CNAP Accuracy in the Bariatric Surgery Population: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for CNAP Accuracy in the Bariatric Surgery Population (NCT01896206), a non-randomized na study. The primary outcome is The Absolute Difference in Mean Arterial Pressure Between the Arterial Catheter and the CNAP..

2. Clinical Question

3. Trial Design

This was a completed, non-randomized, single-group device study with no masking. The registry lists 21 participants and identifies bariatric surgery as the condition. The intervention was the CNAP monitor, used while participants underwent surgery. The study was not structured as a randomized allocation to competing treatment groups, and the listed design does not support interpreting the device groups as like-for-like randomized controls.

The study summary describes an evaluation of CNAP against invasive arterial-line blood pressure and two intermittently oscillometric non-invasive blood pressure devices. The registered primary endpoint is narrower: it is the absolute difference in mean arterial pressure between the arterial catheter and CNAP. The registry record does not give a separate primary endpoint for the two oscillometric devices, nor does it report numerical comparisons with them.

The primary endpoint timeframe is the duration of surgery, with an expected average of 2 hours. This is a monitoring study in an operative setting, rather than a comparison of clinical outcomes over a longer follow-up period. The record does not specify the actual duration of monitoring for individual participants or describe the timing and handling of paired readings.

The initial cohort included 21 patients, but the registry reports that no data were collected for one patient because the finger cuff was expired and that data for two other patients were lost during download to the electronic medical record system. The number included in the endpoint analysis is not reported in the registry record. The record also does not report the recruitment method, eligibility criteria, or the rationale for the planned enrollment. The study dates are listed as 2013-06 for start and 2013-12 for primary completion, and the status is completed.

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

Analysis population for the primary outcome, as defined in the registry: The initial study cohort included 21 patients; however, the finger cuff was expired in 1 patient, resulting in no data collection, and the data from 2 other patients were lost in the download to the electronic medical record system.

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.

6. Primary Endpoint

The registry lists the primary outcome as The Absolute Difference in Mean Arterial Pressure Between the Arterial Catheter and the CNAP., measured over Participants will be followed for the duration of surgery, an expected average of 2 hours..

The primary endpoint is the absolute difference in mean arterial pressure between the arterial catheter and the CNAP monitor, expressed in mmHg. Mean arterial pressure is a continuous measurement, so the endpoint is intended to describe the size of the discrepancy between the two devices rather than to classify participants into categories.

The registry specifies an absolute, rather than directional, difference. As its description explains, a CNAP reading 10 mmHg above or below the arterial catheter reading would contribute a difference of 10 mmHg either way. This expresses magnitude without distinguishing over-reading from under-reading. That choice can make the average discrepancy easier to describe as an error magnitude, but it removes information about the direction of any bias. A signed comparison would be needed to show whether one device tends to read higher or lower.

The endpoint is relevant to a device-accuracy question because it compares the monitor of interest with an invasive arterial catheter reference. The registry states that the endpoint is summarized as a mean with standard deviation. The mean would describe the average absolute discrepancy, while the standard deviation would describe how much the individual discrepancies vary around that average. Neither numerical summary is reported in the registry record. The record also does not state how readings were paired, how many measurements contributed per participant, or how repeated readings were accounted for.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary measure is continuous and is defined as a mean absolute difference in mmHg. A continuous outcome can take values across a measurement scale rather than belonging to a small set of categories. For this endpoint, each difference represents the magnitude of the gap between CNAP and arterial-catheter mean arterial pressure readings, without a positive or negative direction. The registry record does not report the observed mean or standard deviation.

What the mean describes

In general, the mean is a measure of the central value of a set of observations. For an absolute-difference endpoint, it would describe the average size of the discrepancies included in the analysis. It does not describe every individual discrepancy, and it can conceal variation among observations. A reported mean is therefore more informative when accompanied by a measure of spread and, where appropriate, an interval describing uncertainty about the estimate.

What the standard deviation describes

In general, a standard deviation describes how dispersed observations are around their mean. A relatively small spread would indicate that discrepancies are more concentrated around the average; a larger spread would indicate more variability. The registry specifies standard deviation as the dispersion measure but supplies no numerical value. It also does not make clear whether the unit of analysis is an individual participant summary, a paired reading, or another summary of repeated readings. That distinction matters because repeated measurements within a participant are not necessarily independent observations.

Absolute and signed differences

Using an absolute difference focuses on the magnitude of disagreement. It avoids cancellation that can occur when positive and negative signed differences are averaged, but it also discards the direction of each discrepancy. The registry gives an example in which readings 10 mmHg above or below the arterial catheter value both count as a difference of 10 mmHg. This endpoint can describe how large the gaps are on average, but it cannot by itself show a consistent tendency for CNAP to read high or low. No signed-difference result is reported.

Precision and confidence intervals

In general, a confidence interval expresses the uncertainty around an estimated quantity under the assumptions of the analysis. A narrower interval indicates greater precision than a wider interval, while the interval's location indicates the range of values compatible with the data and method. The registry record does not report a confidence interval for the primary endpoint. Without an estimate and an interval, readers cannot assess either the magnitude or the precision of the device comparison from the posted information.

Paired measurements and repeated observations

Device-accuracy studies commonly compare readings taken in relation to the same underlying clinical situation. The registry's endpoint names the arterial catheter and CNAP, but it does not report how readings were paired in time, whether measurements were summarized within each participant, or how repeated measurements were incorporated. In general, if measurements from one participant are correlated, treating every reading as independent can understate uncertainty. Appropriate analysis depends on the data structure and the prespecified method; neither is described in the registry record.

Over-dispersion and variability

In general, over-dispersion means that observed variability is greater than a statistical model assumes. For a continuous monitoring measure, extra variability can arise from differences between participants, changing conditions over time, measurement noise, or other sources. Accounting for variability is important because it affects uncertainty estimates. The registry record does not report an over-dispersion assessment or enough detail to determine whether this issue was considered. This is a general statistical point, not a claim about the study's data.

Analysis plan and interpretation

The registry lists one outcome measure and zero analyses. It does not report a statistical analysis plan, a test procedure, an adjustment method, or a confidence-interval method. Nor does it report a numerical result for the registered mean endpoint. It is therefore not possible to determine whether the observed measurements met any statistical or clinical criterion for acceptable agreement. The endpoint's definition indicates what the investigators intended to measure, not what value they observed.

General statistical interpretation also distinguishes estimation from hypothesis testing. An estimate describes the magnitude of an effect or difference; a test addresses compatibility with a particular null hypothesis under stated assumptions. A p-value, if reported, would not by itself indicate the size or clinical importance of a difference. No p-value or inferential result for this endpoint is reported in the registry record, so neither estimation nor hypothesis-testing conclusions can be drawn from the posted results.

10. Confidence Intervals for Each Arm

No two-arm comparison is shown for this trial, but each arm's mean can be given with a t-based 95% confidence interval computed from the posted mean, spread and number of participants.

R code and output

arm <- c("CNAP Monitor")
n   <- c(18)
m   <- c(7.7)
sp  <- c(4.9)
sd  <- sp      # posted standard deviations
for (i in seq_along(n)) {
  se <- sd[i] / sqrt(n[i]); h <- qt(0.975, n[i] - 1) * se
  cat(sprintf("CARM|%s|%d|%.3f|%.3f|%.3f|%.3f\n", arm[i], n[i], m[i], sd[i], m[i] - h, m[i] + h))
}

CARM  CNAP Monitor  18  7.700  4.900  5.263  10.137

11. Multiplicity, Missing Data and Analysis Populations

The registry lists one outcome measure and zero analyses. It does not report whether additional outcomes were assessed outside the registry entry, whether multiple comparisons were planned, or whether any adjustment for multiplicity was used. The summary mentions comparisons with two intermittently oscillometric non-invasive blood pressure devices, but the registered primary endpoint is the absolute difference between CNAP and the arterial catheter. Numerical results for the other comparisons are not reported in the registry record.

In general, multiplicity arises when several outcomes, comparisons, or analyses are considered. The more opportunities there are to identify a result as noteworthy, the more important it is to understand which analyses were planned and how they were interpreted. An adjustment may be used in some settings, but the appropriate approach depends on the study's aims and analysis plan. No such plan or adjustment is reported, so it would be speculative to describe the study as having used one.

The registry reports three sources of unavailable participant data: one expired finger cuff that led to no data collection and data from two other patients lost during download to the electronic medical record system. The final analysis population, handling of missing measurements, and any sensitivity analysis are not reported in the registry record. The outcome result itself is also not posted. Consequently, it is not possible to assess how missing data affected the estimate or whether any analysis addressed that concern.

In general, an analysis population should be defined clearly, particularly when data are missing or measurements are repeated. The registry record does not state which participants or readings would have been included in the primary analysis, and it does not report a prespecified rule for excluding incomplete data. No conclusion should be drawn about the analysis population beyond the initial cohort and the stated data losses.

12. Safety Results

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

Other (non-serious) adverse events

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

The registry lists 21 participants at risk in the CNAP Monitor group. It records zero serious adverse events and zero other adverse events in that group. These figures describe what is entered in the registry record; they do not provide details about event definitions, event collection, attribution, or follow-up procedures.

The deaths field is null, so deaths are not reported in the registry record. A null entry should not be read as a reported zero. The record also lists no serious adverse event terms. This does not establish that every type of event was systematically assessed or that no event occurred outside the recorded categories.

These safety entries should be interpreted separately from the device-accuracy endpoint. They do not indicate the mean difference between CNAP and the arterial catheter, and they do not show whether the monitor's readings agreed with the reference. The listed group is a single CNAP-monitor group, not a randomized safety comparison. The registry record does not report a comparator-group safety analysis or further event detail.

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

The study addresses a focused measurement question: the absolute difference in mean arterial pressure between a continuous non-invasive CNAP monitor and an arterial catheter during bariatric surgery. The registry describes the intended endpoint and specifies that it was to be summarized as a mean and standard deviation. The posted record does not include those numerical results, so the magnitude of the measured difference and its variability remain unknown.

This absence of a result limits what can be concluded. It does not show that CNAP and the arterial catheter agreed, and it does not show that they disagreed. It also does not establish whether any observed discrepancy was precise, consistent, or important in the surgical monitoring context. Those judgments would require reported estimates and adequate information about the measurement and analysis methods.

The absolute endpoint would describe the magnitude of discrepancies without showing their direction. Even if a mean absolute difference were available, it would not alone indicate whether CNAP tended to read above or below the catheter. The registry record does not report a signed difference or another analysis that resolves direction.

The study is not a randomized comparison of separate groups, and the registry lists a single group monitored with CNAP. Its stated comparator for the primary endpoint is an arterial catheter. The summary's reference to two intermittently oscillometric devices does not supply numerical evidence about those comparisons. The record also does not report clinical outcomes that could be used to assess whether monitoring changed care or outcomes.

Accordingly, the registry supports description of the study question, setting, design, and reported data losses, but not a conclusion about the monitor's accuracy. The missing endpoint result should not be interpreted as a favorable or unfavorable finding. It is simply a limitation of the information reported in the registry record.

14. Why This Trial Matters Statistically

Statistically, the study illustrates the distinction between defining a measurement question and reporting evidence that answers it. The registry provides a specific primary endpoint: the absolute difference in mean arterial pressure between CNAP and the arterial catheter, measured in mmHg. That definition clarifies the intended quantity, but without a numerical estimate and measure of uncertainty, readers cannot evaluate the observed agreement or variability.

The choice of absolute differences also highlights how endpoint construction affects interpretation. It captures discrepancy magnitude while discarding direction. A separate signed comparison would answer a different question about whether readings tended to be higher or lower. Reporting both kinds of information can be useful in measurement studies, but the registry record does not report a signed result.

The study also underscores the importance of transparent data-flow reporting. The initial cohort included 21 patients, while one patient had no data collection because of an expired finger cuff and data from two others were lost during download. The final analysis population and the handling of these missing data are not reported. In a device comparison, such details help readers understand which observations support an estimate and how representative they may be.

Finally, the design and analysis details matter for repeated monitoring data. The registry identifies a single-group, non-randomized study, but does not describe the pairing, frequency, or statistical handling of readings. These details affect how uncertainty should be estimated when observations from a participant may be related. The record's lack of a numerical result and analysis description limits statistical interpretation, even though the measurement question itself is clearly stated.

15. 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 most important limitation of the registry record for statistical interpretation is that it does not report the primary endpoint result. The mean absolute difference and standard deviation are absent, as are confidence intervals and inferential statistics. The stated measurement question can therefore be described, but the observed magnitude and precision cannot be assessed.

The analysis population is unclear. The initial cohort included 21 patients, one of whom had no data collection because the finger cuff was expired; data from two other patients were lost during download to the electronic medical record system. The registry does not report how many participants or observations entered the primary analysis or how these missing records were handled.

The record does not explain how measurements were paired in time, how many readings were collected, or whether repeated readings were summarized per participant or modeled as repeated observations. These details are important for interpreting a device comparison and estimating uncertainty. No analysis method is reported, and the registry lists zero analyses.

The absolute-difference endpoint does not preserve the direction of disagreement. The registry does not report a signed comparison that could show whether CNAP readings tended to be above or below arterial-catheter readings. The summary refers to two intermittently oscillometric devices, but no numerical results for those comparisons are reported either.

Other study details, including eligibility criteria, participant characteristics, recruitment procedures, and the rationale for enrollment, are not reported in the registry record. The record also does not give further details about monitoring procedures or the handling of readings. These limitations should be kept distinct from the study's findings: the absence of reported information does not establish what the unreported data would have shown.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: CNAP Accuracy in the Bariatric Surgery Population — NCT01896206.
• Registry record last updated 2015-02-27; status last verified 2015-02.
• Results first posted 2015-02-27.
• 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.

19. Record Summary

NCT01896206, titled “CNAP Accuracy in the Bariatric Surgery Population,” is listed as completed. It is a non-randomized, unmasked, single-group device study with an enrollment of 21 participants undergoing bariatric surgery. The CNAP monitor was evaluated during surgery, with the primary endpoint defined as the absolute difference in mean arterial pressure between the arterial catheter and CNAP, in mmHg. The expected average follow-up duration was 2 hours.

The registry reports that one patient had no data collection because the finger cuff was expired and that data from two other patients were lost during download. It lists one outcome measure and zero analyses. No numerical primary endpoint result, analysis method, or final analysis population is reported in the registry record. The safety entries list zero serious and zero other adverse events among 21 at risk; deaths are not reported in the registry record.