Lifebox pulse oximeter implementation in Malawi: evaluation of educational outcomes and impact on oxygen desaturation episodes during anaesthesia

Pulse oximetry is an essential monitor for safe anaesthesia but is often not available in low‐income countries. The aim of this study was to determine whether the introduction of pulse oximetry with training was feasible and could reduce the incidence of oxygen desaturation during anaesthesia in a low‐income country. Pulse oximeters were donated, with training, to 83 non‐physician anaesthetists in Malawi. Knowledge was tested immediately before and after training and at follow‐up. Providers were asked to record the lowest peripheral oxygen saturation (SpO₂) for the first 100 cases anaesthetised after training. The primary clinical outcome was the proportion of cases with an oxygen desaturation event (SpO₂ < 90%). Seventy‐seven of 83 (93%) participants completed all pre‐ and post‐training tests. Pulse oximetry knowledge improved after training from a median (IQR [range]) score of 39 (37–42 [28–48]) to 44 (42–46 [35–50]) and this knowledge was maintained for 8 months (p < 0.001). Oxygen saturation data and provider responses were recorded for 4772 cases. The proportion of oxygen desaturation episodes decreased from 17.2% to 6.5%, representing a 36% reduction in the odds of an oxygen desaturation event in the second 50 cases compared with the first 50 (OR 0.64, 95%CI 0.50–0.82, p < 0.001). We conclude that donation of pulse oximeters, with training, in Malawi was feasible, improved knowledge and reduced the incidence of oxygen desaturation events.     

The desaturation response time of finger pulse oximeters during mild hypothermia

Pulse oximeters may delay displaying the correct oxygen saturation during the onset of hypoxia. We investigated the desaturation response times of pulse oximeter sensors (forehead, ear and finger) during vasoconstriction due to mild hypothermia and vasodilation caused by glyceryl trinitrate. Ten healthy male volunteers were given three hypoxic challenges of 3 min duration under differing experimental conditions. Mild hypothermia increased the mean response time of finger oximeters from 130 to 215 s. Glyceryl trinitrate partly offset this effect by reducing the response time from 215 to 187 s. In contrast, the response times of the forehead and ear oximeters were unaffected by mild hypothermia, but the difference between head and finger oximeters was highly significant (p < 0.0001). The results suggest that the head oximeters provide a better monitoring site for pulse oximeters during mild hypothermia.     

Performance of three new-generation pulse oximeters during motion and low perfusion in volunteers

Study ObjectiveTo evaluate pulse oximeter performance during motion and induced low perfusion in volunteers.DesignProspective volunteer study.SettingDirect Observation unit.Subjects10 healthy adult volunteers.InterventionsTen volunteers were monitored with three different pulse oximeters while they underwent desaturation to about 75% oxygen saturation (SpO₂) and performed machine-generated (MG) and volunteer-generated (VG) hand movements with the test hand, keeping the control hand stationary.MeasurementsSpO₂ and pulse rate readings from the motion (test) and stationary (control) hands were recorded as well as the number of times and the duration that the oximeters connected to the test hands did not report a reading. Sensitivity, specificity, performance index for SpO₂, and pulse rate (PR) were calculated for each pulse oximeter by comparing performance of the test hand with the control hand.Main ResultsDuring both MG and VG motion, the Masimo Radical had higher SpO₂ specificity (93% and 97%) than the Nellcor N-600 (67% and 77%) or the Datex-Ohmeda TruSat (83% and 82%). The Masimo Radical also had higher SpO₂ sensitivity (100% and 95%) than the Nellcor N-600 (65% and 50%) or the Datex-Ohmeda TruSat (20% and 15%) during both MG and VG motion. During MG motion, the Masimo Radical had the lowest PR failure rate (0%) compared with the Nellcor N-600 (22.2%) and Datex-Ohmeda TruSat (1.3%). However, during VG motion, the Masimo Radical had the lowest SpO₂ failure rate (0%) of the three devices (Nellcor N-600 16.4% and Datex-Ohmeda TruSat 1.7%). Both the Masimo Radical and the Datex-Ohmeda TruSat had lower PR failure rates (0% and 4.4%) than the Nellcor N-600 (33.9%). There were no significant differences in SpO₂ or PR performance index between the three devices.ConclusionsThe Masimo Radical had higher SpO₂ sensitivity and specificity than the Nellcor N-600 and Datex-Ohmeda TruSat during conditions of motion and induced low perfusion in this volunteer study.     

An assessment of the accuracy of pulse oximeters

Peripheral pulse oximetry has become a core monitoring modality in most fields of medicine. Pulse oximeters are used ubiquitously in operating theatres, hospital wards, outpatient clinics and general practice surgeries. This study used a portable spectrometer (Lightman^®, The Electrode Co. Ltd., Monmouthshire, UK) to measure the emission spectra of the two light emitting diodes within the pulse oximeter sensor and to determine the accuracy of 847 pulse oximeters currently in use in 29 NHS hospitals in the UK. The standard manufacturing claim of accuracy for pulse oximeters is ± 2–3% over the range of 70–100% S_pO₂. Eighty‐nine sensors (10.5%) were found to have a functional error of their electrical circuitry that could cause inaccuracy of measurement. Of the remaining 758 sensors, 169 (22.3%) were found to have emission spectra different from the manufacturers’ specification that would cause an inaccuracy in saturation estimation of > 4% in the range of 70–100% saturation. This study has demonstrated that a significant proportion of pulse oximeter sensors may be inaccurate.     

Utilization of the Walking Oximetry Test to Allow Safe Ambulation After Pulmonary Resection

Supplemental oxygen therapy after pulmonary resection can generally be tapered according to arterial blood gases at rest or pulse oximetry (SpO₂). However, detecting exercise-induced oxygen desaturation can be difficult. We developed the walking oximetry test (WOT) so that thoracotomy patients could be rehabilitated without the risk of undetected ambulatory hypoxemia. The subjects were 58 patients who had undergone pulmonary resection and could walk at the bedside, with oxygen at 3 l/min via a nasal cannula. Patients with a value of more than 100 torr were allowed to walk with assistance for 6 min in the corridor. The oxygen flow rate was kept at 3 l/min and the walking pace was less than 50 m/min. SpO₂ was determined using a wristwatch pulse oximeter. The test was stopped if the SpO₂ fell below 90% or there was a score of 5 or more on the Borg scale (range 1–10). Oxygen desaturation occurred in six patients (10%) during the WOT. These patients underwent ambulatory training with sufficient oxygen supplementation and were then tested again. Patients whose SpO₂ values remained higher than 90% and who showed no more than 5% desaturation were permitted to walk in the corridor with oxygen at 3 l/min via a nasal cannula. All these patients had a Borg score of 4 or lower. The WOT is a reliable, nonvasive method for detecting exercise-induced oxygen desaturation during ambulation after pulmonary resection. Received: December 5, 2000 / Accepted: July 17, 2001     

Effects of skin pigmentation on pulse oximeter accuracy at low saturation

BACKGROUND: It is uncertain whether skin pigmentation affects pulse oximeter accuracy at low HbO2 saturation. METHODS: The accuracy of finger pulse oximeters during stable, plateau levels of arterial oxygen saturation (Sao2) between 60 and 100% were evaluated in 11 subjects with darkly pigmented skin and in 10 with light skin pigmentation. Oximeters tested were the Nellcor N-595 with the OxiMax-A probe (Nellcor Inc., Pleasanton, CA), the Novametrix 513 (Novametrix Inc., Wallingford, CT), and the Nonin Onyx (Nonin Inc., Plymouth, MN). Semisupine subjects breathed air-nitrogen-carbon dioxide mixtures through a mouthpiece. A computer used end-tidal oxygen and carbon dioxide concentrations determined by mass spectrometry to estimate breath-by-breath Sao2, from which an operator adjusted inspired gas to rapidly achieve 2- to 3-min stable plateaus of desaturation. Comparisons of oxygen saturation measured by pulse oximetry (Spo2) with Sao2 (by Radiometer OSM3) were used in a multivariate model to determine the interrelation between saturation, skin pigmentation, and oximeter bias (Spo2 - Sao2). RESULTS: At 60-70% Sao2, Spo2 (mean of three oximeters) overestimated Sao2 (bias +/- SD) by 3.56 +/- 2.45% (n = 29) in darkly pigmented subjects, compared with 0.37 +/- 3.20% (n = 58) in lightly pigmented subjects (P < 0.0001). The SD of bias was not greater with dark than light skin. The dark-light skin differences at 60-70% Sao2 were 2.35% (Nonin), 3.38% (Novametrix), and 4.30% (Nellcor). Skin pigment-related differences were significant with Nonin below 70% Sao2, with Novametrix below 90%, and with Nellcor at all ranges. Pigment-related bias increased approximately in proportion to desaturation. CONCLUSIONS: The three tested pulse oximeters overestimated arterial oxygen saturation during hypoxia in dark-skinned individuals.     

Evaluation of the Ohmeda 3700 pulse oximeter: steady-state and transient response characteristics

The authors determined the accuracy of the Ohmeda 3700 (version J) pulse oximeter in healthy volunteers rendered hypoxic (SaO2 from 60-98%) by breathing mixtures of O2 in N2. When equipped with an ear probe, the pulse oximeter reading (y) reliably predicted arterial saturation (x) under steady-state conditions (y = 1.05x - 4.66, r = 0.98) as well as when oxygen saturation was rapidly decreasing (y = 1.05x - 6.38, r = 0.96). Conversely, when equipped with a finger probe, the oximeter tended to significantly underestimate steady-state arterial saturation (y = 1.21x - 19.1, r = 0.98, P less than 0.001). In response to this information, the manufacturer modified the oximeter's software (version XJ1), resulting in improved agreement between oximeter readings and arterial values (y = 0.96x + 4.59, r = 0.99). Despite the close correlation between steady-state oximeter readings and arterial saturation, the 99% prediction limits for both the ear and finger probes (version XJ1) were +/- 8%. Finger probe readings did not reliably reflect radial arterial oxygenation during rapid desaturation (y = 0.55x + 45.2, r = 0.78). This may be related to the time required to "arterialize" the blood in the finger; during acute resaturation, we found that the ear- to finger-probe delay was 24.0 +/- 2.3 s (means +/- SE, P less than 0.001).     

Notes on the apparent discordance of pulse oximetry and multi-wavelength haemoglobin photometry

Multi-wavelength photometers, blood gas analysers and pulse oximeters are widely used to measure various oxygen-related quantities. The definitions of these quantities are not always correct. This paper gives insight in the various definitions for oxygen quantities. Furthermore, the possible influences of dyshaemoglobins and fetal haemoglobin on the accuracy of pulse oximetry are discussed.
As pulse oximeters are constructed for the determination of arterial oxygen saturation, they should be validated with sample oxygen saturation values and not with the oxyhaemoglobin fraction. The influence of carboxy-haemoglobin is insubstantial over an oxygen saturation range of 0% to 100%. Through the presence of methaemoglobin, pulse oximetry will give an underestimation above 70% and an overestimation below 70% oxygen saturation. The influence of fetal haemoglobin is insignificant in the neonatal use of pulse oximetry, in the range of 75% to 100% arterial oxygen saturation. However, a pulse oximeter underestimates the arterial oxygen saturation at the 25% level with 5%, if the pulse oximeter has been calibrated in human adults. Such a low level of arterial oxygen saturation can be present in the fetus during labor.     

Evaluation of oesophageal pulse oximetry in patients undergoing cardiothoracic surgery

Pulse oximetry probes placed peripherally may fail to give accurate values of blood oxygen saturation when the peripheral circulation is poor. Because central blood flow may be preferentially preserved, we investigated the oesophagus as an alternative monitoring site. A reflectance blood oxygen saturation probe was developed and evaluated in 49 patients undergoing cardiothoracic surgery. The oesophageal pulse oximeter results were in good agreement with oxygen saturation measurements obtained by a blood gas analyser, a CO-oximeter and a commercial finger pulse oximeter. The median (IQR [range]) difference between the oesophageal oxygen saturation results and those from blood gas analysis were 0.00 (-0.30 to 0.30 [-4.47 to 2.60]), and between the oesophageal oxygen saturation results and those from CO-oximetry were 0.75 (0.30 to 1.20 [-1.80 to 1.80]). Bland-Altman analysis showed that the bias and the limits of agreement between the oesophageal and finger pulse oximeters were -0.3% and -3.3 to 2.7%, respectively. In five (10.2%) patients, the finger pulse oximeter failed for at least 10 min, whereas the oesophageal readings remained reliable. The results suggest that the oesophagus may be used as an alternative monitoring site for pulse oximetry even in patients with compromised peripheral perfusion.     

Response time of pulse oximeters assessed using acute decompression.

In human volunteers, the response times of 11 pulse oximeters to a 10% step reduction in arterial oxygen saturation were measured using an acute decompression technique. When finger probes were used, nine oximeters had similar response times and two were significantly slower (P less than 0.05). The ear probe response time was similar on six oximeters assessed, and faster than the finger probes. The response times of the oximeters to an acute increase in arterial saturation were tested by suddenly changing the inspired gas from air to 100% oxygen at an ambient pressure of 380 mm Hg. For ear probes, the response times were similar for all oximeters; for finger probes, three fast-responding and three slow-responding oximeters were identified (P less than 0.05). A faster response could be elicited by placing the probes on the thumb (P less than 0.05). We conclude that if a rapid indication of changes in arterial saturation is required, pulse oximeters with ear probes should be used. If finger probes are used, they should be placed on the thumb. The oximeter used will influence the response time if finger probes are used, but it will have little effect if ear probes are used.     

Effects of Skin Pigmentation on Pulse Oximeter Accuracy at Low Saturation

Background: It is uncertain whether skin pigmentation affects pulse oximeter accuracy at low HbO2 saturation.

Methods: The accuracy of finger pulse oximeters during stable, plateau levels of arterial oxygen saturation (Sao2) between 60 and 100% were evaluated in 11 subjects with darkly pigmented skin and in 10 with light skin pigmentation. Oximeters tested were the Nellcor N-595 with the OxiMax-A probe (Nellcor Inc., Pleasanton, CA), the Novametrix 513 (Novametrix Inc., Wallingford, CT), and the Nonin Onyx (Nonin Inc., Plymouth, MN). Semisupine subjects breathed air-nitrogen-carbon dioxide mixtures through a mouthpiece. A computer used end-tidal oxygen and carbon dioxide concentrations determined by mass spectrometry to estimate breath-by-breath Sao2, from which an operator adjusted inspired gas to rapidly achieve 2- to 3-min stable plateaus of desaturation. Comparisons of oxygen saturation measured by pulse oximetry (Spo2) with Sao2 (by Radiometer OSM3) were used in a multivariate model to determine the interrelation between saturation, skin pigmentation, and oximeter bias (Spo2 - Sao2).

Results: At 60-70% Sao2, Spo2 (mean of three oximeters) overestimated Sao2 (bias +/- SD) by 3.56 +/- 2.45% (n = 29) in darkly pigmented subjects, compared with 0.37 +/- 3.20% (n = 58) in lightly pigmented subjects (P < 0.0001). The SD of bias was not greater with dark than light skin. The dark-light skin differences at 60-70% Sao2 were 2.35% (Nonin), 3.38% (Novametrix), and 4.30% (Nellcor). Skin pigment-related differences were significant with Nonin below 70% Sao2, with Novametrix below 90%, and with Nellcor at all ranges. Pigment-related bias increased approximately in proportion to desaturation.     

RESPONSE OF 10 PULSE OXIMETERS TO AN IN VITRO TEST SYSTEM

Pulse oximeters are often used in situations in which severehypoxaemia may occur. We have developed an in vitro system totest the accuracy of pulse oximeter calibration. The probe of10different oximeters was attached to a model finger in an invitro blood circuit, and pulse oximeter readings (Spo₂) werecompared with multi-wavelength in vitro oximeter readings (So₂over a range of So₂ values from 50 to 100%. The oximeters testedvaried widely in their accuracy and linearity. We conclude thatthe system can test the accuracy, reproducibility and linearityof response of pulse oximeter readings at low oxyhaemoglobinsaturations. ^*Present address, for correspondence: c/o Physiology Department,University of Leicester, University Road, Leicester LEI 7RH     

Accuracy of pulse oximetry in patients with low systemic vascular resistance

In order to assess the accuracy of pulse oximeters in patients with septic shock, we compared 80 paired readings of oxygen saturations taken from pulse oximeters and oxygen saturations obtained from co-oximetry in patients receiving intensive therapy with indwelling pulmonary artery flotation catheters. Comparison between groups with low or normal systemic vascular resistance indices showed a small (1.4%) but significant (p < 0.001) underreading of the saturation from the pulse oximeter in the presence of a low systemic vascular resistance. With normal or high systemic vascular resistance pulse oximeter readings correlated well with co-oximetry. We hypothesise that the main cause of this underreading is because the pulse oximeter is sensing pulsatile venous flow due to the opening of arteriovenous channels in the skin in septic states.     

Continuous pulse oximetry in the general surgical ward: Nellcor N-200 versus Nellcor N-3000

New measurement principles in pulse oximetry have been introduced to decrease the incidence of false movement alarms. Experimental studies have shown that the new Nellcor Symphony N-3000 may reduce the incidence of false alarms when monitoring during different activities. We compared the Nellcor Symphony N-3000 with the Nellcor N-200 pulse oximeter, when monitoring patients in the general surgical ward. Twenty-two patients were monitored during unrestricted ward activities for a total of 275 h with a N-3000 and a N-200 pulse oximeter simultaneously. Data were analysed for lack of concordance between the two pulse oximeters with respect to frequency of registered hypoxaemic episodes and thus the amount of time spent in the alarm state. The median number of desaturation episodes with the N-200 was 18 (range 0-511) compared with four (range 0-476) with the N-3000 (p < 0.0001). The median number of drop-outs (loss of signal) was 13 (range 1-46) with the N-200 compared with nine (2-41) with the N-3000 (p = 0.06). The N-200 registered saturation values of 85% or below for 23% of the observation time compared with 6% of the observation time with the N-3000 pulse oximeter (p < 0.0001). The different working principles of the two generations of oximeters were reflected in the present results derived from a clinical setting. The Nellcor Symphony N-3000 may offer an advantage compared with the Nellcor N-200, because of the reduced frequency of alarms and total time in alarm when monitoring patients in the general surgical ward.     

Continuous auditory monitoring--how much information do we register?

We have studied response times of 30 anaesthetists to a standardized episode of arterial oxygen desaturation in a simulated patient, randomized to the use of either a fixed or variable pitch pulse oximeter. We wished to determine if a variable auditory signal was important in detecting adverse events. A variable pitch pulse signal had a shorter time to recognition of desaturation (P < 0.0001), with a mean response time of 32 s, compared with 129 s for the fixed pitch signal.      

Effects of Motion on the Performance of Pulse Oximeters in Volunteers

Background: Pulse oximetry is considered a standard of care in both the operating room and the postanesthetic care unit, and it is widely used in all critical care settings. Pulse oximeters may fail to provide valid pulse oximetry data in various situations that produce low signal-to-noise ratio. Motion artifact is a common cause of oximeter failure and loss of accuracy. This study compares the accuracy and data dropout rates of three current pulse oximeters during standardized motion in healthy volunteers.

Methods: Ten healthy volunteers were monitored by three different pulse oximeters: Nellcor N-200, Nellcor N-3000, and Masimo SET (prototype). Sensors were placed on digits 2, 3, and 4 of the test hand, which was strapped to a mechanical motion table. The opposite hand was used as a stationary control and was monitored with the same pulse oximeters and an arterial cannula. Arterial oxygen saturation rate was varied from 100% to 75% by changing the inspired oxygen concentration. While pulse oximetry was both constant and changing, the oximeter sensors were connected before and during motion. Oximeter errors and dropout rates were digitally recorded continuously during each experiment.

Results: If the oximeter was functioning before motion began, the following are the percentages of time when the instrument displayed a pulse oximetry value within 7% of control: N-200 = 76%, N-3000 = 87%, and Masimo = 99%. When the oximeter sensor was connected after the beginning of motion, the values were N-200 = 68%, N-3000 = 47%, and Masimo = 97%. If the alarm threshold was chosen as pulse oximetry less than 90%, then the positive predictive values (true alarms/total alarms) are N-200 = 73%, N-3000 = 81%, and Masimo = 100%. In general, N-200 had the greatest pulse oximetry errors and N-3000 had the highest dropout rates.     

Potential errors in pulse oximetry

There is no absolute reference for oxygen saturation, although multiwavelength in vitro oximeters are accepted as the 'gold standard'. Regardless of whether fractional or functional saturation is used by manufacturers to calibrate their oximeters, evaluation against fractional saturation is recommended since this is the clinically relevant variable. The use of standard notation and comparisons based on bias and precision is recommended. The accuracy of pulse oximetry is intrinsically limited by the use of only two wavelengths, and is dependent on the initial calibration population. The empirical algorithms used to convert the signal to its 'readout value' and the quality control of hardware may both be important sources of variability between oximeters. Change in blood temperature may introduce errors in pulse oximeter and in vitro oximeter saturation readings, but these will be clinically insignificant. Changes in blood pH should not decrease pulse oximetry accuracy.     

Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry

The performance of three commercially available pulse oximeters was assessed in five anesthetized dogs in which increasing levels of methemoglobin were induced. Hemoglobin oxygen saturation in each dog was monitored with three pulse oximeters (Nellcor N-100, Ohmeda 3700, and Novametrix 500) and a mixed venous saturation pulmonary artery catheter (Oximetrix Opticath). Arterial and mixed venous blood specimens were analyzed for PaO2, PaCO2, and pHa using standard electrodes. An IL-282 Co-oximeter was used on the same specimens to determine oxyhemoglobin and methemoglobin as percentages of total hemoglobin. Methemoglobin levels of up to 60% were induced by intratracheal benzocaine. As MetHb gradually increased while the dogs were breathing 100% inspired oxygen, the pulse oximeter saturation (SpO2) overestimated the fractional oxygen saturation (SaO2) by an amount proportional to the concentration of methemoglobin until the latter reached approximately 35%. At this level the SpO2 values reached a plateau of 84-86% and did not decrease further. When, at fixed methemoglobin levels, additional hemoglobin desaturation was induced by reducing inspired oxygen fraction, SpO2 changed by much less than did SaO2 (regression slopes from 0.16 to 0.32). Thus, at high methemoglobin levels SpO2 tends to overestimate SaO2 by larger amounts at low hemoglobin saturations. Plots of SpO2 versus functional saturation (oxyhemoglobin/reduced hemoglobin plus oxyhemoglobin) show an improved but still poor relationship (regression slopes from 0.32 to 0.46). The Oximetrix Opticath pulmonary artery catheter behaves similarly but provides somewhat better agreement with functional saturation than do the pulse oximeters in the presence of methemoglobinemia. Pulse oximetry data (SpO2) should be used with caution in patients with methemoglobinemia.     

Potential errors in pulse oximetry

The published studies of pulse oximeter performance under conditions of normal, high and low saturation, exercise, poor signal quality and cardiac arrhythmia are reviewed. Most pulse oximeters have an absolute mean error of less than 2% at normal saturation and perfusion; two-thirds have a standard deviation (SD) of less than 2%, and the remainder an SD of less than 3%. Some pulse oximeters tend to read 100% with fractional saturations of 97-98%. Pulse oximeters may be suitable hyperoxic alarms for neonates if the alarm limit chosen is directly validated for each device. Pulse oximeters are poorly calibrated at low saturations and are generally less accurate and less precise than at normal saturations; nearly 30% of 244 values reviewed were in error by more than 5% at saturations of less than 80%. Ear, nose and forehead probes respond more rapidly to rapid desaturation than finger probes, but are generally less accurate and less precise. Ear oximetry may be inaccurate during exercise. Low signal quality can result in failure to present a saturation reading, but data given with low signal quality warning messages are generally no less accurate than those without. Cardiac arrhythmias do not decrease accuracy of pulse oximeters so long as saturation readings are steady.     

The Effects of Motion on the Performance of Pulse Oximeters in Volunteers (Revised publication)

Background: Pulse oximetry is considered a standard of care in both the operating room and the postanesthetic care unit, and it is widely used in all critical care settings. Pulse oximeters may fail to provide valid SpO2 data in various situations that produce low signal-to-noise ratio. Motion artifact is a common cause of oximeter failure and loss of accuracy. This study compares the accuracy and data dropout rates of three current pulse oximeters during standardized motion in healthy volunteers.

Methods: Ten healthy volunteers were monitored by three different pulse oximeters: Nellcor N-200, Nellcor N-3000, and Masimo SET (prototype). Sensors were placed on digits 2, 3, and 4 of the test hand, which was strapped to a mechanical motion table. The opposite hand was used as a stationary control and was monitored with the same pulse oximeters and an arterial cannula. Arterial oxygen saturation was varied from 100% to 75% by changing the inspired oxygen concentration. While SpO sub 2 was both constant and changing, the oximeter sensors were connected before and during motion. Oximeter errors and dropout rates were digitally recorded continuously during each experiment.

Results: If the oximeter was functioning before motion began, the following are the percentages of time when the instrument displayed an SpO sub 2 value within 7% of control: N-200 = 76%, N-3000 = 87%, and Masimo = 99%. When the oximeter sensor was connected after the beginning of motion, the values were N-200 = 68%, N-3000 = 47%, and Masimo = 97%. If the alarm threshold was chosen SpO2 less than 90%, then the positive predictive values (true alarms/total alarms) are N-200 = 73%, N-3000 = 81%, and Masimo = 100%. In general, N-200 had the greatest SpO2 errors and N-3000 had the highest dropout rates.