Sleep apnea causes involuntary stops in breathing during sleep, up to 400 times a night. About 24 per cent of men and 9 per cent of women experience sleep apnea symptoms. People with this condition are at greater risk for stroke. Normally, we take in oxygen when we inhale and expel carbon dioxide when we exhale. During an apnea episode, breathing temporarily stops, so oxygen is not taken in and carbon dioxide accumulates. When this occurs, blood vessels in the brain expand due to an increase of carbon dioxide in the brain, which leads to an improvement in blood flow reducing the chance of brain damage from insufficient oxygen. However, this mechanism becomes less sensitive with a repeated lack of oxygen or exposure to higher than normal levels of carbon dioxide. Jordan Querido is investigating the combined effect of low oxygen and high carbon dioxide to determine which plays a greater role in decreasing the expansion of blood vessels and increasing the risk of stroke. Querido will also examine whether these repetitive exposures lessen the blood vessels’ ability to dilate during exercise, when extra oxygen is needed. An exercise program is often prescribed for sleep apnea patients, as most are overweight. This research will help to clarify whether patients are at greater risk of stroke during exercise, with the goal of designing safe exercise rehabilitation programs for sleep apnea patients.