#21. Principles of End-tidal Oximetry and Oxygen Gradient

Hi, I am Dr. Robert Bilkovski. Welcome to this podcast series on end-tidal oximetry. In this podcast titled - Principles of End-Tidal Oximetry and the Oxygen Gradient, which is the second podcast in this series, we will take aim at the concept of end-tidal oximetry and the oxygen gradient and how it may be clinically applied.

The core measures associated with monitoring respiratory function include pulse oximetry and end-tidal CO₂. Both are non-invasive and provide minute-by-minute data that can inform the clinician of potentially serious perturbations in respiratory status. As was discussed in our first podcast in this series, end-tidal CO₂ can serve as an estimate for the arterial CO₂ content in the blood and thus inform clinicians on the ventilation and CO₂ elimination status of a patient.¹

However, changes in end-tidal CO₂ reflect the impact of carbon dioxide buffering that occurs within the blood and thus, reflects a delayed view of ventilatory changes.^2,3

A more novel concept is the measurement of end-tidal oxygen, and while the amount of clinical literature on this topic is small compared to end-tidal CO₂, there are some interesting avenues worthy of further study.

First, back in 1989, authors from Finland conducted a series of studies evaluating the association between end-tidal oxygen and the partial pressure of oxygen. From their studies, the term “oxygen gradient” was first used. Simply stated the oxygen gradient is the difference between the fraction of inspired oxygen and the end-tidal oxygen.^{2, 4}

These authors observed a relationship between the partial pressure of oxygen and end-tidal oxygen in their large animal study; notably when the fraction of inspired oxygen was at or below 40%. When ventilation was reduced in this animal study by 50% they observed a 23% decrease in the partial pressure of oxygen while the oxygen gradient increased 112%. More important was that there was no change in pulse oximetry, thereby suggesting that SpO₂ is a less sensitive indicator of hypoventilation than end-tidal oximetry. These authors conducted a human study in subjects undergoing abdominal surgery where similar observations were noted during periods of controlled hypoventilation.

In this 20-subject study, breath-by-breath recording of oxygen, carbon dioxide and oxygen saturation occurred during general anesthesia and immediately during recovery. During periods of apnea, the decreasing alveolar oxygen was detected earlier by the end-tidal oximetry than by SpO₂ or end-tidal capnography. Similarly, during hypoventilation, the end-tidal oximetry changes were identified more rapidly than changes in the other two continuous measures. The authors concluded that the oxygen gradient, which is FiO₂ minus end-tidal O₂, served as a more sensitive index of hypoventilation than either end-tidal CO₂ or pulse oximetry.

Granted, this data from Linko and colleagues is limited, and caution should be noted to avoid making broad conclusions on clinical utility. What is of interest is the potential areas of research and where additional data regarding end-tidal oximetry show promise.  That will be the focus of our next podcast. Until then, this concludes the second podcast in the series and thank you for listening.

References:

[1]J. F. Nunn, Applied Respiratory Physiology, 2d ed. (London, Boston: Butterworths, 1977).

[2]K Linko and M Paloheimo, “Inspiratory End-Tidal Oxygen Content Difference: A Sensitive Indicator of Hypoventilation,” Critical Care Medicine 17, no. 4 (April 1, 1989): 345–48, https://doi.org/10.1097/00003246-198904000-00009.

[3]Harvey A Zar et al., “Monitoring Pulmonary Function with Superimposed Pulmonary Gas Exchange Curves from Standard Analyzers,” n.d.

[4] Kai Link and Markku Paloheimo, "Monitoring of the Inspired and End-tidal Oxygen, Carbon Dioxide, and Nitrous Oxide Concentrations: Clinical Applications during Anesthesia and Recovery", Journal of Clinical Monitoring 5, no.3 (July 1, 1989): 149-56), https://doi.org/10.1007/BF01627446