Noninvasive measurement of pulmonary gas exchange: comparison with data from arterial blood gases

Evaluation of agreement between a noninvasive method for real-time measurement of critical blood values with a standard point-of-care device

The purpose of this work was to investigate the degree of agreement between two distinct approaches for measuring a set of blood values and to compare comfort levels reported by participants when utilizing these two disparate measurement methods. Radial arterial blood was collected for the comparator analysis using the Abbott i-STAT® POCT device. In contrast, the non-invasive proprietary DBC methodology is used to calculate sodium, potassium, chloride, ionized calcium, total carbon dioxide, pH, bicarbonate, and oxygen saturation using four input parameters (temperature, hemoglobin, pO2, and pCO2). Agreement between the measurement for a set of blood values obtained using i-STAT and DBC methodology was compared using intraclass correlation coefficients, Passing and Bablok regression analyses, and Bland Altman plots. A p-value of <0.05 was considered statistically significant. A total of 37 participants were included in this study. The mean age of the participants was 42.4 ± 13 years, most were male (65%), predominantly Caucasian/White (75%), and of Hispanic ethnicity (40%). The Intraclass Correlation Coefficients (ICC) analyses indicated agreement levels ranging from poor to moderate between i-STAT and the DBC's algorithm for Hb, pCO2, HCO3, TCO2, and Na, and weak agreement for pO2, HSO2, pH, K, Ca, and Cl. The Passing and Bablok regression analyses demonstrated that values for Hb, pO2, pCO2, TCO2, Cl, and Na obtained from the i-STAT did not differ significantly from that of the DBC's algorithm suggesting good agreement. The values for Hb, K, and Na measured by the DBC algorithm were slightly higher than those obtained by the i-STAT, indicating some systematic differences between these two methods on Bland Altman Plots. The non-invasive DBC methodology was found to be reliable and robust for most of the measured blood values compared to invasive POCT i-STAT device in healthy participants. These findings need further validation in larger samples and among individuals afflicted with various medical conditions.

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.

Construção e validação do instrumento de verificação da adesão às diretrizes brasileiras de ventilação mecânica

Objetivo: construir e validar um instrumento para verificar a adesão às recomendações das Diretrizes Brasileiras de Ventilação Mecânica dos profissionais de saúde envolvidos na assistência aos pacientes em Ventilação Mecânica Invasiva. Método: estudo metodológico conduzido entre setembro e dezembro de 2019, em um hospital público com 87 pacientes. Adotou-se o Índice de Validação de Conteúdo para a validação de conteúdo, o Coeficiente de Correlação de Pearson para a validade de critério, o alfa de Cronbach para a consistência interna, o Coeficiente Kappa e o Coeficiente de Correlação Intraclasse para a confiabilidade interobservador. Resultados: o instrumento apresentou validade de conteúdo e consistência interna aceitável. A correlação de Pearson, indicou uma correlação do escore de adesão com a saturação (r=0,31; p≤0,005), o escore médio para o observador A e B resultou, respectivamente, em 88,89(±5,23) e 88,86(±5,34) e o intervalo de confiança foi de 0,96. Conclusão: o instrumento se mostrou válido e confiável para verificar a adesão às Diretrizes Brasileiras de Ventilação Mecânica dos profissionais.

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.

Estimating exercise PaCO2 in patients with heart failure with preserved ejection fraction

Patients with heart failure with preserved ejection fraction (HFpEF) exhibit cardiopulmonary abnormalities that could affect the predictability of exercise [Formula: see text] from the Jones corrected partial pressure of end-tidal CO2 (PJCO2) equation (PJCO2 = 5.5 + 0.9 × [Formula: see text] - 2.1 × VT). Since the dead space to tidal volume (VD/VT) calculation also includes [Formula: see text] measurements, estimates of VD/VT from PJCO2 may also be affected. Because using noninvasive estimates of [Formula: see text] and VD/VT could save patient discomfort, time, and cost, we examined whether partial pressure of end-tidal CO2 ([Formula: see text]) and PJCO2 can be used to estimate [Formula: see text] and VD/VT in 13 patients with HFpEF. [Formula: see text] was measured from expired gases measured simultaneously with radial arterial blood gases at rest, constant-load (20 W), and peak exercise. VD/VT[art] was calculated using the Enghoff modification of the Bohr equation, and estimates of VD/VT were calculated using [Formula: see text] (VD/VT[ET]) and PJCO2 (VD/VT[J]) in place of [Formula: see text]. [Formula: see text] was similar to [Formula: see text] at rest (-1.46 ± 2.63, P = 0.112) and peak exercise (0.66 ± 2.56, P = 0.392), but overestimated [Formula: see text] at 20 W (-2.09 ± 2.55, P = 0.020). PJCO2 was similar to [Formula: see text] at rest (-1.29 ± 2.57, P = 0.119) and 20 W (-1.06 ± 2.29, P = 0.154), but underestimated [Formula: see text] at peak exercise (1.90 ± 2.13, P = 0.009). VD/VT[ET] was similar to VD/VT[art] at rest (-0.01 ± 0.03, P = 0.127) and peak exercise (0.01 ± 0.04, P = 0.210), but overestimated VD/VT[art] at 20 W (-0.02 ± 0.03, P = 0.025). Although VD/VT[J] was similar to VD/VT[art] at rest (-0.01 ± 0.03, P = 0.156) and 20 W (-0.01 ± 0.03, P = 0.133), VD/VT[J] underestimated VD/VT[art] at peak exercise (0.03 ± 0.04, P = 0.013). Exercise [Formula: see text] and VD/VT[ET] provides better estimates of [Formula: see text] and VD/VT[art] than PJCO2 and VD/VT[J] does at peak exercise. Thus, estimates of [Formula: see text] and VD/VT should only be used if sampling arterial blood during CPET is not feasible.NEW & NOTEWORTHY [Formula: see text] provides a better estimate of [Formula: see text] than PJCO2 at peak exercise, and VD/VT[ET] provides a better estimate of VD/VT[art] than VD/VT[J] at peak exercise. Although we reported significant correlations, we did not find an identity between [Formula: see text] and estimates of [Formula: see text], nor did we find an identity between VD/VT[art] and estimates of VD/VT[art]. Thus, caution should be taken and estimates of [Formula: see text] and VD/VT should only be used if sampling arterial blood during CPET is not feasible.

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.

Temporal changes in pulmonary gas exchange efficiency when breath-hold diving below residual volume

NEW FINDINGS

What is the central question of this study? How does deep breath-hold diving impact cardiopulmonary function, both acutely and over the subsequent 2.5 hours post-dive? What is the main finding and its importance? Breath-hold diving, to depths below residual volume, is associated with acute impairments in pulmonary gas exchange, which typically resolve within 2.5 hours. These data provide new insight into the behaviour of the lungs and pulmonary vasculature following deep diving.

ABSTRACT

Breath-hold diving involves highly integrative and extreme physiological responses to both exercise and asphyxia during progressive elevations in hydrostatic pressure. Over two diving training camps (Study 1 and 2), 25 breath-hold divers (recreational to world-champion) performed 66 dives to 57 ± 20 m (range: 18-117 m). Using the deepest dive from each diver, temporal changes in cardiopulmonary function were assessed using non-invasive pulmonary gas exchange (indexed via the O₂ deficit), ultrasound B-line scores, lung compliance and pulmonary haemodynamics at baseline and following the dive. Hydrostatically induced lung compression was quantified in Study 2, using spirometry and lung volume measurement, enabling each dive to be categorized by its residual volume (RV)-equivalent depth. From both studies, pulmonary gas exchange inefficiency - defined as an increase in O₂ deficit - was related to the depth of the dive (r²  = 0.345; P < 0.001), with dives associated with lung squeeze symptoms exhibiting the greatest deficits. In Study 1, although B-lines doubled from baseline (P = 0.027), cardiac output and pulmonary artery systolic pressure were unchanged post-dive. In Study 2, dives with lung compression to ≤RV had higher O₂ deficits at 9 min, compared to dives that did not exceed RV (24 ± 25 vs. 5 ± 8 mmHg; P = 0.021). The physiological significance of a small increase in estimated lung compliance post-dive (via decreased and increased/unaltered airway resistance and reactance, respectively) remains equivocal. Following deep dives, the current study highlights an integrated link between hydrostatically induced lung compression and transient impairments in pulmonary gas exchange efficiency.

Case Studies in Physiology: Breath-hold diving beyond 100 meters-cardiopulmonary responses in world-champion divers

In this case study, we evaluate the unique physiological profiles of two world-champion breath-hold divers. At close to current world-record depths, the extreme physiological responses to both exercise and asphyxia during progressive elevations in hydrostatic pressure are profound. As such, these professional athletes must be capable of managing such stress, to maintain performing at the forefront human capacity. In both divers, pulmonary function before and after deep dives to 102 m and 117 m in the open sea was assessed using noninvasive pulmonary gas exchange (indexed via the O₂ deficit, which is analogous to the traditional alveolar to arterial oxygen difference), ultrasound B-line scores, airway resistance, and airway reactance. Hydrostatic-induced lung compression was also quantified via spirometry. Both divers successfully performed their dives. Pulmonary gas exchange efficiency was impaired in both divers at 10 min but had mostly restored within a few hours. Mild hemoptysis was transiently evident immediately following the 117-m dive, whereas both divers experienced nitrogen narcosis. Although B-lines were only elevated in one diver postdive, reductions in airway resistance and reactance occurred in both divers, suggesting that the compressive strain on the structural characteristics of the airways can persist for up to 3.5 h. Marked echocardiographic dyssynchrony was evident in one diver after 10 m of descent, which persisted until resolving at ∼77 m during ascent. In summary, despite the enormous hydrostatic and physiological stress to diving beyond 100 m on a single breath, these data provide valuable insight into the extraordinary capacity of those at the pinnacle of apneic performance.NEW & NOTEWORTHY This study shows that world-champion breath-hold divers demonstrate incredible tolerability to extreme levels of hydrostatic-induced lung compression. Immediately following dives to >100 m, there were acute impairments in pulmonary gas exchange efficiency, mild accummulation of extravascular lung fluid, noticable intrathoracic discomfort, and evident nitrogen narcosis, however, within a few hours, these had all mostly resolved.

Endothelial function and shear stress in hypobaric hypoxia: time course and impact of plasma volume expansion in men

High-altitude exposure typically reduces endothelial function, and this is modulated by hemoconcentration resulting from plasma volume contraction. However, the specific impact of hypobaric hypoxia independent of external factors (e.g., cold, varying altitudes, exercise, diet, and dehydration) on endothelial function is unknown. We examined the temporal changes in blood viscosity, shear stress, and endothelial function and the impact of plasma volume expansion (PVX) during exposure to hypobaric hypoxia while controlling for external factors. Eleven healthy men (25 ± 4 yr, mean ± SD) completed two 4-day chamber visits [normoxia (NX) and hypobaric hypoxia (HH; equivalent altitude, 3,500 m)] in a crossover design. Endothelial function was assessed via flow-mediated dilation in response to transient (reactive hyperemia; RH-FMD) and sustained (progressive handgrip exercise; SS-FMD) increases in shear stress before entering and after 1, 6, 12, 48, and 96 h in the chamber. During HH, endothelial function was also measured on the last day after PVX to preexposure levels (1,140 ± 320 mL balanced crystalloid solution). Blood viscosity and arterial shear stress increased on the first day during HH compared with NX and remained elevated at 48 and 96 h (P < 0.005). RH-FMD did not differ during HH compared with NX and was unaffected by PVX despite reductions in blood viscosity (P < 0.05). The stimulus-response slope of increases in shear stress to vasodilation during SS-FMD was preserved in HH and increased by 44 ± 73% following PVX (P = 0.023). These findings suggest that endothelial function is maintained in HH when other stressors are absent and that PVX improves endothelial function in a shear-stress stimulus-specific manner.NEW & NOTEWORTHY Using a normoxic crossover study design, we examined the impact of hypobaric hypoxia (4 days; altitude equivalent, 3,500 m) and hemoconcentration on blood viscosity, shear stress, and endothelial function. Blood viscosity increased during the hypoxic exposure and was accompanied by elevated resting and exercising arterial shear stress. Flow-mediated dilation stimulated by reactive hyperemia and handgrip exercise was preserved throughout the hypoxic exposure. Plasma volume expansion reversed the hypoxia-associated hemoconcentration and selectively increased handgrip exercise flow-mediated dilation.

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.

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

The efficiency of pulmonary gas exchange is often assessed by the ideal alveolar-arterial partial pressure difference (A-aDO₂). Through a combination of pulse oximetry and rapidly responding gas analyzers to measure the partial pressures of O₂ and CO₂ in expired gas, one can measure the oxygen deficit. Defined as the difference between the measured alveolar Po₂ and the arterial Po₂ calculated from SpO2, the oxygen deficit is a substitute for the alveolar-arterial Po₂ difference. The oxygen deficit is physiologically reasonable in that it increases with age in healthy subjects and is well correlated with the A-aDO₂. To calculate arterial Po₂ from saturation, the saturation should be below the very flat upper part of the O₂-Hb dissociation curve; good estimates can be made provided the arterial O₂ saturation is below ~95%. Since saturations at or above 95% imply reasonably well-maintained gas exchange efficiency, this limitation is of only minor concern. Calculations show that it is necessary to take into account the change in Po₂ at a saturation of 50% of the O₂-Hb dissociation curve based on the measured alveolar Pco₂. As the measurement is designed to be noninvasive, determination of any base excess is not practical, but calculations show that the effect of assuming a zero base excess is modest, with a similar small effect from an abnormal body temperature. Taken together, these results show that a noninvasive assessment of pulmonary gas exchange efficiency can be obtained from subjects with below-normal arterial O₂ saturations through a combination of expired O₂ and CO₂ measurements and SpO2 made during quiet breathing.NEW & NOTEWORTHY The details and limitations of a noninvasive measurement of pulmonary gas exchange efficiency, the oxygen deficit, are described. The oxygen deficit, calculated from expired gas measurements made during quiet breathing coupled with pulse oximetry, is a good surrogate measurement of the ideal alveolar-arterial Po₂ difference and does not require arterial blood gas sampling.