Diagnostic room-air pulse oximetry: effects of smoking, race, and sex

Objective

We sought to determine the distribution of oximetry (Spo₂) values in awake, asymptomatic adults and the effect of personal characteristics on these values.

Methods

Using a cross-sectional design, we sampled oximetry readings in awake, asymptomatic adults in an emergency department setting. Personal characteristics were analyzed using logistic regression, with lower oximetry readings, defined by the 20th percentile, as the dependent variable.

Results

Of 871 eligible subjects, 50 (5.7%) had an Spo₂ value less than 97%, and 13 (1.5%) had an Spo₂ value less than 96%. Lower readings were associated with the following characteristics (odds ratio with 95% confidence interval): male sex, 3.8 (2.5-5.6); age ≥60 years, 2.4 (1.3-4.5); white race, 5.3 (3.6-7.8); obesity, 3.2 (2.1-4.8); history of asthma, 3.2 (1.6-6.2). Smoking was not associated with lower Spo₂ values.

Conclusion

Room-air Spo₂ values less than 97% are rare in asymptomatic, awake adults. White race and male sex are associated with lower Spo₂ readings.     

The ability of pulse oximetry to screen for hypoxemia and hypercapnia in patients breathing room air

This study was designed to determine whether high room-air pulse oximetry can rule out hypoxemia or moderate hypercapnia. Based on retrospective analysis of 513 arterial blood gas results, oxygen saturation cutpoints were derived. Coincidentally, a room-air oxygen saturation (RAO₂ sat) value of 96% was selected as a cutpoint to screen for both hypoxemia (PaO₂ < 70 mm Hg) and moderate hypercapnia (PaCO₂ > 50 mm Hg). These tests were validated prospectively by using a convenience sample of 213 Emergency Department patients in whom room-air arterial blood gas sampling was ordered. To detect hypoxemia, the sensitivity of RAO₂ sat ≤ 96% was 1.0 [0.95–1.0, 95% confidence interval (CI)] and specificity was 0.54 (0.45–0.64, 95% CI). To detect hypercapnia, the sensitivity of RAO₂ sat ≤ 96% was 1.0 (0.7–1.0) and specificity was 0.31 (0.25–0.38, 95% CI). We concluded that RAO₂ sat ≥ 97% rules out hypoxemia and may also rule out moderate hypercapnia.     

The accuracy of pulse oximetry in the emergency department

The objective of this retrospective study was to identify factors affecting the accuracy of pulse oximetry in the ED. Over a 3-year period, 664 consecutive emergency department (ED) patients had simultaneous arterial blood gas (ABG) and pulse oximeter readings taken. Pulse oximeter saturations (SpO2) were compared with ABG CO-oximeter saturations (SaO2) for accuracy. Multiple variables including age, sex, hemoglobin, bicarbonate, pH, and carboxyhemoglobin (COHb) were analyzed to see if they affected SpO2 accuracy. ROC curves were used to determine the best pulse oximeter threshold for detecting hypoxia. Using multivariate analysis, COHb was the only statistically significant factor affecting the accuracy of pulse oximetry. In patients with COHb <2%, SpO2 overestimated SaO2 by more than 4% in 8.4% of cases. In patients with COHb > or = 2%, SpO2 overestimated SaO2 by more than 4% in 35% of cases. The best pulse oximetry threshold for detecting hypoxia is 92%. At this threshold, if COHb is <2%, pulse oximetry has a sensitivity of 0.92 and specificity of 0.90. If COHb is > or =2%, sensitivity is 0.74 and specificity is 0.84. For patients likely to have a COHb < 2, pulse oximetry is an effective screening tool for detecting hypoxia. However, more caution must be exercised when using pulse oximetry in patients likely to have a COHb > or = 2%.     

The pulse in reflectance pulse oximetry: Modeling and experimental studies

Objective.Reflectance pulse oximetry permits the use of alternative monitoring sites such as the face or torso, and is the approach commonly employed in fetal pulse oximetry systems. The purpose of this study is to investigate the impact of assumptions about the nature of arterial pulsatility on the calibration of such systems. Methods. Monte Carlo simulations of reflectance pulse oximetry were run on a six-layer tissue model, varying depth and magnitude of the arterial pulse. SpO₂ readings on and off the femoral artery obtained during desaturation studies in newborn piglets were compared to predictions. Results. Monte Carlo simulation results clarified the difference between deep and shallow pulsatility found with photon diffusion models, agreeing with earlier in vivo observations. Significant overestimation of SpO₂ 75% and slight underestimation > 75% is expected if a sensor is placed on a highly pulsatile site. The on- and off-artery SpO₂ readings recorded during desaturation in the newborn piglet follow the model predictions. Conclusions. The sensitivity of reflectance pulse oximetry calibration to the depth and magnitude of arterial pulsatility reinforces the observation that monitoring site selection is of importance in optimizing reflectance pulse oximetry performance, particularly fetal pulse oximetry. Sites with palpable pulsatility should be avoided. Reuss JL, Siker D. The pulse in reflectance pulse oximetry: Modeling and experimental studies     

Safe use of pulse oximetry

Practice profiles are reflective pieces written by nurses in practice and based on continuing professional development articles. This week Ruth Garner discusses pulse oximetry. Article no. 501. Woodrow P (1999) Pulse oximetry. Nursing Standard. 13, 42, 42-46.     

Limitations of forehead pulse oximetry

During initial clinical tests to calibrate our reflectance pulse oximetry system, we observed serious physiologic limitations to the use of pulse oximetry in the forehead region. We present a case of simultaneous reflectance and transmission mode pulse oximetry monitoring in a child undergoing cardiac surgery for congenital cyanotic heart disease with a large intracardiac shunt. During general anesthesia, when the patient was endotracheally intubated and mechanically ventilated, the transmission mode saturation agreed well with arterial oxygen saturation measurements; but, our reflectance pulse oximeter, with the sensor applied to the forehead, displayed spuriously lower (–18%) oxygen saturations. Before and after anesthesia and surgery, there was fine agreement between reflectance and transmission mode saturation values. We suggest that the difference was caused by vasodilatation and pooling of venous blood due to compromised venous return to the heart, and a combination of arterial and venous pulsations in the forehead region. This means that the reflectance pulse oximeter measured a mixed arterial-venous oxygen saturation.This work was supported by a grant from the Desirée and Nils Yde Foundation, Zurich, Switzerland.     

The correct use of pulse oximetry in measuring oxygen status

Pulse oximetry is a valuable method of monitoring a patient's oxygenation levels and is an increasingly common test. It is important that any health-care professional conducting pulse oximetry is aware of the various factors that may influence the results obtained, ranging from incorrect use of the equipment to the effect of the patient's condition.     

The use of pulse oximetry to exclude pneumonia in children

The objective of this study was to determine whether pulse oximetry alone or in conjunction with the clinical examination is predictive of pneumonia in children who present to the emergency department with respiratory complaints. A retrospective comparison of children with radiographic pneumonia with children with respiratory complaints and negative chest radiography was used. The study took place in an emergency department of a large academic, tertiary care hospital. All children less than 24 months of age who presented with a respiratory complaint and underwent chest radiography during a 1-year period were included. Charts of children with radiographic pneumonia were compared with charts of children without pneumonia, retrospectively. Data abstracted onto data collection forms included: pulse oximetry measurement, vital signs, general appearance, lung examination, and final radiology interpretation of chest radiographs. Pneumonia was defined as a chest radiograph showing any opacity consistent with pneumonia as read by a board-prepared or -certified radiologist. A total of 803 children qualified for the study. Radiograph interpretations were available for 762, and 10.5% were found to have radiographic pneumonia. The median pulse oximetry reading of children with radiographic pneumonia was 97% (interquartile range 95th-98th percentile) compared with 98% (interquartile range 96th-99th percentile) in the control group. Forty-five percent (35 of 78) of the children with radiographic pneumonia showed oxygen saturations of 98% or higher with greater than 10% (8 of 78) displaying oxygen saturations of 100%. By using logistic regression, pulse oximetry was not found to be a statistically significant predictive variable for radiographic pneumonia. Pulse oximetry could not be used to rule out the presence of radiographic pneumonia in children less than 2 years of age who presented with respiratory complaints.     

The role of pulse oximetry in the accident and emergency department.

Prompt recognition and treatment of hypoxia is an important part of management in the accident and emergency (A & E) department. Until recently the only reliable method of detecting hypoxia was by estimation of the arterial blood gases (ABG). Continuous monitoring of the arterial oxygen saturation (Sao2) is possible using an infra-red pulse oximeter. This study assessed the usefulness of this instrument in the A&E setting. The Sao2 was measured in 50 patients using a pulse oximeter. In 15 patients simultaneous ABG estimations were obtained. The Sao2 correlated closely with calculated values for Sao2. The use of the oximeter identified 21 patients (42%) with clinically unsuspected hypoxia. The pulse oximeter proved simple to use, accurate and a useful addition to our resuscitation equipment.     

The utility of iPhone oximetry apps: A comparison with standard pulse oximetry measurement in the emergency department

ObjectivesTo determine if a correlation exists between 3 iphone pulse ox applications' measurements and the standard pulse oximetry (SpO2) and whether these applications can accurately determine hypoxia.MethodsThree applications reportedly measuring SpO2 were downloaded onto an iPhone 5s. Two of these applications used the onboard light and camera lens “Pulse Oximeter” (Pox) and “Heart Rate and Pulse Oximeter” (Ox) and one used an external device that plugged into the iphone (iOx). Patients in the ED were enrolled with chief complaints of cardiac/pulmonary origin or a SpO2 ≤ 94%. All measurements were compared to controls. Concordance correlation coefficients, sensitivity, and specificity were calculated.ResultsA total of 191 patients were enrolled. The concordance correlation of iOx with control was 0.55 (CI 0.46, 0.63), POx was 0.01 (CI −0.09, 0.11), and Ox was 0.07 (CI −0.02, 0.15). 68/191 patients (35%) were found to have hypoxemia. Sensitivities for detecting hypoxia were 69%, 0%, and 7% for iOx, POx, and Ox, respectively. Specificities were 89%, 100%, and 89%. Even iOx (the most accurate) 21 (11%) were incorrectly classified nonhypoxic, and 22 (12%) were incorrectly classified hypoxic.ConclusionsWhile iOx has modest concordance with control, Ox and POx showed almost none. The iOx device was best in correctly identifying hypoxia patients, but almost 1/4 of patients were incorrectly classified. The three apps provided inaccurate SpO2 measurements and had limited to no ability to accurately detect hypoxia. These apps should not be relied upon to provide accurate SpO2 measurements in emergent, even austere conditions.     

Accuracy of pulse oximetry in the intensive care unit

OBJECTIVE: Pulse oximetry (SpO2) is a standard monitoring device in intensive care units (ICUs), currently used to guide therapeutic interventions. Few studies have evaluated the accuracy of SpO2 in critically ill patients. Our objective was to compare pulse oximetry with arterial oxygen saturation (SaO2) in such patients, and to examine the effect of several factors on this relationship. DESIGN: Observational prospective study. SETTING: A 26-bed medical ICU in a university hospital. PATIENTS: One hundred two consecutive patients admitted to the ICU in whom one or serial arterial blood gas analyses (ABGs) were performed and a reliable pulse oximeter signal was present. INTERVENTIONS: For each ABG, we collected SaO2, SpO2, the type of pulse oximeter, the mode of ventilation and requirement for vasoactive drugs. MEASUREMENTS AND RESULTS: Three hundred twenty-three data points were collected. The mean difference between SpO2 and SaO2 was -0.02% and standard deviation of the differences was 2.1%. From one sample to another, the fluctuations in SpO2 to arterial saturation difference indicated that SaO2 could not be reliably predicted from SpO2 after a single ABG. Subgroup analysis showed that the accuracy of SpO2 appeared to be influenced by the type of oximeter, the presence of hypoxemia and the requirement for vasoactive drugs. Finally, high SpO2 thresholds were necessary to detect significant hypoxemia with good sensitivity. CONCLUSION: Large SpO2 to SaO2 differences may occur in critically ill patients with poor reproducibility of SpO2. A SpO2 above 94% appears necessary to ensure a SaO2 of 90%.     

Increasing clinical use of pulse oximetry

Changing the system from measuring blood gases through invasive measures to using noninvasive pulse oximetry is a challenge in the Critical Care Unit where invasive techniques are taken for granted. The authors report a project that was successful in increasing the use of noninvasive monitoring techniques by critical care nurses in a Surgical Intensive Care Unit. This clinical project became an important aspect of incorporating the staff in a change to more extensive use of pulse oximetry. This study defines the change in nursing practice with the use of pulse oximetry. The authors discuss three areas: (1) demonstration of the correlation between O2Sat as measured by the pulse oximeter and arterial blood gas saturations; (2) introduction of the pulse oximeter as a reliable alternative to ABGs when monitoring oxygenation; and (3) the establishment of guidelines for using pulse oximetry within the clinical setting.