Continuous TAP Blocks: Relative Effects of a Basal Infusion vs. Repeated Bolus Doses: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Continuous TAP Blocks: Relative Effects of a Basal Infusion vs. Repeated Bolus Doses (NCT02662023), a randomized phase 4 study. The primary outcome is Sensory Deficit (Cold Roller), Mid-axillary Line.

2. Clinical Question

3. Trial Design

The registry identifies this as a randomized, quadruple-masked, active-control study and lists the model as parallel. The official title, however, describes a split-body, crossover study. The arm descriptions also specify bilateral TAP catheters and opposite right-left assignments for bolus and basal administration. These details point to a within-person, side-based comparison, but the record does not explain how the design labels are reconciled or how the analysis accounted for the design.

One listed arm received the bolus regimen on the right and basal regimen on the left; the other received basal on the right and bolus on the left. Both regimens used ropivacaine 0.2%. The bolus schedule was two doses of 24 mL each at time point zero and 3 hours later. The basal schedule was 8 mL/h from time point zero for the following 6 hours. Further details for the opposite-side administration are not reported in the registry record.

The registry lists phase 4, postoperative pain, an enrollment of 24, and University of California, San Diego as sponsor. The study is marked completed. Its start is recorded as 2016-02 and primary completion as 2016-06. Eligibility criteria, recruitment setting, participant demographics, and the rationale for the enrollment target are not reported in the registry record. Although randomization and quadruple masking are recorded, the record does not describe the randomization procedure or identify the masked roles.

The endpoint comparison is organized by regimen, with 24 recorded in each arm. These arm counts should not be interpreted as evidence that the study enrolled separate parallel groups: the official title and bilateral side assignments describe a split-body comparison. The record does not state how participant-level pairing or side assignment was incorporated into the reported estimates.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: Since it's a split body study, right side of each participant's body was randomized to bolus or basal arm. Left side of same participant's body simultaneously received the opposite treatment of what the right side of the body was randomized to( for example: if right side of one participants body was randomized to Bolus doses then the left side of the same participant's body received ropivacaine administration as basal infusion).

6. Primary Endpoint

The registry lists the primary outcome as Sensory Deficit (Cold Roller), Mid-axillary Line, measured over 6 hours following the initiation of local anesthetic administration.

The primary endpoint is the distance of sensory deficit to cold, in centimeters, measured along the mid-axillary line at 6 hours following initiation of local anesthetic administration. A cold roller, described as a metal roller that can be brought to freezing, is moved along the line until the sensory deficit is identified; the distance is then measured. The measure captures the extent of an area with reduced cold sensation after local anesthetic administration.

Sensory mapping offers a way to assess whether local anesthetic administration reaches nerves supplying the abdominal wall. In this comparison, the measured distance is an indicator of the distribution of sensory change, not a direct measure of pain, patient comfort, surgical recovery, or medication use. Those outcomes are not identified as the primary endpoint in the registry record.

The registry lists four secondary sensory outcomes: cold-roller deficit at the mid-axillary line and transverse horizontal line, and von Frey hair deficit at the transverse horizontal line and mid-axillary line. Together, these measures assess sensory deficit using different testing methods or locations. Their posted results are not described in the record excerpt. The primary endpoint’s measurement details do not establish how reliable or clinically important a given distance is, and a threshold for a meaningful difference is not reported in the registry record.

7. Results Posted in the Registry

• Basal: mean 9.30 versus 9.20 on control; difference 0.10 (95% CI -2.93 to 3.13), standardized effect d = 0.02; z-test p = 0.948.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary endpoint is continuous: it records a measured distance in centimeters. The registry reports the mean and standard deviation for each regimen, followed by a mean difference and a confidence interval. These statistics summarize the center and variability of the recorded measurements and the estimated contrast between regimens.

What does the mean difference represent?

The reported contrast is basal relative to bolus. Its estimate of 0.1 cm expresses the difference between the arm means in the direction specified by that comparison. A positive value indicates a larger mean distance for basal in the posted summaries; a negative value would indicate a smaller mean distance. The estimate describes the observed contrast, but does not by itself show whether the difference is precise, clinically important, or causal under the study’s design.

What does the standard deviation describe?

The standard deviations, 5.6 cm for bolus and 5.1 cm for basal, describe variation in individual measurements around each arm’s mean. They are not confidence intervals and do not describe uncertainty in the mean difference directly. A standard deviation reflects the spread of observations, while a confidence interval reflects uncertainty about an estimated quantity under the statistical model. Considerable variation in individual measurements can coexist with a mean difference close to zero.

How should the confidence interval be read?

The 95% confidence interval for the mean difference extends from -2.93 to 3.13 cm. In general, a confidence interval gives a range of estimates compatible with the observed data and the assumptions used to construct it. It is not a statement that the true value has a 95% probability of falling within this particular interval. Its width conveys precision: a wider interval leaves more uncertainty about the magnitude and direction of the contrast.

What does the p-value add?

The unadjusted p-value is 0.948. In general, a p-value measures how unusual data at least as inconsistent with a null hypothesis would be if that hypothesis and the analysis assumptions were correct. It is not the probability that the null hypothesis is true, nor a measure of the probability that one regimen is better. A high p-value does not prove equal effects. The registry record does not report an adjusted p-value or the full statistical analysis plan.

Why does the design matter?

In general, a split-body design can compare treatments within a participant, potentially accounting for characteristics shared by the two sides. The corresponding analysis may need to reflect that pairing. The registry’s official title and arm descriptions indicate bilateral side-based treatment, whereas its model field says parallel. Although the reported group sizes and means are available, the record does not describe the analysis model, whether observations were treated as paired, or how side assignment was handled. The posted contrast should therefore be interpreted with that reporting limitation in mind rather than assuming a particular model.

The comparison is marked unadjusted. No covariate adjustment, repeated-measures model, or handling of side or period effects is described in the registry record. The difference between regimens may be summarized by the reported estimate, but the reader cannot assess from the record whether additional design features entered the analysis.

Why can over-dispersion matter?

In general, over-dispersion occurs when observed variability is greater than a statistical model expects. A model that understates variability may make uncertainty appear smaller than it is. This endpoint is continuous and the registry reports standard deviations; it does not report an over-dispersion assessment or a model for the measurement distribution. That general issue does not establish that over-dispersion occurred in this trial.

The mean and standard deviation are useful summaries, but they do not reveal the full distribution of distances. The registry record does not report medians, ranges, participant-level measurements, or diagnostic checks. Consequently, the shape of the measurements and the influence of unusually large or small values cannot be assessed from the summary statistics alone.

The record lists five outcome measures, including the primary endpoint and four secondary sensory outcomes, and reports no analysis entries. It does not describe an analysis population, missing-data method, multiplicity adjustment, or prespecified endpoint hierarchy beyond identifying the primary outcome. Those omissions matter when assessing how the estimate was selected and how confidently it can be interpreted alongside other outcomes.

10. Confidence Intervals and Precision

The estimate of the basal-minus-bolus mean difference is 0.1 cm. Its 95% confidence interval ranges from -2.93 to 3.13 cm. Because the interval crosses zero and includes values on both sides, the posted data do not determine the direction of the underlying difference with precision. The interval also includes contrasts larger in magnitude than the point estimate, so a near-zero estimate should not be treated as a precise finding of no difference.

In general, confidence intervals show the uncertainty associated with an estimate under the model used to calculate them. Greater precision is reflected in a narrower interval; less precision is reflected in a wider one. Here, the interval leaves room for a range of possible effects on the measured distance. Whether any part of that range would be clinically important cannot be determined because the registry record does not report a threshold for a meaningful difference.

The standard deviations of 5.6 cm for bolus and 5.1 cm for basal indicate substantial spread in individual observations relative to the small difference between the means. They do not substitute for the confidence interval. The registry record does not provide participant-level data or a detailed description of the interval calculation, including whether within-participant pairing was incorporated. The discrepancy between the parallel model label and split-body description further limits independent assessment of the uncertainty calculation.

The unadjusted p-value of 0.948 is consistent with the absence of strong evidence against a zero-difference null under the analysis used. It does not resolve the uncertainty expressed by the interval and is not evidence of equivalence. The precision of conclusions about the primary endpoint is limited by the interval’s breadth and by the lack of reported detail about the analysis model.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The sample-size figures are planning illustrations based on the observed effect, not the trial’s original design calculation. The record states that the illustration uses a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is set as a stated fraction of the observed effect. The trial’s own original sample-size assumptions and justification are not reported in the registry record.

These figures are participants per arm, not total events. The 80% power column describes the planned number of participants per arm for an analysis designed to have an 80% chance of detecting the specified effect under the stated assumptions, if that effect is real. The 90% power column uses a higher target chance of detecting that same specified effect. Power is a property of a proposed design under assumptions, not a measure of what happened in the completed trial.

The rows indicate planning for 100% of the observed effect, 75% of the observed effect, and 50% of the observed effect. As the assumed effect becomes smaller, the required number of participants per arm increases. The figures for the smaller fractions are therefore much larger than those for planning around the full observed effect. This illustrates how a small target difference requires more information to distinguish from random variation.

These values are not observed power. They are hypothetical planning estimates based on the observed difference, which is close to zero, and should not be read as evidence about the probability that the completed trial detected a true effect. The table and curve can help compare planning scenarios, but they do not establish that the observed effect is the true effect or that the registry used these assumptions when designing the study.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = 0.100; pooled standard deviation σ = 5.356.
2. n = 2 × (1.96 + 0.842)² × 5.356² / 0.100² = 45,029.0, rounded up to 45,030 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
delta <- 0.100000000000001                      # observed difference in means
sigma <- 5.35583793630838                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(2662023)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 45,030 (80% power), 60,282 (90% power)
 75% of observed effect: n per arm = 80,052 (80% power), 107,167 (90% power)
 50% of observed effect: n per arm = 180,117 (80% power), 241,125 (90% power)
power.t.test: 45030 
Simulated power at n = 45,030 per arm: 0.800 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Multiplicity, Missing Data and Analysis Populations

The registry identifies one primary endpoint and four secondary sensory outcomes: cold-roller sensory deficit at the mid-axillary line and transverse horizontal line, and von Frey hair sensory deficit at the transverse horizontal line and mid-axillary line. It lists five outcome measures in total but no analysis entries. The posted numerical results described for this comparison concern the primary cold-roller, mid-axillary endpoint.

When a study tests multiple outcomes, the chance of at least one apparently notable result can increase. In general, multiplicity methods may address that issue by adjusting thresholds or by specifying an order in which outcomes are tested. The registry record does not report whether such a method was planned or used, whether secondary outcomes were formally tested, or whether any hierarchy beyond the primary endpoint was prespecified. No adjustment should be assumed.

The record also does not report missing-data counts or methods, the number analyzed for each endpoint, exclusions after enrollment, or an analysis-population definition. Thus it is not possible to establish from the posted record whether all enrolled participants contributed to the primary summary or how any unavailable measurements were handled. The listed arm counts alone do not answer those questions.

Further details of the statistical analysis plan are not reported in the registry record. In particular, the record does not specify whether the analysis accounted for the bilateral, split-body structure, how side assignment was treated, or whether any period or carryover effects were considered. These details are relevant to understanding the model behind the uncertainty interval and p-value. The registry’s parallel model label and split-body title and arm descriptions do not resolve how the analysis was conducted.

14. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

The serious adverse event figures list 24 at risk in the bolus group and 24 at risk in the basal group. The record lists zero participants affected by serious adverse events in each group. It also lists zero participants affected by other adverse events in each group, and zero deaths in each group.

These are counts as recorded, not estimates of comparative risk. Zero recorded events does not establish that an event is impossible or that the regimens have no safety concerns. The registry record does not report the safety observation period, event definitions, event collection procedures, or whether adverse events were judged related to treatment. It also does not provide further descriptive information about events beyond the listed figures; no serious adverse event terms are listed.

The bilateral, split-body description and parallel model label make the relationship between the recorded groups and participants unclear. The adverse-event counts should therefore be read as registry group summaries, without inferring a separate set of participants for each side or regimen.

15. What the Result Does, and Does Not, Mean

The posted primary summaries show mean cold sensory-deficit distances of 9.2 cm for bolus and 9.3 cm for basal. The estimated mean difference is 0.1 cm in favor of a larger measured distance for basal, but the confidence interval extends from -2.93 to 3.13 cm. The data therefore support a description of similar observed means, while leaving considerable uncertainty about the underlying difference.

That description is limited to the measured sensory endpoint at 6 hours following initiation of local anesthetic administration. It is not a direct result about postoperative pain intensity, duration of pain relief, functional recovery, or medication use. The registry’s summary presents a rationale involving postoperative pain and continuous TAP blocks, but those broader outcomes are not the primary endpoint reported in the posted results.

The result does not establish that the regimens are equivalent. Equivalence requires a prespecified margin defining what range of differences is acceptable and an analysis showing that the uncertainty interval lies within that margin. Such a margin is not reported in the registry record. Nor does the high unadjusted p-value show that the regimens have the same effect or rule out meaningful differences.

The endpoint is a measure of sensory distribution, and the record does not state what size of difference in centimeters would matter to patients or clinicians. The four secondary sensory outcomes are identified, but their results are not reported in the posted summaries. Conclusions about other sensory tests or patient-centered outcomes cannot be drawn from the primary comparison alone.

Interpretation also depends on the study structure. The official title and arm descriptions indicate a split-body design, but the model is labeled parallel and the analysis details are not reported. Without knowing how the analysis handled the paired sides, the uncertainty calculation cannot be fully evaluated. The soundest reading is therefore narrow: the recorded group means are close, the estimated difference is small, and the interval remains compatible with differences in either direction.

16. Why This Trial Matters Statistically

This trial raises an important statistical issue: the design description and analysis label are not fully aligned. A split-body comparison uses bilateral treatment assignments, while a parallel-group analysis ordinarily describes distinct groups. The record contains both a parallel model label and a split-body title, with arm descriptions assigning bolus and basal regimens to opposite sides. Making the analysis structure explicit is essential because it determines what information contributes to the estimated difference and its uncertainty.

The primary contrast is a mean difference, but its interpretation depends on the measurement and design. The outcome is a sensory distance, not a direct report of pain. A careful statistical reading keeps the measurement-specific finding separate from the broader clinical motivation for studying TAP blocks. This prevents a sensory mapping result from being treated as evidence about outcomes that were not measured or reported as primary.

The interval also illustrates why a near-zero point estimate is not the same as a precise null finding. The estimate is 0.1 cm, but the 95% confidence interval extends from -2.93 to 3.13 cm. Without a clinically meaningful threshold, an equivalence framework, and clear analysis details, the interval cannot be translated into a conclusion that any possible difference is unimportant.

Finally, the record demonstrates the value of reporting analysis populations, missing-data handling, multiplicity plans, and design-specific models. Such details help readers distinguish a descriptive summary from a fully assessable inferential analysis. Their absence does not invalidate the posted measurements, but it narrows what can be concluded from them.

17. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record does not report participant characteristics, eligibility criteria, the surgical setting, or a detailed account of recruitment. Generalizability to particular patient groups or procedures therefore cannot be assessed from the record. It also does not report the original sample-size rationale, so the intended precision or design target cannot be evaluated against the completed study.

The design description is internally difficult to interpret: the model is labeled parallel, but the official title says split-body crossover and the arm descriptions assign the regimens to opposite sides with bilateral catheters. The record does not explain the randomization process, the masking procedures, or the statistical treatment of paired sides and side assignment. It also does not describe period or carryover considerations.

The results are limited to arm means, standard deviations, a mean difference, confidence interval, and unadjusted p-value for the primary endpoint. Participant-level data, distribution summaries, analysis diagnostics, and a detailed model description are not reported. The record also does not report a clinically meaningful threshold for the sensory-distance measure, making clinical interpretation of the interval uncertain.

The four secondary outcomes are named but their results are not included in the posted summary, and no analysis entries are listed. Missing-data counts, analysis populations, and multiplicity procedures are not reported. Safety figures include zero affected participants in the listed categories, but event definitions and observation time are not reported. These limitations constrain interpretation of both the primary estimate and the wider evidence from the trial.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Continuous TAP Blocks: Relative Effects of a Basal Infusion vs. Repeated Bolus Doses — NCT02662023.
• Registry record last updated 2021-03-18; status last verified 2021-02.
• Results first posted 2021-03-18.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

21. Record Summary

The registry record describes a completed phase 4 randomized, quadruple-masked, active-control study sponsored by University of California, San Diego. Enrollment is listed as 24, with a study start of 2016-02 and primary completion of 2016-06. The primary outcome is cold-roller sensory deficit measured along the mid-axillary line at 6 hours following initiation of local anesthetic administration.

The comparison uses ropivacaine 0.2% delivered through bilateral TAP catheters as either repeated bolus doses or a basal infusion. The official title describes a split-body crossover design, while the model field says parallel. The posted primary means are 9.2 cm for bolus and 9.3 cm for basal; the mean difference is 0.1 cm, with a 95% confidence interval from -2.93 to 3.13 cm and an unadjusted p-value of 0.948. The record does not report detailed analysis methods, missing-data handling, or results for the listed secondary outcomes.