Effect of Location of Tetanic Stimuli on Photoplethysmogram: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Effect of Location of Tetanic Stimuli on Photoplethysmogram (NCT03648853), a na na study. The primary outcome is Photoplethysmography (PPG).

2. Clinical Question

The study asks whether the location of a tetanic stimulus is associated with a change in photoplethysmography (PPG) signals in patients under general anesthesia. The registry describes the stimulus as causing vasoconstriction, which can be recorded as a decrease in the PPG AC/DC signal. The endpoint is the maximum decrease from baseline following stimulation.

The clinical context is the assessment of blood-vessel tone while patients are anesthetized. The registry describes baseline AC/DC as a measure of relative vessel tone and states that higher values indicate greater vasodilation. The outcome is therefore a physiological signal response to a stimulus, rather than a patient-reported outcome or a clinical event.

The planned study enrolled 12 participants and used one intervention group; all participants received the same intervention. Although the title and brief description focus on stimulus location, the registry record does not identify the specific locations tested or give results by location. It therefore does not establish from the posted information whether responses differed according to stimulus location.

The central statistical question is how the PPG signal changed after tetanic stimulation, and whether the response can be described with enough precision to characterize it. The registry record lists one continuous primary outcome, measured in arbitrary units, with a mean and standard deviation specified as the summary format. However, it reports no outcome values, estimates, confidence intervals, or analyses. The direction and magnitude of the observed response, and any uncertainty around it, are not reported in the registry record.

3. Trial Design

This is a completed, single-group study sponsored by the University of California, San Francisco. The registry lists the phase as NA, allocation as NA, and masking as none. Enrollment is recorded as 12. All participants received the same intervention, identified as a tetanic stimulus and classified as a device intervention. There is no separate control group described.

The registry dates the study start to 2018-09-01 and primary completion to 2018-11-30. These dates describe the record’s study timeline; they do not provide the timing of each participant’s measurement or establish how long participants were followed. The study status is completed.

The design does not support a randomized, between-group comparison: allocation is listed as NA, and the record describes a single group. The title refers to stimulus location, but the registry does not state how many locations were used, what those locations were, or how participants were assigned to them. It also does not explain whether each participant received stimulation at one location or more than one. Those details are not reported in the registry record.

The primary outcome is continuous, with the mean and standard deviation designated as its summary measures. It concerns the maximum decrease in PPG AC/DC from baseline after stimulation, with a stated measurement timeframe of 1-2 minutes. The description says measurements were collected immediately before stimulation and for 2 min afterward. No analysis results are recorded, and the registry lists zero analyses. The study’s protocol, any prespecified statistical analysis plan, and the rationale for the enrollment target are not reported in the registry record.

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.

6. Primary Endpoint

The registry lists the primary outcome as Photoplethysmography (PPG), measured over 1-2 minutes.

The primary endpoint is the change in the photoplethysmography (PPG) signal following a tetanic stimulus. It is measured in arbitrary units. The registry defines the outcome as the maximum decrease in AC/DC from baseline. AC/DC is described as a relative measure of blood-vessel tone: higher baseline values indicate a more vasodilated state. The registry’s rationale is that tetanic stimulation causes vasoconstriction, which can be reflected as a decrease in the PPG signal.

Measurements were collected immediately before the stimulus to establish baseline and for 2 min afterward. The registry states that the maximum decrease typically occurs 30-60 seconds after stimulation, while the outcome timeframe is listed as 1-2 minutes. It also notes that AC/DC values return to baseline within a few minutes after stimulation. These statements describe the measurement rationale; they are not reported study results.

Using the maximum decrease focuses the endpoint on the strongest recorded signal response during the post-stimulation observation period. A baseline-referenced change can help distinguish the response from the participant’s initial signal level, which the registry says can vary from close to 0 to above 10 in anesthetized patients. No participant-level values, group mean, standard deviation, or estimate of the change are reported in the registry record. Consequently, the observed response and its precision cannot be assessed from the posted results.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry describes one continuous primary outcome: the maximum decrease in PPG AC/DC from baseline after tetanic stimulation. The specified summary format is a mean with a standard deviation. No analysis results are reported, so the statistical methods below explain how these measures are generally interpreted rather than describing an analysis documented for this study.

What does a mean describe?

For a continuous measure, the mean describes the central value across observations included in an analysis. In this study, a mean change would summarize the maximum decrease in the PPG signal from baseline among the analyzed observations. It would not, by itself, show whether all participants had similar responses. A few unusually large or small responses can influence a mean, and the pattern of individual values matters when judging whether the mean is representative. The registry does not provide individual observations or a reported mean.

What does the standard deviation describe?

The standard deviation describes the spread of observations around their mean. A smaller standard deviation indicates observations are more closely clustered, while a larger one indicates more variability. It is a description of variation among observations, not a direct measure of how precisely the mean is estimated. Precision of a mean is commonly conveyed with a confidence interval, which reflects uncertainty in the estimate under the analysis assumptions. No standard deviation or confidence interval is reported in the registry record.

Why does the measurement window matter?

The endpoint is defined as a maximum decrease after stimulation, and the registry says the maximum decrease typically happens 30-60 seconds after the stimulus. Measurements were collected immediately before stimulation and for 2 min after it; the listed timeframe is 1-2 minutes. A maximum is dependent on how and when measurements are taken. The observation window needs to be applied consistently, because a different window could capture a different point in the signal’s response. The registry does not give further detail about the measurement schedule, signal processing, or how the maximum was selected.

How should a baseline-referenced change be understood?

A baseline-referenced endpoint compares a post-stimulation signal with the pre-stimulation value. This can account for differences in initial signal level when describing a response. The record notes that baseline AC/DC can vary substantially in anesthetized patients and that higher values correspond to greater vasodilation. The outcome is a maximum decrease from each baseline, but the registry does not report how baseline values were summarized or whether any statistical adjustment was used. A within-participant change is not the same as a controlled treatment effect: in a single-group study, change over time can also reflect influences other than the intervention.

What can a confidence interval add?

In general, a confidence interval gives a range of values compatible with the observed data and the method used to calculate it. It helps readers assess precision alongside the point estimate. It does not ensure that a result is free from bias, and it does not capture limitations caused by a study design that lacks a control group. The registry record reports no interval estimates, so precision cannot be evaluated from its posted results.

What do the planning assumptions mean?

The registry describes a sample-size planning illustration based on the observed effect. Its stated assumptions are a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. The planning target is described in terms of detecting a stated fraction of the observed effect. These are assumptions for a hypothetical planning calculation, not evidence about the actual design or conduct of this single-group study. The record does not report the calculated planning sizes in the registry record of this record. Such calculations are planning illustrations, not observed power, and the two-arm equal-allocation assumption does not match the described single-group design.

More generally, sample-size planning depends on the outcome variability, the effect considered important to detect, the desired error rates, and the analysis design. The registry does not report the study’s own sample-size justification or indicate that the planning illustration formed part of the original protocol. It would therefore be inappropriate to infer the study’s design rationale from that illustration.

What analysis details are unavailable?

The registry lists zero analyses. It does not report a statistical model, hypothesis test, adjustment for covariates, handling of repeated measurements, or treatment of outliers. It also does not report whether any interim analyses were planned or conducted. These details matter because analysis choices can affect estimates and their uncertainty. In the absence of reported results and methods, the endpoint definition and summary format can be described, but an inferential analysis cannot be reconstructed from the record.

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("Intervention")
n   <- c(12)
m   <- c(3.3)
sp  <- c(1.6)
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  Intervention  12  3.300  1.600  2.283  4.317

11. Multiplicity, Missing Data and Analysis Populations

The registry lists one primary outcome measure and zero analyses. It does not report a statistical analysis plan, hypothesis tests, or any adjustment for multiple comparisons. Since the record does not identify the stimulus locations or describe location-specific analyses, it is not possible to determine whether multiple comparisons were planned or performed. Any multiplicity strategy is not reported in the registry record.

The registry does not describe missing outcome data, the number of participants with evaluable measurements, exclusions from analysis, or reasons for missing measurements. Enrollment is recorded as 12, but that figure alone does not establish how many observations contributed to an analysis. The analysis population is not described, and the record does not say whether all enrolled participants were included in the endpoint summary.

There is also no reported method for handling missing values, incomplete signal recordings, or measurements outside the stated observation window. No sensitivity analysis is reported. These omissions limit assessment of whether the available data represent all enrolled participants and whether missingness could have influenced a result. Because no outcome estimates are posted, it is not possible to evaluate the effect of these uncertainties on a finding.

12. 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 registry reports zero serious adverse events among 12 at risk and zero other adverse events among 12 at risk. It also reports zero deaths. No serious adverse-event terms are listed. These are the counts recorded for the intervention group; they should be read as registry-reported event counts, not as proof that no unrecorded or unreported events occurred.

The record does not describe the adverse-event collection period, definitions, severity grading, or how events were assessed. It also does not provide narrative safety details. The reported figures therefore give a limited account of events in this study and do not establish a rate that can be compared with another population or study. With no control group described, the record does not support a comparative safety assessment. These figures concern event reporting and do not determine the PPG endpoint result.

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

The record documents a completed study intended to examine the PPG response to tetanic stimulation in patients under general anesthesia. It defines the primary outcome as the maximum decrease in AC/DC from baseline and gives the measurement timeframe and a physiological rationale for using that signal. These details clarify what investigators intended to measure.

They do not reveal the observed result. No mean, standard deviation, confidence interval, or analysis result is reported in the registry record. It is therefore not possible to say whether the signal decreased in the enrolled participants, how large any decrease was, or how much responses varied. The record also does not provide results by stimulus location, even though location is central to the study title.

The design is described as a single group in which all participants received the same intervention. Allocation is listed as NA and masking as none. Without a separate control group or reported location-specific comparison, the posted information cannot establish that a signal change was caused by stimulation or that one location produced a different response from another. The specific locations and the way the stimulus was administered across them are not reported.

The endpoint is a physiological measurement rather than a clinical outcome, and the registry gives no threshold for interpreting a change as clinically meaningful. A physiological signal can be useful for characterizing a response, but its meaning depends on measurement quality, reproducibility, and how it relates to outcomes of interest. Those matters are not addressed by the posted results.

The appropriate conclusion is limited: the registry describes the question, intervention, and endpoint, but it does not report findings that answer the question. The absence of a reported result is not evidence that there was no response; it means the magnitude, direction, and precision of the response cannot be assessed from this record.

14. Why This Trial Matters Statistically

This study is statistically interesting because it focuses on a short-lived physiological response measured against a participant’s baseline. The registry explains why the maximum PPG decrease was selected and gives the expected timing of that response. This makes the endpoint definition and measurement window central to interpreting any estimate: an endpoint based on a maximum depends on when measurements are taken and how the signal is processed.

The record also illustrates the distinction between observing a change and estimating a causal effect. A single-group before-and-after measurement can describe a signal’s movement over time, but without a control condition it cannot separate the intervention’s contribution from other influences. The title raises a further comparative question about stimulus location, yet the registry does not specify the locations or provide location-based results. A precise comparison would require those design and analysis details.

Another statistical issue is that baseline signal levels can vary. The registry notes that AC/DC values in anesthetized patients can range from close to 0 to above 10, and that higher values indicate a more vasodilated state. A baseline-referenced change is relevant in that setting, but readers would need the actual distribution of baseline values and response measurements to assess variability and precision. These results are not reported.

The record’s planning assumptions also contrast with its described design: the illustration uses equal allocation between two arms, while the trial is listed as single-group. This is a reminder that a planning calculation is meaningful only in relation to its assumptions and intended design. The registry record does not report a study-specific sample-size justification.

Ultimately, the statistical value of the record is in its endpoint and design description. Without posted estimates or methods, it cannot establish the size or reliability of the response or answer whether location matters.

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 posted record is that it contains no outcome estimates or analysis results. Although it identifies a primary endpoint and reports enrollment, it does not provide a mean, standard deviation, confidence interval, or test result. The response’s direction, magnitude, variability, and precision are therefore unknown.

The study is described as single-group, with all participants receiving the same intervention. Allocation is NA, and no control group is described. A change from baseline in this design would not, by itself, distinguish the effect of stimulation from other changes during the measurement period. Masking is listed as none, and the record does not describe any safeguards against measurement or assessment bias.

The location question cannot be evaluated from the posted details. The specific stimulus locations, the number of locations, and the approach to assigning or comparing locations are not reported. The record also does not explain whether stimulation at multiple locations was repeated within participants. Thus the design underlying the title’s location comparison is unclear.

Measurement details are limited. The endpoint definition and collection window are given, but signal-processing procedures, measurement frequency, handling of artifacts, and selection of the maximum are not described. The record does not report missing measurements, exclusions, analysis populations, or methods for handling incomplete data.

Finally, no protocol, study-specific sample-size justification, statistical analysis plan, or interim-analysis information is reported in the registry record. The adverse-event counts are reported without details about collection periods or assessment procedures. These gaps restrict assessment of both the endpoint analysis and the completeness of safety reporting.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Effect of Location of Tetanic Stimuli on Photoplethysmogram — NCT03648853.
• Registry record last updated 2020-05-22; status last verified 2020-05.
• Results first posted 2020-04-30.
• 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

The registry record describes NCT03648853, a completed, single-group study sponsored by the University of California, San Francisco. Enrollment is recorded as 12, and the intervention is a tetanic stimulus. The study concerns the effect of stimulus location on PPG signals during general anesthesia. Allocation is listed as NA and masking as none.

The primary continuous outcome is the maximum decrease in PPG AC/DC from baseline, measured in arbitrary units. The registry describes measurement immediately before stimulation and for 2 min afterward, with a stated timeframe of 1-2 minutes. It lists one outcome measure and zero analyses.

No outcome estimates or analysis results are reported. The record reports zero serious adverse events, zero other adverse events, and zero deaths. The specific stimulus locations, analysis methods, missing-data handling, and study-specific sample-size justification are not reported in the registry record.