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Description: This study will investigate autonomic reactivity and end-tidal CO2 (etCO2) levels in patients with persistent somatic symptoms (PSS) compared to healthy controls. While acute and chronic stress, along with dysregulation in the autonomic nervous system, are believed to contribute to the genesis and perpetuation of persistent somatic/functional symptoms and syndromes, existing literature heavily relies on heart rate variability analysis and sympathetic skin response for assessing sympathetic function. However, breathing, being sensitive to both internal and external states, provides an avenue to explore how symptom perception may also contribute to symptom maintenance. Lower etCO2 levels have been associated with symptoms frequently observed in PSS, such as dizziness, nausea, fatigue, and heart palpitations, often linked to shallow and rapid breathing. Yet, following episodes of hyperventilation, whether acute or chronic, the body may persist in producing these symptoms as part of a feed-forward predictive behavioral mechanism. The current study therefore seeks to examine differential autonomic reactivity through a mild respiratory challenge and etCO2 levels in patients with PSS and healthy controls, alongside disparities in self-reported measures of breathing and objective outcomes between groups.