The researchers, from Boston Children's Hospital and Pendar Technologies, used resonance Raman spectroscopy to measure whether enough oxygen is reaching the mitochondria, the organelles that provide cells with energy.

In critically ill patients with compromised circulation or breathing, oxygen delivery is often impaired. And when a cell's oxygen levels are too low, electrons start to build up in certain cellular proteins - haemoglobin, myoglobin and mitochondrial cytochromes. This energy shift reduces or shuts down mitochondrial energy production and can also trigger cell death - setting the stage for organ injury or even cardiac arrest.

Raman spectroscopy works by detecting wavelength shifts in laser light shone onto tissue and inelastically scattered by - in this study - the mitochondrial proteins. Varying oxygenation conditions cause differences in the bending and stretching of chemical bonds in these proteins, scattering the light differently and thus generating distinct spectral signatures. The team used a regression algorithm to analyse the spectral information in real time.

"With current technologies, we cannot predict when a patient's heart will stop," says John Kheir, of Boston Children's Heart Center, who co-led the study. "We can examine heart function on the echocardiogram and measure blood pressure, but until the last second, the heart can compensate quite well for low oxygen conditions. Once cardiac arrest occurs, its consequences can be life-long, even when patients recover."

The team defined a metric, the resonance Raman reduced mitochondrial ratio (3RMR), that uses the Raman spectra to quantify oxygenation and mitochondrial function in real time. In tests on oxygen deprived rats undergoing open-heart surgery, the researchers found that 3RMR increased as mitochondria became oxygen deficient. Elevations of more than 40%, measured after 10 minutes of low-oxygen conditions, predicted cardiac arrest with 97% specificity and 100% sensitivity, outperforming all other measurement techniques.

The team further tested the device during heart surgery in a pig model. They showed that 3RMR values increased in pig hearts that received insufficient blood flow, then returned to baseline levels when the animals recovered.

"Our likely first application of this device will be to monitor oxygen delivery during and after heart surgery," says Kheir. The technology could also be used to monitor tissue viability during other types of surgery. Potential applications include monitoring organs intended for transplantation and detection of dangerously reduced blood flow in limbs.

The team's goal is to seek FDA approval and commercialize a bedside monitor of mitochondrial oxygenation. In the meantime, Kheir and colleagues plan to seek approval to test the device to monitor heart patients.