Abstract
Background and Aims:
Dural sac cross-sectional area (DSCSA)-based intrathecal dosing aims to optimise block spread and improve haemodynamic stability during spinal anaesthesia (SA). Ultrasound-guided DSCSA measurement helps in local anaesthetic (LA) dose reduction compared with conventional fixed-dose practice. The primary objective was to determine LA dosage proportional to DSCSA reduction, while the secondary objectives included comparison of block characteristics and intra-operative haemodynamic parameters [heart rate and mean arterial pressure (HR and MAP)] between the two approaches.
Methods:
This prospective randomised study included 60 American Society of Anesthesiologists I–III patients aged >65 years undergoing orthopaedic surgery under SA. Patients were allocated to an ultrasound-guided DSCSA-based dosing group (US, n = 30) or a control group (C, n = 30). In Group US, DSCSA was calculated from ultrasound-measured dural sac diameter at L3–L4, and hyperbaric bupivacaine dosing was individualised, while Group C received a fixed 11 mg dose. Sensory and motor block characteristics, HR, MAP, and surgical duration were recorded.
Results:
Group US had a mean dural sac diameter of 11.99 (0.76) mm and a DSCSA of 112.98 (13.42) mm², resulting in a mean calculated dose of 1.79 (0.14) mL. Both groups achieved a maximum motor block score of 3. Group C showed significantly greater reductions in HR, with more frequent bradycardia, and a significant decrease in MAP between 10 and 30 minutes. Surgical duration was comparable between the groups.
Conclusion:
Ultrasound-guided DSCSA-based dosing improves haemodynamic stability and reduces bradycardia and hypotension while maintaining comparable block quality to conventional fixed-dose SA.
Keywords: Geriatric anaesthesia, local anaesthetic, spinal anaesthesia
INTRODUCTION
Spinal anaesthesia (SA) in elderly patients is technically challenging due to age-related anatomical changes such as spinal arthrosis, facet joint hypertrophy, and narrowing of interspinous and interlaminar spaces.[1] Lumbar central canal spinal stenosis, a common cause of disability in this population, results from facet joint hypertrophy, disc degeneration, and ligamentum flavum thickening, leading to reduced spinal canal dimensions and altered dural sac morphology.[2] The dural sac cross-sectional area (DSCSA) is a sensitive indicator of lumbar canal stenosis, with affected patients often demonstrating a 30% reduction.[3]
Age-related anatomical and physiological changes significantly influence the pharmacokinetics and intrathecal spread of local anaesthetics (LAs). Reduced hepatic metabolism, decreased plasma clearance, diminished cerebrospinal fluid (CSF) volume, and dural sac compression can result in unpredictable and extensive block spread, increasing the risk of high spinal block and toxicity.[4,5,6] Additionally, a 25% reduction in systemic vascular resistance makes elderly patients more susceptible to haemodynamic instability following SA.[3] Consequently, minimising and individualising intrathecal LA dosing is essential in this population.
Ultrasonography has emerged as a reliable bedside tool for identifying spinal landmarks and assessing dural sac dimensions, with the paramedian sagittal approach providing accurate DSCSA measurements.[5] Given that DSCSA is the most sensitive marker of lumbar canal stenosis, dose adjustment based on DSCSA may optimise the safety and efficacy of SA in elderly patients.[3]
This study evaluates the utility of ultrasound-guided DSCSA measurement to individualise intrathecal LA dosing in elderly patients undergoing orthopaedic surgery. The primary objective was to calculate LA dosage proportional to DSCSA reduction. Secondary objectives included comparing sensory and motor block characteristics, two-segment regression time, intra-operative mean arterial pressure (MAP), heart rate (HR), and the incidence of hypotension and bradycardia between ultrasound-guided and conventional dosing techniques.
METHODS
This prospective randomised study included 60 patients of either gender, aged >65 years, with American Society of Anesthesiologists (ASA) physical statuses I–III, scheduled for orthopaedic surgery of anticipated duration <2 hours under SA. The study was conducted over 1 year from June 2024 to May 2025 after institutional ethics committee approval (vide approval number BREC/24/346) registered with the Clinical Trials Registry- India (vide registration number CTRI/2024/05/067353). Patients with contraindications to SA (e.g., neurological disorders, local infections, allergy to LAs, coagulopathy), marked spinal deformity, or previous spinal surgery or those unwilling to participate were excluded. Informed consent was obtained from all participants. The study adhered to the Declaration of Helsinki (2013) and Good Clinical Practice guidelines, and the manuscript followed Consolidated Standards of Reporting Trials (CONSORT) guidelines [Figure 1]. Randomisation was performed by a fellow anaesthesiologist not involved in the study using a computer-generated table. Patients were allocated into two groups: Group US (n = 30), in which the sagittal anteroposterior diameter of the dural sac was measured at the L3–L4 interspace in the sitting position using ultrasound, and Group C (n = 30, control), which received standard SA.
Figure 1.
Consolidated standards of Reporting trials (CONSORT) flow diagram
Blinding of the anaesthesiologist performing the spinal block was not feasible due to the use of ultrasound in Group US. However, randomisation and allocation concealment were ensured using sequentially numbered, opaque, sealed envelopes. Sensory and motor block characteristics and haemodynamic parameters were assessed by an independent anaesthesiologist blinded to group allocation, and the proceduralist was not involved in outcome assessment or data analysis.
In Group US, the sagittal anteroposterior dural sac diameter at the L3–L4 interspace was measured in the sitting position using ultrasound, and the DSCSA was calculated as A = π (D/2)². The dose of 0.5% hyperbaric bupivacaine was individualised based on DSCSA using a baseline dose of 11 mg adjusted proportionally (±1 mg per ± 10 mm²), referenced to a standard DSCSA of 150 mm². Although the relationship between DSCSA and intrathecal spread is not strictly linear, this pragmatic approach allowed clinically feasible dose individualisation. In Group C, a fixed dose of 2.2 mL (11 mg) of 0.5% hyperbaric bupivacaine was administered assuming a standard DSCSA of 150 mm².
All ultrasound measurements were performed by a single experienced consultant anaesthesiologist to minimise inter-observer variability using a portable Sonosite Edge II total system with a curved array probe (2–5 MHz). A standardised scanning protocol was used in all patients to reduce intra-observer variation. Patients were seated with an arched back on a trolley. The scanning protocol involved imaging the paramedian sagittal oblique (PSO) plane. In the PSO plane, key structures identified included the posterior complex (ligamentum flavum, epidural space, posterior dura mater), the anterior complex (anterior dura mater, posterior longitudinal ligament, vertebral body/intervertebral disc), and the dural sac appearing as an anechoic area between these complexes. The PSO plane was used for ultrasound assessment as it provides consistent visualisation of the anterior and posterior spinal canal complexes, particularly in elderly patients with degenerative changes where transverse interlaminar views may be limited. The anteroposterior diameter of the dural sac, measured between the anterior and posterior complexes, was used to estimate the DSCSA using A = π (D/2)². Although the dural sac is anatomically elliptical, this circular approximation based on the reliably visualised anteroposterior diameter offers a practical and validated surrogate for relative comparison and dose adjustment. Ultrasound depth and gain settings were standardised, and all measurements were performed by a single experienced operator following a pre-defined protocol.
In the operating room, standard ASA monitoring (electrocardiography, non-invasive blood pressure, and pulse oximetry) was applied, and baseline HR, blood pressure, and peripheral oxygen saturation were recorded. Under aseptic precautions, SA was performed at the L3–L4 interspace in the sitting position using a 25-gauge Quincke–Babcock needle. Hyperbaric 0.5% bupivacaine was administered intrathecally, with the dose calculated based on DSCSA in Group US and fixed at 2.2 mL in Group C. Patients were then positioned supine.
Sensory and motor block assessments were performed by a single blinded observer at pre-defined intervals to minimise observer variability. Sensory block was assessed using an alcohol-soaked cotton swab every 2.5 minutes up to 5 minutes and every 5 minutes thereafter until 20 minutes. Outcomes included time to onset of sensory block (T12), which was defined as the time to achieve T12 dermatome level from intrathecal injection, maximum sensory level achieved, time to maximum block height, and two-segment regression to L2. Motor block was assessed concurrently using the Modified Bromage scale, noting time to maximum block and time to regression to Bromage zero. Adequate block was defined as attainment of at least T12 sensory level within 20 minutes; failure to do so was considered technique failure and managed with general anaesthesia.
Haemodynamic parameters, namely, MAP and HR, were recorded at baseline, every 2 minutes for the first 10 minutes, every 5 minutes until 30 minutes, and every 15 minutes thereafter until the end of surgery. Complications including bradycardia, hypotension, shivering, nausea, and vomiting were documented. Bradycardia was defined as HR <50 beats per minute (bpm) or a ≥20% decrease from baseline and treated with 0.6 mg intravenous (IV) atropine. Hypotension was defined as an MAP drop >20 mmHg and managed with 5 mg IV ephedrine. Nausea and vomiting were treated with 4 mg IV ondansetron. The total surgical duration was recorded for all the patients.
The sample size estimation was based on the expected difference in drug dose (mg) between the study groups.[3] For the calculation, a mean difference (δ) of 2.9 mg with a standard deviation (SD) of 3.95 mg was assumed. The sample size was calculated using a two-sided significance level (α) of 0.05, 80% statistical power (1−β), and a 95% confidence interval. Pooled standard deviation was defined as= 
where S1 and S2 represent the standard deviations of group 1 and group 2, respectively. The required sample size per group (N) was calculated using the following formula for comparing two independent means. SD = standard deviation = 3.95; δ = mean difference = 10-7.1 = 2.9
Zα/2 = Z0.05/2 = Z0.025 = 1.96 — From Z table at type I error of 5
Zβ = Z0.20 = 0.84 — at 80% power
Substituting these values into the formula:
= 2 (1.96 + 0.84)2 (3.95)2/(2.9)2 = 29.09
Thus, the required sample size was rounded up to 30 participants in each group.
Data were coded and recorded in the MS Excel spreadsheet program and analysed using Statistical Package for Social Sciences (SPSS) version 25.0, (SPSS Inc., Chicago, IL, United States of America). Descriptive statistics was elaborated in the form of means/SDs and medians/interquartile ranges (IQRs) for continuous variables, and frequencies and percentages for categorical variables. Data were presented in a graphical manner wherever appropriate for data visualisation using histograms/box-and-whisker plots/column charts for continuous data and bar charts/pie charts for categorical data.
Categorical variables were presented in number and percentage (%). Normality was assessed using the Shapiro–Wilk test. Normally distributed variables are presented as mean ± SD and non-normally distributed variables as median (IQR). Analyses were performed on an intention-to-treat basis. Non-parametric test was used if the normality was rejected. A P value of <0.05 was considered statistically significant.
RESULTS
No patients were lost to follow-up or required conversion to general anaesthesia. The primary outcome was the total dose of 0.5% hyperbaric bupivacaine required to achieve an adequate sensory block. In Group US, the dosing was individualised based on DSCSA, whereas Group C received a fixed dose of 11 mg (2.2 mL). Secondary outcomes included sensory and motor block characteristics, haemodynamic parameters, and adverse events. The demographic variables were comparable between the groups [Table 1]. In Group US, the mean dural sac diameter was 11.99 (0.76) mm and the mean DSCSA was 112.98 (13.42) mm². The calculated LA dose in Group US was 1.79 (0.14) mL, with a median (IQR) of 1.8 (1.76–1.8) mL. Sensory and motor block characteristics are summarised in Table 2. All patients in both groups achieved a maximum motor block of Bromage grade 3.
Table 1.
Demographic profile
| Parameter | Group US (n=30) | (95% CI) | Group C (n=30) | (95% CI) | P |
|---|---|---|---|---|---|
| Age (years) Mean (SD) | 72.67 (7.02) | (70.06–75.28) | 72.10 (7.04) | (69.48–74.72) | 0.800 |
| Gender, n (%) | |||||
| Male | 10 (33.3%) | 13 (43.3%) | 0.426 | ||
| Female | 20 (66.7%) | 17 (56.7%) | |||
| Weight (kg) Mean (SD) | 63.17 (8.29) | (60.08–66.26) | 64.63 (3.92) | (63.16–66.10) | 0.386 |
| ASA Grade (n %) | |||||
| II | 18 (60.0%) | 17 (56.7%) | 0.793 | ||
| III | 12 (40.0%) | 13 (43.3%) |
Values are presented as number (%) and are presented as mean±SD. ASA: American Society of Anesthesiologists; CI: Confidence interval; SD: Standard deviation
Table 2.
Sensory and motor block characteristics with 95% confidence intervals
| Parameter | Group US (n=30) | (95% CI) | Group C (n=30) | (95% CI) | χ 2 | P |
|---|---|---|---|---|---|---|
| Sensory block onset to T12 (min), mean (SD) | 3.45 (1.32) | (2.96–3.94) | 3.37 (1.07) | (2.97–3.77) | 0.698 | |
| Time to maximum sensory block (min), mean (SD) | 9.03 (2.48) | (8.10–9.96) | 8.80 (1.65) | (8.18–9.42) | 0.482 | |
| Maximum block height, n (%) | <0.001* | |||||
| T4 | 1 (3.3%) | 21 (70.0%) | 29.439 | |||
| T6 | 26 (86.7%) | 9 (30.0%) | ||||
| T8 | 3 (10.0%) | 0 (0.0%) | ||||
| Twosegment regression time (min), mean | 90.77 (8.73) | (87.52–94.02) | 99.77 (12.75) | (95.00–104.54) | 0.009* | |
| Motor block onset (min), mean (SD) | 9.70 (1.86) | (9.01–10.39) | 9.20 (2.41) | (8.30–10.10) | 0.484 | |
| Motor block regression to Bromage 0 (min), mean (SD) | 122.63 (16.43) | (116.49–128.77) | 140.50 (12.34) | (135.90–145.10) | <0.001* |
Values are presented as number (%). Statistically significant values with P<0.05 are indicated with*; χ2: Chi-squared. SD: Standard deviation; n: Number of patients
Significant differences in HR were observed between the groups at multiple time points from 6 to 45 minutes post-block, with Group C exhibiting a higher incidence of bradycardia [Figure 2]. MAP showed a significant decline in Group C between 10 and 30 minutes [Figure 3]. The adverse events are summarised in Table 3. The mean duration of surgery was comparable between the groups (95.10 [6.29] vs. 98.47 [11.17] minutes; P = 0.124).
Figure 2.
Heart rate variation between the groups
Figure 3.
Mean arterial pressure variation between the groups
Table 3.
Adverse events
| Adverse events | Group US (n=30) | Group C (n=30) | P |
|---|---|---|---|
| Hypotension | 2 (6.7%) | 7 (23.3%) | 0.407 |
| Bradycardia | 2 (6.7%) | 5 (16.7%) | 0.605 |
| Nausea | 2 (6.7%) | 6 (20.0%) | 1.000 |
| Shivering | 1 (3.3%) | 0 (0.0%) | 1.000 |
Values are presented as number (%). n: Number of patients
DISCUSSION
The present study demonstrated that ultrasound-guided, DSCSA-based intrathecal dosing provides effective SA in elderly patients with improved haemodynamic stability and more predictable block spread, while maintaining adequate sensory and motor blockade and facilitating faster recovery compared with fixed-dose techniques.
The mean sagittal anteroposterior dural sac diameter at L3–L4 in the US group closely aligns with previously reported measurements in elderly populations (~11.6–12 mm) and lower than those described in younger adults.[5,6,7] This age-related reduction reflects degenerative changes such as ligamentum flavum thickening, facet joint hypertrophy, disc height loss, and reduced CSF fluid volume, contributing to spinal canal narrowing.[2,8,9] The consistency of these findings across studies supports the clinical relevance of dural sac assessment for individualised intrathecal dosing in elderly patients.
The mean DSCSA in the US group was 112.98 ± 13.42 mm², notably lower than the 150 mm² reference used in the conventional group. This aligns with previous reports showing age-related reduction and variability in DSCSA, often approaching levels associated with spinal stenosis.[3,10] As a sensitive marker of canal narrowing,[2] DSCSA is useful for guiding individualised intrathecal dosing and optimising SA in the elderly. Ultrasound-guided DSCSA-based dosing significantly reduced intrathecal bupivacaine requirements compared with conventional fixed dosing. This aligns with evidence showing lower minimum effective doses in elderly patients, with 50% effective dose (ED50) values near 6–6.5 mg significantly reducing hypotensive events in elderly patients undergoing hip fracture surgery.[9,11] Similarly, studies employing ultrasound-guided spinal techniques have consistently demonstrated lower intrathecal dose requirements in the range of approximately 6.7–7.5 mg compared with conventional fixed-dose regimens of about 10 mg.[3,10,12] Given age-related cardiovascular changes such as diminished baroreceptor sensitivity and reduced vascular compliance, elderly patients are particularly susceptible to sympathetic blockade-induced haemodynamic instability. Accordingly, lower intrathecal doses of bupivacaine limit sympathetic block and contribute to reduced incidence and severity of hypotension, reinforcing the clinical value of DSCSA-guided dose individualisation in this population.
In the present study, reduced intrathecal dosing in the US group did not delay sensory block onset, consistent with the pharmacodynamics of hyperbaric bupivacaine. Ultrasound-guided dosing resulted in controlled sensory block heights, whereas conventional dosing more frequently led to excessive cephalad spread in the current investigation. Previous studies have shown that excessively high sensory levels increase the risk of sympathetic blockade-related complications, such as hypotension and bradycardia, especially in elderly patients with limited cardiovascular reserve.[13,14,15] Previous studies similarly show that ultrasound-guided, individualised dosing yields safer, more predictable block levels.[6,10,13,16]
The time to reach peak sensory blockade was similar between both the groups, which is consistent with previous studies using hyperbaric local anesthetic solutions.[10,11,16] In contrast, DSCSA-guided dosing was associated with earlier sensory regression, suggesting a shorter duration of block without compromising intraoperative anaesthetic adequacy. Faster regression may facilitate earlier neurological recovery and post-operative mobilisation, which are particularly advantageous in elderly patients.
Motor block onset and intensity were similar between groups in the present study, indicating that reduced intrathecal dosing did not compromise motor blockade adequacy for surgery. However, ultrasound-guided DSCSA-based dosing led to faster motor regression. Earlier motor recovery is clinically advantageous in elderly patients as it facilitates timely mobilisation, reduces the risk of thromboembolic events, and enhances discharge readiness. Previous studies report similar benefits with reduced or ultrasound-guided SA, supporting functional advantages of individualised dosing.[3,17]
In the current study, ultrasound-guided DSCSA-based dosing provided better haemodynamic stability than conventional fixed-dose SA. Patients in the US group demonstrated improved HR and MAP, likely due to reduced sympathetic blockade from limited intrathecal spread. This is particularly relevant in elderly patients with limited cardiovascular reserve and impaired compensatory mechanisms. These findings are consistent with previous studies showing that individualised or ultrasound-guided SA reduces haemodynamic fluctuations, highlighting the clinical benefit of DSCSA-based dose adjustment in this population.[3,10]
Although conventional dosing was associated with a higher incidence of bradycardia, hypotension, and nausea, the overall complication rates were comparable between the groups in the present study, aligning with previous evidence that ultrasound-guided and conventional SA have comparable safety profiles.[13] Surgical duration was similar in both groups, confirming that low-dose hyperbaric bupivacaine provides adequate anaesthesia for procedures of roughly 90–100 minutes, with supplemental techniques needed only for longer surgeries.[12,15,16]
These findings indicate that ultrasound-guided, DSCSA-based intrathecal dosing enhances haemodynamic predictability without increasing adverse events. Although intrathecal spread is affected by factors such as height, body habitus, spinal curvature, CSF volume, and drug baricity, DSCSA served as a practical surrogate for spinal anatomy in this study. Future studies should explore multifactorial dosing models that integrate these variables to further individualise SA in elderly patients.
With regards to comparison with the study of Pula et al.,[3] the present study involved elderly patients with a mean DSCSA of 112.98 ± 13.42 mm², comparable to Pula et al. (110.45 ± 23.09 mm²). The calculated intrathecal bupivacaine dose in the present study (8.9 mg) was higher than Pula et al. (7.1 ± 1.42 mg), reflecting the longer surgical duration in our orthopaedic population (95.10 ± 6.29 min vs. 50.9 ± 7.65 min). Regarding block characteristics, Pula et al. achieved a lower maximal sensory level (T8 vs. T6) with faster two-segment regression, whereas in our study, DSCSA-guided dosing produced a controlled sensory block predominantly at T6, avoided excessive cephalad spread (T4 in 70% in conventional dosing), and showed prolonged two-segment regression, making it more suitable for longer procedures. Based on these findings, we recommend a slightly higher DSCSA-guided dose for successful completion of longer orthopaedic surgeries, while lower doses are sufficient for surgeries under 60 minutes, as in Pula et al. Importantly, despite the higher dose, our regimen maintained good haemodynamic stability, with fewer episodes of hypotension and bradycardia, demonstrating that DSCSA-based individualisation allows safe dose adjustment according to surgical duration.
The present study has limitations, including a small, homogeneous sample, single-centre design, lack of anatomical measurements in the control group, and absence of a validated non-linear dosing model. Larger, multicentric studies with representative populations and standardised follow-up are needed to confirm and extend these findings.
CONCLUSION
The study demonstrates that ultrasound-guided, DSCA-based dosing allows effective individualisation of intrathecal bupivacaine in elderly patients, resulting in a significant reduction in LA dose compared with conventional fixed dosing. Despite dose reduction, adequate sensory and motor blockade was consistently achieved, with improved haemodynamic stability in higher-risk elderly patients. These findings support DSCSA-guided dosing as a practical and safer approach to spinal anaesthesia in the elderly.
Presentation at conferences/CMEs and abstract publication
This manuscript was not presented at any conference.
Study data availability
The data for this original research may be requested with reasonable justification from the corresponding author and shall be shared upon request.
Disclosure of use of artificial intelligence (AI)-assistive or generative tools
The AI tools have not been utilised in the manuscript, except it has been used for grammar corrections.
Declaration of use of permitted tools
All figures and tables are freely available and not subject to copyright.
Conflicts of interest
There are no conflicts of interest.
Acknowledgement
Nil.
Funding Statement
Nil.
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