Continuous, noninvasive, and localized microvascular tissue oximetry using visible light spectroscopy

Background The authors evaluated the ability of visible tight spectroscopy (VLS) oximetry to detect hypoxemia and ischemia in human and animal subjects. Unlike near-infrared spectroscopy or pulse oximetry (Spo(2)), VLS tissue oximetry uses shallow-penetrating visible light to measure microvascular hemoglobin oxygen saturation (Sto(2)) in small, thin tissue volumes. Methods: In pigs, Sto(2) was measured in muscle and enteric mucosa during normoxia, hypoxemia (Spo(2) = 40-96%), and ischemia (occlusion, arrest). In patients, Sto(2) was measured in skin, muscle, and oral/enteric mucosa during normoxia, hypoxemia (SPo2 = 60-99%), and ischemia (occlusion, compression, ventricular fibrillation). Results: In pigs, normoxic Sto(2) was 71 +/- 4% (mean +/- SD), without differences between sites, and decreased during hypoxemia (muscle, 11 +/- 6%; P < 0.001) and ischemia (colon, 31 +/- 11%; P < 0.001). In patients, mean normoxic Sto(2) ranged from 68 to 77% at different sites (733 measures, 111 subjects); for each noninvasive site except skin, variance between subjects was low (e.g., colon, 69% +/- 4%, 40 subjects; buccal, 77% +/- 3%, 21 subjects). During hypoxemia, Sto(2) correlated with Spo(2) (animals, r(2) = 0.98; humans, r(2) = 0.87). During ischemia, Sto(2) initially decreased at -1.3 +/- 0.2%/s and decreased to zero in 3-9 min (r(2) = 0.94). Ischemia was distinguished from normoxia and hypoxemia by a widened pulse/VLS saturation difference (Delta < 30% during normoxia or hypoxemia vs. Delta > 35% during ischemia). Conclusions: VLS oximetry provides a continuous, noninvasive, and localized measurement of the Sto(2), sensitive to hypoxemia, regional, and global ischemia. The reproducible and narrow Sto(2) normal range for oral/enteric mucosa supports use of this site as an accessible and reliable reference point for the VLS monitoring of systemic flow.