◊ BACKGROUND:
Previous studies have shown more extensive cephalad sensory blockade in women receiving combined spinal-epidural (CSE) anesthesia compared with single-shot spinal (SSS) anesthesia for elective cesarean delivery. It has been postulated that introduction of the epidural needle during CSE disturbs the negative pressure in the epidural space, resulting in relatively greater cerebrospinal fluid (CSF) pressure and increased spread of intrathecal local anesthetic. We tested the hypothesis that CSE results in more extensive cephalad sensory blockade than SSS anesthesia and that loss-of-resistance during initiation of CSE anesthesia increases CSF pressure compared with SSS.

◊ METHODS:
Thirty parturients scheduled for elective cesarean delivery were enrolled in this randomized, double-blind study. Patients received either SSS or CSE anesthesia with equal doses of intrathecal anesthetic (hyperbaric bupivacaine 12 mg, fentanyl 10 microg and morphine 200 microg). Before the intrathecal injection, the CSF pressure was measured with a fiberoptic pressure sensor. Maximum cephalad sensory blockade to pinprick, cold and touch was measured. The total dose of phenylephrine required to maintain baseline arterial blood pressure was also recorded.

◊ RESULTS:
There were no significant differences in the median (interquartile range) pinprick sensory block height [T4 (T4-2) vs T3 (T4-1)] or CSF pressures [6 (4-12) vs 9 (8-12) mm Hg] between the SSS and CSE groups. There were no significant correlations between CSF pressure and block height or total dose of phenylephrine.

◊ CONCLUSION:
The SSS and CSE techniques inserted in the lateral decubitus position resulted in similar extent of sensory blockade and CSF pressure. These findings suggest that altering the intrathecal dose is not necessary and that any difference in intrathecal pressure associated with initial placement of an epidural needle in the epidural space during CSE anesthesia is clinically inconsequential.

◊ Reviewed by P. Rosenberg, MD, PhD
Professor of Anaesthesiology,Department of Anaesthesiology and Intensive Care Medicine
Meilahti Hospital, Helsinki University Hospital, Helsinki, Finland

Hypothesis - not so well founded
The authors hypothesized that in comparison with single-shot spinal anesthesia (SSS), the sensory block produced in spinal anesthesia spreads more cephalad when an epidural puncture with a Tuohy needle is performed just before the intrathecal puncture and local anesthetic administration; possibly because mechanical dural compression may influence the volume of the CSF. The hypothesis was primarily based on two small-scale studies on Singaporean female patients (height 153 – 158 cm, on average) (1,2) using a similar approach, but a smaller bupivacaine dose as was used in the present study on Californian women (height 161 cm, on average).

Main result
There was no difference in the maximum spread of sensory block between the groups, i.e. mechanical manipulation of the epidural pressure by introducing a Tuohy needle and injecting 1 ml or air into the epidural space prior to the intrathecal bupivacaine (+ opioids) injection did not affect the spread of the block.

Confounding factors
In the present study on spinal anesthesia for Cesarean section, the dose of hyperbaric bupivacaine was 12 mg, mixed with 10 µg fentanyl and 200 µg morphine. The dose of bupivacaine appears to be higher than what is normally considered to be required for Cesarean section and, therefore, it may not be surprising that in 4 of 30 patients, the sensory block spread to the cervical dermatomal level. Probably, the main reason why no difference in the sensory block spread was found between the SSS and the spinal+epidural technique is that the used amount of bupivacaine was too large. Correspondingly, it is not possible to achieve a unilateral spinal block of the lower extremities with bupivacaine if the dose is too large.
It seems that the presence of 1 ml or air (epidural test dose) and the tip of the Tuohy needle in the epidural space did not compress the dural sac, as indicated indirectly by the observation of no effect on the lumbar CSF pressure. The role of the volume of the injected air for an epidural test dose on epidural and CSF pressure remains open. Interestingly, in the two studies that initiated the present one, the test volume was two or four times larger.
A general confounding factor in comparisons of spinal anesthesia studies from different clinical centers is the inexactness of the testing of the spread of sensory block due to  the lack of uniform definition (or maps) of the distribution of the spinal nerve dermatomal innervation of the skin. Thus, there was no reference to any textbook or acknowledged publication in association with the description of sensory block testing in the present paper, nor in the two referenced studies. Typically, difficulties are seen in determining the sensory block of the upper most thoracic and the lower cervical dermatomes.

Ethics
Is it ethical to expose a patient (here a pregnant patient) to an invasive anesthetic technique which will not be used for the care of the patient? In the present study, aiming at spinal anesthesia for Cesarean section, the patients received an epidural puncture with a 17G Tuohy needle without medical need. Although the risks of complications related to epidural punctures are low, they do exist. It is not mentioned in the Methods-section of the present paper what patients were told, i.e. did they know that the second needle puncture was only for research?

Conclusion
The study is not useful for guidance in clinical practice. Definite conclusions cannot be drawn, because there were too few patients and the interventions were not directly relevant to clinical practice. The study is also problematic from an ethical point of view.

◊ References.

1. Goy RW, Sia AT.
   Sensimotor anesthesia and hypotension after subarachnoid block: combined spinal-epidural versus single-shot spinal technique. Anesth Analg   2004;98:491-6
   
   
2. Ithnin F, Lin Y, Sia AT, Ocampo CE.
   Combined spinal epidural causes higher level of block than equivalent single-shot spinal anesthesia in elective Cesarean patients. Anesth Analg 2006;102:577-80.