Pulse oximetry in methaemoglobinaemia

The results of pulse oximetry saturation in a patient with a high level of methaemoglobinaemia, who subsequently underwent intravenous methylene blue treatment, are presented. The reasons for the erroneously low values after treatment are explained. Pulse oximeters currently available are not helpful in patients treated with methylene blue and should be used with caution in patients who present with cyanosis of unknown origin.     

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.     

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.     

Nasal pulse oximetry overestimates oxygen saturation

Ten surgical patients were monitored with nasal and finger pulse oximetry (Nellcor N-200) for five study periods with alternating mouth and nasal breathing and switching of cables and sensors. Nasal pulse oximetry was found to overestimate arterial oxygen saturation by 4.7 (SD 1.4%) (bias and precision).     

Finger blood content, light transmission, and pulse oximetry errors

The changes in light emitting diode current necessary to maintain a constant level of light incident upon a photodetector were measured in 20 volunteers at the two wavelengths employed by pulse oximeters. Three states of finger blood content were assessed; exsanguinated, hyperaemic, and normal. The changes in light emitting diode current with changes in finger blood content were small and are not thought to represent a significant source of error in saturation as measured by pulse oximetry.     

Identification of autonomic dysfunction with a pulse oximeter

Chart recordings of the pulse oximeter waveform were taken during the performance of a standard Valsalva manoeuvre, in one subject with and another without autonomic dysfunction. They demonstrate that certain types of pulse oximeter may be used for the rapid pre-operative identification of patients who have an autonomic neuropathy in which the cardiovascular responses to stress are impaired.     

Pulse oximetry in the recovery room

Haemoglobin oxygen saturation (SO2) was measured percutaneously with a pulse oximeter, in a group of 150 healthy ambulant volunteers to determine the range of normal values. The measuring site was not arterialised in advance. SO2 was below 94% in 13.3% of cases, while in no case was it below 90%. SO2 was then measured in 350 patients in the recovery room after a variety of surgical and anaesthetic procedures. Only 1.1% of patients who received additional oxygen following general anaesthesia exhibited an SO2 below 90%, compared to 16.7% of similar cases who did not receive additional oxygen. Administration of additional oxygen raised the SO2 above 90% in all the latter cases. In only 55.3% of those who did not receive additional oxygen was the SO2 above 94%, compared to 86.7% of normal volunteers and 73.9% of patients who received additional oxygen. This study reiterates the need to administer supplemental oxygen to all patients in the recovery room, unless facilities are available to measure SO2. A lower alarm limit of 90% is appropriate for the peri-operative period when using the Criticare Systems 501 oximeter.     

Arterial flow waveforms from pulse oximetry compared with measured Doppler flow waveforms

This study compared derived arterial flow waveforms, extracted from pulse oximeter waveforms, with Doppler flow waveforms. Fingertip pulse oximeter waveforms and radial artery Doppler flow waveforms were measured simultaneously in volunteers. The pulse oximeter waveforms were processed to extract the arterial flow waveforms and these were compared with the measured Doppler waveforms. They were very similar.     

Pulse oximetry compared with Doppler ultrasound for assessment of collateral blood flow to the hand

Ischaemic injury to the hand after arterial cannulation is a rare but well documented complication and routine testing of the adequacy of collateral circulation is widely advocated. The widespread availability of the pulse oximeter in the operating theatre. its applicability in circumstances where the patient is unable to cooperate, and its dependence on pulsatile blood flow suggest that this device could potentially be usefully applied to the assessment of collateral blood flow. The reliability of the pulse oximeter to detect the presence or absence of collateral circulation was prospectively compared to Doppler ultrasound in 109 hands from 64 adult patients. Nine hands demonstrated inadequate ulnar collateral flow, one hand demonstrated inadequate radial collateral flow and a persistent median artery was found in one hand. In all patients the results of pulse oximeter testing (probe placed on the thumb correlated precisely with the results obtained with the Doppler device (probe located over the lateral aspect of the superficial palmar arch). These results demonstrate pulse oximetry to be a reliable method of assessing collateral blood flow to the hand before arterial cannulation.     

Comparison of the Nellcor N-200 and N-3000 pulse oximeters during simulated postoperative activities

Twenty-six healthy volunteers were monitored simultaneously with the Nellcor N-200 and N-3000 pulse oximeters during nonhypoxaemic simulated postoperative activity. The overall number of registered events (hypoxaemic episodes or loss of signal) was fewer with the N-3000 than with the N-200 (8 vs. 32, p < 0.00005). Episodes of 'desaturation' of ≥5% from baseline were significantly fewer with the N-3000 than with the N-200 (5 vs. 19, p =0.0001), and lowest values below 90% occurred nine times on the N-200, but were not seen with the N-3000 (p <0.00005). Furthermore, episodes owing to loss of signal were significantly rarer with the N-3000 than with the N-200 (3 vs. 13, p =0.001). The Nellcor N-3000 oximeter may offer an advantage over the N-200 model when monitoring patients in the postoperative period.     

The response times during anaesthesia of pulse oximeters measuring oxygen saturations during hypoxaemic events

Episodes of desaturation were recorded simultaneously by computer from two Ohmeda Biox 3700 pulse oximeters, one with an ear and one with a finger probe, on patients undergoing anaesthesia. Over a period of 6 months, 28 episodes of desaturation were detected. Analysis of the recordings showed the mean minimum saturations recorded for ear and finger probes were 86.3% and 83.5% respectively (p less than 0.01). The mean delay for finger compared to ear pulse oximetry was 4.4 s (p less than 0.01). Analysis at different saturation levels showed finger probe responses to be significantly slower than ear probe responses at saturations equal to and above 91% (p less than 0.05). At saturation levels of 90% or less no significant difference in probe response times were found.     

Intra-operative pulmonary embolism Detection by pulse oximetry

A case is presented of a 49-year-old patient who had a pulmonary embolism during ovarian cystectomy. The only evidence of its occurrence was a sudden, marked reduction in arterial oxygen saturation as detected by a pulse oximeter. The diagnosis was confirmed 24 hours later by a chest radiograph and a ventilation perfusion scan. Anticoagulant treatment was instituted.     

Intraoperative pulse oximetry: Frequency and distribution of discrepant data

Study Objective: To determine the types of discrepant data during intraoperative pulse oximetry and their frequency and duration.

Design: Prospective study.

Setting: University medical center.

Patients: 46 consecutive ASA physical status I–III patients undergoing general anesthesia for elective surgical operations.

Measurements and Main Results: With an integrated computer algorithm on the pulse oximeter and another computer linked to it, data were screened and the frequency and distribution of the following oximeter signals recorded: absent; low quality or interrupted, as detected by the pulse oximeter algorithm, nonphysiologic, identified by the personal computer as a heart rate change greater than 10 beats per minute within 2 consecutive 2-second samples, with no similar abrupt change reported simultaneously on ECG. The number of episodes per hour of discrepant oximeter data and the duration of the episodes were recorded by phase of anesthesia: induction, maintenance, and emergence. Discrepant data occurred most frequently and lasted longest during emergence (p < 0.05); the majority of episodes of discrepant data during emergence lasted less than 12 seconds. Excluding discrepant data that lasted less than 12 seconds decreased the frequency of discrepant data by 63% and excluding those that lasted less than 30 seconds decreased the frequency of discrepant data by 93%.

Conclusions: Pulse oximeters frequently report discrepant data intraoperatively, most frequently during emergence from anesthesia. An alarm delay triggered by discrepant data and lasting 12 to 30 seconds would keep most discrepant data from becoming false alarms and, thus, may reduce distracting sound pollution in the operating room.     

Measurement of pulse oximetry,capnography and pH

The measurement of arterial oxygen saturations, end-tidal carbon dioxide and pH are all key to modern anaesthetic practice. They can all be measured in a variety of ways, but the most common are discussed in this article. The understanding of the underlying physical principles and how the anaesthetist monitors function to measure these variables is discussed in this article, including limitations and inaccuracies of each technique.     

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.