Investigation of the human oesophagus as a new monitoring site for blood oxygen saturation

Pulse oximeter probes placed peripherally may fail to give accurate values of arterial blood oxygen saturation (SpO(2)) when peripheral perfusion is p oor. Since central blood flow may be preferentially preserved, the oesophag us was suggested as an alternative monitoring site. A reflectance oesophage al photoplethysmographic (PPG) probe and a multiplexed data acquisition sys tem, operating simultaneously at two wavelengths and incorporating an exter nal three-lead electrocardiogram (ECG) reference channel, has been develope d. It has been used to investigate the suitability of the oesophagus as a p ossible monitoring site for SpO(2) in cases of compromised peripheral perfu sion. Oesophageal PPG signals and standard ECG traces were obtained from 16 anaesthetized patients and displayed on a laptop computer. Measurable PPG signals with high signal-to-noise ratios at both infrared and red wavelengt hs were obtained from all five oesophageal depths investigated. The maximum PPG amplitude occurred at 25 cm from the upper incisors in the mid-oesopha gus. The measured pulse transit times (PTTs) to the oesophagus were consist ent with previous measurements at peripheral sites and had a minimum value of 67 +/- 30 ms at a depth of 30 cm. There was broad agreement between the calculated values of oesophageal SpO(2) and those from a commercial finger pulse oximeter.