Effects of Acute, Profound Hypoxia on Healthy Humans: Implications for Safety of Tests Evaluating Pulse Oximetry or Tissue Oximetry Performance.

Extended periods of oxygen deprivation can produce acidosis, inflammation, energy failure, cell stress, or cell death. However, brief profound hypoxia (here defined as SaO2 50%-70% for approximately 10 minutes) is not associated with cardiovascular compromise and is tolerated by healthy humans without apparent ill effects. In contrast, chronic hypoxia induces a suite of adaptations and stresses that can result in either increased tolerance of hypoxia or disease, as in adaptation to altitude or in the syndrome of chronic mountain sickness. In healthy humans, brief profound hypoxia produces increased minute ventilation and increased cardiac output, but little or no alteration in blood chemistry. Central nervous system effects of acute profound hypoxia include transiently decreased cognitive performance, based on alterations in attention brought about by interruptions of frontal/central cerebral connectivity. However, provided there is no decrease in cardiac output or ischemia, brief profound hypoxemia in healthy humans is well tolerated without evidence of acidosis or lasting cognitive impairment.

Accurate and continuous respiratory rate using touchless monitoring technology.

PURPOSE: Respiratory rate is a commonly used vital sign with various clinical applications. It serves as a crucial marker of acute health issues and any significant alteration in respiratory rate may be an early warning sign of major issues such as infections in the respiratory tract, respiratory failure, or cardiac arrest. Timely recognition of changes in respiratory rate enables prompt medical action, while neglecting to detect a change may lead to adverse patient outcomes. Here, we report on the performance of respiratory rate determined using a depth sensing camera system (RRdepth) which allows for continuous, non-contact 'touchless' monitoring of this important vital sign. METHODS: Thirty adult volunteers undertook a range of set breathing rates to cover a target breathing range of 4-40 breaths/min. Depth information was acquired from the torso region of the subjects using an Intel D415 RealSense camera positioned above the bed. The depth information was processed to generate a respiratory signal from which RRdepth was calculated. This was compared to a manually scored capnograph reference (RRcap). RESULTS: An overall RMSD accuracy of 0.77 breaths/min was achieved across the target respiratory rate range with a corresponding bias of 0.05 breaths/min. This corresponded to a line of best fit given by RRdepth = 1.01 x RRcap - 0.22 breaths/min with an associated high degree of correlation (R = 0.997). A breakdown of the performance with respect to sub-ranges corresponding to respiratory rates or ≤7, >7-10, >10-20, >20-30, >30 breaths/min all exhibited RMSD accuracies of less than 1.00 breaths/min. We also had the opportunity to test the performance of spontaneous breathing of the subjects which occurred during the study and found an overall RMSD accuracy of 1.20 breaths/min with corresponding accuracies ≤1.30 breaths/min across each of the individual sub-ranges. CONCLUSIONS: We have conducted an investigative study of a prototype depth sensing camera system for the non-contact monitoring of respiratory rate. The system achieved good performance with high accuracy across a wide range of rates including both clinically important high and low rates.

Pulse Oximeter Performance, Racial Inequity, and the Work Ahead.

It has long been known that many pulse oximeters function less accurately in patients with darker skin. Reasons for this observation are incompletely characterized and potentially enabled by limitations in existing regulatory oversight. Based on decades of experience and unpublished data, we believe it is feasible to fully characterize, in the public domain, the factors that contribute to missing clinically important hypoxemia in patients with darkly pigmented skin. Here we propose 5 priority areas of inquiry for the research community and actionable changes to current regulations that will help improve oximeter accuracy. We propose that leading regulatory agencies should immediately modify standards for measuring accuracy and precision of oximeter performance, analyzing and reporting performance outliers, diversifying study subject pools, thoughtfully defining skin pigmentation, reporting data transparently, and accounting for performance during low-perfusion states. These changes will help reduce bias in pulse oximeter performance and improve access to safe oximeters.

Maximizing the laboratory setting for testing devices and understanding statistical output in pulse oximetry.

Maximizing the laboratory setting for testing baseline pulse oximetry accuracy in an arterial desaturation study requires a study design that considers management of several aspects in the physiology of the test subject, special attention to the device under test, and great care in the preanalytical (sample handling) and analytical (Co-oximeter) phases. Statistics used to describe the resulting SpO2 performance include Precision (size of the data cloud), Bias (offset of the data cloud), and A(rms) (accuracy root mean square), which combines the size and offset of the data cloud in one number. The A(rms) is the primary statistic required by regulatory organizations to describe general performance over the entire saturation range. It does not describe any one point, but is a compilation of all points over the range tested. Most pulse oximeters in use today specify an A(rms) of 2%. To meet this specification, two-thirds of the readings will be within 2% of the Co-oximeter reference; however, some individual readings can be as inaccurate as 6% or more. The A(rms) statistic does not have the capacity to represent all pulse oximeter behavior. Saturation pop-ups, drop-downs, frozen readings, and periods of no reading are not portrayed by the A(rms). The next steps in the advancement of regulatory validation testing would be to develop standards that include an expanded analysis of pulse oximeter performance by assessment of pop-ups, dropouts, frozen readings, and periods of no reading through assessment of sensitivity/specificity and possibly a "Performance Index" similar to the approach taken by Barker.

Individual-Specific, Beat-to-beat Trending of Significant Human Blood Loss: The Compensatory Reserve.

Current monitoring technologies are unable to detect early, compensatory changes that are associated with significant blood loss. We previously introduced a novel algorithm to calculate the Compensatory Reserve Index (CRI) based on the analysis of arterial waveform features obtained from photoplethysmogram recordings. In the present study, we hypothesized that the CRI would provide greater sensitivity and specificity to detect blood loss compared with traditional vital signs and other hemodynamic measures. Continuous noninvasive vital sign waveform data, including CRI, photoplethysmogram, heart rate, blood pressures, SpO2, cardiac output, and stroke volume, were analyzed from 20 subjects before, during, and after an average controlled voluntary hemorrhage of ∼1.2 L of blood. Compensatory Reserve Index decreased by 33% in a linear fashion across progressive blood volume loss, with no clinically significant alterations in vital signs. The receiver operating characteristic area under the curve for the CRI was 0.90, with a sensitivity of 0.80 and specificity of 0.76. In comparison, blood pressures, heart rate, SpO2, cardiac output, and stroke volume had significantly lower receiver operating characteristic area under the curve values and specificities for detecting the same volume of blood loss. Consistent with our hypothesis, CRI detected blood loss and restoration with significantly greater specificity than did other traditional physiologic measures. Single measurement of CRI may enable more accurate triage, whereas CRI monitoring may allow for earlier detection of casualty deterioration.

Validation of a conical cuff on the forearm for estimating radial artery blood pressure.

INTRODUCTION: There has been a growing need for and interest in measuring noninvasive blood pressure (NIBP) in obese patients. In many situations, available rectangular upper arm blood pressure cuffs do not fit properly, closing in a crisscross manner or overlapping the elbow. To address this issue, GE Healthcare has designed a conically shaped cuff for use on the forearm to estimate radial arterial blood pressure. This study evaluated using this forearm cuff with an oscillometric NIBP algorithm compared with an invasive radial arterial blood pressure reference. PATIENTS AND METHODS: Thirty-four patients were enrolled in the study with an aim to acquire a minimum of 150 paired measurements. Blood pressure was measured and recorded invasively from the radial artery of each patient, while noninvasive oscillometric measurements were acquired from the same limb using a conically shaped cuff placed on the patient's forearm. ANALYSIS: NIBP values were compared with the invasive blood pressure values acquired in the combined 30-s period preceding cuff inflation and the 30-s period following cuff deflation. The acceptance criteria for measurement accuracy were defined in accordance with the ANSI/AAMI/ISO 81060-2:2009 standard, which requires an absolute average error of 5 mmHg or less, with an SD of 8 mmHg or less. RESULTS: The systolic mean error (-0.82 mmHg) and SD (6.08 mmHg) and the diastolic mean error (1.53 mmHg) and SD (3.83 mmHg) were within the 81060-2 acceptance criteria. CONCLUSION: The statistical results show that oscillometric NIBP measurements taken with the conically shaped cuff placed on the forearm give an accurate estimation of radial arterial blood pressure.

Effects of tissue outside of arterial blood vessels in pulse oximetry: a model of two-dimensional pulsation.

We describe a new model of pulse oximetry that addresses the disagreement between theoretical calibration curves based on Beer-Lambert's Law and test results based on human test subjects. Sources of this discrepancy include variability among human subjects, experimental conditions and the effect of optical radiation propagating in tissue surrounding arteries. Unlike the conventional model, our model considers the change in the relative proportion of light that does or does not pass through blood in pulsating vessels in addition to the change in the path length of optical radiation through the blood. Theoretical calibration curves based on this model agree with human test results and help to explain the variability between in vitro and in vivo test conditions.

A characterization of motion affecting pulse oximetry in 350 patients.

As part of an oximetry research effort in which plethysmographic data were collected from moving patients, a wide variety of patient motion affecting pulse oximetry was observed and characterized by the clinical incidence, type, severity and duration of patient motion. 350 patients were observed for movement in clinical settings, including ICU, SICU, MICU, PICU, NICU, OR and PACU, at 4 hospitals and on an ambulance. 20% (70) of the patients exhibited motion. Half (35) of the moving patients were instrumented to record oximetric, plethysmographic and/or acceleration information. 31% of NICU infants moved compared to only 7% of adults in ICUs. The most common noisy oximetry signals were caused by motion characterized by extending/flexing (/kicking in infants) and by clenching/pressing/rubbing. In infants, these motion types accounted for 53% and 11% of motion, respectively. Less common infant motion types were patient cares, shifting body position and cough/cry. The most common adult motions were equally divided among extend/flex, clench/press/rub, twitch/shake and transport motion types. Extend/flex motions typically demonstrated high plethysmographic waveform modulation (71.5% maximum) and high acceleration. Clench/press/rub motions typically also had high modulations, but low G-force. With one high-G exception, twitch/shake motions had little or no effect on oximetry readings. Less common adult motion types affecting pulse oximetry included cough/cry, strain/posture, tremors and tap/bump. Most recorded motions were aperiodic and short-lived, 62% lasting less than 10 seconds, only 5% lasting over 1 minute. NICU patients made the longest lasting continuous series of motions, while adults made 86% of the motions lasting a second or less.