Following the institutional research ethics board (IRB) approval and written informed consents, we enrolled two groups of parturients with 25 in each group. Group NL were 25 nonlaboring parturients undergoing elective cesarean delivery, and Group L were 25 parturients in spontaneous active labor (having regular uterine contractions and cervical dilation greater than 3cm) undergoing cesarean delivery for failure to progress. All parturients were between 20 and 40 years of age, ASA physical status I-II with a singleton pregnancy at greater than 36 weeks gestation, and received a lower segment transverse incision for cesarean delivery. The decision for cesarean delivery was made by the obstetric team, independent of the study investigators. Exclusion criteria included parturient refusal, having received any analgesic treatment, any contraindication to CSE or general anesthesia, height less than 150cm or greater than 175cm, body weight greater than 100kg, obstetric and/or medical comorbidities such as preeclampsia, any cardiac, renal, neurologic, or other systemic diseases, unilateral block, or maximal pinprick sensory block level below T10 at 20min postspinal injection.

Each parturient was preloaded with 500mL of lactated Ringer's solution intravenously (IV). On arrival in the operation room, baseline vital signs were recorded and the initial severity of labor pain for a laboring parturient was assessed on a 10cm visual analog scale (VAS) before CSE anesthesia was placed. The degree of cervical dilations for laboring parturient was also recorded.

One anesthesiologist performed all CSE procedures following previously published methodology [2, 3]. With the parturient in the right lateral decubitus position, epidural space was identified at the L3-4 interspace with a 17-gauge Tuohy needle using the loss of resistance to air technique. The L3-4 interspace was identified by the line connecting the iliac crests to cross the spine. The volume of air used was limited to no more than 2mL. Using the needle-through-needle technique, a 25-gauge Whitacre spinal needle (BD Durasafe) was advanced via the epidural needle with the orifice facing cephalad direction. When cerebrospinal fluid was detected, 10mg plain bupivacaine (2mL of 0.5% w/v bupivacaine), which was the same as the previously published studies by Ithnin and Lim et al. [2, 3], was injected over 10 seconds without barbotage or aspiration. Immediately after intrathecal injection, the epidural needle was removed without inserting an epidural catheter. All parturients were promptly placed in a supine position with left uterine displacement.

A second anesthesiologist recorded vital signs at two-minute intervals for 20 minutes. IV phenylephrine 100μg was administrated for a decrease in systolic blood pressure (SBP) to less than 90mmHg or a 30% reduction in SBP and was repeated as necessary. Bradycardia with less than 50 beats per minute was treated with IV atropine 0.3mg. Sensory level of the block was assessed bilaterally at two-minute intervals using alcohol swab (sensory blockade level to cold ) and 25-gauge hypodermic needle (sensory blockade level to pinprick) in the cephalad direction. The degree of motor blockade was evaluated bilaterally using the modified Bromage scale [5]. The peak sensory and motor blockade and the amount of time taken to achieve it were compared. Other side effects such as shivering, nausea, and vomiting were recorded.

Skin incisions were made 20 minutes after intrathecal injection. Prior to delivery of the fetuses, any reaction to surgical stimulation (including any verbal complaint of pain or withdrawal of extremities to pinprick) was supplemented with IV ketamine at a dose of 0.5mg/kg each time until adequate anesthesia was achieved. After childbirth, midazolam 0.03mg/kg was administrated to all parturients, and IV pethidine 0.5mg/kg each time was also utilized according to parturient's reaction to ensure adequate anesthesia. General anesthesia was reserved as a back-up plan. Time intervals from skin incision to uterine incision, from uterine incision to fetus delivery, and surgical time were collected. The Apgar scores and parturients' satisfaction with anesthesia care were recorded. All parturients were followed for three days postoperatively.

All parturients had similar bilateral sensory block level, and all the peak pinprick sensory block levels were above T10. Median peak dermatomal sensory block level to pinprick and cold sensation and the time taken to achieve it in 20min after spinal injection were comparable between the two study groups (Table 2).

Spearman's correlation coefficients were 0.07 and −0.44, respectively, for sensory blockade level to pinprick in relation with cervical dilation and labor pain scores (P = 0.81 and P = 0.10, resp.). In addition, Spearman's correlation coefficients for sensory blockade level to cold in relation with cervical dilation and labor pain scores were 0.22 and 0.17, respectively (P = 0.43 and P = 0.54, resp.). During surgery, there were separately 14 nonlaboring parturients and 8 laboring parturients required rescue analgesic (P = 0.15). The median rescue ketamine dosage was 30mg (IR: 0–50mg) for nonlaboring parturients and 0mg (IR: 0–40mg) for laboring parturients (P = 0.21). The median rescue pethidine dosage was 30mg (IR: 0–40mg) for nonlaboring parturients and 0mg (IR: 0–40mg) for laboring parturients (P = 0.19). No conversion to general anesthesia was warranted.

With the same CSE technique, the median peak sensory level in the present study achieved was no higher than T5, while previous studies using same amount of hyperbaric bupivacaine reported a median peak sensory level between C6 [2] and T3 [3] on nonlaboring and laboring parturients, respectively. The only difference among the above three studies was the baricity of bupivacaine. It has been widely accepted that, under the influence of gravity, hyperbaric anesthetic solutions injected into spinal space at the L3-4 level (the peak of lumbar lordosis) are more likely to spread into and halt at the lower thoracic region (the nadir of thoracic kyphosis). Hirabayashi et al. demonstrated flattening of both lumbar lordosis and thoracic kyphosis during later stages of pregnancy using magnetic resonance imaging in Japanese women [7]. Interestingly, both the present study and the studies by Ithnin and Lim et al. were conducted on Asian parturients. We believe that the flattening of the curvatures of the spinal column, especially the disappearing of the upward slope in the high thoracic spine, could enhance the cephalad spread of intrathecally administered hyperbaric anesthesia solutions, leading to the unusually high blockade reported by Ithnin et al. Such effect was eliminated by using plain bupivacaine solution when block level was much less affected by curvatures of the spine and body position. We are not certain if these anatomic changes were specific to Asian parturients, as previous studies on western parturient showed that baricity had no differences in subarachnoid spread of bupivacaine, in which bupivacaine was used at higher dose or combined with opioids [8, 9].

Hypotension during spinal anesthesia primarily results from decreased systemic vascular resistance after blockade of preganglionic sympathetic fibers, and patients with a higher sympathetic drive are more prone to develop hypotension [10]. So, a laboring parturient with spinal anesthesia would be more prone to hypotension than a nonlaboring parturient because both epinephrine and norepinephrine levels are increased during labor [11]. In this study, we also found a greater decrease of SBP in laboring parturients. However, the compensation of the autologous transfusion with uterine contractions [12] and higher baseline SBP of laboring parturients resulted in similar incidence of hypotension or the phenylephrine dosage.

The choice of the dose of bupivacaine (10mg) in the present study was based on previous studies [2, 3], which provided comparability among the studies. The median sensory blockade to pinprick was lagged at T7 in this study, which could not provide an absolute pain-free situation for parturients during cesarean operation. However, the inadequate blockade level was easily rectified by ketamine and pethidine as stated in the materials section; no significant adverse effect on the clinical outcome of parturient and neonate was noticed. Followups of parturients were satisfactory.

Selection bias was one of the concerns that could not be excluded completely from this study because of the non-randomized design. Laboring parturients were shorter in height (2.7cm difference in average) and lighter in weight (3.6kg difference in average) than those of nonlaboring parturients, which might in part explain the reason for their failure to progress resulting in cesarean delivery. However, it has been demonstrated that height, weight, and bodymass index did not influence the spread of sensory blockade after subarachnoidal injection of bupivacaine [13, 14]. The obese and nonobese parturients also had a similar effective dose of intrathecal bupivacaine for cesarean delivery [15]. Of course, a carefully designed randomized controlled trial would eliminate these possible confounding factors. Although the gestational age had a significant difference between two groups, all parturients we enrolled had pregnancy greater than 36 weeks. To avoid the potential interference in judgment of level of sensory loss, laboring parturients who had received analgesics prior to cesarean delivery were excluded from this study.