Ultrasound has revolutionarily improved the efficacy and safety of many regional anesthetic and intensive care procedures. Preprocedural ultrasound has been used to assist the conventional epidural catheterization technique based on anatomical landmark palpation. It precisely identifies the intervertebral spaces, determines the proper needle insertion point, determines the optimum angle of needle insertion (by observing the angle of the probe obtaining the best sonographic image), and predicts the distance eventually traveled by the epidural needle (by measuring the distance to the ligamentum flavum-dura mater complex).1,2
Several studies have examined the use of preprocedural ultrasound for epidural catheterization and compared it with the conventional landmark technique; many of these studies were performed on an obstetric population.3,4 However, these studies greatly differ in their design regarding the patients’ characteristics (obese or nonobese, easy or difficult landmarks), the expertise of the operator, the performance of the ultrasound and subsequent epidural technique by the same or different operators, and the primary outcome (first-attempt success or procedural duration). In addition, most of these studies were not blinded.3,4
The aim of this study was to compare preprocedural ultrasound with conventional palpation for epidural catheterization performed by a single experienced anesthesiologist as a component of double-interspace combined spinal–epidural (CSE) anesthesia for cesarean delivery. We hypothesized that the use of preprocedural ultrasound would increase the success rate of epidural catheterization at the first needle pass.
After obtaining local ethics committee approval and registering the study at ClinicalTrials.gov (NCT02410226) in March 2015, the study was conducted on American Society of Anesthesiologists physical status II parturients with full-term singleton pregnancy undergoing elective cesarean delivery using double-interspace CSE anesthesia. Exclusion criteria were age <19 or >40 years, body mass index ≥35 kg/m2, women presenting in labor or having any contraindication to neuraxial anesthesia (coagulopathy, uncorrected hypovolemia, increased intracranial pressure, or local skin infection), marked spinal deformity, previous spinal surgery, or impalpable anatomical landmarks. Before patient enrollment, the ease of landmark palpation was assessed by a single investigator using a 4-point scale (easy = the iliac crests and/or the spines can be visualized; moderate = the landmarks can be identified by light palpation; difficult = the landmarks can be identified by deep palpation; impalpable = the landmarks cannot be identified). The difficulty grade for each patient was recorded, and patients with impalpable landmarks were excluded from the study.
Written informed consent was obtained from eligible subjects after the preanesthetic interview on the day of delivery. The study subjects were randomly assigned to 1 of 2 groups (palpation and ultrasound) using a computer-generated table of random numbers. Sequentially numbered, sealed opaque envelopes containing group allocation were opened only by the primary investigator after patient enrollment. All the procedures were performed by the primary investigator who had 10 years of experience in clinical anesthesia and 4 years of experience in ultrasound-assisted neuraxial techniques. The outcome data of the epidural procedure (number of needle passes and skin punctures, duration and complications of the procedure, and patient satisfaction) were recorded by a blinded investigator who entered the operating room after the primary investigator had finished the preprocedural spinal examination and skin marking. This blinded investigator also performed postoperative epidural injection, assessed the adequacy of the analgesic block, and observed the occurrence of backache.
Standard monitors were applied, and patients were positioned sitting during landmark identification and throughout the anesthetic procedure. In the palpation group, a sham procedure was performed by moving the ultrasound probe on the patient’s back with the machine in the freeze position. Conventional palpation of the anatomical landmarks was performed and the intercristal (Tuffier) line was assumed to cross the spine at L4 spinous process or L3–L4 interspace. The skin was marked with horizontal and vertical lines at the L3–L4 and L2–L3 interspaces in a manner similar to that used in the ultrasound group to blind the outcome assessor.
In the ultrasound group, a systematic 7-step approach was performed for all cases using a 5-2 MHz curved array probe (ClearVue 350®; Philips, Bothell, WA):
- The probe was longitudinally placed on the bottom of the spine, with the probe mark pointing cephalad, to obtain the longitudinal view of the sacrum and the paramedian sagittal oblique view of the L5–S1 interspace.
- The operator counted up the intervertebral levels to obtain the paramedian sagittal oblique view of the L3–L4 interspace.
- The probe was rotated 90° anticlockwise to obtain the transverse interlaminar view of the L3–L4 interspace, which was used in the next steps.
- The skin was marked with horizontal and vertical lines at the midpoints of the probe’s short and long sides, respectively; the intersection point of the 2 lines represented the needle insertion point.
- The angle of the probe obtaining the best sonographic image was observed.
- The distance from the skin surface to the ventral aspect of the ligamentum flavum-dura mater complex was measured; this corresponded to the depth of the epidural space.
- The probe was moved 1 interspace cephalad to obtain the transverse interlaminar view of the L2–L3 interspace and steps 4 to 6 were repeated.
In both groups, the widest space and/or that with the best sonographic image was chosen for the first attempt, whereas the other marked space was used only if needed. If the 2 spaces were of equal width and/or image quality, the L3–L4 interspace was first attempted. The puncture site was infiltrated with lidocaine, and the epidural space was located using a midline approach with loss of resistance to air using an 18-gauge Tuohy needle (Perifix®; Braun, Melsungen, Germany). In the ultrasound group, the angle of the probe obtaining the best sonographic image determined the angle of needle insertion, and the loss-of-resistance test was started when the epidural needle had traveled a distance 0.5 cm less than the pre-estimated epidural space depth.
A maximum of 6 passes (the first attempt and 5 redirections) of the Tuohy needle were allowed for each skin puncture and a maximum of 3 skin punctures were allowed. Catheterization failure was defined if identification of the epidural space was not possible after adequate needle attempts at 3 separate skin punctures; the patient was managed per the judgment of the attending anesthesiologist. After successful identification of the epidural space, a 20-gauge multiorifice epidural catheter was inserted through the Tuohy needle up to 5 cm into the epidural space. The number of performed needle passes (defined as any forward introduction of the Tuohy needle after its incomplete withdrawal, including the primary attempt) and skin punctures (defined as any separate skin puncture by the needle after its complete withdrawal, including the primary attempt) and the duration of the epidural procedure (time from the start of the first skin puncture by the Tuohy needle to the end of epidural catheter placement) were recorded by a blinded investigator. To allow for blinding, the duration of preprocedural spinal examination (by palpation or ultrasound) was not recorded.
The catheter was aspirated for cerebrospinal fluid or blood, and a test dose of 3 mL lidocaine 1% was injected to exclude intrathecal placement. The occurrence of unintentional dural or vascular puncture was recorded. Patient satisfaction from the procedure was assessed immediately after epidural catheterization using a 5-point scale (1 = very unsatisfied; 2 = unsatisfied; 3 = fair; 4 = satisfied; and 5 = very satisfied). After catheter placement, spinal anesthesia was administered using a 27- or 25-gauge spinal needle at the interspace below that used for epidural catheterization; intrathecal 12.5 mg bupivacaine and 10 µg fentanyl were administered.
No drugs were injected in the epidural catheter until the first postoperative analgesic request when 10 mL bupivacaine 0.2% and 20 µg fentanyl were incrementally injected. Thirty minutes after the epidural injection, the extent of the block was assessed by loss of sensation to pinprick starting from the lateral thighs and moved cephalad and caudad on both sides, and the occurrence of unilateral or patchy block (defined as a significant difference in analgesia between both sides or missed segments, respectively) was observed. Severity of pain was assessed using an 11-point verbal rating scale (0 = no pain; 10 = the worst pain possible). If there was no loss of sensation to pinprick and/or the pain score was ≥4, another equal epidural dose was injected. If no loss of sensation to pinprick and/or the pain score persisted ≥4 thirty minutes after the second dose, failed analgesia was diagnosed. The occurrence of backache was observed at 24 hours after delivery by a yes/no question.
The primary outcome was the rate of successful epidural catheterization at the first needle pass. Secondary outcomes were the rate of successful epidural catheterization at the first skin puncture, number of performed needle passes and skin punctures, duration of the epidural procedure, patient satisfaction from the procedure, and complications of the procedure (incidence of unintentional dural and vascular punctures, failed block, unilateral or patchy block, and backache).
Before patient enrollment, we performed a pilot study using the conventional palpation technique and found a 60% success rate of epidural catheterization at the first needle pass. Assuming α = 0.05 and β = 0.2 (80% power) and using the χ2 test, 49 patients were required in each group to demonstrate an absolute 25% increase in the success rate using preprocedural ultrasound. To allow for subject dropout, 55 patients were randomized to each group.
Data were analyzed using the R software, version 3.2.2 (R Foundation for Statistical Computing, Vienna, Austria).Continuous data were tested for normality using the histogram and the Kolmogorov-Smirnov test. Normally distributed data are presented as mean ± SD and were compared using the Student t test. Nonnormally distributed data are presented as median (range) and were compared using the Mann-Whitney U test. In addition, the data for the duration of the epidural procedure (nonnormally distributed, positively skewed) are presented as mean ± SD and were compared using the Student t test with unequal variances. Categorical data are presented as number (percentage) and were compared using the χ2 test or Fisher exact test as appropriate. A 2-tailed P value of <0.05 was considered statistically significant. The 95% confidence interval of the difference between the 2 groups in proportions of the first-pass success rate was calculated using the “prop.test” function in R.
One hundred ten patients were randomized, and data from 108 patients (55 patients in the palpation group and 53 patients in the ultrasound group) were analyzed (Figure 1). Two patients in the ultrasound group were excluded from the study. At the start of the epidural procedure, 1 patient declined to complete the study and requested conventional spinal anesthesia, and another patient experienced a vasovagal episode and received spinal anesthesia in the lateral position after her stabilization.
Patients’ baseline characteristics were similar in the 2 groups (Table 1). The rate of successful epidural catheterization at the first needle pass was 60% in the palpation group and 58.5% in the ultrasound group (95% confidence interval of the difference in proportions between groups is −18.5% to 21.6%; P > 0.99). There were no significant differences between the 2 groups in the success rate at the first skin puncture, the number of needle passes and skin punctures, or patient satisfaction (Table 2).
The median (range) duration of the epidural procedure was 185 (57–680) seconds in the ultrasound group and 215 (114–720) seconds in the palpation group; the difference between the 2 groups was significant (P = 0.036) when the data were compared using the Mann-Whitney U test and nonsignificant (P = 0.083) when the data were compared using the Student t test with unequal variances (Table 2).
The overall rate of complications of the procedure (unintentional dural and vascular punctures, failed block, unilateral or patchy block, and backache) was low (Table 3). One unintentional dural puncture occurred in the ultrasound group at the L3–L4 interspace; subsequently, location of the epidural space and catheter placement were successfully performed through the L2–L3 interspace. There were 7 cases of unilateral/patchy block (4 cases in the palpation group and 3 cases in the ultrasound group). Location of the epidural space failed in 1 patient in the palpation group after 3 unsuccessful skin punctures; general anesthesia was administered as requested by the patient. This patient was excluded from the analysis of the duration of the epidural procedure, patient satisfaction, interspace of successful catheterization, and backache.
This double-blind, randomized controlled trial failed to demonstrate a significant improvement in the rate of successful epidural catheterization at the first needle pass when using preprocedural ultrasound compared with the conventional palpation technique. There was a median 30-second reduction in the duration of the epidural procedure in the ultrasound group. Other outcomes (success rate at the first skin puncture, number of needle passes and skin punctures, and patient satisfaction) were similar in the 2 groups. The sample size was inadequate to determine whether there were differences in complications between groups.
Since its first reported use in the English literature by Cork et al in 1980,5 preprocedural ultrasound has been demonstrated by several studies to facilitate epidural catheterization and decrease its complication rate in the obstetric population compared with the conventional landmark technique. Some of these studies specifically addressed obese parturients6 or those with difficult landmarks.7 Epidural catheterization was either performed by resident trainees8 or single7,9,10 or multiple11 expert anesthesiologists. In 2 recent meta-analyses3,4 of the randomized studies comparing ultrasound-assisted with standard palpation techniques for lumbar punctures, spinal anesthesia, and epidural catheterization, Shaikh et al3 found a reduced number of insertion attempts and a reduced risk of failed or traumatic procedures with ultrasound, and Perlas et al4 concluded that ultrasound identifies lumbar intervertebral levels more accurately than landmark palpation, accurately predicts the depth of the epidural or intrathecal space, and decreases the risk of failure and the number of needle punctures in patients with normal or difficult anatomical landmarks.
However, in a recent randomized controlled trial, not included in the previous meta-analyses, in which epidural catheterization was performed by a cohort of anesthesiologists with different expertise in laboring women with easily palpable landmarks, Arzola et al12 demonstrated no significant improvement with ultrasound in the ease and duration of the technique. Moreover, in another randomized controlled trial in which spinal anesthesia for cesarean delivery was performed by 3 experienced anesthesiologists in nonobese parturients with easily palpable landmarks, Ansari et al13 demonstrated no significant improvement with ultrasound in the duration and the success rate of the procedure.
The difference of our results compared with those of the previous older studies may be explained by the exclusion of obese parturients and those with impalpable landmarks and the performance of all the procedures by 1 anesthesiologist experienced in both the conventional and ultrasound-assisted neuraxial techniques.
The ultrasound-estimated epidural depth correlates well with the actual depth traveled by the epidural needle using both the paramedian sagittal oblique view8,14,15 and the transverse interlaminar view.8,16–18 However, ultrasound tends to slightly underestimate the actual epidural depth as a result of tissue compression by the probe and differences between the angle of the probe used for distance measurement and the actual angle of needle insertion.3,4 Based on this information, we measured the distance from the skin to the ventral aspect of the ligamentum flavum-dura mater complex in the transverse interlaminar view and started the loss-of-resistance test when the epidural needle had traveled a distance 0.5 cm less than the ultrasound-measured distance. This added modification was assumed to decrease the duration of the epidural procedure in the ultrasound group. Although the duration of the epidural procedure was significantly shorter in the ultrasound group when the data were compared using the Mann-Whitney U test, it was not significantly different when the data were compared using the Student t test with unequal variances. Moreover, patient satisfaction was similar in the 2 groups, and lastly, because the duration of preprocedural spinal examination was not recorded, the impact of ultrasound on the total required time is unknown. In their study, Arzola et al12 set a 60-second reduction in the mean epidural procedural duration to be the minimum clinically significant effect and found a “trend” (P = 0.06) toward a shorter epidural procedural duration with ultrasound in a subgroup analysis involving resident trainees.
Although our results question the benefits of the ultrasound-assisted technique in parturients with normal anatomical landmarks, its use has been greatly beneficial in patients with significant spinal pathologies,19 deformities,20,21 or previous spinal surgery and instrumentation.22,23 In a recent randomized controlled trial, Chin et al24 demonstrated improved spinal anesthetic technique with preprocedural ultrasound in nonobstetric patients with difficult anatomical landmarks. Grau et al7 studied parturients with difficult landmarks and demonstrated a significant benefit for ultrasound use; however, recent large studies specifically addressing these conditions in an obstetric population are lacking.
A validated standardized scoring system for the difficulty of surface landmarks relevant to neuraxial blocks is lacking. In this study, we used a 4-point scale with prespecified definitions and excluded patients with impalpable landmarks. This should be considered when comparing our patient population with those in other studies, because the presumed moderate or difficult landmarks in this study may be considered by other authors to be easy as long as the landmarks are palpable.
Obese parturients were excluded from our study because they present difficulty in both palpation25 and ultrasound26 techniques and thus should be studied separately. Sahin et al27 and Wang et al6 demonstrated an improved first-attempt success rate and reduced number of needle attempts with ultrasound in spinal and CSE techniques, respectively, in obese parturients undergoing cesarean delivery.
Patient satisfaction from the procedure was assessed immediately after catheter placement before administering the spinal anesthetic to avoid other variables that might have affected patient satisfaction, including the quality of anesthesia and postoperative analgesia, backache, and neonatal outcome. We used the double-interspace CSE technique because it may be associated with higher success rate28,29 and less paresthesia30 compared with the needle-through-needle technique.
Our study has some limitations. First, the sample size is relatively small, because it was calculated assuming a 25% difference in the success rate of epidural catheterization at the first needle pass, which is a relatively large clinical effect; thus, it is unknown whether there is a smaller yet clinically significant difference between groups. Second, the current study design did not allow blinding of the operator; moreover, blinding of the patients cannot be totally guaranteed because conventional palpation was not performed in the ultrasound group. Third, because the duration of spinal examination preceding the epidural procedure was not recorded, we were unable to compare the total required time in the 2 groups. Last, the performance of all the procedures by a single operator increases the internal validity of the study but decreases its external validity (generalizability),31 so it is unknown whether the results of this study can be applied to other anesthesiologists with different expertise.
In conclusion, this study failed to demonstrate a significant improvement in epidural catheterization technique with preprocedural ultrasound performed by a single experienced anesthesiologist in parturients with palpable landmarks undergoing cesarean delivery. Although the duration of the epidural procedure may be decreased, an actual clinical benefit cannot be deduced.
Name: Mohamed Mohamed Tawfik, MD.
Contribution: This author helped design the study, conduct the study, collect the data, analyze the data, and prepare the manuscript.
Name: Magdy Mamdouh Atallah, MD.
Contribution: This author helped design the study, conduct the study, and collect the data.
Name: Walaa Safaa Elkharboutly, MD.
Contribution: This author helped design the study, conduct the study, analyze the data, and prepare the manuscript.
Name: Nasser Sameh Allakkany, MD.
Contribution: This author helped design the study and prepare the manuscript.
Name: Mostafa Abdelkhalek, MSc.
Contribution: This author helped conduct the study, collect the data, analyze the data, and prepare the manuscript.
This manuscript was handled by: Cynthia A. Wong, MD.
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