Ultrasound-assisted technique versus the conventional landmark location method in spinal anesthesia for cesarean delivery in parturients with class 3 obesity: a randomized controlled trial

Abstract

Background

Spinal anesthesia with the conventional landmark technique can be challenging in parturients with morbid obesity. In the present study, the researchers examined the effects of ultrasound-assisted pre-puncture anesthesia in parturients with morbid obesity with difficult topographic anatomy on the success of the first injection attempt. It was hypothesized that the pre-procedural ultrasonographic examination would increase the successful first attempt rate compared with manual palpation.

Method

A total of 80 parturients with class 3 obesity (WHO classification), BMI ≥ 40 kg/m², ASA physical status classification 3 and 18-to 45 years old scheduled for elective cesarean section with spinal anesthesia were included in the study. Localization techniques were randomized into conventional landmark technique (group L, n = 40) and pre-puncture ultrasound(USG) assisted technique (group U, n = 40). The ultrasound technique utilized both longitudinal parasagittal and transverse midline views, employing a convex probe. The injection site was marked, and spinal injection was performed with the patient in the lateral decubitus position. The primary outcome was the single-shot successful dura-subarachnoid membrane puncture rate. Secondary outcomes were skin puncture, number of needle passes, location marking and procedure times, patient satisfaction scores, and incidence of complications.

Results

The single-shot dura-subarachnoid puncture success rate was significantly higher in group U (10% vs. 42.5%; p = 0.002). No parturients required > 10 needle passes that was described difficult spinal anesthesia in group U (p = 0.0026), and there were fewer skin puncture attempts and needle passes than in group L (p < 0.05). Although the pre-procedural evaluation (p < 0.001) and total time (p = 0.017) were longer in group U compared with group L, the spinal injection time (p < 0.001) was shorter.

Conclusions

In the lateral position, ultrasound used to determine the needle insertion point provided a high first-attempt dura-subarachnoid success rate and reduced needle passages (skin puncture + needle redirections) and puncture attempts in parturients with class 3 obesity who underwent spinal anesthesia.

Trial registration

The study was registered prospectively at clinicaltrials.gov (NCT05342922) in 2022-04-18.

Background

The number of parturients with obesity is increasing worldwide, and cesarean delivery is becoming more common in these patients [1]. The World Health Organization (WHO) describes class 3 obesity as body mass index (BMI) exceeding 40 kg/m² [2]. In these parturients, oxygen reserve is limited due to low functional residual capacity and high metabolic demand. Therefore, the manipulation time required to ensure a safe airway during potentially difficult intubation is shorter than expected [3]. Therefore, despite technical difficulties, neuraxial techniques may accepted as a preferred anesthesia for elective cesarean delivery.

The conventional landmark technique, palpating the iliac crests and spinous processes constituting the anatomic landmarks, is used in spinal anesthesia to determine the needle insertion site [4]. However, the effectiveness of this method is likely reduced in parturients with class 3 obesity due to obesity and pregnancy-related changes, such as increased lumbar lordosis, soft-tissue edema, and increased adipose tissue [5]. Hence, the determination of landmarks using ultrasonography(USG) before neuraxial block is valuable in eliminating this ambiguity in these parturients [5].

In addition, when performing spinal anesthesia, it is essential to find the appropriate space in the first trial, perform the procedure on the first attempt, to provide successful surgical anesthesia [6]. In a meta-analysis, it was determined that preprocedural use of USG while application of neuraxial anesthesia in parturients increased the first-attempt success rate that was described a single needle insertion with no redirections in obstetric patients [7]. However, the quality of evidence for this outcome was rated as low [7]. By contrast, in another meta-analysis, pre-procedural spinal USG did not increase the first-attempt success rate in parturients whose spinous processes were easily palpable. Nevertheless, it increased the success rate in patients whose spinous processes were difficult to palpate [7, 8]. The European Society of Anesthesiology and Intensive Care Guidelines on peri-operative use of ultrasound for regional anesthesia (PERSEUS regional anesthesia) recommends the use of perioperative USG for regional anesthesia to select the correct intervertebral interspace for neuraxial anesthesia in patients with difficult anatomy to facilitate the procedure [9]. On the other hand, the PERSEUS Taskforce members could not find evidence to support the use of USG in the reduction of success or complications, and thus could not make any recommendations [9].

This randomized study hypothesized that USG scanning before spinal anesthesia would facilitate the procedure and increase the first-attempt success in parturients with class 3 obesity with possible difficult topographic anatomy. This study’s secondary outcomes were the number of skin punctures, needle passes, time to mark the location and perform spinal anesthesia, patient satisfaction, and the incidence of complications.

Method

Study design

The present study was planned in a prospective, randomized, controlled design. The approval of the Institutional Review Board (02–2022/02) was obtained on March 8^th, 2022. The study was registered prospectively before the first patient enrolment at clinicaltrials.gov (NCT05342922, registration date: April 18^th, 2022) and designed following the principles specified in the Declaration of Helsinki. Written informed consent was received from all participants regarding the enrollment and interventions in the study. This manuscript adheres to the applicable Consolidated Standards for Reporting Studies (CONSORT) guidelines. The CONSORT checklist was used for patient registration.

Eighty term parturients recruited for elective cesarean delivery under spinal anesthesia with American Society Anesthesiologists (ASA) physical status classification 3, with an uncomplicated singleton pregnancy, and BMI ≥ 40 kg/m² were included. Emergency cesarean sections, multiple gestations, local anesthetic allergy, existing contraindications of spinal anesthesia, BMI < 40 kg/m², history of lumbar surgery and lumbar spinal diseases, and refusal to participate in this trial were accepted as exclusion criteria.

Patient randomization

In this study, the patients were randomly assigned into group U (prepuncture USG-assisted technique, n = 40) and group L (conventional landmark method, n = 40) according to a computer-generated randomization table produced by an independent researcher. A random code concealed in a sealed envelope was assigned to each patient (n = 80) involved in the study to ensure blindness. The patient, the anesthetist administering spinal anesthesia, and the outcome-evaluating investigator were blind to the patient group assignment. The needle insertion sites were marked for both groups by the assessor while the performer of the spinal anesthesia waited outside the operating room.

Two operators with more than five years of regional anesthesia training performed spinal anesthesia. Both anesthetists accepted an equal number of patients in the groups. USG examinations were performed by an assessor who performed at least 60 USG-assisted and/or guided neuraxial blocks [10].

Two investigators, blinded to the study groups, entered the operating room after the pre-procedural spine examination and skin marking were completed and recorded the characteristics of the procedure and the study results.

Anesthesia application

A nurse in the ward measured the height and weight of the parturients before arrival to the operation room, their BMIs were calculated, and intravenous cannulation was established. Patients were transported to the operating room in the left lateral position. Standard ASA-guided monitoring was instituted inside the operation room. The baseline parameters were recorded in a 15–20° left lateral position to prevent aortocaval compression. Marking the injection site and spinal injection was performed with the parturient in the lateral position.

Group L

The conventional landmark palpation method, in which the spinal injection site was determined through palpation of the anterior superior iliac spine and spinous processes, was used in the study. The line connecting the upper level of both iliac crests was marked with L4 vertebra or L3-L4 intervertebral space with a marker pen. The midline was determined through palpation of the spinous processes, and L2-L3 and L3-L4 spaces were marked using a pen as midline insertion sites after a sham USG scan procedure was performed by moving the USG probe on the patient’s back with a frozen screen before the locations were marked.

The interspinous level was identified using USG following spinal anesthesia.

Group U

The same (1.0–7.0 MHz) convex USG transducer and USG (SamsungRS85,CA1-7 A, Hongcheon, Korea) device was used in group U. Prepuncture USG-assisted technique, scans of longitudinal parasagittal and transverse midline views were performed using the convex probe of the USG device according to the findings of previous studies [11]. Needle insertion points were determined as the intersection points of the longitudinal and transverse lines corresponding to the L2-3 and L3-4 interspaces providing the best USG image created with an appropriate probe-tilt angle with a convex low frequency probe. The cross-point of these planes was subsequently marked as the site of insertion site for both interspaces in midline. After the skin was marked, the patient was asked not to move, full aseptic precautions were applied, and a subarachnoid puncture was performed immediately.

Outcome measures and data collection

All subarachnoid injections were performed with a midline approach using a 25-gauge 90-mm Quincke-tip needle. After the free flow of the Cerebrospinal fluid (CSF) was observed, 2.1 mL of 0.5% heavy bupivacaine was administered in the cephalad direction of the needle tip. The L3-L4 interspace was selected for the first trial, and the L2-3 interspace, at a higher level, was used for the subsequent trials. The transition to the second interspace was allowed if a maximum of three skin puncture attempts were unsuccessful (i.e.,the needle was advanced again after being wholly withdrawn from the skin). The limit for each skin puncture attempt was set at a maximum of five-needle passes (i.e., the needle was directed again and advanced without withdrawing from the skin completely).

If the dura-subarachnoid membrane puncture failed after the attempts at the L2-L3 interspace in both groups, the operator was allowed to find the correct space for spinal injection by using different methods, such as the USG-guided or paramedian approaches. After successful puncture, the parturients were placed in the supine position with a 15-degree left lateral tilt. Spinal anesthesia was accepted as successful after confirmation of bilateral T4 block at the postprocedure 5^th minute [12].

Hypotension was defined as a 25% drop from baseline systolic blood pressure below 90 mmHg Ephedrine (5 mg) was administered in cases of predefined hypotension. All hypotension episodes were recorded. Other complications (e.g., blood on the spinal needle or paresthesia) were also recorded by an independent observer who was blinded to the group allocation.

The primary outcome was a successful dura-subarachnoid membrane puncture rate with free flow of CSF on the first insertion of the needle with no redirections. Secondary outcomes were the number of skin punctures (each skin puncture was considered a separate procedure), the number of needle passes (skin puncture + needle redirections), site marking times, procedure time and total time, patient satisfaction scores, and incidences of complications.

The time needed to determine the needle insertion site was described as the time between applying the USG probe to the skin of the back and marking the planned insertion point in group U. In group L, it was accepted as the time between when the operator first touched the patient and the completion of marking the needle insertion point. The time to administer spinal anesthesia in the groups was from the needle touching the skin to visualizing the CSF. The total procedure time was defined as the sum of the time required to locate the needle insertion site and administer spinal anesthesia.

More than 10 needle passes were defined as a difficult spinal injection. Also, the level of block evaluated by the loss of cold sensation, the number of levels perforated with a needle (the number of transitions to a second lumbar interspace), the failure rate of spinal anesthesia (transition to general anesthesia or the need for rescue analgesia), and the satisfaction of the parturient with the procedure were recorded. The evaluation of the satisfaction of the patients was made immediately after the intrathecal injection using a scale of “Very satisfied,” “Satisfied,” or “Not satisfied.” All parturients were also evaluated for spinal anesthesia complications such as radicular pain, paresthesia, and post-dural puncture headache 24 h after the surgery.

Statistical analysis

The statistical analysis was performed using the R 4.2.2. program. Mean and standard deviation statistics are given for numerical variables, and frequency and percentage statistics are given for categorical variables. Normality testing was performed using Q-Q plots and the Shapiro-Wilk W Statistic. The t-test was used to analyze the numerical variables, the Chi-square and Fisher’s exact tests were used in categorical variables, and p < 0.05 was considered significant.

The sample size of the study was calculated to find the first-attempt success rates in parturients with BMI ≥ 40 kg/m² by using ClinCalc.com [13]. In a previous study, the first attempt rate during spinal anesthesia administration was reported as 65% using USG and 32% with conventional palpation in patients with difficult spinal anatomy [4]. Therefore, a sample size of 35 patients per group was needed for 80% power with an α error of 5%. Assuming a 10% dropout rate, we recruited 80 patients.

Results

In the present study, 80 eligible patients were enrolled between May 2022 and January 2023 in two groups, group U(n = 40) and group L (n = 40) (Fig. 1). Demographic data are given in Table 1. There was no statistically significant difference between the groups.

Fig. 1.

CONSORT diagram of patient recruitment (L: Land-mark; U; Ultrasound)

Table 1.

Demographic data

	Group L, n = 40*	Group U, n = 40*	p-value†
Age (y)	31.85 ± 5.19	30.32 ± 4.97	0.2
Weight (kg)	106.58 ± 7.25	107.28 ± 8.01	0.7
Height (cm)	159.57 ± 5.58	160.68 ± 5.72	0.4
BMI (kg/m2)	41.82 ± 1.15	41.53 ± 1.83	0.4
Surgical time (min)	50.50 ± 9.87	50.85 ± 7.66	0.9
OR time (min)	56.40 ± 9.56	56.92 ± 7.56	0.8

BMI Body Mass Index, L Land-mark, SD Standard deviation, U Ultrasound

Surgical time (min) Time between skin incision and closure of skin, OR time(min) Time between entering and leaving the operating room

^*Mean ± SD, n (%), ^†Independent Samples t test with mean ± SD (%)

Comparisons of spinal procedure-related data in the groups are shown in Table 2. The first-attempt success rate was significantly higher in group U than in group L (42.5% vs. 10%; p = 0.002). Fewer skin punctures were required to obtain free CSF flow in group U than in group L (p < 0.001) (Table 2). The number of cases evaluated as difficult(> 10 needle passes) was six in group L, and there were none in group U (p = 0.026) (Table 2). Although the pre-procedural evaluation (p < 0.001) and total time (p = 0.017) were longer in group U compared with group L, the spinal injection time (p < 0.001) was shorter (Table 2). No statistical differences in patient satisfaction scores were detected between the groups (p = 0.4) (Table 3).

Table 2.

Comparisons of procedure-related data between groups

	Group L, n = 40*	Group U, n = 40*	p-value
First attempt success rate	4 (10%)	17 (42.5%)	0.002†
Number of skin punctures	1.85 ± 0.74	1.20 ± 0.41	< 0.001‡
Number of needle pass	7.4 ± 3.68	2.58 ± 1.99	0.005‡
Difficult spinal injection (> 10 needle passes)	6 (15%)	0 (0%)	0.026†
Time interval to determine needle insertion site (sec)	27.88 ± 9.79	85.15 ± 16.10	< 0.001‡
Time taken for spinal injection (sec)	70.60 ± 29.44	29.05 ± 12.78	< 0.001‡
Total procedure time (sec)	99.55 ± 31.40	114.10 ± 20.36	0.017‡
Level of spinal space			0.5†
L3-L4	38 (95%)	40 (100%)
L2-L3	2 (5.0%)	0 (0%)
Number of puncture another level	2 (5.0%)	0 (0%)	0.5†

L Land-mark, U Ultrasound

^*Mean± Standard deviation; n (%);^†Fischer's exact test; ^‡Welch Two Sample t-test

Table 3.

Incidence of complications and satisfaction scores between groups

	Group L, n = 40*	Group U, n = 40*	p-value†
Incidence of complications during puncture	27 (68%)	28 (70%)	0.8
Type of complication			0.7
Hypotension (only)	19 (48%)	23 (57%)
Bloody tap	2 (5.0%)	1 (2.5%)
Bloody tap with hypotension	6 (15%)	4 (10%)
Parturient satisfaction			0.4
Dissatisfied	7 (18%)	4 (10%)
Satisfied	18 (45%)	16 (40%)
Very satisfied	15 (38%)	20 (50%)

L Land-mark, U Ultrasound

^*Mean± Standard deviation; n (%), ^†Pearson's Chi-squared test

To assess the potential value of pre-procedural USG evaluations at different levels of BMI, a scatterplot of patients with different BMI values was created (Fig. 2).

Fig. 2.

Scatterplots of BMI and Time (to locate midline) with regression line (BMI: Body Mass Index; L: Land-mark; U; Ultrasound)

Eight patients in group L and six in group U had a bloody tap; however, no statistically significant difference was found between the groups (Table 3). No patients in either group required conversion to general anesthesia or rescue analgesia. Also, complications such as postoperative headache, radicular pain, or paresthesia were not observed in any patients in either group.

Discussion

In this randomized controlled clinical trial, we compared the first-attempt success rates using the conventional landmark method and pre-procedural USG assistance in spinal anesthesia in parturients with class 3 obesity undergoing elective cesarean section. The first-attempt success rate (42.5%) under USG imaging was found to be higher than the landmark method (10%). Despite extending the total preparation time, the successful spinal anesthesia procedure time showed more than 50% shortening with USG assistance compared with the conventional manual palpation technique.

In spinal anesthesia, the chance of success in the first attempt was found to be closely associated with ease of palpation of anatomic landmarks [6]. However, the palpation of bony structures and Tuffier’s line to determine landmarks in parturients with obesity for the administration of regional anesthesia can be difficult due to relatively unidentifiable landmarks [14]. Moreover, it was shown in radiologic and cadaveric studies that Tuffier’s line could show different levels from L5-S1 to L3-L4 [5, 15]. However, spinal anesthesia may be facilitated by USG guidance in morbid obesity, where palpation of anatomic landmarks is difficult [16, 17]. It has been determined that neuraxial USG has a role in increasing the effectiveness of lumbar neuraxial anesthesia, accurately predicting the depth of the target [18]. In several studies, pre-procedure USG examinations were found to determine the intervertebral level more accurately (68–71%) than palpation [19, 20]. The improvement in the accuracy rate was also highly correlated with the frequency of use of USG guidance in neuraxial procedures [20]. In our study, the assessor with experience of at least 60 cases of neuraxial USG recruited the parturients to obtain a higher accuracy rate for the detection of interspinous spaces [18, 21].

The result of our study differs from some studies claimed that USG might not create additional benefits for first-attempt success rates. On the other hand, in these studies, patients had easily palpable spines or were administered epidural anesthesia [22, 23]. Moreover, Tawfik et al. obtained similar first-attempt success rates with preprocedural USG and conventional palpation [24]. In this study, all procedures, including pre-procedure spinal examinations and spinal anesthesia applications, were performed by a single operator, and spinal anesthesia was performed in the sitting position in both groups. This methodologic difference may explain the difference compared with our results. Also, in explaining the differences between these data and those of Tawfik et al., it is inescapable to consider differences in USG and/or neuraxial procedural skills and, most importantly, unconscious bias as factors. However, the sitting position gives an advantage for palpation of midline structures among patients with obesity. The incidence of hypotension rises in the management of parturients with spinal anesthesia in contrast to lateral positioning, in which sensory and motor blocks occur faster with a high patient satisfaction rate [25]. Therefore, in our clinic, we prefer to have parturients in the left lateral position while performing neuroaxial anesthesia.

As reported in previous studies with patients with BMI ≥ 35 kg/m², the number of skin punctures and needle passages was found to be lower with USG assistance, as in the present study [16, 26]. Nevertheless, the results of our study are also compatible with other published meta-analyses [7, 21, 27]. The nobility of the present study was that all our patients were in the class 3 obese parturient. Therefore, the exact interpretation of the benefit of USG-guidance on these population can be made accordingly. In comparison with the number of skin punctures, the results of our study indicates that conventional palpation method leads to multiple attempts similar to other studies [26, 28, 29]. Although we did not face with any problems related with neuronal injury at the postoperative period, fewer needle passes/punctures may reduce the incidence of these type of complications [21]. The overall number of needle passes was reported to be higher in Chin et al.’s study in nonobstetric patients with BMI > 35 than in the present study [4]. This may have been due to differences in the patient populations. When the procedure times were compared, the results of the present study were compatible with the results of many previous studies [28–30]. In our study and the literature, the time taken to determine the needle insertion site and the total time in group U was more prolonged than in group L, and the spinal anesthesia performance time was shorter [30]. It was possibly due to the shorter duration of spinal intervention with the success of USG imaging in determining the vertebral structures and the needle insertion site. On the other hand, Li et al. found that the time taken for site determination to be similar in both USG and landmark-assisted techniques and the total processing time to be longer with the palpation technique [16]. The difference between the present study was the characteristics of the patient population in which all the parturients had class 3 obesity in contrast to a small number recruited by Li et al. [16]. This might result in difficulty in determining an appropriate acoustic window during USG imaging in our cases. In the present study, patient satisfaction scores were similar in both groups. Based on this result, although the landmark method seems advantageous in terms of time, it suggests that pre-procedure USG for determining needle location is acceptable to patients.

There were some limitations in the present study. The effectiveness of the paramedian approach, which was claimed to be superior to the midline approach, was not examined when spinal anesthesia with USG or the landmark method was performed [31]. In our study, spinal anesthesia was not performed using real-time USG guidance. Also, the depth of the spinal interspace level was not measured. Moreover, the study participants’ blindness cannot be guaranteed. The palpation group underwent a sham USG procedure, but the USG group did not receive sham palpation. Furthermore, the accuracy of the interspace spaces examined using USG was not confirmed using other radiologic techniques. Lastly, spinal anesthesia was performed in the lateral position, so the study did not provide adequate information about the sitting position.

Conclusion

The pre-puncture USG-assisted technique increased the success rate at the first attempt in pregnant women with class 3 obesity who underwent spinal anesthesia in the lateral position and reduced the number of puncture attempts, needle passages, and need for punctures from different levels. Therefore, pre-puncture USG imaging in spinal anesthesia is useful for parturients with class 3 obesity whose anatomic landmarks are unclear in conventional landmark palpation.

Abbreviations

WHO

The World Health Organization

BMI

Body Mass Index

USG

Ultrasonography

PERSEUS regional anesthesia

Peri-operative Use of Ultrasound for regional anesthesia

CSF

Cerebrospinal fluid

CONSORT

Consolidated Standards for Reporting Studies

ASA

American Society Anesthesiologists

Authors’ contributions

All of the authors (AB, BB) made substantial contributions to conception and design, acquisition of data, or analysis and interpretation of data; they have been involved in drafting the manuscript or revising it critically for important intellectual content; have given final approval of the version to be published.

Funding

No funding.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Institutional Review Board approval (02-2022/02) of Karamanoglu Mehmetbey University Faculty of Medicine, Turkey, was obtained on March 8^th, 2022. A written informed consent form was obtained from all patients who agreed to participate in the study. All methods were carried out in accordance with the Declaration of Helsinki.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.