ALTHOUGH continuous spinal anesthesia (CSA) can be traced back to a 1907 publication in which Dean1left the needle in place to titrate anesthesia, it was rarely used before the 1940s, when Lemmon2popularized the technique with the development of indwelling 17- and 18-gauge malleable German silver needles. In 1944, Hinebaugh and Lang3applied this technique to the parturient, achieving pain relief in 48 of 50 patients. To overcome the obvious limitations in patient positioning and the substantial problems with needle dislodgement, Tuohy4modified the technique, substituting a ureteral catheter passed through a 15-gauge needle. Although well received, the high incidence of post–dural puncture headache (PDPH) continued to temper enthusiasm. Accordingly, the most significant advancement in CSA during the past half-century has been the incremental reduction in catheter/needle diameter. Most recently, in the late 1980s, three manufacturers introduced 27- to 32-gauge “microcatheters” capable of passage through standard 22- to 26-gauge needles. Experience with these devices was just gaining momentum in obstetrics when the occurrence of neural injuries led to their withdrawal from the US market.5,6As evolving research identified anesthetic toxicity as the cause of injury,7,8interest in microcatheters was rekindled. In 1996, seeking regulatory approval for their reintroduction, Arkoosh et al. organized a multicenter study comparing the safety and efficacy of a 28-gauge spinal catheter with a standard epidural for labor analgesia, a rather challenging undertaking in light of the previous injuries associated with microcatheters. The long-awaited results of this evaluation are now reported in this issue of Anesthesiology.9
The primary focus of the study was to investigate the incidence of neurologic sequelae. However, as the authors appropriately note, the study was powered to establish that the incidence of persistent or permanent deficit is less than 1% at the 95% confidence level. Although clearly insufficient to establish adequate safety, it was deemed sufficient to support premarket approval, with a slow introduction of the device, coupled with extensive postmarket surveillance.
Although there were no permanent neurologic deficits, minor or transient changes did occur, the significance of which is unclear. Two patients (one in each group) developed an abnormal gait believed to be secondary to postoperative pain. Fifteen patients receiving spinal anesthesia were noted to have mild changes from baseline. In 11 of these cases, changes were restricted to deep tendon reflexes, one of which was attributed to preeclampsia. Only one patient had findings significant enough to warrant neurologic consultation, and these were considered consistent with nerve compression from the fetal head. Reports of postpartum weakness or loss of sensation occurred equivalently in 4% and 6% of the spinal and epidural groups, further underscoring the difficulties interpreting detailed sequential neurologic assessments in the obstetric population, given the unstable physiology compounded by the often underestimated potential for neurologic sequelae from the birthing process.
Patients receiving CSA had a higher incidence of PDPH (9% vs. 4%), although this did not achieve statistical significance. However, this lack of significance likely reflects the study's limited power, and the higher than expected rate of PDPH associated with the epidural procedure. It thus remains to be seen whether this is the true incidence and, if so, whether modifications in design or technique (e.g. , leaving the catheter in place for a longer period) might reduce this to a more acceptable rate. One of the important target populations for CSA, morbidly obese parturients, is thought to have a lower incidence of PDPH, so at least in this application, PDPH may be less problematic. Similarly, PDPH poses less of a barrier to the use of these devices in the general surgical population.
As catheter diameter decreases, so does tensile strength. The force required for withdrawal decreases as well, but unfortunately, not to the extent that tensile strength diminishes. Consequently, catheter breakage has been an important consideration since the introduction of these devices into clinical practice. Care in their removal is mandatory, but there are also material modifications that can potentially reduce risk.
In terms of efficacy, CSA performed well against the epidural technique, resulting in more immediate pain control and greater satisfaction at 24 h. These findings are more impressive when one considers that the epidural technique might have had an unfair advantage. Years had passed since the clinical use of microcatheters, which had been limited to start, while the experience during this study was diluted among 17 investigators at seven institutions, and the device itself required modification after initiation of the study. The tightly controlled protocol mandated a single drug for spinal infusion, in contrast to the epidural's combination therapy. In addition, there was limited previous experience with CSA for labor on which to base the treatment algorithm, contrasting sharply with the epidural technique that has been fine-tuned over decades. Conversely, while CSA had faster onset and produced greater early satisfaction, equivalent results are likely achievable with the combined spinal–epidural technique.
There are several potential advantages of CSA that are not evident in this study. Dosage is minimized, eliminating concern for systemic toxicity or fetal exposure, even with a misplaced catheter. In contrast, the epidural dosing carries significant risk of systemic toxicity with intravascular injection, while intrathecal misplacement can result in complete spinal block and neural injury from anesthetic neurotoxicity. In contrast to identification of the epidural space, spinal placement is facilitated by the well-defined endpoint, permitting use in patients with altered or abnormal neuraxial anatomy, and CSA has an additional potential advantage in patients with a current or previously failed, one-sided or patchy epidural block. CSA has also been used successfully in a number of patients with significant comorbidities who might not have tolerated the hemodynamic changes associated with the less controllable lumbar epidural anesthesia. Because of the more rapid onset with spinal administration, transition to surgical anesthesia can be achieved more readily, potentially circumventing the need for general anesthesia. One of the difficult problems in obstetric anesthesia is what to do after a failed attempt to convert from labor epidural analgesia to surgical anesthesia. In this setting, spinal anesthesia is somewhat unpredictable and carries an increased risk of high or total block. The availability of CSA can be a welcome option in this not-uncommon situation. Finally, of course, CSA is the clear answer for the anxious Oral Boards candidate when faced with the semimythical case of the morbidly obese, severely preeclamptic, asthmatic parturient with the class 4 airway presenting for urgent cesarean delivery.
With respect to CSA, size does matter, particularly with regard to regulatory restrictions, because practitioners currently remain at liberty to use large-bore catheters for this purpose.6Indeed, many resort to this technique after inadvertent dural puncture during attempted epidural placement. More fundamentally, there are important differences in subarachnoid distribution between injections made through large- and small-bore catheters,10although these result from differences in flow rate, which will be blurred with drugs administered by slow infusion.
Because of the substantial challenges and obstacles in conducting a study of this nature, the current data are likely the best that will be collected anytime soon, which is unfortunate given the numerous questions that remain. Among the most critical, identification of the optimal combination of analgesic/anesthetic agents and the optimal method of delivery has yet to be determined. It is well established that slow infusion potentiates restricted distribution,10and a reduction in required dosage, improved analgesia, and reduced risk of anesthetic neurotoxicity might be achievable if an anesthetic is administered by repetitive bolus injection. However, the extent to which this can be realized with these high-resistance catheters also remains a question.
In their 1944 report of CSA for labor and delivery, Hinebaugh and Lang3concluded: “While no serious complications occurred in this series, further trial is necessary to evaluate its future place in obstetrical anesthesia.” These words are perhaps as relevant now as they were 60 years ago.
*Department of Anesthesia, University of California, San Francisco, California. firstname.lastname@example.org. †Columbia University, Columbia University Medical Center, New York, New York.