Fluorescent light interferes with pulse oximetry

Arterial oxygen saturation (SaO₂) values displayed on the pulse oximeter dropped dramatically in 3 children undergoing neurosurgical procedures when a hand-held fluorescent light was used to observe the patients. Pulse rates were unchanged on both the electrocardiograph and pulse oximeter. Electromagnetic interference was excluded as the cause of desaturation. A great deal of energy was emitted by the hand-held light in the 660-nm region, which is one of the wavelengths used by the oximeter. False readings of pulse rate and SaO₂ values caused by ambient light could be avoided if oximeter probes were manufactured of black opaque material that does not transmit light or enclosed in an opaque plastic housing.     

Pulse oximetry: Analysis of theory,technology, and practice

Interest in two-wavelength classic, that is, nonpulse, oximetry began early in the 20th century. Noninvasive in vivo measurements of oxygen saturation showed promise, but the methods were beset by several problems. The pulse oximetry technique, by focusing on the pulsatile arterial component, neatly circumvented many of the problems of the classic nonpulse arterial approach. Today's pulse oximeter owes a good measure of its success to the technologic advances in light emission and detection and the ready availability of microcomputers and their software. Many clinicians have recognized how valuable the assessment of the patient's oxygenation in real time can be. This appreciation has propelled the use of pulse oximeters into many clinical fields, as well as nonclinical fields such as sports training and aviation. Understanding how and what pulse oximetry measures, how pulse oximetry data compare with data derived from laboratory analysis, and how the pulse oximeter responds to dyshemoglobins, dyes, and other interfering conditions must be understood for the correct application and interpretation of this revolutionary monitor.     

Skin reflectance pulse oximetry: In vivo measurements from the forearm and calf

This study describes the results from a series of human experiments demonstrating the ability to measure arterial hemoglobin oxygen saturation (SaO₂) from the forearm and calf using a reflectance pulse oximeter sensor. A special optical reflectance sensor that includes a heating element was interfaced to a temperature controller and a commercial Data-scope ACCUSAT pulse oximeter that was adapted for this study to perform as a reflectance pulse oximeter. The reflectance pulse oximeter sensor was evaluated in a group of 10 healthy adult volunteers during steady-state hypoxia. Hypoxia was induced by gradually lowering the inspired fraction of oxygen in the breathing gas mixture from 100 to 12%. Simultaneous SaO₂ measurements obtained from the forearm and calf with two identical reflectance pulse oximeters were compared with SaO₂ values measured by a finger sensor that was interfaced to a standard Datascope ACCUSAT transmittance pulse oximeter. The equations for the best-fitted linear regression lines between the percent reflectance, SpO₂(r), and transmittance, SpO₂(t), values in the range between 73 and 100% were SpO₂(r)=–7.06+1.09 SpO₂(t) for the forearm (n=91,r=0.95) and SpO₂(r)=7.78+0.93 SpO₂(t) for the calf (n=93,r=0.88). The regression analysis of the forearm data revealed a mean ± SD error of 2.47±1.66% (SaO₂=90–100%), 2.35±2.45% (SaO₂=80–89%), and 2.42±1.20% (SaO₂=70–79%). The corresponding regression analysis of the calf data revealed a mean ± SD error of 3.36±3.06% (SaO₂=90–100%), 3.45±4.12% (SaO₂=80–89%), and 2.97±2.75% (SaO₂=70–79%). This preliminary study demonstrated the feasibility of measuring SaO₂ from the forearm and calf in healthy subjects with a heated skin reflectance sensor and a pulse oximeter.Financial support for this study was provided in part by the Datascope Corporation and NIH Grant R15 GM36111-01A1.The authors would like to acknowledge the clinical assistance of Albert Shahnarian, PhD, Gary W. Welch MD, PhD, and Robert M. Giasi, MD, Department of Anesthesiology, University of Massachusetts Medical Center, Worcester, MA. We also thank Paul A. Nigromi, Datascope Corporation, Paramus, NJ, and Kevin Hines, Semiconductor Division, Analog Devices, Wilmington, MA, for technical assistance. The skillful art work by Yi Wang is also greatly appreciated.     

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.     

A new system to record reliable pulse oximetry data from the nellcor N-200 and its applications in studies of variability in infant oxygenation

We have developed a simple system for internal validation of oximetry data collected over many hours from the Nellcor N-200 pulse oximeter (Nellcor, Inc., Hayward, CA). This system uses signals from the oximeter alone and a validation algorithm that is based in a computer connected to the oximeter. Unlike other validation systems, this system does not require connections to other monitors. The system was tested on 10 acutely ill newborns in an intensive care nursery over 16 hr of continuous recording for each infant (birthweight, 2.50±0.73 kg; age, 3.4±3.2 days). Oximetry data were accepted as valid using the new system if they surpassed a minimum level of quality (empirically derived, and equal to a 60% fractional success in pulse detection). The validated oximetry data were compared to data obtained using a conventional compared to the electrocardiogram (ECG) algorithm. For the new and the conventional algorithms, the distributions of validated SpO₂ percents were nearly identical, with data rejection rates of 28.9% for the new system and 37.3% for the conventional system. In the newborns, the new system was used to demonstrate that as the mean saturations decreased, there were striking increases in variability about the reported mean saturation (p < 0.001). While variability in infant SpO₂ is a well-known phenomenon, the amount seen here was unexpected. For example, the range of true saturations frequently recorded was quite wide at a reported mean SpO₂ of 90% (from 81 to 94%; but, the range was only from 92 to 98% at a mean SpO₂ of 96%). These findings demonstrate the usefulness of the new system and, if substantiated in more detailed studies, have important implications for the use of pulse oximeters to assess oxygenation in newborns.     

Pulse oximetry

The pulse oximeter, a widely used noninvasive monitor of arterial oxygen saturation, has numerous applications in anesthesiology and critical care. Although pulse oximetry is considered sufficiently accurate for many clinical purposes, there are significant limitiations on the accuracy and availability of pulse oximetry data. This article reviews both the clinical uses of the pulse oximeter and the limitations on its performance. The pulse oximeter is generally acknowledged to be one of the most important advances in the history of clinical monitoring.     

Pulse oximetry

The pulse oximeter, a widely used noninvasive monitor of arterial oxygen saturation, has numerous applications in anesthesiology and critical care. Although pulse oximetry is considered sufficiently accurate for many clinical purposes, there are significant limitiations on the accuracy and availability of pulse oximetry data. This article reviews both the clinical uses of the pulse oximeter and the limitations on its performance. The pulse oximeter is generally acknowledged to be one of the most important advances in the history of clinical monitoring.     

Evaluation of the Datascope ACCUSAT pulse oximeter in healthy adults

Arterial hemoglobin oxygen saturation measured with the Datascope ACCUSAT pulse oximeter was compared with simultaneous arterial hemoglobin oxygen saturation measurements in healthy adult volunteers. One hundred thirty-five arterial blood samples ranging in saturation from 63 to 100% were obtained from 15 adults, aged 20 to 43 years. These subjects had different skin pigmentation, hematocrit, and smoking habits. Steady-state hypoxia was achieved by varying the inspired oxygen concentration between 10 and 21%. Readings from the Datascope ACCUSAT pulse oximeter and the Hewlett-Packard 47201A ear oximeter were compared with arterial blood samples analyzed by the Instrumentation Laboratories IL 282 in vitro CO-Oximeter. The equation for the best fitted linear regression line between the ACCUSAT pulse oximeter and the reference IL 282 CO-Oximeter was: ACCUSAT=1.08(IL)–6.86. The linear regression analysis revealed a high degree of correlation (r=0.99), and a small standard error of the estimate (SEE=1.29%). Simultaneous comparisons between arterial hemoglobin oxygen saturation measured with the ACCUSAT pulse oximeter and the Hewlett-Packard ear oximeter also showed a close correlation (r=0.99, SEE=1.47%). A similar comparison between the ACCUSAT and the Ohmeda 3700 pulse oximeter revealed good correlation (r=0.99, SEE=1.72%). We found that the ACCUSAT pulse oximeter is an accurate instrument for measuring arterial hemoglobin oxygen saturation noninvasively in the range between 60 and 100%.     

Diffusion hypoxia: A reappraisal using pulse oximetry

Sixty healthy surgical patients were monitored during surgery with a pulse oximeter. At the completion of the operation, nitrous oxide and oxygen were discontinued abruptly in 50 of these patients. During air breathing, a small drop in arterial hemoglobin oxygen saturation (SaO₂), to about 4% below preoperative values, was observed in all patients. In 10 patients, only oxygen was given before removal of the mask. There was no sudden drop in SaO₂ in these patients, but by 5 minutes after discontinuation there was no difference between the two groups of patients—SaO₂ was reduced 2 to 3% below preoperative values in both groups. For patients without cardiopulmonary disease, the phenomenon of diffusion hypoxia is a mild and transient event. Clinically significant hypoxemia (SaO₂<90%) after removal of nitrous oxide/oxygen at the completion of the anesthetic occurred in 3 patients (6%) and was associated with airway obstruction in each case.     

Failure rate of pulse oximetry in the postanesthesia care unit

Objective. The objective of this study was to prospectively examine the incidence of patient-related failure of pulse oximetry in the postanesthesia care unit (PACU).Methods. We studied 2,937 patients who, after receiving anesthesia, were admitted to the PACU at the University of Washington Medical Center from December 1989 through May 1990. Pulse oximetry readings were recorded using a Nellcor N-200 oximeter without electrocardiographic synchronization. Failure was defined as the inability to obtain a pulse oximetry reading for 2 or more 15-minute periods after eliminating probe position or mechanical malfunctions.Results. The overall failure rate in our study was 0.64%, with 19 patient-related pulse oximetry failures from 2,937 cases. Patients on whom the device failed were significantly older (62±18 vs 46±19 yr [mean ± SD];p<0.01), had higher median American Society of Anesthesiologists status (3 vs 2), and had longer operations than nonfailure patients (328±182 vs 185±127 min;p<0.01). There was no difference in the duration of PACU times for both groups.Conclusions. The failure rate and patient characteristics compare favorably with a previously published study of intraoperative pulse oximetry failure. We conclude that while the pulse oximeter is a reliable instrument for the measurement of blood oxygenation, there is a small but consistent incidence of patient-related failure with this monitoring device in the PACU.

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Kurzfassung Ziel. Ziel dieser Studie war es, das Auftreten von zu erwartenden patientenbezogenen Fehlern bei der Pulsoximetrie im Rahmen der Überwachung nach der Anästhesie zu untersuchen.Methoden. Wir untersuchten 2937 Patienten, die von Dezember 1989 bis Mai 1990 nach einer Anästhesie am University of Washington Medical Center überwacht wurden. Die Pulsoximetrie-Meßwerte wurden mit einem Nellcor N-200-Oximeter ohne elektrokardiographische Synchronisation aufgezeichnet. Ein Fehler wurde definiert als das Unvermögen, Pulsoximetrie-Meßwerte über zwei oder mehr 15-minütige Zeiträume zu erhalten, nachdem Störungen der Fühlerposition oder mechanische Störungen ausgeschaltet worden waren.Ergebnisse. Die Gesamtfehlerrate in unserer Studie betrug 0,64% bei 19 patientenbezogenen Pulsoximetriefehlern in 2937 Fällen. Die Patienten, bei denen ein Fehler am Gerät auftrat, waren signifikant älter (62±18 zu 46±19 Jahre [Mitte] ± SD];p<0,01), besaßen einen höheren mittleren American Society of Anesthesiologists-Status (3 zu 2) und hatten längere Operationen als die Patienten, bei denen kein Gerätefehler aufgetreten war. Es gab keinen Unterschied zwischen den beiden Gruppen in der Dauer der Überwachung nach der Anästhesie.Schlußfolgerung. Die Fehlerrate und die Patientenmerkmale sind mit denen einer früher veröffentlichten Untersuchung über intraoperative Pulsoximetriefehler vergleichbar. Wir schließen daraus, daß, während das Pulsoximeter ein verläßliches Instrument zur Messung der Sauerstoffsättigung ist, im geringen, aber konsistenten Umfang patientenbezogene Fehler bei diesem Überwachungsgerät während der Überwachung nach der Anästhesie auftreten.

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Resumen Objetivo. El objetivo de este estudio fue examinar prospectivamente la incidencia de fracasos relacionados al paciente de la oximetría de pulso en la unidad de cuidados postanestésicos (PACU).Métodos. Estudiamos 2937 pacientes, quienes luego de recibir anestesia fueron admitidos a la PACU del Centro Médico de la Universidad de Washington, entre Diciembre de 1989 y Mayo de 1990. Fracaso fue definido como la imposibilidad de obtener lectura de oximetría de pulso por dos o más períodos de 15 minutos, una vez corregidos los problemas debidos a mala posición del sensor o problemas mecánicos.Resultados. La incidencia global de fracasos relacionados al paciente de la oximetría de pulso en este estudio fue 0.64%, con 19 fracasos en 2,937 casos. Los pacientes en quienes no se pudo obtener señal eran significativamente mayores (62±18 vs 46±19 años [promedio ± DS]; p<0.01), presentaban mayor mediana en el valor del indice de estado físico de la Sociedad Americana de Anestesiología (3 vs 2), y habían sido sometidos a operaciones más prolongadas (328±182 vs 185±127 minutos; p<0.01), que los pacientes en quienes se obtuvo adecuada señal. No hubo diferencia en la duración de la estadía en PACU entre ambos grupos.Conclusiones. La incidencia de fracaso en obtener oximetría de pulso y las características de los pacientes se comparan favorablemente con un estudio previo de fracaso de oximetría de pulso duante el período intraoperatorio. Concluimos que, si bien el oxímetro de pulso es un instrumento confiable para la medición de la oxigenación sanguínea, existe una pequeña pero constante incidencia de fracaso de medición relacionado al paciente usando este equipo de monitorización en PACU.

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Résumé Objectifs. Le but de cette étude est d'examiner de manière prospective l'incidence des éches de l'oxymétrie de pouls dûs au patient dans une unité de réveil.Méthodes. Nous avons étudié 2937 patients en période post-anesthésique admis dans l'unite de réveil deUniversity of Washington Medical Center, de Décember 1989 jusqu'à Mai 1990. La saturation artérielle en oxygène était mesurée à l'aide d'un oxymètre de pouls Nellcor N-200 sans synchronisation électrocardiographique. Un échec était défini comme l'incapacité à obtenir une valeur de saturation artérielle en oxygène durant plus d'une (deux ou plus) périodes de quinze minutes, en dehors de problèmes de positionnement du capteur ou de dysfonctionnements méchaniques.Resultats. le taux global d'échec dans notre étude a été de 0,64%, avec 19 échecs de l'oxymétrie de pouls dûs aux patients parmi les 2937 patients étudiés. Les patients chez lesquels l'oxymétrie de pouls était défaillante étaient significativement plus vieux (62±18 vs 46±19; moyenne ± écart-type; p<0,01), avaient une classification ASA plus élevée (3 vs 2), et bénéficiaient d'interventions plus longues (328±182 vs 185±127 minutes; p<0,01) que les patients sans échec de monitorage. Il n'existait aucune différence dans la durée des séjours en unité de réveil dans les deux groupes.Conclusions. Le taux d'échec et les caractéristiques des patients sont comparables avec ceux issus d'une étude préalable portant sur les échecs de l'oxymètrie de pouls durant la période opératoire. Nous concluons que, alors que l'oxymètre de pouls est un unstrument fiable de mesure de l'oxygénation du sang, il existe une fiable, mais non négligeable, incidence d'échecs dûs au patient de ce monitorage en unité de réveil.

     

Can people hear the pitch change on a variable-pitch pulse oximeter?

The introduction of the variable-pitch feature on pulse oximeters in 1983 by the Nellcor Corporation (Hayward, CA) allowed users to rapidly detect changes in oxygen saturation by listening for changes in the pitch of the tones emitted by the pulse oximeter. A few individuals have reported that they have been unable to detect a change in pitch when oxygen saturation changes. To these individuals, the variable-pitch feature of these pulse oximeters has not been beneficial. Using the pitches from one manufacturer of oximeters, we created a computer program to simulate the pitches that accompanied various oxygen saturations. The pitches were recorded onto a tape player and played for 75 volunteer subjects unfamiliar with the pitches of a variable-pitch pulse oximeter. Of our sample, 67% were able to detect a single change in pitch corresponding to a 1% fall in oxygen saturation, and 11% of the population could not detect a change in pitch until there was a change in pitch with every beat. We suggested four alternative designs that may prove beneficial to this group of individuals.     

Can people hear the pitch change on a variable-pitch pulse oximeter?

The introduction of the variable-pitch feature on pulse oximeters in 1983 by the Nellcor Corporation (Hayward, CA) allowed users to rapidly detect changes in oxygen saturation by listening for changes in the pitch of the tones emitted by the pulse oximeter. A few individuals have reported that they have been unable to detect a change in pitch when oxygen saturation changes. To these individuals, the variable-pitch feature of these pulse oximeters has not been beneficial. Using the pitches from one manufacturer of oximeters, we created a computer program to simulate the pitches that accompanied various oxygen saturations. The pitches were recorded onto a tape player and played for 75 volunteer subjects unfamiliar with the pitches of a variable-pitch pulse oximeter. Of our sample, 67% were able to detect a single change in pitch corresponding to a 1% fall in oxygen saturation, and 11% of the population could not detect a change in pitch until there was a change in pitch with every beat. We suggested four alternative designs that may prove beneficial to this group of individuals.     

Pulse oximetry during cardiac catheterization in children with congenital heart disease

One hundred and five children with congenital heart disease were monitored by pulse oximetry during cardiac catheterization. Excellent correlation (r = 0.95) was found between oxygen saturation values obtained with pulse oximetry and those obtained from arterial blood in 133 data pairs. This correlation was described by the regression equation y = 0.91 x + 8.1. The correlation was also excellent in 47 data pairs with saturation values of less than 90% (r = 0.94, y = 0.93x + 6.0) from 36 cyanotic children. The clinical usefulness of pulse oximetry in the early recognition of decreased pulmonary blood flow or partial airway obstruction was demonstrated. Early diagnosis of changes in oxygenation was especially helpful in children with cyanotic congenital heart disease, in whom small changes in arterial oxygen tension may cause large changes in oxygen saturation.     

Interference in a pulse oximeter from a fiberoptic light source

A healthy patient who was undergoing cystoscopy suddenly showed a rapid increase in heart rate on a pulse oximeter. The cause was determined to be incident light from the light source for the cystoscope.     

Simple and noninvasive indicator of pulmonary gas exchange impairment using pulse oximetry

We postulated that the fractional inspired oxygen concentration (FiO₂) required to achieve a certain value of arterial oxygen saturation (SaO₂) can be used as an indicator of pulmonary gas exchange impairment in patients during mechanical ventilation. We tested this hypothesis in 20 patients. By reducingFiO₂ in increments of 10 vol% of capacity while monitoring SaO₂ with pulse oximetry, we could determineFi ₉₈,Fi ₉₇,Fi ₉₆, andFi ₉₅; that is, the yields 98, 97, 96, and 95% SaO₂, respectively. On the basis of our data, we choseFi ₉₈ as the most appropriate index, as an SaO₂ of 97% or below could not be achieved even with a lowFiO₂ in some of the patients. To test the significance of the newly proposed index, we comparedFi ₉₈ with the alveolar-arterial oxygen tension difference, P(A–a)O₂, and with the respiratory index, which are routinely used elsewhere. The correlation betweenFi ₉₈ and P(A–a)O₂ was excellent: P(A–a)O₂=490.5*Fi ₉₈+117.2 with a correlation coefficient of 0.906 (P<0.01).Fi ₉₈ also correlated significantly with the respiratory index: respiratory index=4.354*Fi ₉₈–0.776 (r=0.889,P<0.01).We conclude thatFi ₉₈ may be used as a simple index for the rough estimation of pulmonary gas exchange impairment without the need for invasive procedures. However, further studies are needed to confirm the validity of our method in hemodynamically unstable patients or when other brands of pulse oximeters are used.     

Does measurement of systolic blood pressure with a pulse oximeter correlate with conventional methods?

The pulse oximeter is commonly used in the operating room. We evaluated the use of a pulse oximeter to monitor systolic blood pressure in 20 healthy volunteers and 42 anesthetized patients. We compared the pulse oximeter method of measuring systolic blood pressure with the cuff methods using Korotkoff sounds and Doppler ultrasound as well as with direct pressure measurement through an intraarterial cannula. Systolic blood pressure values obtained by pulse oximeter correlated well with values obtained by other conventional methods. The best correlation was found with Doppler ultrasound (r = 0.996) and the worst with arterial cannulation (r = 0.880). We conclude that this method can be used intraoperatively to measure systolic blood pressure.     

Pulse oximetry and finger blood pressure measurement during open-heart surgery

Pulse oximeter arterial hemoglobin oxygen saturation (SpO₂) and finger arterial pressure (FINAP) were continuously monitored before, during, and after cardiopulmonary bypass in 15 male patients. SpO₂ was monitored simultaneously with two pulse oximeters, a Nellcor N-100 and an Ohmeda Biox III. The readings obtained from the two pulse oximeters were compared with arterial blood measurements obtained using a CO-oximeter. FINAP was monitored by a prototype device (Finapres) based on the Peaz volume-clamp method. FINAP was correlated with intraarterial pressure (IAP). Both pulse oximeters functioned well before cardiopulmonary bypass. The correlations with CO-oximeter values were 0.927 for the N-100 and 0.921 for the Biox III. Immediately after the onset of cardiopulmonary bypass, the N-100 pulse oximeter stopped displaying values. The Biox III pulse oximeter continued to display values during the cardiopulmonary bypass period; the correlation with CO-oximeter values was 0.813. After cardiopulmonary bypass, the N-100 began displaying values in 2 to 10 minutes. After cardiopulmonary bypass the correlation with CO-oximeter values was 0.792 for the N-100 and 0.828 for the Biox III pulse oximeter. The Finapres finger blood pressure device functioned well in 13 of 15 patients before cardiopulmonary bypass. The mean bias ± precision of FINAP-IAP for mean pressure was 8.3±10.2 mm Hg (SD) and the correlation coefficient was 0.814. During cardiopulmonary bypass, the Finapres device functioned well in 10 of 15 patients. The mean bias precision of FINAP-IAP, for mean pressure in these 10 patients was 6.6±8.7 mm Hg and the correlation coefficient was 0.902. Immediately after cardiopulmonary bypass, the Finapres functioned well in 11 of 15 patients. The mean bias ± precision of FINAP-IAP for mean pressure was 8.6±14.1 mm Hg and the correlation coefficient was 0.533. This study documented that devices for continuous noninvasive monitoring can usually function well under the extreme conditions seen during open-heart surgery. Pulse oximeters may find a place in the monitoring of patients during open-heart surgery, although they cannot totally replace the invasive techniques. Under the conditions of diminished pulsatile peripheral blood flow we observed some differences between the two pulse oximeters.     

The prevalence of hypoxemia detected by pulse oximetry during recovery from anesthesia

Pulse oximetry was used to assess the prevalence of hypoxemia (arterial oxygen saturation of 90% or less) at various times in the immediate postoperative period: five minutes after arrival, 30 minutes later, and just before discharge. Among 149 inpatients studied, one or more hypoxemic measurements were made in 21 (14%) during their postoperative course. Of 92 outpatients, 1 (1%) was found to be hypoxemic. For inpatients, the prevalence of hypoxemia preoperatively, 5 minutes after arrival in recovery, 30 minutes later, and at discharge was 2%, 4%, 6%, and 9%, respectively. Patient factors associated with a significantly higher prevalence of hypoxemia were obesity (22%), body cavity surgical procedures (24%), age over 40 years (18%), American Society of Anesthesiologists physical status (I, 7%; II, 17%; III, 18%; IV, 100%), duration of anesthesia longer than 90 minutes (18%), and intraoperative administration of greater than 1,500 ml of fluid (20%). Unrecognized hypoxemia in postsurgical inpatients with or without these risk factors is common. Therefore routine monitoring of these patients with a pulse oximeter is suggested.     

Experimental and clinical evaluation of a noninvasive reflectance pulse oximeter sensor

The objective of this study was to evaluate a new reflectance pulse oximeter sensor. The prototype sensor consists of 8 light-emitting diode (LED) chips (4 at 665 nm and 4 at 820 nm) and a photodiode chip mounted on a single substrate. The 4 LED chips for each wavelength are spaced at 90-degree intervals around the substrate and at an equal radial distance from the photodiode chip. An optical barrier between the photodiode and LED chips prevents a direct coupling effect between them. Near-infrared LEDs (940 nm) in the sensor warm the tissue. The microthermocouple mounted on the sensor surface measures the temperature of the skin-sensor interface and maintains it at a preset level by servoregulating the current in the 940-nm LEDs. An animal study and a clinical study were performed. In the animal study, 5 mongrel dogs (weight, 10–20 kg) were anesthetized, mechanically ventilated, and cannulated. In each animal, arterial oxygen saturation (SaO₂) was measured continuously by a standard transmission oximeter probe placed on the dog's earlobe and a reflectance oximeter sensor placed on the dog's tongue. In the first phase of the experiment, signals from the reflectance sensor were recorded while the dog was immersed in ice water until its body temperature decreased to 30°C. In the second phase, the animal's body temperature was normal, and the oxygen content of the ventilator was varied to alter the SaO₂. In the clinical study, 18 critically ill patients were monitored perioperatively with the prototype reflectance sensor. The first phase of the study investigated the relationship between local skin temperature and the accuracy of oximeter readings with the reflectance sensor. Each measurement was taken at a high saturation level as a function of local skin temperature. The second phase of the study compared measurements of oxygen saturation by a reflectance oximeter (SpO₂[r]) with those made by a co-oximeter (SaO₂[IL]) and a standard transmission oximeter (SpO₂[t]). Linear regression analysis was used to determine the degree of correlation between (1) the pulse amplitude and skin temperature; (2) SpO₂(r) and SaO₂(IL); and (3) SpO₂(t) and SaO₂(IL). Student'st test was used to determine the significance of each correlation. The mean and standard deviation of the differences were also computed. In the animal study, pulse amplitude levels increased concomitantly with skin temperature (at 665 nm,r=0.9424; at 820 nm,r=0.9834;p<0.001) and SpO₂(r) correlated well with SaO₂(IL) (r=0.982; SEE=2.54%;p<0.001). The results of the clinical study are consistent with these findings. The proto-type reflectance pulse oximeter sensor can yield accurate measurements of oxygen saturation when applied to the forehead or cheek. It is, therefore, an effective alternative to transmission oximeters for perioperative monitoring of critically ill patients.This research was partially supported by a grant in aid from Nippon Colin Electronics, Komaki, Japan. The authors also acknowledge the Southwest Research Institute in San Antonio for assembling the prototype optical sensor reported in this paper.     

Intraoperative pulse oximetry in peripheral revascularization in an infant

Vascular patency after reimplantation has been evaluated by numerous methods. A patient is described in whom pulse oximetry was used for this purpose. Other techniques of evaluating vascular patency are mentioned, and the physics of pulse oximetry are briefly discussed.