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Research Article

Normalized Pulse Volume as a Superior Predictor of Respiration Recovery and Quantification of Nociception Anti-nociception Balance Compared to Opioid Effect Site Concentration: A Prospective, Observational Study

[version 1; peer review: 1 approved, 1 not approved]
PUBLISHED 27 Mar 2024
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Abstract

Background

Quantifying pain and the balance between nociception and anti-nociception (NANB) in sedated patients is challenging. Traditional opioid titration methods overlook individual differences, while existing indices like the Noxious Stimulation Response Index (NSRI) lack correlation with effect-site concentration (Ce). The Normalized Pulse Volume (NPV), used in polygraphs, has potential for pain quantification but is underexplored. This study aimed to assess NPV’s efficacy as a pain monitoring tool compared to Ce and to explore its potential in various clinical settings.

Methods

The study included 39 patients undergoing surgery under total intravenous anesthesia from July 2013 to May 2014. Selection criteria were an American Society of Anesthesiologists physical status classification system (ASA score) of 1 or 2 and surgeries with minimal fluid resuscitation or blood loss. Exclusion criteria were significant posture changes, massive hemorrhage, and high perfusion index variation. NPV and Ce were measured using the Masimo SET adult SpO2 sensor.

Results

Out of 39 patients, 9 were excluded. NPV at recovery of spontaneous respiration (RoR) was 2.62 (95% CI: 2.26–2.98) with a coefficient of variation (CoV) of 36.3%, while total Ce was 1.48 ng/ml (95% CI: 1.14–1.84) with a CoV of 62.4%. NPV showed a narrower CoV than Ce (p < 0.05, 1.93*10−5), indicating less variability. NPV outperformed Ce in predicting RoR, suggesting a more accurate reflection of NANB balance. Its superiority in stable measurement underlines its potential as a reliable pain indicator. The study’s limitations include temporal differences in NPV and Ce calculations, affecting comparative analysis.

Conclusion

NPV demonstrates promise as an objective, reliable indicator of pain or NANB, showing a strong correlation with Ce. Its application could improve pain assessments in clinical settings, optimizing patient care and analgesic administration. Future research should integrate NPV with other vital signs for a comprehensive pain monitoring system.

Keywords

Nociception, anti-nociception, normalized pulse volume, objective pain monitoring

Introduction

Objective quantification of pain, or more specifically the balance between nociception and anti-nociception (NANB), has long been a challenging task, especially in sedated or anesthetized patients.1 The absence of a universally accepted, objective pain monitoring system renders titration of analgesics, particularly opioid analgesics, highly challenging in diverse clinical settings—ranging from the operating room under general anesthesia to intensive care units, and extending to special populations like pediatrics, geriatrics, unconscious patients, non-native speakers, and those undergoing various forms of local anesthesia such as epidural, spinal, and nerve block procedures.25

Traditionally, the titration of opioid analgesics was empirical; however, several contemporary medical facilities now rely on drug pharmacokinetic simulations to estimate the effect-site concentration (Ce).69 While these simulations provide a standardized approach, they often overlook individual variations in drug sensitivity, body composition, and distribution volume. Furthermore, intraoperative fluid resuscitation or blood loss can introduce additional variables that the simulations may not account for. This can inadvertently compromise patient experience, leading to either inadequate analgesia or excessive drug effects, such as respiratory depression with opioids or local anesthetic systemic toxicity.

While attempts have been made to numerically quantify pain or NANB, such as the Noxious Stimulation Response Index (NSRI) and the Surgical Pleth Index (SPI), their adoption in clinical practice remains limited.1013 Notably, despite the potential advantages of these indices, previous studies have not compared them against the established standard of Ce. This may partly explain their limited widespread acceptance. Moreover, the accuracy and relevance of these indices in the face of hemodynamic events have been debated, further impeding their universal adoption.1421

One notable parameter is the normalized pulse volume (NPV), which represents the ratio of the pulsatile to non-pulsatile component in pulse oximetry. Historically used in polygraphs,22,23 recent research has shown its potential to reflect mental stress. This parameter has also been explored in contexts like peripheral nerve block success and fluid volume assessment.2426 However, its utility as an indicator for pain or NANB quantification remains largely unexplored.

Therefore, our study aimed to delve deeper into the realm of objective pain monitoring, exploring the efficacy, limitations, and potential of existing and emerging technologies.

While the challenges and limitations of current pain monitoring systems have been recognized, the potential use of NPV as a marker for the NANB during sedation or anesthesia remains largely unexplored. NPV represents the ratio of the pulsatile (AC) component to the non-pulsatile (DC) component in pulse plethysmography. Historically, this ratio has been used in fields such as physiological psychology, where it is known to decrease with psychological stress, and is thus applied in tools such as polygraphs. In anesthesiology, companies such as Masimo have utilized this measure as the Perfusion Index (PI) to assess circulatory plasma volume. This suggests that under conditions where changes in circulatory plasma volume are minimal, NPV could potentially serve as a reliable indicator of physical stress.

Several studies have observed changes in blood flow in areas targeted by local anesthetics such as epidural, spinal, and nerve blocks, with some research tracking these effects using NPV or PI. However, the application of NPV as a measure of analgesic efficacy under general anesthesia has not been explored. Although some evidence suggests that NPV might better reflect vascular resistance and sympathetic nerve α action than other parameters, its exclusion from initial parameters in tools such as the Surgical Stress Index (SSI) indicates that its potential in this realm is yet to be fully realized.

Given this background, our study hypothesized that NPV accurately reflects the NANB. We aimed to validate the utility of NPV as a novel indicator for the balance of nociceptive stimulation and analgesic efficacy. Additionally, our research explored the relationship between NPV and Ce, which is the standard titrating protocol. Furthermore, by utilizing multiple wavelengths, spanning both infrared and visible light, we strived to refine NPV measurements, ensuring its applicability even under challenging clinical scenarios such as severe anemia, hypovolemia, extreme peripheral vasoconstriction, varying skin pigmentation, and external light interference.

In essence, our goal was to ascertain the validity of NPV in quantifying pain or NANB and to elucidate the relationship between NPV and Ce in a clinical setting.

Methods

Study approval

The design and protocols were reviewed and approved by Tokyo Metropolitan Bokutoh Hospital Ethics Committee the local ethics committee in March 2017, under reference number 29-11.

Patient selection

For the analysis of the relationship between NPV and the Ce, we recruited 39 patients scheduled to undergo surgery under total intravenous anesthesia between July 2013 and May 2014 [Table 1-3].

Table 1. Demographics of cases.

Cases included29
Age [dimensionless] (SD)51.5 (17.8)
Male [dimensionless] (per cent)18 (62.1)
Height [cm] (SD)164.5 (9.5)
Weight [kg] (SD)66.9 (10.9)
BMI [kg/m2] (SD)24.6 (2.9)
ASA 1 [dimensionless] (per cent)20 (68.9)
Room duration [min] (SD)182.7 (82.2)
Anesthesia duration [min] (SD)171.7 (82.3)
Operation duration [min] (SD)108.4 (78.5)
Operation end to anesthesia end [min] (SD)23.7 (10.6)
Anesthesia end to discharge from operation room [min] (SD)5.8 (3.3)
Fentanyl used [mcg] [Q1, Q3]600 [500, 800]
Remifentanil used [mcg] [Q1, Q3]2050.4 [1367.0, 3156.8]
Colloid intravenous infusion [ml] [Q1, Q3]1000 [1000, 1000]
Serous intravenous infusion [ml] [Q1, Q3]500 [250, 900]

Table 2. Demographics of diseases.

DiseaseNumber of cases
Chronic Sinusitis9
Chronic Tonsillitis7
Nasal Septal Deviation6
Allergic Rhinitis4
Chronic Hypertrophic Rhinitis3
Pansinusitis3
Maxillary Osteonecrosis2
Maxillary Cyst2
Nasolacrimal Duct Obstruction2
Parotid Gland Tumor2
Sleep Apnea Syndrome1
Epiglottic Cyst1
Cervical Mass1
Suspected Malignant Lymphoma1
Nasopharyngeal Cyst1
Sinus Mycosis or Sinus Fungal Infection1
Vocal Cord Polyp1

Table 3. Demographics of procedures.

ProcedureNumber of cases
Endoscopic Sinus Surgery13
Palatine Tonsillectomy7
Septoplasty6
Turbinectomy6
Radical Pan-sinus Surgery6
Parotid Gland Tumor Excision2
Lacrimal Duct Silicon Tube Insertion2
Partial Maxillectomy2
Dacryocystorhinostomy2
Direct Laryngoscopy Laryngeal Tumor Excision1
Cervical Lymph Node Dissection1
Nasopharyngeal Tumor Excision1
Nasal Mucosal Cauterization1
Maxillary Bone Tumor Excision1
Intranasal Ethmoidectomy1
Endoscopic Nasal Abscess Drainage1
Direct Laryngoscopy Vocal Cord Polyp Removal1

The inclusion criteria were as follows: patients with an ASA score of 1 or 2, indicating healthy individuals or those with only mild systemic diseases. Only patients undergoing surgeries where there would be no changes in the position of the surgical field relative to the sensor, minimal fluid resuscitation or blood loss, and no concomitant use of epidural, spinal, or nerve block anesthesia were included.2733 At our institution, this primarily pertained to otolaryngology and dental and oral surgeries.

The exclusion criteria were as follows: patients undergoing surgeries that might involve significant posture changes or that have the potential for massive hemorrhage. Cases with high perfusion index variation (PVI > 25) or low perfusion index (PI < 1) during induction were excluded.3444 Cases with insufficient data sets or outliers were also excluded. Of the 38 initially recruited patients, 29 patients are included as 9 were excluded based on the above criteria: 3 due to an insufficient data set, 3 with a high PVI at extubation, and 3 with a low PI at induction.

Procedure protocol

Patients recruited for this segment of the study were equipped with the Masimo SET® Pulse Oximetry adult SpO2 adhesive sensor (Masimo Corporation, Irvine, CA, United States of America) to measure NPV, in addition to routine monitoring. Anesthesia was induced using a bolus of fentanyl and a continuous infusion of remifentanil and propofol, ensuring that the Bispectral Index™ (BIS™, Medtronic, Minneapolis, MN United States of America) remained between 40 and 60. Fentanyl was titrated to approximately 1 ng/ml towards the end of the operation, with remifentanil adjusted accordingly.45

A respiratory rate of eight breaths per minute was maintained until the recovery of spontaneous respiration (RoR). Both NPV and Ce for fentanyl and remifentanil were recorded upon RoR. The estimated Ce was derived using the built-in pharmacokinetic simulations—Shafer model for fentanyl and Minto/Schnider model for remifentanil—provided by the Fortec ORSYS perioperative patient information system (Philips, Netherlands). The total Ce was adjusted considering remifentanil’s greater potency, in alignment with the previous study. The coefficient of variation (CoV) for both NPV and total Ce was evaluated.

Measurement details

Patients were instrumented with the Masimo RainbowSET® Technology adhesive sensor on their fingertips. Continuous recordings were conducted in conjunction with other vital signs from the time of entry to the operating room. The Ce of the drugs was calculated using the built-in functions of the Philips Orsys anesthesia chart, employing the Shafer method for fentanyl and the Minto/Schnider method for remifentanil. Fentanyl was titrated to achieve an estimated Ce of 1 ng/ml at the end of surgery. We ensured the absence of respiratory depression from sedatives or muscle relaxants. The recovery of spontaneous respiration was defined at 8 breaths/minute, and at this juncture, we compared the NPV with the estimated Ce. For this comparison, the respiratory depressive potency ratio between fentanyl and remifentanil was set at 1:2, based on previous research findings,45 and the sum of the effective site concentration of remifentanil and 0.5 times the effective site concentration of fentanyl was recorded (henceforth referred to as CE).

Statistical analysis

Python 3.10 was used to calculate the demographics of cases, the multivariate outlier detection by Mahalanobis distance, the difference in CoV with Z-test, and to bootstrap the distribution of the CoV and the significance level.

Results

Multivariate outlier detection by Mahalanobis distance revealed no case corresponding to outliers as the critical value for the Chi-square distribution with 95% confidence was 5.99. Demographics of the included cases are shown in Tables 1, 2, and 3. NPV on RoR was 2.62 [dimensionless number] with a 95% confidence interval of 2.26–2.98, and a CoV of 36.3%. Conversely, CE on RoR was 1.48 [1.14–1.84] [ng/ml], with a CoV of 62.4% [Figure 1]. The Z test and the test for CoV showed that NPV on RoR was narrower than CE with a significance level of < 0.05 (1.93*10−5) [Figures 24]. In the bootstrapped distribution, the mean and median coefficients of variation for CE were higher than those for NPV, indicating greater relative variability. The confidence interval for NPV was relatively narrow, indicating that the estimation of the coefficient of variation is stable. Alternatively, the confidence interval for CE was wide, indicating a significant uncertainty in the estimation of the coefficient of variation. Although there were areas where the confidence intervals overlapped, differences in the central values were evident. These findings suggest that CE is more variable than NPV [Figure 5].

968c62e0-ed5c-4909-a51d-18e805e96f39_figure1.gif

Figure 1. Scatter plot of NPV and CE.

CE: Weighted Sum of Estimated Effect Site Concentration of Fentanyl and Remifentanil; NPV: Normalized Pulse Volume.

968c62e0-ed5c-4909-a51d-18e805e96f39_figure2.gif

Figure 2. Distribution of NPV and CE.

CE: Weighted Sum of Estimated Effect Site Concentration of Fentanyl and Remifentanil; NPV: Normalized Pulse Volume.

968c62e0-ed5c-4909-a51d-18e805e96f39_figure3.gif

Figure 3. Standardized distribution of NPV and CE.

CE: Weighted Sum of Estimated Effect Site Concentration of Fentanyl and Remifentanil; NPV: Normalized Pulse Volume.

968c62e0-ed5c-4909-a51d-18e805e96f39_figure4.gif

Figure 4. Box plot of CoV of PI and CE.

CE: Weighted Sum of Estimated Effect Site Concentration of Fentanyl and Remifentanil.

968c62e0-ed5c-4909-a51d-18e805e96f39_figure5.gif

Figure 5. Bootstrapped distribution of CoV for PI and CE.

CE: Weighted Sum of Estimated Effect Site Concentration of Fentanyl and Remifentanil.

Discussion

The presence of pain is harmful and associated with both all-cause and cancer-related mortality.4650 Conversely, narcotic analgesics used for pain relief have a negative impact on cancer progression and survival.51,52 Moreover, drug abuse is a social issue.5355 Therefore, there is a great social demand for the proper use of analgesics and the quantification of pain, or NANB, as the basis for the use of analgesics, as well as the quantification of NANB and other sensory information.56,57

Comparison with previous studies

A range of potential bioindicators for quantifying and othering pain include autonomic parameters such as electromyography, heart rate variability, and skin conductance,58 as well as biomarkers in psychiatric disorders, including neuroimaging, metabolite levels, and gene expression changes.59 Other potential bioindicators include functional near-infrared spectroscopy (fNIRS) machine learning,60 and mechanistic, translational, and quantitative pain assessment tools.61 Brain-based biomarkers for pain, particularly those identified through neuroimaging and machine learning, have also been proposed.62 However, the lack of validated objective markers for nociception and pain remains a challenge.63

Comparative performance of NPV and CE

The NPV demonstrated a superior predictive capability for the RoR, compared with the CE. In the context of assessing NANB, the NPV provided a more accurate reflection than drug simulation techniques. This suggests a stronger correlation between NPV and the balance of noxious stimuli and analgesia.

Potential for multimodal approaches

Building on these findings, there is potential to develop a multimodal approach that integrates the strengths of NPV with other measurements. Such a holistic approach could further enhance the accuracy and reliability of pain assessments in diverse clinical settings.

Study limitations

It is important to note that while NPV is calculated on a beat-to-beat basis, CE is computed only every minute. This temporal discrepancy might influence the comparative analysis between NPV and CE.

Future studies

Our team is keen on advancing this research by testing the efficacy of a pain monitoring system that employs NPV as its primary parameter, combined with other vital signs. Such a comprehensive instrument could offer a more nuanced and accurate pain assessment in clinical settings, further optimizing patient care and ensuring optimal analgesic administration. As this study was conducted with only one focus, RoR, it is necessary to study the predictive effect of stimuli that change over time.64

Ethical considerations

This design and protocols were reviewed and approved by Tokyo Metropolitan Bokutoh Hospital Ethics Committee the local ethics committee in March 2017, under reference number.2911 Written consent to participate in the study was obtained from the participants at the anaesthesiology pre-operative consultation, not on the day of surgery, and it was explained that there would be no disadvantage in withdrawing. The entire study was conducted in adherence with the Declaration of Helsinki.65

Author contribution

OT: Conceptualization, Data curation, Formal Analysis, Methodology, Writing – original draft, Writing – review & editing

YO: Writing – original draft, Writing – review & editing

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Tatsuki O and Onishi Y. Normalized Pulse Volume as a Superior Predictor of Respiration Recovery and Quantification of Nociception Anti-nociception Balance Compared to Opioid Effect Site Concentration: A Prospective, Observational Study [version 1; peer review: 1 approved, 1 not approved]. F1000Research 2024, 13:233 (https://doi.org/10.12688/f1000research.146215.1)
NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article.
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Reviewer Report 26 Jun 2024
Mika Särkelä, GE Healthcare Finland Oy, Helsinki, Finland 
Not Approved
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Tatsuki and Onishi have compared normalized pulse volume and estimated analgesic effect site concentration in 39 surgical patients. I regret my delayed report, but that is partly caused by inaccuracies in the manuscript. 

1. Term "normalized pulse ... Continue reading
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Särkelä M. Reviewer Report For: Normalized Pulse Volume as a Superior Predictor of Respiration Recovery and Quantification of Nociception Anti-nociception Balance Compared to Opioid Effect Site Concentration: A Prospective, Observational Study [version 1; peer review: 1 approved, 1 not approved]. F1000Research 2024, 13:233 (https://doi.org/10.5256/f1000research.160275.r273068)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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Reviewer Report 10 May 2024
Tatsuhiko Arafune, Division of Electronic Information and Biomedical Engineering, School of Schience and Engineering, Tokyo Denki University, Saitama, Japan 
Approved
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This study evaluated the validity of normalized pulse volume (NPV) as a novel measure for quantifying pain and the balance between nociception and antinociception in sedated patients. 39 total venous anesthesia surgical patients were compared to NPV and ... Continue reading
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Arafune T. Reviewer Report For: Normalized Pulse Volume as a Superior Predictor of Respiration Recovery and Quantification of Nociception Anti-nociception Balance Compared to Opioid Effect Site Concentration: A Prospective, Observational Study [version 1; peer review: 1 approved, 1 not approved]. F1000Research 2024, 13:233 (https://doi.org/10.5256/f1000research.160275.r260349)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.

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Alongside their report, reviewers assign a status to the article:
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