In a development that could reshape how clinicians track one of the most elusive vital signs in critical care medicine, researchers at Uppsala University have shown that a small optical sensor pressed against the breastbone can reliably follow changes in mixed venous oxygen saturation, a measurement that normally requires threading a catheter deep into the heart and lungs. The proof-of-concept study, conducted in anesthetized pigs and published in Medical & Biological Engineering & Computing, demonstrates that photoplethysmography, the same light-based principle that powers the fingertip pulse oximeter, can be adapted to sense the oxygen content of blood flowing through the marrow of the sternum, offering a non-invasive window on the balance between oxygen delivery and oxygen consumption across the entire body.
Mixed venous oxygen saturation, abbreviated SvO2, is measured in blood taken from the pulmonary artery, the vessel that carries deoxygenated blood from the right side of the heart back to the lungs. Because that blood is the confluence of venous return from every tissue bed in the body, its oxygen saturation represents a weighted average of how much oxygen the whole organism has extracted from what the circulation delivered. When supply outstrips demand, SvO2 rises; when tissues are starved of perfusion or metabolically stressed, it falls. This makes SvO2 a uniquely informative parameter in conditions such as circulatory shock, sepsis, major heart surgery, and the optimization of fluid therapy and vasoactive drugs. Yet despite its clinical value, SvO2 remains a rarity at the bedside. Obtaining it demands advanced cannulation of intrathoracic vessels through a balloon-tipped pulmonary artery catheter, a procedure that carries real risks of complications, including arterial rupture, and requires equipment that many standard intensive care units do not possess.
The Swedish team, led by Erik Näslund together with Gunnar Strandberg, Jacob Karlsson, Stephanie Franzén and Robert Frithiof, set out to test whether this invasive gold standard could be approximated from the outside. Their approach builds on earlier work from the group showing that the oxygen saturation of blood within sternal bone marrow, a compartment they call SsO2, tracks changes in global oxygenation more closely than arterial saturation does. The sternum occupies a privileged anatomical position: it houses actively perfused red marrow, receives blood from multiple arteries, features a mixed arteriovenous flow with a sinusoidal microcirculation, and sits centrally enough that changes in whole-body oxygen balance register there quickly. It is also, crucially, easily accessible through intact skin.
The probe itself is a wireless device developed by the Swedish company RespiHeart AB, which loaned the hardware to the investigators but took no part in the study design, analysis, or interpretation. The sensor emits red light at 660 nanometers and infrared light at 810 nanometers into the caudal, flat surface of the sternum in reflectance mode, catching the returning photons with a photodiode positioned 22.5 millimeters away. That unusually wide separation between source and detector is deliberate: it allows light to penetrate far deeper into tissue than the geometry of a conventional fingertip pulse oximeter permits, reaching the blood-rich marrow within the bone. A start-up routine automatically adjusts light intensity and detector gain to maximize signal quality, and the 100-hertz raw signal streams over Bluetooth to a tablet, where a second-order Butterworth filter with a 0.4-hertz cutoff isolates the non-pulsatile DC component of the photoplethysmographic waveform.
Where a standard pulse oximeter calculates its saturation quotient from the ratio of pulsatile (AC) to steady (DC) signal components, thereby isolating the arterial pulse, the sternal probe does something different: it forms a quotient value, dubbed SQV, from the DC components of the red and infrared channels alone. This means the device measures the total light absorbance of the illuminated tissue volume, an approach that edges toward near-infrared spectroscopy methodology while retaining the pulsatile AC signal, which can be harvested separately to derive pulse and respiratory rate. Because the automatic gain adjustment made each animal’s starting quotient converge near one regardless of its actual saturation, a single population-wide calibration curve was not feasible. Instead, the researchers calibrated each subject individually, regressing SQV against SvO2 values obtained simultaneously from pulmonary artery blood samples analyzed on a porcine-calibrated ABL800 CO-oximeter. The resulting estimates of non-invasive sternal oxygen saturation, which the team named nSsO2, were computed as nSsO2 equals intercept plus slope times SQV for each animal.
The experimental protocol subjected fifteen Norwegian Landrace/Hampshire/Yorkshire pigs, averaging 33.1 kilograms, to two distinct physiological challenges. In the first, progressive hypoxemia was induced by incrementally enriching the inspired gas with nitrogen, driving peripheral pulse oximetry down through plateaus of 95 to 100, 85 to 90, 80 to 85, and finally 70 to 80 percent, with paired arterial and mixed venous blood samples drawn at each stage. After a recovery period, the second challenge delivered controlled hemorrhage: first a withdrawal of 20 percent of the estimated circulating blood volume of 70 milliliters per kilogram, followed by another 20 percent five minutes later. Cardiac output was measured by thermal dilution, and blood gases were sampled at baseline, immediately after each bleed, and five minutes thereafter. One animal could not complete the protocol due to hemodynamic instability; the remaining fourteen yielded 177 valid paired measurements between SQV and SvO2.
The statistical architecture reflected the repeated-measures nature of the data. A linear mixed-effects model treated SQV and SvO2 as fixed effects with subject-specific slopes and intercepts as random effects, yielding a conditional R-squared of 0.71, meaning that 71 percent of the variance in SvO2 was explained once between-animal differences were accounted for. The marginal R-squared, reflecting only the fixed effects, was 0.37, a figure the team attributes largely to the individualized calibration imposed by the probe’s automatic start-up routine. Ordinary least-squares regression on each animal’s raw data confirmed strong individual linear relationships, and a subanalysis revealed that when two outlier animals were excluded, the remaining subjects shared a statistically indistinguishable common slope, a finding with encouraging implications for future universal calibration curves.
The more clinically consequential test was whether nSsO2 could track the direction of change in SvO2, not merely correlate with it at rest. Using four-quadrant concordance analysis, the standard method for validating trend monitors, and applying a 10 percent exclusion zone derived from the known precision of the reference CO-oximetry method, the researchers found that nSsO2 moved in the same direction as SvO2 in 87 percent of qualifying data pairs (95 percent confidence interval, 79.1 to 95.0). Breaking the results down by provocation, concordance reached 90.6 percent during hypoxemia and 83.8 percent during hemorrhage. Although this fell short of the conventional 92 percent threshold for excellent trending, the pooled result approached it within statistical uncertainty, and a post hoc sensitivity analysis excluding the most severe, 40 percent blood-loss data points pushed concordance to 92.2 percent.
That divergence during profound hemorrhage is itself physiologically revealing. The authors argue that SvO2, as a global weighted average, responds acutely to the surge in systemic oxygen extraction triggered by rapid blood loss and the accompanying neurohumoral activation, whereas the sternal signal reflects a localized marrow compartment whose highly vascularized, slow-transit sinusoidal microcirculation, and whose preferential perfusion under sympathetic blood-flow redistribution, may buffer its oxygen saturation against rapid decline. The DC-based optical method may also lose sensitivity when local blood volume shrinks, since less hemoglobin within the sampled tissue means smaller proportional changes in the light-absorbance ratio, producing a plateau despite continued systemic desaturation. Notably, arterial saturation remained constant throughout the bleeding, so arteriovenous admixture within the bone may likewise have attenuated the sternal signal’s fall.
The team is careful to position the technology correctly. Sternal photoplethysmography does not measure SvO2 directly; it reflects regional tissue oxygenation in the sternum, much as cerebral near-infrared spectroscopy reflects regional brain oxygenation, and the two quantities cannot be considered interchangeable in absolute terms. But the consistency of the individual SQV–SvO2 slopes suggests that, for trend monitoring, the method captures clinically meaningful swings in global oxygen utilization. The researchers envision the probe as a parallel complement to conventional pulse oximetry: if both readings fall together, hypoxic hypoxemia is the likely culprit, but if the finger probe holds steady while the sternal signal drops, that divergence could signal rising tissue oxygen extraction, as in occult hypovolemia, potentially hours before basic circulatory monitors sound an alarm.
Significant hurdles remain before the sternum sensor reaches human patients. The animals were young pigs whose sternal anatomy, ossification, subcutaneous tissue thickness, and high metabolic turnover differ from adult humans, and the probe was not explicitly designed for porcine geometry; some animals even displayed chest-wall deformities resembling pectus excavatum that may have degraded light coupling. The automatic calibration that forced each subject’s starting quotient near one currently precludes comparisons between individuals, and the authors acknowledge that recreating a universal calibration curve will likely require rethinking that feature, perhaps by widening the source-detector separation further to increase penetration depth. Conditions beyond hypoxemia and hemorrhage, including early sepsis, fever, and thyroid dysfunction, have yet to be tested, and how the calibration translates to awake, unanesthetized patients remains unknown.
Still, the central message of the study stands: a sensor resting on intact skin over the breastbone, using nothing more exotic than two wavelengths of light and a Bluetooth link, captured the direction and rough magnitude of changes in a physiological variable that medicine has long tethered to pulmonary artery catheters. For the intensive care units, operating theaters, and trauma bays where early recognition of deteriorating oxygen balance can decide outcomes between life and death, the sternum may prove to be the most informative few square centimeters of the body, and it has been sitting in plain sight all along.
Subject of Research: Animals; Anesthesia; Blood; Heart Disease; Shock; Trauma
Subject of Research: Technology and Engineering
Article Title: Monitoring changes in mixed venous oxygen saturation using photoplethysmography on the sternum – a proof of concept study
Article References: Näslund, E., Strandberg, G., Karlsson, J., Franzén, S., & Frithiof, R. (2026). Monitoring changes in mixed venous oxygen saturation using photoplethysmography on the sternum – a proof of concept study. Medical & Biological Engineering & Computing. https://doi.org/10.1007/s11517-026-03648-3
Image Credits: AI Generated
DOI: 10.1007/s11517-026-03648-3
Keywords: Mixed venous oxygen saturation, photoplethysmography, sternum, non-invasive monitoring, hypovolemia, hypoxemia, critical care, pulse oximetry, near-infrared spectroscopy, bone marrow oxygenation, concordance analysis, linear mixed-effects model
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Denise Maddox. (September 10, 2026). Sternum photoplethysmography shows promise for tracking mixed venous oxygen saturation. Scienmag. https://scienmag.com/sternum-photoplethysmography-shows-promise-for-tracking-mixed-venous-oxygen-saturation/
Denise Maddox. “Sternum photoplethysmography shows promise for tracking mixed venous oxygen saturation.” Scienmag, 10 September 2026, https://scienmag.com/sternum-photoplethysmography-shows-promise-for-tracking-mixed-venous-oxygen-saturation/. Accessed 10 September 2026.
Denise Maddox. “Sternum photoplethysmography shows promise for tracking mixed venous oxygen saturation.” Scienmag. September 10, 2026. https://scienmag.com/sternum-photoplethysmography-shows-promise-for-tracking-mixed-venous-oxygen-saturation/
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