Advanced pulse oximeter signal processing technology compared to simple averaging. I. Effect on frequency of alarms in the operating room

Observational study on the influence of averaging time on oximetry results in infants and children

AIM

Oximetry values are influenced by the averaging time (AT) used. We aimed to evaluate the effect of different ATs on number, duration, mean single event and total integral of desaturations in preterm infants and children to convert between parameters obtained with different ATs.

METHODS

In a prospective observational study, 49 children underwent sleep laboratory-based polysomnography and 15 preterm infants were studied in the intensive care unit. Their raw red-to-infrared-saturation-data were reprocessed using seven different ATs (3-16 seconds). Desaturation thresholds were <80% (infants) and <90% (children), conversion formulas and their median percentage errors were calculated.

RESULTS

We found a linear relationship between the logarithms of the ATs and those of the desaturation parameters, leading to a conversion formula with different exponents. Based on this relationship, the number of desaturations decreased from AT = 3s to AT = 16s by factor 0.28 (children) and 0.18 (infants); total oxygen saturation integral decreased by factor 0.72 (children) and 0.48 (infants). The desaturation duration increased by factor 1.89 (children) and 3.34 (infants).

CONCLUSION

The number and total integral decreased, but the duration and mean single event integral increased with increasing AT. These changes were stronger in infants. Conversion formulas may facilitate comparisons between studies using different averaging times.

The Impact of Averaging Window Length on the"Desaturation Indexes during Overnight Pulse Oximetry at High-Altitude"

STUDY OBJECTIVES

To determine the impact of averaging window-length on the "desaturation" indexes (DIs) obtained via overnight pulse oximetry (SpO2) at high altitude.

DESIGN

Overnight SpO2 data were collected during a 10-day sojourn at high altitude. SpO2 was obtained using a commercial wrist-worn finger oximeter whose firmware was modified to store unaveraged beat-to-beat data. Simple moving averages of window lengths spanning 2 to 20 cardiac beats were retrospectively applied to beat-to-beat SpO2 datasets. After SpO2 artifacts were removed, the following DIs were then calculated for each of the averaged datasets: oxygen desaturation index (ODI); total sleep time with SpO2 < 80% (TST < 80), and the lowest SpO2 observed during sleep (SpO2 low).

SETTING

South Base Camp, Mt. Everest (5,364 m elevation).

PARTICIPANTS

Five healthy, adult males (35 ± 5 y; 180 ± 1 cm; 85 ± 4 kg).

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

49 datasets were obtained from the 5 participants, totalling 239 hours of data. For all window lengths ≥ 2 beats, ODI and TST < 80 were lower, and SpO2 low was higher than those values obtained from the beat-to-beat SpO2 time series data (P < 0.05).

CONCLUSIONS

Our findings indicate that increasing oximeter averaging window length progressively underestimates the frequency and magnitude of sleep disordered breathing events at high altitude, as indirectly assessed via the desaturation indexes.

Effect of concurrent oxygen therapy on accuracy of forecasting imminent postoperative desaturation

Episodic postoperative desaturation occurs predominantly from respiratory depression or airway obstruction. Monitor display of desaturation is typically delayed by over 30 s after these dynamic inciting events, due to perfusion delays, signal capture and averaging. Prediction of imminent critical desaturation could aid development of dynamic high-fidelity response systems that reduce or prevent the inciting event from occurring. Oxygen therapy is known to influence the depth and duration of desaturation epochs, thereby potentially influencing the accuracy of forecasting of desaturation. In this study, postoperative pulse oximetry data were retrospectively modeled using autoregressive methods to create prediction models for [Formula: see text] and imminent critical desaturation in the postoperative period. The accuracy of these models in predicting near future [Formula: see text] values was tested using root mean square error. The model accuracy for prediction of critical desaturation ([Formula: see text] [Formula: see text]) was evaluated using meta-analytical methods (sensitivity, specificity, likelihood ratios, diagnostic odds ratios and area under summary receiver operating characteristic curves). Between-study heterogeneity was used as a measure of reliability of the model across different patients and evaluated using the tau-squared statistic. Model performance was evaluated in [Formula: see text] patients who received postoperative oxygen supplementation and [Formula: see text] patients who did not receive oxygen. Our results show that model accuracy was high with root mean square errors between 0.2 and 2.8%. Prediction accuracy as defined by area under the curve for critical desaturation events was observed to be greater in patients receiving oxygen in the 60-s horizon ([Formula: see text] vs. [Formula: see text]). This was likely related to the higher frequency of events in this group (median [IQR] [Formula: see text] [Formula: see text]) than patients who were not treated with oxygen ([Formula: see text] [Formula: see text]; [Formula: see text]). Model reliability was reflected by the homogeneity of the prediction models which were homogenous across both prediction horizons and oxygen treatment groups. In conclusion, we report the use of autoregressive models to predict [Formula: see text] and forecast imminent critical desaturation events in the postoperative period with high degree of accuracy. These models reliably predict critical desaturation in patients receiving supplemental oxygen therapy. While high-fidelity prophylactic interventions that could modify these inciting events are in development, our current study offers proof of concept that the afferent limb of such a system can be modeled with a high degree of accuracy.

A conversion formula for comparing pulse oximeter desaturation rates obtained with different averaging times

Objective

The number of desaturations determined in recordings of pulse oximeter saturation (SpO₂) primarily depends on the time over which values are averaged. As the averaging time in pulse oximeters is not standardized, it varies considerably between centers. To make SpO₂ data comparable, it is thus desirable to have a formula that allows conversion between desaturation rates obtained using different averaging times for various desaturation levels and minimal durations.

Methods

Oxygen saturation was measured for 170 hours in 12 preterm infants with a mean number of 65 desaturations <90% per hour of arbitrary duration by using a pulse oximeter in a 2–4 s averaging mode. Using 7 different averaging times between 3 and 16 seconds, the raw red-to-infrared data were reprocessed to determine the number of desaturations (D). The whole procedure was carried out for 7 different minimal desaturation durations (≥1, ≥5, ≥10, ≥15, ≥20, ≥25, ≥30 s) below SpO₂ threshold values of 80%, 85% or 90% to finally reach a conversion formula. The formula was validated by splitting the infants into two groups of six children each and using one group each as a training set and the other one as a test set.

Results

Based on the linear relationship found between the logarithm of the desaturation rate and the logarithm of the averaging time, the conversion formula is: D₂ = D₁ (T₂/T₁)^c, where D₂ is the desaturation rate for the desired averaging time T₂, and D₁ is the desaturation rate for the original averaging time T₁, with the exponent c depending on the desaturation threshold and the minimal desaturation duration. The median error when applying this formula was 2.6%.

Conclusion

This formula enables the conversion of desaturation rates between different averaging times for various desaturation thresholds and minimal desaturation durations.

Comprehensive Oxygen Management for the Prevention of Retinopathy of Prematurity: the pediatrix experience

Comprehensive oxygen management, focused on avoiding hyperoxia and repeated episodes of hypoxia-hyperoxia in very low birth weight infants, has been successfully used for the reduction of retinopathy of prematurity. Building on this experience, the Comprehensive Oxygen Management for the Prevention of Retinopathy of Prematurity quality improvement initiative was developed to facilitate the spread and refinement of these techniques. The initiative focused on staff education and evaluation and redesign of the processes and practices involving oxygen use. Monitoring of the effectiveness of the system changes was supported through audits of clinical practice changes, use of oxygen saturation trending data, and the incidence of retinopathy of prematurity.

The problem of artifacts in patient monitor data during surgery: a clinical and methodological review

Artifacts are a significant problem affecting the accurate display of information during surgery. They are also a source of false alarms. A secondary problem is the inadvertent recording of artifactual and inaccurate information in automated record keeping systems. Though most of the currently available patient monitors use techniques to minimize the effect of artifacts, their success is limited. We reviewed the problem of artifacts affecting patient monitor data during surgical cases. Methods adopted by currently marketed patient monitors to eliminate and minimize artifacts due to technical and environmental factors are reviewed and discussed. Also discussed are promising artifact detection and correction methods that are being investigated. These might be used to detect and eliminate artifacts with improved accuracy and specificity.

New-Generation Pulse Oximetry in the Care of Critically Ill Patients

• Objective To review the published, peer-reviewed studies to date on use of the new-generation pulse oximeters, which are manufactured with algorithms to filter out patients’ body motions, and describe the application of new-generation pulse oximetry in clinical practice in critical care.

• Methods MEDLINE was used to locate appropriate articles on pulse oximetry for the years 1995 to 2003. Each article was examined for scientific merit, content, and applicability to clinical practice.

• Results A total of 17 relevant articles on the clinical performance of the new-generation pulse oximeters were reviewed, and the data were organized into a table.

• Conclusions The combination of studies done in both clinical and laboratory settings did not provide any strong and convincing evidence that the performance of any single new-generation device was superior to that of any other new-generation device. However, the clinical performance of all the new-generation pulse oximetry devices was better than that of earlier devices.

Human factors error and patient monitoring

A wide range of studies have shown that human factors errors are the major cause of critical incidents that threaten patient safety in the medical environments where patient monitoring takes place, contributing to approximately 87% of all such incidents. Studies have also shown that good cognitively ergonomic design of monitoring equipment for use in these environments should reduce the human factors errors associated with the information they provide. The purpose of this review is to consider the current state of knowledge concerning human factors engineering in its application to patient monitoring. It considers the prevalence of human factors error, principles of good human factors design, the effect of specific design features and the problem of the measurement of the effectiveness of designs in reducing human factors error. The conclusion of the review is that whilst the focus of human factors studies has, in recent years, moved from instrument design to organizational issues, patient monitor designers still have an important contribution to make to improving the safety of the monitored patient. Further, whilst better psychological understanding of the causes of human factors errors will in future guide better human factors engineering, in this area there are still many practical avenues of research that need exploring from the current base of understanding.