- Question: Does prepuncture ultrasound scanning facilitate neuraxial blockade and improve the first-attempt success rate in obese parturients with difficult topographic anatomy?
- Findings: In obese parturients, prepuncture ultrasound examination can facilitate spinal anesthesia insertion in the lateral position.
- Meaning: The outcomes of our study add important evidence to the current knowledge regarding the usefulness of prepuncture ultrasound examination in obese parturients in whom the conventional manual palpation technique is challenging.
Spinal anesthesia is one of the most common modes of anesthesia for cesarean deliveries.1,2 The needle insertion site is traditionally determined by manual palpation using the Tuffier line (the horizontal line connecting the superior iliac crests) as a surface landmark, which is a “blind technique.” However, in patients with poorly palpable surface landmarks and positioning challenges, particularly overweight parturients, neuraxial blocks can be difficult to perform.3,4 The edema, weight gain, and exaggerated lordosis associated with pregnancy can induce anatomical and physiological changes that increase the difficulty in discerning anatomic landmarks. Moreover, pregnant patients, especially obese parturients, may find it difficult to achieve adequate flexion of the lumbar spine. Therefore, identification of anatomic landmarks by palpation can be difficult in such patients. The resultant multiple attempts to administer spinal anesthesia can also increase the risk of postdural puncture headache, paresthesia, and spinal hematoma.5–9
According to the Second American Society of Regional Anesthesia Consensus on Neuraxial Anesthesia and Anticoagulation, the ideal approach to provide spinal anesthesia is via a single skin puncture with no needle redirection.10 In this context, neuraxial ultrasound examinations may improve patient assessments before spinal anesthesia. Although preoperative ultrasound scanning has been shown to assist epidural placement in parturients,11 there is limited literature regarding its usefulness for spinal anesthesia in obese parturients in the lateral position. Although the sitting position is generally preferred for obese women in many centers, there are situations (advanced labor, difficulty holding still) where the lateral position is advantageous as long as the procedure can be completed expeditiously. Therefore, we conducted this study to investigate whether an ultrasound-assisted technique for guiding spinal anesthesia in the lateral position for cesarean delivery in obese parturients was better than the blind technique using manual palpation. We hypothesized that preprocedural ultrasound scanning would facilitate neuraxial blockade and improve the first-attempt success rate in obese parturients with difficult topographic anatomy. Our secondary outcome measures were the number of skin punctures, number of needle passes, procedure times, parturient satisfaction, changes in the intended interspace, and incidence of complications.
This prospective, randomized controlled trial was conducted in accordance with the principles of the Declaration of Helsinki after obtaining approval from our Institutional Hospital Ethics Committee (Ref: SHSY-IEC-KY-4.0/17–9/01; November 20, 2016). This study adheres to the applicable Consolidated Standards of Reporting Trials (CONSORT) guidelines and was registered before patient enrollment at http://www.chictr.org.cn (identifier: ChiCTR-INR-16009962; principal investigator: M.L.; date of registration: November 22, 2016).
After providing informed consent, a total of 80 parturients scheduled to undergo nonemergency cesarean delivery under spinal anesthesia between November 2016 and December 2016 were recruited. Inclusion criteria were as follows: ≥18 years of age, a normal singleton pregnancy with gestational age of ≥37 weeks, and body mass index (BMI) ≥30 kg/m2 based on weight measured on the day before delivery). Patients were excluded if they were pregnant with twins, unable to provide consent, refused spinal anesthesia, had marked spinal deformities or a history of spinal surgery, were contraindicated for subarachnoid blocks (infection at the puncture site, coagulopathy, allergy to local anesthetic, hypovolemia, or abnormal spinal anatomy), or were undergoing urgent or emergency cesarean deliveries. The degree of obesity was classified according to the World Health Organization categories (class I: BMI, 30–34.9 kg/m2; class II: BMI, 35–39.9 kg/m2; and class III: BMI, ≥40 kg/m2).
Using computer-generated random numbers, all eligible subjects were randomized to 2 groups: a prepuncture ultrasound-guided spinal anesthesia group (ultrasound group, n = 40) and a conventional landmark-guided spinal anesthesia group (landmark group, n = 40). The patients, anesthesiologist performing the spinal anesthesia, and outcome assessors were all blinded to the patients’ group allocation.
Perioperative anesthesia management was performed according to departmental guidelines. The day before delivery, the height, weight, and BMI of the parturients were measured by a nurse in the ward. After the eligible parturients arrived in the operating room, baseline monitoring (noninvasive blood pressure, pulse oximetry, and 3-lead electrocardiography) and intravenous access were established. Baseline characteristics of the patients were recorded, such as age, gestational week, height, weight, and BMI. Monitoring was performed with the patients in a 15°–20° left lateral position to prevent aortocaval compression. No sedation was performed before or during administration of spinal anesthesia.
Three anesthesiologists with 3 years of clinical experience in spinal anesthesia were selected as operators. Ultrasonography examinations were performed by a single investigator who was trained in this technique and had performed >150 ultrasound-guided neuraxial blocks. Identification of the injection site and spinal injection were conducted with patients in the lateral position.
In the landmark group, the injection site was determined using the conventional method of palpating the posterior superior iliac spine. The line connecting both posterior iliac crests was used as a surface landmark for the L4 vertebral body or the L3–L4 space. The midline was established by palpating the tips of the spinous processes. Midline needle insertion sites were marked on the patients’ skin over the L2–L3 and L3–L4 spaces. After completion of the spinal anesthetic injection, rapid identification of the interspinous level was performed using ultrasound.
Ultrasound Imaging Protocol
In the ultrasound group, ultrasound scans were performed with an M-Turbo ultrasound machine (GE, Solingen, Germany) with a low-frequency (2–5 MHz) curvilinear probe. Scans were performed in the longitudinal parasagittal and transverse midline views in accordance with the findings of previous studies.12,13 The needle insertion sites at the L2–L3 and L3–L4 interspaces were determined as the intersection of the longitudinal and transverse lines (Figure 1). For both groups, the needle insertion sites were marked while the spinal puncture operator was outside the room. After skin marking, the parturient was asked to stay still and the subarachnoid puncture was conducted immediately.
Before subarachnoid puncture, full aseptic precautions were exercised. All spinal anesthetics were performed through a midline approach with a 25-gauge, 90-mm pencil-point needle inserted through a 20-gauge introducer needle. After observing free flow of the cerebrospinal fluid after subarachnoid puncture, 0.5% hyperbaric bupivacaine (2 mL of a mixture of 2 mL of 0.75% hyperbaric bupivacaine and 1 mL of 10% glucose) was administered. The slope of the needle point was oriented cephalad, and the injection rate was approximately 0.1 mL/s. The L3–L4 interspace was chosen for the first attempt, and the L2–L3 interspace was chosen for subsequent attempts. A maximum of 3 skin-puncture attempts (needle withdrawn from the skin and then readvanced) were allowed for 1 interspace and a maximum of 5 needle passes (needle withdrawn and readvanced without complete withdrawal from the skin) were allowed for each skin puncture attempt. If dural puncture was unsuccessful after attempts at the L2–L3 interspace in the landmark group, the operator was allowed to use other means to locate a lumbar interlaminar space, including a paramedian or ultrasound-guided approach. Parturients were then immediately placed in the left-tilted supine position after spinal anesthesia. Successful spinal anesthesia was defined by a bilateral T4 block 5 minutes after injection.
The incidence of hypotension (mean blood pressure below 90 mm Hg or systolic pressure reduction of >25% from the initial value) was recorded. Other complications, such as bloody tap or paresthesia, were also recorded by an independent observer blinded to the group allocation. A blinded attending anesthesiologist recorded all the outcomes.
The primary outcome was the rate of successful dural puncture on the first attempt. Secondary outcomes were the number of skin punctures required (each skin puncture was considered a separate attempt), number of needle passes (skin punctures + needle redirections), and the procedure time. Two time periods were recorded. In the ultrasound group, the time required to identify the insertion site was defined as the interval between placement of the ultrasound probe on the skin and marking the intended insertion point. In the landmark group, this was defined as the interval between the time when the operator first touched the patient and the completion of insertion point marking. For both groups, the needle insertion time was the time from contact of the local anesthetic needle with the skin to visualization of cerebrospinal fluid in the spinal needle. The total procedure time was the sum of the time required to identify the insertion site and the needle insertion time.
A difficult spinal injection was defined by >10 needle passes. In addition, parturient satisfaction, level of block (loss-of-cold sensation), number of puncture levels (moving to a second interspace after 3 needle insertion attempts), failure rate of spinal anesthesia (need for rescue analgesia or conversion to general anesthesia), and immediate complications were recorded. Patients rated their satisfaction as very satisfied, satisfied, or dissatisfied immediately after the procedure. All patients were interviewed 24 hours postprocedure regarding spinal anesthesia complications such as paresthesia, radicular pain, backache, and postdural puncture headache.
The sample size was determined using G*Power 3.1.2 software (Franz Faul, University of Kiel, Kiel, Germany). In a previous study,14 the first-attempt success rate in patients with obscured anatomic landmarks was 65% with an ultrasound-assisted approach and 32% with a conventional manual palpation technique. With an α error of 5% and a β error of 20% (80% power), a sample size of 35 patients per group was required. We increased the target sample size to 40 parturients per group to allow for dropouts.
Statistical analysis was performed using SPSS Version 22 (IBM, Armonk, NY). Continuous data were tested for normality using Q-Q plots and the Shapiro–Wilk W statistic. Normally distributed outcome data were summarized as mean (standard deviation) values and were compared between groups using the independent-measures t test. In contrast, the duration variables, which had large minimum–maximum ranges, were presented as medians with the tenth and 90th percentile values to provide information on the spread. Categorical data were analyzed using the χ2 test or Fisher exact test. The primary outcome (ie, the first-attempt success rate) was analyzed using the χ2 test, while Fisher exact test was used in subgroup analyses for subgroups with <40 patients. A 2-tailed P value <.05 was considered statistically significant.
Between November 2016 and December 2016, a total of 80 parturients were recruited and enrolled in the study: 40 each in the ultrasound and landmark groups. All participants completed the study and were included in the data analysis, and no patients were missing data or lost to follow-up. No clinically important intergroup differences were noted with respect to age, gestational age, height, weight, BMI, or American Society of Anesthesiologists classification (Table 1).
Data related to the spinal anesthesia procedures performed in both groups are shown in Table 2. Significantly higher first-attempt success rate was observed in the ultrasound group (Figure 2A). The average number of skin-puncture attempts in the landmark group was approximately 3 times that of ultrasound group, and the mean number of needle passes in the landmark group was approximately 7 times that of the ultrasound group (Table 2). The number of cases with >10 needle passes was also higher in the landmark group (Table 2). There was no statistically significant difference in the time taken to identify the needle insertion site between the 2 groups (Table 2); however, the spinal injection time and the total procedure time were longer in the landmark group (Table 2). Fewer cases in the ultrasound group required an attempt at a second interspace level (Table 2). Patient satisfaction scores were also higher in the ultrasound group (Table 2).
To evaluate the potential value of preprocedural ultrasound assessments at different levels of obesity, we performed subgroup analysis of patients in different BMI ranges and these results are presented in Table 3 and Figure 2. Among patients with BMI between 30 and 34.9 kg/m2, the first-attempt success rate and number of cases with >10 needle passes were similar in both groups (Table 3). More time was required to identify the needle insertion site in the ultrasound group (P < .001; Figure 2B); however, no differences in the spinal injection time or total procedure time were observed between the 2 groups in patients with BMI between 30 and 34.9 kg/m2 (P = .081 and P = .729, respectively; Figures 2C, D). In contrast, for patients with BMI ≥35 kg/m2, the between-group differences were similar to those of the whole study cohort. The first-attempt success rate was significantly higher, and the number of cases with >10 needle passes was significantly lower in the ultrasound group (Table 3). All procedure times for patients with BMI ≥35 kg/m2, including the time required to identify the needle insertion site (P < .001; Figure 2B), the spinal injection time (P < .001; Figure 2C), and the total procedure time (P < .001; Figure 2D) were significantly longer in the landmark group.
In 8 patients in the landmark group, the dural puncture was unsuccessful in the L2–L3 interspace. The paramedian approach was used in 4 of these cases, and ultrasound was used in 2 cases. Two patients in the landmark group had a bloody tap; however, the corresponding intergroup difference was not statistically significant (P = .494). Each group had an unsuccessful block and required rescue analgesia. However, no patient in either group experienced postoperative headache or backache, and none of the patients had neurological complications such as paresthesia or radicular pain.
In this randomized controlled trial, we evaluated the impact of preprocedural ultrasound guidance for spinal anesthesia in the lateral position compared with the conventional manual palpation technique for obese parturients undergoing elective cesarean deliveries.
Neuraxial anesthesia can be uniquely challenging in obese parturients using the traditional manual palpation technique because of the difficulty in identifying bony landmarks.15,16 Moreover, palpation using anatomic landmarks has repeatedly been shown to be inaccurate at identifying corresponding interspaces,17–21 which can lead to unintended intracord injection, resulting in spinal cord injury and permanent neurological sequelae.22,23 As reported in a recent meta-analysis, ultrasound guidance has improved the precision and efficacy of neuraxial anesthetic techniques.24
Similar to several previous studies, the results of this investigation confirm the effectiveness of ultrasound guidance in facilitating spinal anesthesia in obese parturients, even among experienced anesthesiologists. In a study of spinal anesthesia in obese parturients in the sitting position, Sahin et al25 observed no difference in the rate of first-pass insertion success (8/25 vs 7/25), but a decrease in the need for ≥3 needle redirections (1/25 vs 10/25), after preprocedural ultrasound, when compared with the landmark technique.
In the comparison of procedure times, the results of our study differ from those of previous studies. Time taken to identify the needle insertion site was comparable in both groups, and the total procedure time was longer in the landmark group in this study. Because parturients included in this study were all obese, palpation of obscured bony markers to ensure accuracy of the needle insertion site may have taken more time and the insertion site may have needed to be confirmed repeatedly. These 2 factors are likely responsible for the longer time required to identify the needle insertion site in the landmark group in the present study.
The ultrasound-guided technique was associated with better patient satisfaction, which could be attributed to the shorter time taken to perform spinal injections.
Our subgroup analysis showed that ultrasound guidance was not superior to conventional manual palpation in parturients with BMI <35 kg/m2. Conversely, in patients with BMI ≥35 kg/m2, ultrasound guidance significantly improves the first-attempt success rate, reduces the number of needle passes, and reduces all procedure-related times. This subgroup analysis provides new evidence about how the utility of prepuncture ultrasound guidance depends on the degree of obesity in parturients.
Because no patients in this study had BMI >43 kg/m2, we could not determine the value of ultrasound guidance in parturients with more extreme degrees of obesity. Superobese women have increased reflective interfaces, exaggerated attenuation, and phase aberration, which can make ultrasound imaging more difficult, but not uniformly impossible. As reported in a case report by Morimoto et al,26 spinal anesthesia was successfully performed with prepuncture ultrasound guidance in a superobese patient with BMI of 50 kg/m2 (weight, 110 kg; height, 148 cm). Further studies to evaluate the efficacy of ultrasound guidance in morbidly obese parturients and the efficacy of real-time ultrasound imaging for spinal anesthesia would be helpful.
Performing spinal anesthesia in the lateral position is the standard practice in our hospital; however, few previous studies have investigated the role of ultrasound guidance in neuraxial blockade in the lateral position. Although the sitting position can bring the intervertebral space closer to the skin and facilitate identification of the midline,27,28 the lateral position is still commonly used during spinal anesthesia in many countries, including China, because this position is more convenient and comfortable than other positions, especially for patients who are ill, in advanced labor, or have difficulty holding still.
The study has several limitations. First, although we selected 2 comparable anesthesiologists, both with 3 years of clinical experience in spinal anesthesia, the results may still be affected by differences in their skill levels. Second, for the manual palpation technique, we did not study the effectiveness of the paramedian approach, which has been shown to be superior to the midline approach in some studies.29–31
In conclusion, prepuncture ultrasound examination can facilitate spinal anesthesia in the lateral position in obese parturients (35 kg/m2 ≤ BMI ≤ 43 kg/m2) by improving the first-attempt success rate, reducing the number of needle passes and puncture attempts, and shortening the total procedure time. The present findings highlight the usefulness of prepuncture ultrasound guidance in obese parturients in whom the conventional blind manual palpation technique is challenging.
The authors thank the patients who participated in the study.
Name: Mengzhu Li, MD.
Contribution: This author helped design the study and write the manuscript.
Name: Xiu Ni, MD.
Contribution: This author helped collect and analyze the data.
Name: Zhendong Xu, PhD.
Contribution: This author helped conduct the study.
Name: Fuyi Shen, MD.
Contribution: This author helped conduct the study.
Name: Yingcai Song, MD.
Contribution: This author helped analyze the data.
Name: Qian Li, MD.
Contribution: This author helped conduct the study.
Name: Zhiqiang Liu, PhD.
Contribution: This author helped conduct the trial and design the study.
This manuscript was handled by: Jill M. Mhyre, MD.
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