dashfield 4 found that psychomotor abilities were poor determinants of the initial proficiency or rate of progress during the early phase of learning obstetrical epidurals but did impact on their performance after the initial training. 

2) Patient Factors

Multiple patient characteristics and factors have been identified that may have a negative impact on the success of epidural analgesia. Some of these include a body mass index > than 30, various musculoskeletal abnormalities including even mild degrees of scoliosis, previous spinal surgery, a posterior fetal presentation, and a duration of labor > than 6 hours 5. Additionally, Collier 6 reported on a patient who was taking heroin and methadone and in whom the clinicians were unable to provide adequate analgesia after 2 punctures and large doses of local anesthetic. Although tolerance to epidural opioids would be expected in this patient, the authors speculated that the development of a competitive block at the spinal cord receptors may have been responsible for the poor effect of the local anesthetic. 

3) Surgical Factors

Uterine exteriorization and diaphragmatic irritation from blood and amniotic fluid can often contribute to patient discomfort during Cesarean Section.  

4) Anatomical Factors

The diameter of the spinal roots may impact on the quality of analgesia. The larger sacral roots have been notoriously difficult to block with a reported failure rate of up to 18%. 

The presence of an obstructive midline barrier has been confirmed by numerous studies including epidurograms, CT scans and epiduroscopy. However, the actual composition is presently a contentious issue.  Using epidurograms, Collier 7 was able to demonstrate flow restricted to one side in 5 of 11 patients with inadequate analgesia secondary to unilateral or missed segments. 

5) Equipment

Epidural catheters can become blocked from blood and other tissue debris as well as during the manufacturing process. This is more likely with terminal eye catheters which have been well known to be associated with unilateral and missed segment blocks. Moreover, both single & terminal eye catheters have been associated with an inability to extend the block high enough for Cesarean Section in up to 14% of cases.  

6) Methodology

The technique utilized by the operator may impact on the quality of the epidural block. For example, loss of resistance to saline as compared to air has been associated with a lower incidence of inadequate analgesia (19% versus 36%). Additional factors that increase the risk of poor pain control include inadequate analgesia from the initial dose9, radicular pain during insertion 9 and catheter  misplacement 7. In regards to the latter, the most common catheter malpositions included transforaminal escape, lateral catheter placement, and paravertebral. Of these, transforaminal escape is the most common and, in some cases, contrast has been noted to outline the psoas muscle resulting in a “psoasagram”. During early labor some of these patients may have adequate, although limited, analgesia that is thought to be due to diffusion of the local anesthetic through to the lumbar plexus that lies within the posterior part of the psoas muscle. In contrast, with a paravertebral block, one tends to get a more extensive block which may allow the diagnosis to be made on clinical grounds. In addition, in two cases, Collier found a catheter positioned in a posterior compartment of the epidural space that did not allow adequate anterior spread of the contrast to reach the nerve roots at the intervertebral foramina. Both cases had little sacral analgesia. Catheters may also be inadvertently misplaced into the subdural space. Although accidental subdural cannulations are usually thought to cause a more extensive sensory block than normal, Collier found 4 subdural catheters in patients with inadequate epidural blocks.  
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