Detecting oxygenation changes after hypoxia: pulse oximetry vs. near-infrared spectroscopy

Introduction: Pulse oximetry is commonly used in critical care to monitor changes in arterial oxygen saturation (SpO2). However, studies have reported that decreases in SpO2 may lag behind the actual clinical event. Previous studies have demonstrated that cerebral oxygenation monitoring using near-infrared spectroscopy (NIRS) can detect alterations in oxygenation earlier than pulse oximetry. Here, we compare responses of NIRS monitoring of spinal cord tissue oxygenation (TOI) to pulse oximetry SpO2 during hypoxia. Methods: During a study on optical monitoring of spinal cord hemodynamics in an animal model of spinal cord injury (SCI), episodes of acute (70-80% SpO2) hypoxia were induced. Six anesthetized Yucatan miniature pigs were studied. A standard pulse oximeter was attached to the ear of the animal and a custom-made NIRS sensor was placed extradurally on the spinal cord. Hypoxia was induced by removing the ventilator from the animal and reattaching it once SpO2 reached 70% or 80% as reported by the pulse oximeter. Results: 21 episodes of acute hypoxia were analyzed. Upon the start of hypoxia, NIRS TOI responded in 1.8 ± 0.5 seconds, while pulse oximetry SpO2 responded in 11.4 ± 0.6 seconds (p > 0.0001). Conclusion: NIRS can detect the effects of hypoxia on spinal cord tissue earlier than pulse oximetry can detect arterial oxygenation changes in the periphery. The NIRS sensor may be used as an earlier detector of oxygen saturation changes in the clinical setting than the standard pulse oximeter.