Aneurysmal subarachnoid hemorrhage (aSAH) is a severe type of stroke associated with high mortality and morbidity. Although neurocritical care has improved over recent decades, delayed cerebral ischemia (DCI) remains a common yet difficult-to-predict complication with current clinical tools. Affecting approximately 30% of patients, it contributes substantially to poor outcome.

The aim of this thesis is to investigate cerebrovascular reactivity (CVR) as a method for assessing cerebral blood flow regulation and identifying patients at risk of ischemic complications after aSAH.

To enable studies across the full patient population, a novel method for controlled carbon dioxide challenges during both mechanical ventilation and spontaneous breathing was developed and applied in subsequent work. CVR measured with magnetic resonance imaging was evaluated in healthy volunteers to systematically investigate methodological factors affecting repeatability. Finally, CVR was assessed using transcranial Doppler ultrasound and magnetic resonance imaging in patients with aSAH to evaluate feasibility, safety, and potential clinical utility.

The developed system for controlled carbon dioxide challenges proved reliable and robust in achieving predefined stimulus levels during both mechanical ventilation and spontaneous breathing. Several important methodological considerations affecting the performance and interpretation of CVR measurements were identified. Magnetic resonance-based CVR using controlled inspired carbon dioxide was shown to be feasible in sedated and mechanically ventilated patients with aSAH. Preliminary results further suggest that CVR measurements may allow monitoring of ischemic evolution and early detection of DCI following aSAH.

By developing a method for controlled carbon dioxide challenges in mechanically ventilated patients, this work enables CVR investigations in previously understudied patient populations beyond aSAH. The systematic methodological investigations also contribute to greater transparency and standardization within the field. Although the clinical utility of CVR for risk stratification after aSAH could not be fully established, this thesis provides a methodological framework for future studies.