Comparison of a prototype esophageal oximetry probe with two conventional digital pulse oximetry monitors in aortocoronary bypass patients

Maximizing the laboratory setting for testing devices and understanding statistical output in pulse oximetry

Maximizing the laboratory setting for testing baseline pulse oximetry accuracy in an arterial desaturation study requires a study design that considers management of several aspects in the physiology of the test subject, special attention to the device under test, and great care in the preanalytical (sample handling) and analytical (Co-oximeter) phases. Statistics used to describe the resulting SpO2 performance include Precision (size of the data cloud), Bias (offset of the data cloud), and A(rms) (accuracy root mean square), which combines the size and offset of the data cloud in one number. The A(rms) is the primary statistic required by regulatory organizations to describe general performance over the entire saturation range. It does not describe any one point, but is a compilation of all points over the range tested. Most pulse oximeters in use today specify an A(rms) of 2%. To meet this specification, two-thirds of the readings will be within 2% of the Co-oximeter reference; however, some individual readings can be as inaccurate as 6% or more. The A(rms) statistic does not have the capacity to represent all pulse oximeter behavior. Saturation pop-ups, drop-downs, frozen readings, and periods of no reading are not portrayed by the A(rms). The next steps in the advancement of regulatory validation testing would be to develop standards that include an expanded analysis of pulse oximeter performance by assessment of pop-ups, dropouts, frozen readings, and periods of no reading through assessment of sensitivity/specificity and possibly a "Performance Index" similar to the approach taken by Barker.

Pulse oximetry in the oesophagus

Pulse oximetry has been one of the most significant technological advances in clinical monitoring in the last two decades. Pulse oximetry is a non-invasive photometric technique that provides information about the arterial blood oxygen saturation (SpO(2)) and heart rate, and has widespread clinical applications. When peripheral perfusion is poor, as in states of hypovolaemia, hypothermia and vasoconstriction, oxygenation readings become unreliable or cease. The problem arises because conventional pulse oximetry sensors must be attached to the most peripheral parts of the body, such as finger, ear or toe, where pulsatile flow is most easily compromised. Since central blood flow may be preferentially preserved, this review explores a new alternative site, the oesophagus, for monitoring blood oxygen saturation by pulse oximetry. This review article presents the basic physics, technology and applications of pulse oximetry including photoplethysmography. The limitations of this technique are also discussed leading to the proposed development of the oesophageal pulse oximeter. In the majority, the report will be focused on the description of a new oesophageal photoplethysmographic/SpO(2) probe, which was developed to investigate the suitability of the oesophagus as an alternative monitoring site for the continuous measurement of SpO(2) in cases of poor peripheral circulation. The article concludes with a review of reported clinical investigations of the oesophageal pulse oximeter.

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.

Esophageal pulse oximetry utilizing reflectance photoplethysmography

Peripheral perfusion is often poor and barely pulsatile in patients undergoing prolonged major surgery. Hence, the arterial blood oxygen saturation (SpO2) readings from commercial finger pulse oximeters can become unreliable or cease when they are most needed. To overcome this limitation, the esophagus has been investigated as an alternative measurement site, as perfusion may be preferentially preserved centrally. A reflectance esophageal pulse oximeter probe, and a processing system implemented in LabVIEW were developed. The system was evaluated in clinical measurements on 49 cardiothoracic surgery patients. The SpO2 values from the esophagus were in good agreement with arterial blood oxygen saturation (SaO2) values obtained from blood gas analysis and CO-oximetry. The means (+/-SD) of the differences between the esophageal SpO2 and SaO2 results from blood gas analysis and CO-oximetry were 0.02 +/- 0.88% and -0.73 +/- 0.72%, respectively. In five (10.2%) of the patients, the finger pulse oximeter failed for at least 10 min while the esophageal SpO2 readings remained reliable. The results confirm that the esophagus may be used as an alternative monitoring site for pulse oximetry even in patients with compromised peripheral perfusion.