A History of Mild Traumatic Brain Injury Affects Peripheral Pulse Oximetry during Normobaric Hypoxia

Physiological and emotional stressors increase symptoms of concussion in recently injured individuals and both forms of stress-induced symptoms in people recovering from mild traumatic brain injury (mTBI), but who are asymptomatic when not stressed or are at rest.

Healthy asymptomatic adults (25.0 ± 5.1 years) with a history of mTBI (n = 36) and matched healthy controls (HC) (n = 36) with no mTBI history were exposed to three levels of normobaric hypoxic stress generated with the Reduced Oxygen Breathing Device (ROBD) (Environics, Inc., Tollande, CT, USA), which reduced the percent O₂ by mixing sea level air with nitrogen. The ROBD reduced the percent O₂ in the breathable air from the normal 21% to 15.5% O₂, 14% O₂, and 13% O₂. Under these conditions: (a) a standard pulse oximeter recorded peripheral oxygen saturation (SpO₂) and pulse rate (beats per minute) and (b) the Functional Impairment Tester (FIT) (PMI, Inc., Rockville, MD, USA) recorded saccadic velocity and pupillary response dynamics to a brief light flash.

For all three hypoxic stress conditions, the mTBI group had significantly higher SpO₂ during the final minute of exposure than did the controls [F(2.17,151.8) = 5.29, p < 0.001, η² = 0.852] and the rate of SpO₂ change over time was significantly shallower for the mTBI than for the controls [F(2.3,161.3) = 2.863, p < 0.001, η² = 0.569], Greenhouse-Geisser corrected. Overall, mTBI had lower pulse rate but the difference was only significant for the 14% O₂ condition. FIT oculomotor measures were not sensitive to group differences. When exposed to mild or moderate normobaric hypoxic stress (15% O₂): (1) SpO₂ differences emerged between the mTBI and matched HC groups, (2) heart rate trended lower in the mTBI group, and (3) FIT measures were not sensitive to group differences.

A relatively minor hypoxic challenge can reveal measurable differences in SpO₂ and heart rate in otherwise asymptomatic individuals with a history of mTBI.