Deriving the arterial Po2 and oxygen deficit from expired gas and pulse oximetry

Noninvasive Assessment of Impaired Gas Exchange with the Alveolar Gas Monitor Predicts Clinical Deterioration in COVID-19 Patients

BACKGROUND AND OBJECTIVE

The COVID-19 pandemic magnified the importance of gas exchange abnormalities in early respiratory failure. Pulse oximetry (SpO2) has not been universally effective for clinical decision-making, possibly because of limitations. The alveolar gas monitor (AGM100) adds exhaled gas tensions to SpO2 to calculate the oxygen deficit (OD). The OD parallels the alveolar-to-arterial oxygen difference (AaDO2) in outpatients with cardiopulmonary disease. We hypothesized that the OD would discriminate between COVID-19 patients who require hospital admission and those who are discharged home, as well as predict need for supplemental oxygen during the index hospitalization.

METHODS

Patients presenting with dyspnea and COVID-19 were enrolled with informed consent and had OD measured using the AGM100. The OD was then compared between admitted and discharged patients and between patients who required supplemental oxygen and those who did not. The OD was also compared to SpO2 for each of these outcomes using receiver operating characteristic (ROC) curves.

RESULTS

Thirty patients were COVID-19 positive and had complete AGM100 data. The mean OD was significantly (p = 0.025) higher among those admitted 50.0 ± 20.6 (mean ± SD) vs. discharged 27.0 ± 14.3 (mean ± SD). The OD was also significantly (p < 0.0001) higher among those requiring supplemental oxygen 60.1 ± 12.9 (mean ± SD) vs. those remaining on room air 25.2 ± 11.9 (mean ± SD). ROC curves for the OD demonstrated very good and excellent sensitivity for predicting hospital admission and supplemental oxygen administration, respectively. The OD performed better than an SpO2 threshold of <94%.

CONCLUSIONS

The AGM100 is a novel, noninvasive way of measuring impaired gas exchange for clinically important endpoints in COVID-19.

Validation of Noninvasive Assessment of Pulmonary Gas Exchange in Patients with Chronic Obstructive Pulmonary Disease during Initial Exposure to High Altitude

Investigation of pulmonary gas exchange efficacy usually requires arterial blood gas analysis (aBGA) to determine arterial partial pressure of oxygen (mPaO₂) and compute the Riley alveolar-to-arterial oxygen difference (A-aDO₂); that is a demanding and invasive procedure. A noninvasive approach (AGM100), allowing the calculation of PaO₂ (cPaO₂) derived from pulse oximetry (SpO₂), has been developed, but this has not been validated in a large cohort of chronic obstructive pulmonary disease (COPD) patients. Our aim was to conduct a validation study of the AG100 in hypoxemic moderate-to-severe COPD. Concurrent measurements of cPaO₂ (AGM100) and mPaO₂ (EPOC, portable aBGA device) were performed in 131 moderate-to-severe COPD patients (mean ±SD FEV₁: 60 ± 10% of predicted value) and low-altitude residents, becoming hypoxemic (i.e., SpO₂ < 94%) during a short stay at 3100 m (Too-Ashu, Kyrgyzstan). Agreements between cPaO₂ (AGM100) and mPaO₂ (EPOC) and between the O₂-deficit (calculated as the difference between end-tidal pressure of O₂ and cPaO₂ by the AGM100) and Riley A-aDO₂ were assessed. Mean bias (±SD) between cPaO₂ and mPaO₂ was 2.0 ± 4.6 mmHg (95% Confidence Interval (CI): 1.2 to 2.8 mmHg) with 95% limits of agreement (LoA): -7.1 to 11.1 mmHg. In multivariable analysis, larger body mass index (p = 0.046), an increase in SpO₂ (p < 0.001), and an increase in PaCO₂-PETCO₂ difference (p < 0.001) were associated with imprecision (i.e., the discrepancy between cPaO₂ and mPaO₂). The positive predictive value of cPaO₂ to detect severe hypoxemia (i.e., PaO₂ ≤ 55 mmHg) was 0.94 (95% CI: 0.87 to 0.98) with a positive likelihood ratio of 3.77 (95% CI: 1.71 to 8.33). The mean bias between O₂-deficit and A-aDO₂ was 6.2 ± 5.5 mmHg (95% CI: 5.3 to 7.2 mmHg; 95%LoA: -4.5 to 17.0 mmHg). AGM100 provided an accurate estimate of PaO₂ in hypoxemic patients with COPD, but the precision for individual values was modest. This device is promising for noninvasive assessment of pulmonary gas exchange efficacy in COPD patients.

Mouse Subcutaneous BCG Vaccination and Mycobacterium tuberculosis Infection Alter the Lung and Gut Microbiota

The objective of this study was to characterize the effect of Bacillus Calmette-Guérin (BCG) vaccination and M. tuberculosis infection on gut and lung microbiota of C57BL/6 mice, a well-characterized mouse model of tuberculosis. BCG vaccination and infection with M. tuberculosis altered the relative abundance of Firmicutes and Bacteroidetes phyla in the lung compared with control group. Vaccination and infection changed the alpha- and beta-diversity in both the gut and the lung. However, lung diversity was the most affected organ after BCG vaccination and M. tuberculosis infection. Focusing on the gut-lung axis, a multivariate regression approach was used to compare profile evolution of gut and lung microbiota. More genera have modified relative abundances associated with BCG vaccination status at gut level compared with lung. Conversely, genera with modified relative abundances associated with M. tuberculosis infection were numerous at lung level. These results indicated that the host local response against infection impacted the whole microbial flora, while the immune response after vaccination modified mainly the gut microbiota. This study showed that a subcutaneous vaccination with a live attenuated microorganism induced both gut and lung dysbiosis that may play a key role in the immunopathogenesis of tuberculosis. IMPORTANCE The microbial communities in gut and lung are important players that may modulate the immunity against tuberculosis or other infections as well as impact the vaccine efficacy. We discovered that vaccination through the subcutaneous route affect the composition of gut and lung bacteria, and this might influence susceptibility and defense mechanisms against tuberculosis. Through these studies, we can identify microbial communities that can be manipulated to improve vaccine response and develop treatment adjuvants.

Non-invasive Measurement of Pulmonary Gas Exchange Efficiency: The Oxygen Deficit

The efficiency of pulmonary gas exchange has long been assessed using the alveolar-arterial difference in PO₂, the A-aDO₂, a construct developed by Richard Riley ~70years ago. However, this measurement is invasive (requiring an arterial blood sample), time consuming, expensive, uncomfortable for the patients, and as such not ideal for serial measurements. Recent advances in the technology now provide for portable and rapidly responding measurement of the PO₂ and PCO₂ in expired gas, which combined with the well-established measurement of arterial oxygen saturation via pulse oximetry (SpO₂) make practical a non-invasive surrogate measurement of the A-aDO₂, the oxygen deficit. The oxygen deficit is the difference between the end-tidal PO₂ and the calculated arterial PO₂ derived from the SpO₂ and taking into account the PCO₂, also measured from end-tidal gas. The oxygen deficit shares the underlying basis of the measurement of gas exchange efficiency that the A-aDO₂ uses, and thus the two measurements are well-correlated (r²~0.72). Studies have shown that the new approach is sensitive and can detect the age-related decline in gas exchange efficiency associated with healthy aging. In patients with lung disease the oxygen deficit is greatly elevated compared to normal subjects. The portable and non-invasive nature of the approach suggests potential uses in first responders, in military applications, and in underserved areas. Further, the completely non-invasive and rapid nature of the measurement makes it ideally suited to serial measurements of acutely ill patients including those with COVID-19, allowing patients to be closely monitored if required.

Case Studies in Physiology: Untangling the cause of hypoxemia in a patient with obesity with acute leukemia

Diagnosing the cause of hypoxemia and dyspnea can be complicated in complex patients with multiple comorbidities. This "Case Study in Physiology" describes an man with obesity admitted to the hospital for relapse of acute lymphoblastic leukemia, who experienced progressive hypoxemia, shortness of breath, and dyspnea on exertion during his hospitalization. After initial empirical treatment with diuresis and antibiotics failed to improve his symptoms and because an arterial blood gas measurement was not readily available, we applied a novel, recently described physiological method to estimate the arterial partial pressure of oxygen from the peripheral saturation measurement and calculate the alveolar-arterial oxygen difference to discern the source of his hypoxemia and dyspnea. Using basic physiological principles, we describe how hypoventilation, anemia, and the use of a β blocker and furosemide, collaborated to create a "perfect storm" in this patient that impaired oxygen delivery and limited utilization. This case illustrates the application of innovative physiology methodology in medicine and provides a strong rationale for continuing to integrate physiology education in medical education.NEW & NOTEWORTHY Discerning the cause of dyspnea and hypoxemia in complex patients can be difficult. We describe the "real world" application of an innovative methodology to untangle the underlying physiology in a patient with multiple comorbidities. This case further demonstrates the importance of applying physiology to interrogate the underlying cause of a patient's symptoms when treatment based on probability fails.

Measuring the efficiency of pulmonary gas exchange using expired gas instead of arterial blood: comparing the "ideal" Po2 of Riley with end-tidal Po2

When using a new noninvasive method for measuring the efficiency of pulmonary gas exchange, a key measurement is the oxygen deficit, defined as the difference between the end-tidal alveolar Po₂ and the calculated arterial Po₂. The end-tidal Po₂ is measured using a rapid gas analyzer, and the arterial Po₂ is derived from pulse oximetry after allowing for the effect of the Pco₂ on the oxygen affinity of hemoglobin. In the present report we show that the values of end-tidal Po₂ and Pco₂ are highly reproducible, providing a solid foundation for the measurement of the oxygen deficit. We compare the oxygen deficit with the classical ideal alveolar-arterial Po₂ difference (A-aDO₂) as originally proposed by Riley, and now extensively used in clinical practice. This assumes Riley's criteria for ideal alveolar gas, namely no ventilation-perfusion inequality, the same Pco₂ as arterial blood, and the same respiratory exchange ratio as the whole lung. It transpires that, in normal subjects, the end-tidal Po₂ is essentially the same as the ideal value. This conclusion is consistent with the very small oxygen deficit that we have reported in young normal subjects, the significantly higher values seen in older normal subjects, and the much larger values in patients with lung disease. We conclude that this noninvasive measurement of the efficiency of pulmonary exchange is identical in many respects to that based on the ideal alveolar Po₂, but that it is easier to obtain.

Oxygen deficit is a sensitive measure of mild gas exchange impairment at inspired O2 between 12.5% and 21

The oxygen deficit (OD) is the difference between the end-tidal alveolar Po₂ and the calculated Po₂ of arterial blood based on measured oxygen saturation that acts as a proxy for the alveolar-arterial Po₂ difference. Previous work has shown that the alveolar gas meter (AGM100) can measure pulmonary gas exchange, via the OD, in patients with a history of lung disease and in normal subjects breathing 12.5% O₂. The present study measured how the OD varied at different values of inspired O₂. Healthy subjects were split by age (young 22-31; n = 23; older 42-90; n = 13). Across all inspired O₂ levels (12.5, 15, 17.5, and 21%), the OD was higher in the older cohort 10.6 ± 1.0 mmHg compared with the young -0.4 ± 0.6 mmHg (P < 0.0001, using repeated measures ANOVA), the difference being significant at all O₂ levels (all P < 0.0001). The OD difference between age groups and its variance was greater at higher O₂ values (age × O₂ interaction; P = 0.002). The decrease in OD with lower values of inspired O₂ in both cohorts is consistent with the increased accuracy of the calculated arterial Po₂ based on the O₂-Hb dissociation curve and with the expected decrease in the alveolar-arterial Po₂ difference due to a lower arterial saturation. The persisting higher OD seen in older subjects, irrespective of the inspired O₂, shows that the measurement of OD remains sensitive to mild gas exchange impairment, even when breathing 21% O₂.

Validation of a Noninvasive Assessment of Pulmonary Gas Exchange During Exercise in Hypoxia

BACKGROUND

Pulmonary gas exchange efficiency, determined by the alveolar-to-arterial Po₂ difference (A-aDo₂), progressively worsens during exercise at sea-level; this response is further elevated during exercise in hypoxia. Traditionally, pulmonary gas exchange efficiency is assessed through measurements of ventilation and end-tidal gases paired with direct arterial blood gas (ABG) sampling. Because these measures have a number of caveats, particularly invasive blood sampling, the development of new approaches for the noninvasive assessment of pulmonary gas exchange is needed.

RESEARCH QUESTION

Is a noninvasive method of assessing pulmonary gas exchange valid during rest and exercise in acute hypoxia?

STUDY DESIGN AND METHODS

Twenty-five healthy participants (10 female) completed a staged maximal exercise test on a cycle ergometer in a hypoxic chamber (Fio₂ = 0.11). Simultaneous ABGs via a radial arterial catheter and noninvasive gas-exchange measurements (AGM100) were obtained in 2-minute intervals. Noninvasive gas exchange, termed the O₂ deficit, was calculated from the difference between the end-tidal and the calculated Pao₂ (via pulse oximetry and corrected for the Bohr effect by using the end-tidal Pco₂). Noninvasive O₂ deficit was compared with the traditional alveolar to arterial oxygen difference (A-aDo₂), using the traditional Riley analysis.

RESULTS

Under conditions of rest at room air, hypoxic rest, and hypoxic exercise, strong correlations between the calculated gPao₂ and directly measured Pao₂ (R² = 0.97; P < .001; mean bias = 1.70 mm Hg) were observed. At hypoxic rest and exercise, strong relationships between the estimated and directly measured Pao₂ (R² = 0.68; P < .001; mean bias = 1.01 mm Hg) and O₂ deficit with the traditional A-aDo₂ (R² = 0.70; P < .001; mean bias = 5.24 mm Hg) remained.

INTERPRETATIONS

Our findings support the use of a noninvasive measure of gas exchange during acute hypoxic exercise in heathy humans. Further studies are required to determine whether this approach can be used clinically as a tool during normoxic exercise in patients with preexisting impairments in gas exchange efficiency.