Ultrasound Versus Palpation for Determining Spinal Puncture Level in Cesarean Section

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Background and Rationale

Spinal anesthesia is the technique of choice for elective cesarean delivery. The intended puncture site is conventionally identified by palpating Tuffier's line, an imaginary line joining the iliac crests, which is assumed to cross the L4 vertebral body or the L4-L5 interspace. Anatomical studies and imaging data have shown that this assumption is unreliable, particularly in pregnancy, where lumbar lordosis, soft tissue changes and altered pelvic tilt shift the landmark cephalad in a substantial proportion of women. Levels selected by palpation are therefore frequently one or more interspaces higher than intended.

Preprocedural ultrasound allows direct visualization of the sacrum and the lumbar interlaminar spaces, and the selected level can be marked on the skin before needle insertion. Whether the improvement in level accuracy translates into clinically meaningful differences in technical success and maternal hemodynamic response has not been established consistently in the obstetric population.

Study Design

This is a prospective, randomized, controlled, parallel-group, single-center trial conducted at the Department of Anesthesiology and Reanimation, Kahramanmaraş Sütçü İmam University Health Practice and Research Hospital.

Sixty ASA II-III women aged 18-45 years with singleton pregnancies scheduled for elective cesarean section under spinal anesthesia will be enrolled after written informed consent. Randomization will be performed using computer-generated block randomization with an allocation ratio of 1:1 (30 per group).

Interventions

Ultrasound group: Before the procedure, the lumbar spine will be scanned with a low-frequency curvilinear probe in the patient's sitting position. The sacrum will be identified in the paramedian sagittal oblique plane and interspaces counted cephalad. The target interspace will be marked on the skin, and the transverse midline plane will be used to confirm the midline. Spinal anesthesia will then be performed at the marked level.

Palpation group: The puncture level will be determined by palpation of Tuffier's line according to standard clinical practice, without ultrasound assistance.

In both groups, spinal anesthesia will be performed with the patient seated, using a midline approach and a pencil-point spinal needle. Hyperbaric bupivacaine with fentanyl will be administered as a single dose, calculated according to the Harten table based on maternal height. A maximum of three puncture attempts is permitted; if unsuccessful, the case will be managed at the discretion of the attending anesthesiologist and recorded as a failure of the allocated technique.

After completion of the block, and without any modification of clinical management, ultrasonography will be used in both groups purely for observational purposes to verify the intervertebral level at which the dural puncture was performed. This exploratory assessment allows quantification of level accuracy in both arms.

Measurements

Baseline maternal characteristics (age, height, weight, body mass index, gestational age, ASA class, parity) will be recorded. Procedure-related variables include the number of skin punctures, number of needle redirections, and procedure duration measured from needle-skin contact to visualization of cerebrospinal fluid.

Non-invasive blood pressure and heart rate will be recorded at baseline and every 3 minutes thereafter until delivery and subsequently until the end of surgery. Hypotension is defined as a mean arterial pressure below 65 mmHg or a decrease of 20% or more from the baseline value, and will be treated with intravenous ephedrine according to institutional protocol; total ephedrine dose will be recorded.

Sensory block level will be assessed bilaterally at 10 and 15 minutes after intrathecal injection. Intraoperative nausea and vomiting, paresthesia during needle insertion, bloody tap, incomplete block, requirement for supplemental analgesia, and conversion to general anesthesia will be documented. Patients will be assessed at 24 hours postoperatively for post-dural puncture headache, back pain, and satisfaction using a visual analogue scale.

Sample Size

The sample size was calculated using G*Power 3.1, based on success rates reported by Li et al. (2019). For the comparison of proportions between two independent groups using Fisher's exact test, with a significance level of 0.05, power of 0.80, and an effect size of h = 0.79, a minimum of 30 participants per group is required, giving a total of 60 participants.

Statistical Analysis

Distribution of continuous variables will be assessed with the Shapiro-Wilk test and visual methods. Normally distributed data will be presented as mean ± standard deviation and non-normally distributed data as median with interquartile range. Between-group comparisons will use the independent samples t-test or Mann-Whitney U test for continuous variables, and the chi-square or Fisher's exact test for categorical variables. Changes in hemodynamic parameters over time will be analyzed using repeated measures analysis (mixed model or repeated measures ANOVA, according to assumption checks). Statistical significance will be accepted at p < 0.05. Analyses will primarily follow the intention-to-treat principle, and missing data will be reported.

Inclusion criteria

• Scheduled for elective cesarean section under spinal anesthesia
• Age 18 to 45 years
• ASA physical status II or III
• Singleton pregnancy
• No contraindication to spinal anesthesia
• Normotensive
• Provides written informed consent

Exclusion criteria

• Emergency cesarean section
• Morbid obesity (body mass index 40 kg/m² or higher)
• History of lumbar spine surgery
• Severe spinal deformity
• Local infection at the puncture site
• Coagulopathy or ongoing anticoagulant therapy
• Anatomical abnormality expected to require multiple puncture attempts