Accuracy and Interpretation of Transcutaneous Carbon Dioxide Monitoring in Critically Ill Children.

OBJECTIVES: Transcutaneous carbon dioxide (Tc co2 ) monitoring can noninvasively assess ventilation by estimating carbon dioxide ( CO2 ) levels in the blood. We aimed to evaluate the accuracy of Tc co2 monitoring in critically ill children by comparing it to the partial pressure of arterial carbon dioxide (Pa co2 ). In addition, we sought to determine the variation between Tc co2 and Pa co2 acceptable to clinicians to modify patient care and to determine which patient-level factors may affect the accuracy of Tc co2 measurements. DESIGN: Retrospective observational cohort study. SETTING: Single, quaternary care PICU from July 1, 2012, to August 1, 2020. PATIENTS: Included participants were admitted to the PICU and received noninvasive ventilation support (i.e., continuous or bilevel positive airway pressure), conventional mechanical ventilation, or high-frequency oscillatory or percussive ventilation with Tc co2 measurements obtained within 15 minutes of Pa co2 measurement. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Three thousand four hundred seven paired arterial blood gas and Tc co2 measurements were obtained from 264 patients. Bland-Altman analysis revealed a bias of -4.4 mm Hg (95% CI, -27 to 18.3 mm Hg) for Tc co2 levels against Pa co2 levels on the first measurement pair for each patient, which fell within the acceptable range of ±5 mm Hg stated by surveyed clinicians, albeit with wide limits of agreement. The sensitivity and specificity of Tc co2 to diagnose hypercarbia were 93% and 71%, respectively. Vasoactive-Infusion Score (VIS), age, and self-identified Black/African American race confounded the relationship between Tc co2 with Pa co2 but percent fluid overload, weight-for-age, probe location, and severity of illness were not significantly associated with Tc co2 accuracy. CONCLUSIONS: Tc co2 monitoring may be a useful adjunct to monitor ventilation in children with respiratory failure, but providers must be aware of the limitations to its accuracy.

Oximetry and carbon dioxide screening for ventilatory requirements in children with spinal muscular atrophy type 1-3.

INTRODUCTION: Despite disease modifying treatments (DMT), assisted ventilation is commonly required in children with Spinal Muscular Atrophy (SMA). Guidelines suggest screening with oximetry and transcutaneous carbon dioxide (TcCO2) for sleep disordered breathing (SDB). AIM: To determine the utility of pulse oximetry and TcCO2 as a screen for SDB and the need for Non-Invasive Ventilation (NIV) in children with SMA type 1-3. METHODS: A prospective cohort study was conducted in Queensland, Australia. Full diagnostic PSG was completed in DMT naïve children with SMA. Pulse oximetry and TcCO2 were extracted from PSG. Apnoea-hypopnoea indices (AHI) criteria were applied to PSG results to define the need for NIV. Abnormal was defined as: ≤3 months of age [mo] AHI≥10 events/hour; >3mo AHI ≥5 events/hour. Receiver operating characteristic curves were calculated for abnormal PSG and pulse oximetry/TcCO2 variables, and diagnostic statistics were calculated. RESULTS: Forty-seven untreated children with SMA were recruited (type 1 n = 13; 2 n = 21; 3 n = 13) ranging from 0.2 to 18.8 years old (median 4.9 years). Oxygen desaturation index ≥4 % (ODI4) ≥20events/hour had sensitivity 82.6 % (95 % CI 61.2-95.0) and specificity of 58.3 % (95 % CI 36.6-77.9). TcCO2 alone and combinations of oximetry/TcCO2 had low diagnostic ability. The same methodology was applied to 36 children who were treated (type 1 n = 7; type 2 n = 17; type n = 12) and oximetry±TcCO2 had low diagnostic ability. CONCLUSION: ODI4 ≥20events/hour can predict the need for NIV in untreated children with SMA. TcCO2 monitoring does not improve the PPV. If normal however, children may still require a diagnostic PSG. Neither oximetry nor TcCO2 monitoring were useful screening tests in the children treated with DMT.

Changes of transcutaneous oxygen pressure in compressed areas of surgical patients: A prospective study of influencing factors.

OBJECTIVE: To compare the transcutaneous oxygen pressure (tcpO2) measurement values and changes in compressed areas of surgical patients before and after surgery and to explore the related factors influencing the tcpO2 changes before and after surgery. METHODS: Researchers selected 100 patients who underwent elective surgery in a tertiary comprehensive hospital from November 2021 to September 2022. A self-designed general information questionnaire was used to collect patient general information and disease-related data, including gender, age, smoking and drinking history, hypertension, diabetes, local skin temperature and humidity, related biochemical indicators, and activities of daily living score. Researchers used a transcutaneous oxygen pressure meter to measure and record the tcpO2 of the compressed areas (sacrococcygeal area, scapula area, and heel area) before and after surgery. RESULTS: Among the 100 patients, 37.00 % (37/100) developed type I/II pressure ulcers after surgery, and 30 patients (81.08 %) showed regression within 2 h after surgery. There was no statistically significant difference in the preoperative tcpO2 measurement values of the scapula and heel areas between the group with and without pressure ulcers, but the preoperative tcpO2 measurement value of the sacrococcygeal area in the group without pressure ulcers was higher than that in the group with pressure ulcers (P < 0.01). The factors affecting the preoperative tcpO2 measurement value of the sacrococcygeal area were smoking and surgical type. After surgery, the tcpO2 measurement values of the three areas in the group with pressure ulcers were significantly lower than those in the group without pressure ulcers (P < 0.01). Comparing the tcpO2 values of different areas, it was found that the tcpO2 value was lowest in the sacrococcygeal area, followed by the heel area, and the tcpO2 value in the scapula area was highest both before and after surgery (P < 0.01). The main factors affecting the postoperative tcpO2 measurement value were diabetes, Glassgow score, surgical time, and intraoperative red blood cell transfusion. CONCLUSION: The measurement of tcpO2 is related to the incidence of surgically acquired pressure ulcers, and this technology may become an important tool for quantitative assessment of the risk of pressure ulcers.

Screening Strategies for Sleep-Disordered Breathing in Patients With Spinal Cord Injury in a Tertiary Care Rehabilitation Center.

BACKGROUND: Sleep-disordered breathing (SDB) is frequent in patients with spinal-cord injury (SCI). However, SDB is frequently underdiagnosed due to limited access to diagnostic testing and knowledge about the condition. Moreover, SDB heterogeneity (sleep apnea, obstructive sleep apnea or central sleep apnea and nocturnal alveolar hypoventilation) implies complex evaluation of both nocturnal respiratory effort and hypercapnia. The aim of this study was to compare different screening strategies for an SDB diagnosis in patients with SCI. METHODS: This was a retrospective analysis of data from subjects with SCI followed up in a tertiary-care rehabilitation center with a specialized sleep unit. Subjective (questionnaires) and objective data (polysomnography [PSG]), [Formula: see text] extracted from the PSG, morning blood gases, and nocturnal transcutaneous CO2 (PtcCO2 ) were collected and analyzed. A retrospective comparison of different strategies for SDB screening was carried out. Each strategy was compared (alone and in combination) with the standard of care for sleep apnea (PSG) and nocturnal alveolar hypoventilation (PtcCO2 ) diagnosis. The performance of the usual cutoff and visual analysis was studied. RESULTS: Among 190 subjects with SCI who underwent a full night's PSG, data were available for 104 questionnaires and 162 with oximetry. Nocturnal alveolar hypoventilation was screened by PtcCO2 and blood gases in 52 subjects with SCI. Questionnaires (the modified Screening for Obstructive Sleep Apnea in Tetraplegia and the Epworth Sleepiness Scale) had poor performance for identifying sleep apnea and did not identify nocturnal alveolar hypoventilation. [Formula: see text] (oxygen desaturation index score ≥ 13) and visual analysis of [Formula: see text] were good at identifying sleep apnea but insufficient to identify nocturnal alveolar hypoventilation. Diurnal blood gases were poor predictors of nocturnal alveolar hypoventilation. CONCLUSIONS: Questionnaires were of limited use in subjects with SCI, but the oxygen desaturation index derived from oximetry performed well for sleep apnea screening. Both diurnal blood gases and oximetry visual analysis were insufficient for nocturnal alveolar hypoventilation screening. PtcCO2 monitoring should be mandatory and ideally combined with PSG given the heterogeneity of SDB phenotypes and associated sleep comorbidities of patients with SCI.

The relation of lactate level and carbon dioxide pressure discrepancies between transcutaneous and arterial measurements.

Introduction: Partial carbondioxide pressure of the arterial blood (PaCO2) is used to evaluate alveolar ventilation. Transcutaneous carbon dioxide pressure (TcCO2) monitoring has been developed as a non-invasive (NIV) alternative to arterial blood gas analysis (ABG). Studies have shown that decreased tissue perfusion leads to increased carbondioxide (CO2). The use of transcutaneous capnometry may be unreliable in patients with perfusion abnormalities. In this study, we aimed to evaluate the relation between TcCO2-PaCO2 and lactate level which is recognized as a marker of hypoperfusion. Materials and Methods: In this prospective cohort study in critical care patients with hypercapnic respiratory failure (PaCO2 ≥45 mmHg) who received NIV between April 2019 and January 2020 in the intensive care unit were enrolled in the study. Patients' simultaneously measured TcCO2 and PaCO2 values of hypercapnic patients were recorded. Each paired measurement was categorized into two groups; normal lactate (<2 mmol/L) and increased lactate (≥2 mmol/L). Result: A total of 116 paired TcCO2 and PaCO2 measurements of 29 patients were recorded. Bland-Altman analysis showed the mean bias between the TcCO2 and PaCO2 and 95% limits of agreement (LOA) in all measurements (1.75 mmHg 95% LOA -3.67 to 7.17); in the normal lactate group (0.66 mmHg 95% LOA -1.71 to 3.03); and in the increased lactate group (5.17 mmHg 95% LOA -1.63 to 11.97). The analysis showed a correlation between lactate level and the difference between TcCO2 and PaCO2 (r= 0.79, p< 0.001) and a negative correlation between mean blood pressure and the difference between TcCO2 and PaCO2 (r= -0.54, p= 0.001). Multiple regression analysis results showed that lactate level was independently associated with increased differences between TcCO2 and PaCO2 (Beta= 0.875, p< 0.001). Conclusions: TcCO2 monitoring may not be reliable in patients with increased lactate levels. TcCO2 levels should be checked by ABG analysis in these patients.

Correlation Between Pedal Acceleration Time and Transcutaneous Oxygen Pressures - A Prognostic Tool for Wound Healing in the Diabetic Foot Population.

BACKGROUND: Pedal arch interrogation by ultrasound through systolic acceleration time measurement (Pedal Acceleration Time [PAT]) is a recently described technique that offers valuable insights into foot hemodynamics. Previous studies have demonstrated a correlation between PAT and the ankle-brachial index and PAT has been proposed as a prognostic tool for wound healing in patients with peripheral artery disease. This study aims to assess the correlation between PAT and Transcutaneous Pressure of Oxygen (TcPO2) in the diabetic foot population. METHODS: In this single-center cross-sectional study PAT and TcPO2 were measured in diabetic patients with ischemic foot ulcers. The assessment of wound status was performed between the 12th and 16th week after the first evaluation. The primary outcome was to assess the correlation between the values obtained for PAT and TcPO2. The secondary outcome was to establish a cut-off value for wound healing. RESULTS: Sixty limbs with ischemic foot ulcers were evaluated. The results showed a correlation between PAT and TcPO2 [F (1.52) = 16.928; R2 = 0.246; P < 0.001]. The receiver operating characteristics curve analysis showed an optimal cut-off value at PAT >186 ms for predicting a nonhealing ulcer with a 96% specificity (area under the curve = 0.774; confidence interval 0.648-0.872; P < 0.001). CONCLUSIONS: PAT showed a significant correlation with TcPO2 with high specificity to identify nonhealing foot ulcers due to insufficient foot perfusion with a potential prognostic value in the diabetic foot population.

Transcutaneous oximetry values in chronic ulcer patients during Hyperbaric treatment at 1.4 ATA compared to 2 ATA.

Chronic wounds have a significant impact on a patient's quality of life. Different pathologies, such as poor blood supply and tissue breakdown, may lead to inadequate oxygenation of the wound. Hyperbaric oxygen (HBO2) is a widely used treatment for an increasing number of medical practices. A new so-called "hyperbaric treatment" trend has emerged. The use of low-pressure, soft-sided, or inflatable chambers represents a growing trend in hyperbaric medicine. Used in professional settings as well as directly marketed to individuals for home use, they are promoted as equivalent to clinical hyperbaric treatments provided in medical centers. However, these chambers are pressurized to 1.3 atmospheres absolute (ATA) on either air or with an oxygen concentrator, both generate oxygen partial pressures well below those used in approved hyperbaric centers for UHMS-approved indications. A total of 130 consecutive patients with chronic ulcers where tested. TcPO2 was measured near the ulcer area while the patient was breathing 100% O2 at 1.4 ATA for five and 10 minutes. The average TcPO2 at 1.4 ATA after 10 minutes of O2 breathing was 161 mmHg (1-601 mmHg, standard deviation 137.91), compared to 333 mmHg in 2 ATA (1-914±232.56), p < 0.001. Each electrode tested was also statistically significant, both after five minutes of O2 breathing and after 10 minutes. We have not found evidence supporting the claim that 1.4 ATA treatment can benefit a chronic ulcer patient. The field of HBO2 is constantly evolving. We have discovered new ways to treat previously incurable ailments. Nevertheless, it is important to note that new horizons must be examined scientifically, supported by evidence-based data. The actual effect of 1.4 ATA on many ailments is yet to be determined.

Comparative evaluation of transcutaneous oxygen tension and ankle-brachial index as predictors of reoperation following below-knee amputation.

OBJECTIVE: Decision-making regarding level of lower extremity amputation is sometimes challenging. Selecting an appropriate anatomic level for major amputation requires consideration of tradeoffs between postoperative function and risk of wound complications that may require additional operations, including debridement and/or conversion to above-knee amputation (AKA). We evaluated the utility of common, non-invasive diagnostic tests used in clinical practice to predict the need for reoperations among patients undergoing primary, elective, below knee-amputations (BKAs) by vascular surgeons. METHODS: Patients undergoing elective BKA over a 5-year period were identified using Current Procedural Terminology codes. Medical records were reviewed to characterize demographics, pre-amputation testing transcutaneous oxygen tension (TcPO2), and ankle-brachial index (ABI). The need for ipsilateral post-BKA reoperation (including BKA revision and/or conversion to AKA) regardless of indication was the primary outcome. Associations were evaluated using univariable and multivariable logistic regression models. Cutpoints for TcPO2 values associated with amputation reoperation were evaluated using receiver operating characteristic curves. RESULTS: We identified 175 BKAs, of which 46 (26.3%) required ipsilateral reoperation (18.9% BKA revisions and 14.3% conversions to AKA). The mean age was 63.3 ± 14.8 years. Most patients were male (65.1%) and White (72.0%). Mean pre-amputation calf TcPO2 was 40.0 ± 20.5 mmHg, and mean ABI was 0.64 ± 0.45. In univariable models, post-BKA reoperation was associated with calf TcPO2 (odds ratio [OR], 0.97; 95% confidence interval [CI], 0.94-0.99; P = .013) but not ABI (OR, 0.53; 95% CI, 0.19-1.46; P = .217). Univariable associations with reoperation were also identified for age (OR, 0.97; 95% CI, 0.94-0.990; P = .003) and diabetes (OR, 0.43; 95% CI, 0.21-0.87; P = .019). No associations with amputation revision were identified for gender, race, end-stage renal disease, or preoperative antibiotics. Calf TcPO2 remained associated with post-BKA reoperation in a multivariable model (OR, 0.97; 95% CI, 0.94-0.99; P = .022) adjusted for age (OR, 0.98; 95% CI, 0.94-1.01; P = .222) and diabetes (OR, 0.98; 95% CI, 0.94-1.01; P = .559). Receiver operating characteristic analysis suggested a TcPO2 ≥38 mmHg as an appropriate cut-point for assessing risk for BKA revision (area under the curve = 0.682; negative predictive value, 0.91). CONCLUSIONS: Reoperation after BKA is common, and reoperation risk was associated with pre-amputation TcPO2. For patients undergoing elective BKA, higher risk of reoperation should be discussed with patients with an ipsilateral TcPO2 <38 mmHg.

Sleep disordered breathing assessment in patient with slowly progressive neuromuscular disease.

BACKGROUND: Sleep-disordered breathing (SDB) is common in patients with neuromuscular diseases (NMD). Focusing on hypercapnia may lead to the neglect of other SDB such as obstructive and/or central sleep apnea syndrome (SAS). Our objectives were to assess the risk of inappropriate SDB management according to different screening strategies and to evaluate the prevalence and determinants of isolated and overlapping sleep apnea in patients with slowly progressive NMD. METHODS: This monocentric, cross-sectional, retrospective study analyzed medical records of adult NMD patients referred to a sleep department. Diagnostic strategies, including respiratory polygraphy (RP), nocturnal transcutaneous capnography (tcCO2), and blood gases (BG), were assessed for their performance in diagnosing SDB. Demographics and pulmonary function test results were compared between patients with or without SDB to identify predictors. RESULTS: Among the 149 patients who underwent a full diagnostic panel (RP + tcCO2 + BG), 109 were diagnosed with SDB. Of these, 33% had isolated SAS, and central apneas were predominant. Using single diagnostic strategies would lead to inappropriate SDB management in two thirds of patients. A combination of 2 diagnostic tools resulted respectively in 21.1, 22.9 and 42.2 % of inappropriate SDB management for RP + tcCO2, RP + BG and tcCO2 + BG. CONCLUSION: The significant prevalence of sleep apnea syndrome in patients with slowly progressive NMD highlights the need for increased awareness among clinicians. Improved diagnostics involve a systematic approach addressing both sleep apnea and diurnal and nocturnal alveolar hypoventilation to avoid inappropriate management and limit the consequences of SDB.

Use of consecutive transcutaneous oxygen measurement when assessing the need for revascularization and association with the outcomes of ischemic diabetic ulcers.

This study compared the ankle-brachial index (ABI) with transcutaneous oxygen pressure (TcPO2 ) in assessing peripheral vascular disease (PVD) prevalence in 100 diabetic foot ulcer (DFU) patients. Patients were categorized into vascular or nonvascular reconstruction groups and underwent both ABI and TcPO2 measurements four times over 6 months. Predictive validity for PVD diagnosis was analysed using the area under the receiver-operating characteristic curve (AUC). The study found TcPO2 to be a superior predictor of PVD than ABI. Among the DFU patients, 51 with abnormal TcPO2 values underwent vascular reconstruction. Only TcPO2 values showed significant pretreatment differences between the groups and increased post-reconstruction. These values declined over a 6-month follow-up, whereas ABI values rose. For those with end-stage renal disease (ESRD), TcPO2 values saw a sharp decrease within 3 months. Pre-reconstruction TcPO2 was notably lower in amputation patients versus limb salvage surgery patients. In conclusion, TcPO2 is more effective than ABI for evaluating ischemic limb perfusion and revascularization necessity. It should be prioritized as the primary follow-up tool, especially for ESRD patients.

Implementation Techniques for Transcutaneous Carbon Dioxide Monitoring: Approaches for Wearable Smart Health Applications.

Wearable smart health applications aim to continuously monitor critical physiological parameters without disrupting patients' daily activities, such as giving a blood sample for lab analysis. For example, the partial pressure of arterial carbon dioxide, the critical indicator of ventilation efficacy reflecting the respiratory and acid-base status of the human body, is measured invasively from the arteries. Therefore, it can momentarily be monitored in a clinical setting when the arterial blood sample is taken. Although a noninvasive surrogate method for estimating the partial pressure of arterial carbon dioxide exists (i.e., transcutaneous carbon dioxide monitoring), it is primarily limited to intensive care units and comes in the form of a large bedside device. Nevertheless, recent advancements in the luminescence sensing field have enabled a promising technology that can be incorporated into a wearable device for the continuous and remote monitoring of ventilation efficacy. In this review, we examine existing and nascent techniques for sensing transcutaneous carbon dioxide and highlight novel wearable transcutaneous carbon dioxide monitors by comparing their performance with the traditional bedside counterparts. We also discuss future directions of transcutaneous carbon dioxide monitoring in next-generation smart health applications.

Test-retest Reliability and Minimal Detectable Change in Exercise Oximetry in Claudicants.

BACKGROUND: Exercise transcutaneous oxygen pressure measurement (Exercise-TcPO2) can be used to diagnose Lower Extremity Artery Disease (LEAD) and allows the quantification of limb ischemia during exercise on treadmill. Exercise-TcPO2 test-retest reliability in patients with LEAD and severe walking impairment is unknown. The aim of this study was to evaluate the test-retest reliability, standard error of measurement (SEM), and Minimal Detectable Change (MDC) of exercise-TcPO2 in patients with claudication. METHODS: Data were collected from patients that performed 2 treadmill tests within a 1-month interval. Delta from Rest of Oxygen Pressure (DROP) values were measured at both buttocks (proximal) and both calves (distal). Test-retest reproducibility was assessed by recording transcutaneous oximetry measurements twice and expressed as SEM and intra-class correlation coefficients. MDC was calculated using the formula MDC = SEM x 1.96 x √ 2. RESULTS: Twenty eight LEAD patients (61 ± 9 years old) were included. Intra-class correlation coefficients were 0.66 [0.50, 0.79] and 0.65 [0.49, 0.79] for the proximal and distal levels, respectively. The SEM of DROP at the proximal and distal levels were 7 [6, 9] mm Hg and 9 [8, 11] mm Hg, respectively. The SEM for all (proximal and distal) DROP values was 8 [7, 10] mm Hg and the MDC of DROP was 23 mm Hg. CONCLUSIONS: Exercise-TcPO2 with measurement of DROP values has a moderate test-retest reliability in LEAD patients with a maximal walking distance ≤ 300m. For an individual, an improvement or deterioration in DROP of ≥ 23 mm Hg after an intervention would be required to be 95% confident that the change is significant. It should be considered in evaluating the impact of treatment in patients with claudication.

The Precision Between Transcutaneous Carbon Dioxide Versus PaCO2 in Infants Undergoing Therapeutic Hypothermia.

BACKGROUND: Infants with hypoxic-ischemic encephalopathy are often treated with therapeutic hypothermia and high-frequency ventilation. Fluctuations in PaCO2 during therapeutic hypothermia are associated with poor neurodevelopmental outcomes. Transcutaneous CO2 monitors offer a noninvasive estimate of PaCO2 represented by transcutaneously measured partial pressure of carbon dioxide (PtcCO2 ). We aimed to assess the precision between PtcCO2 and PaCO2 values in neonates undergoing therapeutic hypothermia. METHODS: This was a retrospective chart review of 10 neonates who underwent therapeutic hypothermia requiring respiratory support over 2 y. A range of 2-27 simultaneous PtcCO2 and PaCO2 pairs of measurements per neonate were analyzed via linear mixed models and a Bland-Altman plot for multiple observations per neonate. RESULTS: A linear mixed-effect model demonstrated that PtcCO2 and PaCO2 (controlling for sex) were similar. The 95% CI of the mean difference ranged from -2.3 to 5.7 mm Hg (P = .41). However, precision was poor as the PtcCO2 ranged from > 18 mm Hg to < 13 mm Hg than PaCO2 values for 95% of observations. CONCLUSIONS: The neonates' PtcCO2 was as much as 18 mm Hg higher to 13 mm Hg lower than the PaCO2 95% of the time. Transcutaneous CO2 monitoring may not be a good trending tool, nor is it appropriate for estimating PaCO2 in patients undergoing therapeutic hypothermia.

Optimizing Transcutaneous Oxygen Measurement Sites on Humans.

This study utilizes an optical method of transcutaneous oxygen sensing that has the potential to revolutionize at-home care. This technique is based on quenching the luminescence of a platinum porphyrin film. Since oxygen quenches luminescence, its lifetime is further measured to assess the partial pressure of transcutaneous oxygen diffusing through the skin. Unlike conventional transcutaneous oxygen monitors that use electrochemical sensors, the luminescence-based sensor allows the use of dry electrodes that do not require heating and reduce the risk of accidental skin irritations or burns. These properties not only improve patient safety but also allow the creation of miniature wearable transcutaneous oxygen sensors for continuous and accurate remote respiratory monitoring. To this end, it is critical to assess the efficiency of the wearable sensor by determining the optimal location for its placement on the body. Depending on the location on the body, physiological factors such as blood flow rate and skin thickness affect dermal perfusion of transcutaneous oxygen. In this work, four healthy volunteers participated in subject testing. We assessed each participant at the following locations: thumb, top of the wrist, forearm, thigh, and shin. All locations consistently reported accurate and reliable data. Among them, the thumb demonstrated shorter settling times and the most uniform luminescence lifetime values.

Transcutaneous carbon dioxide measurements in anesthetized apneic patients with BMI > 35 kg/m2.

Transcutaneous carbon dioxide measurement (TcCO2) offers the ability to continuously and non-invasively monitor carbon dioxide (CO2) tensions when end-tidal monitoring is not possible. The accuracy of TcCO2 has not been established in anesthetized apneic patients with obesity. In this secondary publication, we present a methods comparison analysis of TcCO2 with the gold standard arterial PCO2, in adult patients with body mass index (BMI) > 35kg/m2 who were randomized to receive high flow or low flow nasal oxygenation during post-induction apnea. Agreement between PaCO2 and TcCO2 at baseline, the start of apnea and the end of apnea were assessed using a non-parametric difference plot. Forty-two participants had a median (IQR) BMI of 52 (40-58.5) kg/m2. The mean (SD) PaCO2 was 33.9 (4.0) mmHg at baseline and 51.4 (7.5) mmHg at the end of apnea. The bias was the greatest at the end of apnea median (95% CI, 95% limits of agreement) 1.90 mmHg (-2.64 to 6.44, -7.10 to 22.90). Findings did not suggest significant systematic differences between the PaCO2 and TcCO2 measures. For a short period of apnea, TcCO2 showed inadequate agreement with PaCO2 in patients with BMI > 35 kg/m2. These techniques require comparison in a larger population, with more frequent sampling and over a longer timeframe, before TcCO2 can be confidently recommended in this setting.

Limitations of transcutaneous carbon dioxide monitoring in apneic oxygenation.

BACKGROUND: High-flow nasal oxygenation is increasingly used during sedation procedures and general anesthesia in apneic patients. Transcutaneous CO2 (ptcCO2)-monitoring is used to monitor hypercapnia. This study investigated ptcCO2-monitoring during apneic oxygenation. METHODS: We included 100 patients scheduled for elective surgery under general anesthesia in this secondary analysis of a randomized controlled trial. Before surgery, we collected ptcCO2 measured by TCM4 and TCM5 monitors and arterial blood gas (ABG) measurements every two minutes during 15 minutes of apnea. Bland-Altman plots analyzed agreement between measurement slopes; linear mixed models estimated the different measuring method effect, and outlined differences in slope and offset between transcutaneous and arterial CO2 partial pressures. RESULTS: Bland-Altman plots showed a bias in slope (95% confidence intervals) between ABG and TCM4-measurements of 0.05mmHg/min (-0.05 to 0.15), and limits of agreement were -0.88mmHg/min (-1.06 to -0.70) and 0.98mmHg/min (0.81 to 1.16). Bias between ABG and TCM5 was -0.14mmHg/min (-0.23 to -0.04), and limits of agreement were -0.98mmHg/min (-1.14 to -0.83) and 0.71mmHg/min (0.55 to 0.87). A linear mixed model (predicting the CO2-values) showed an offset between arterial and transcutaneous measurements of TCM4 (-15.2mmHg, 95%CI: -16.3 to -14.2) and TCM5 (-19.1mmHg, -20.1 to -18.0). Differences between the two transcutaneous measurements were statistically significant. CONCLUSIONS: Substantial differences were found between the two transcutaneous measurement systems, and between them and ABG. Transcutaneous CO2 monitoring cannot replace arterial CO2-monitoring during apneic oxygenation. In clinical settings with rapidly changing CO2-values, arterial blood gas measurements are needed to reliably assess the CO2-partial pressure in blood. TRIAL REGISTRATION: ClinicalTrials.gov (NCT03478774).

A Transcutaneous Carbon Dioxide Monitor Based on Time-Domain Dual Lifetime Referencing.

The partial pressure of arterial carbon dioxide plays a critical role in assessing the acid-base and respiratory status of the human body. Typically, this measurement is invasive and can only be taken momentarily when an arterial blood sample is drawn. Transcutaneous monitoring is a noninvasive surrogate method that provides a continuous measure of arterial carbon dioxide. Unfortunately, current technology is limited to bedside instruments mainly used in intensive care units. We developed a first-of-its-kind miniaturized transcutaneous carbon dioxide monitor that utilizes a luminescence sensing film and a time-domain dual lifetime referencing method. Gas cell experiments confirmed the monitor's ability to accurately identify changes in the partial pressure of carbon dioxide within the clinically significant range. Compared to the luminescence intensity-based technique, the time-domain dual lifetime referencing method is less prone to measurement errors caused by changes in excitation strength, reducing the maximum error from  âˆ¼ 40% to  âˆ¼ 3% and resulting in more reliable readings. Additionally, we analyzed the sensing film by investigating its behavior under various confounding factors and its susceptibility to measurement drift. Finally, a human subject test demonstrated the effectiveness of the applied method in detecting even slight changes in transcutaneous carbon dioxide, as small as  âˆ¼ 0.7%, during hyperventilation. The prototype, which consumes 30.1 mW of power, is a wearable wristband with compact dimensions of 37 mm× 32 mm.

A Prototype Wearable Device for Noninvasive Monitoring of Transcutaneous Oxygen.

Transcutaneous oxygen monitoring is a noninvasive method for measuring the partial pressure of oxygen diffusing through the skin, which strongly correlates with changes in dissolved oxygen in the arteries. Luminescent oxygen sensing is one of the techniques for assessing transcutaneous oxygen. Intensity- and lifetime-based measurements are two well-known methods used in this technique. The latter is more immune to optical path changes and reflections, making the measurements less vulnerable to motion artifacts and skin color changes. Although the lifetime-based method is promising, the acquisition of high-resolution lifetime data is crucial for accurate transcutaneous oxygen measurements from the human body when skin is not heated. We have built a compact prototype along with its custom firmware for the lifetime estimation of transcutaneous oxygen with a provision of a wearable device. Furthermore, we performed a small experiment study on three healthy human volunteers to prove the concept of measuring oxygen diffusing from the skin without heating. Lastly, the prototype successfully detected changes in lifetime values driven by the changes in transcutaneous oxygen partial pressure due to pressure-induced arterial occlusion and hypoxic gas delivery. The prototype resolved a minimum change of 1.34 ns in a lifetime that corresponds to 0.031 mmHg in response to slow changes in the oxygen pressure in the volunteer's body caused by hypoxic gas delivery. The prototype is believed to be the first in the literature to successfully conduct measurements in human subjects using the lifetime-based technique.