Quantifying pulse oximeter accuracy during hypoxemia and severe anemia using an in vitro circulation system.

Anemia and hypoxemia are common clinical conditions that are difficult to study and may impact pulse oximeter performance. Utilizing an in vitro circulation system, we studied performance of three pulse oximeters during hypoxemia and severe anemia. Three oximeters including one benchtop, one handheld, and one fingertip device were selected to reflect a range of cost and device types. Human blood was diluted to generate four hematocrit levels (40%, 30%, 20%, and 10%). Oxygen and nitrogen were bubbled through the blood to generate a range of oxygen saturations (O2Hb) and the blood was cycled through the in vitro circulation system. Pulse oximeter saturations (SpO2) were paired with simultaneously-measured O2Hb readings from a reference CO-oximeter. Data for each hematocrit level and each device were least-squares fit to a 2nd-order equation with quality of each curve fit evaluated using standard error of the estimate. Bias and average root mean square error were calculated after correcting for the calibration difference between human and in vitro circulation system calibration. The benchtop oximeter maintained good accuracy at all but the most extreme level of anemia. The handheld device was not as accurate as the benchtop, and inaccuracies increased at lower hematocrit levels. The fingertip device was the least accurate of the three oximeters. Pulse oximeter performance is impacted by severe anemia in vitro. The use of in vitro calibration systems may play an important role in augmenting in vivo performance studies evaluating pulse oximeter performance in challenging conditions.

Multi-analyte calibration and verification of a multi-parameter laser-based pulse oximeter.

Almost since its introduction pulse oximetry was known to overestimate oxygen saturation in cases of carbon monoxide poisoning or elevated methemoglobin (metHb) levels. To eliminate this dangerous behavior some manufacturers have added additional LED emitters to try to increase the number of measured hemoglobin species and to improve measurement accuracy, but have not been very successful. We hypothesized that the use of narrow-band laser light sources would make accurate and precise measurement of the four primary species of hemoglobin possible, even in cases of elevated levels of carboxyhemoglobin (COHb). Calibration and verification studies were performed on a tissue simulator that employed an artificial finger pulsating with whole human blood. This simulator allowed safe generation of 165 different combinations of the levels of oxyhemoglobin (O2Hb), COHb, metHb, and reduced hemoglobin (RHb) for calibration of the laser-based pulse oximeter. A follow-on study used 56 mixed hemoglobin levels for verification and statistical analysis of the performance of this device. This laser-based pulse oximeter measured all four species of hemoglobin accurately and precisely (ARMS ≤ 1.8%) for metHb levels in the clinically normal range. At elevated metHb levels the device continued to provide accurate and precise measurements of metHb and RHb (ARMS ≤ 1.7%). The use of monochromatic laser light sources can create a new generation of highly accurate, multi-parameter, pulse oximeters.

Respiratory waveform variation can prevent pulsus paradoxus measurement by sphygmomanometry.

BACKGROUND: Pulsus paradoxus (PP) represents increased fluctuation of systolic pressure during the respiratory cycle. PP increases in pathologic conditions, including asthma and other obstructive airways diseases. Respiratory waveform variation (RWV) represents arterial-waveform baseline variability resulting from intra-pleural pressure changes during the respiratory cycle in the presence of airway obstruction. It is not known whether RWV influences manual PP measurement using a sphygmomanometer and stethoscope. METHODS: We performed an observational study in six healthy adults. Participants performed tidal-breathing through a breathing apparatus with pre-determined inspiratory (0-45.6 cm H2O) and expiratory (0-24.4 cm H2O) resistance levels for a total of 23 data sets per participant. PP was measured from continuous radial artery pressure recordings as the absolute difference between maximum and minimum systolic pressure levels during a complete respiratory cycle. RESULTS: In this study, PP values measured without applied airway resistance exceeded 10 mmHg, the traditional definition of PP, in five of the six participants. Manual measurement of PP would not be possible at greater RWV because the maximum diastolic pressure exceeded minimum systolic pressure during RWV. CONCLUSIONS: PP in normal adults may exceed 10 mmHg, and RWV may be of sufficient magnitude to preclude manual PP measurement.

Interrater reliability of emergency physician-performed ultrasonography for diagnosing femoral, popliteal, and great saphenous vein thromboses compared to the criterion standard study by radiology.

PURPOSE: To assess the interrater reliability and test characteristics of lower limb sonographic examination for the diagnosis of deep venous and proximal great saphenous vein thrombosis when performed by Emergency Physicians (EPs) as compared to that by the Department of Radiology (Radiology). The secondary objective was to assess the effects of patient body mass index and EP satisfaction with bedside ultrasound on sensitivity and specificity. METHODS: A prospective study was conducted for patients with clinical suspicion for lower extremity thrombus. EPs evaluated for venous thrombosis in the common femoral vein, femoral vein of the thigh, popliteal vein, and proximal great saphenous vein. Subsequently, all patients received ultrasounds by Radiology, the criterion standard. RESULTS: One hundred ninety-seven patients (257 individual legs) were evaluated. There was 90-95% agreement between EP and Radiology, moderate kappa agreement for common femoral vein, and femoral vein of the thigh and fair kappa agreement for great saphenous vein and popliteal vein. The sensitivity and specificity of EP ultrasounds compared with criterion standard were lower than previously reported. There was no trend in patient body mass index or provider satisfaction influencing the test characteristics. CONCLUSIONS: Our study suggests that point-of-care sonography should not replace Radiology-performed scans. The required amount of training for EPs to be competent in this examination needs further investigation. © 2016 Wiley Periodicals, Inc. J Clin Ultrasound 44:360-367, 2016.

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.